Schizophrenia Research Forum - A Catalyst for Creative Thinking

Hidden Complexity Seen in Serotonin Signaling

29 January 2008. Two recent papers out in PNAS focus on G protein-coupled serotonin receptors as targets for psychoactive drugs. Like other GPCRs, the serotonin receptors wire into multiple downstream signaling pathways, G protein-dependent and -independent. Which road they take depends on the cells in which they reside. Now, researchers are beginning to understand that the nature of the ligand can also influence the choice of signaling pathways. This concept is dramatically illustrated in work from Laura Bohn’s lab at Ohio State University College of Medicine, Columbus, Ohio, which reveals that serotonin (5-hydroxytryptamine, 5-HT) and a synthetic mimic act at the 5-HT2A receptor in vivo to trigger different signaling cascades. Although both ligands elicit the same behavioral response in mice, the effects of 5-HT require the β-arrestin-2 accessory protein, while the synthetic ligand uses a β-arrestin-independent pathway. The findings have major implications for drug discovery aimed at GPCRs: on the one hand, they suggest that all agonists are not created equal, and that current screening paradigms for GPCR agonists may misjudge their in vivo biological activities. On the other hand, the findings open up new possibilities for fine-tuning the specificity of GPCR agonists using ligands that target different signaling pathways.

The serotonin receptor is of interest in schizophrenia, where many atypical antipsychotic drugs antagonize the 5-HT2A receptor in addition to the dopamine D2 receptor. It has been proposed that these drugs’ actions include synergistic effects on the two systems. The second paper, from Karel Vales and Jan Bures of the Institute of Physiology at the Czech Academy of Science in Prague, carves out a possible role for the 5-HT2A/2C receptor subtypes in a specific action of antipsychotic drugs to improve cognitive symptoms in schizophrenia. They suggest that their work could explain why some antipsychotics appear more effective against cognitive symptoms of schizophrenia than others do.

A multitasking receptor
In the first paper, Bohn and coauthors Cullen Schmid and Kirsten Raehal set out to understand the signaling pathways behind the hallucinogenic effects of 5-HT2A agonists. (The receptor is the target for LSD, among other hallucinogens.) In particular, they wanted to find the role of β-arrestin, a scaffolding protein that associates with GPCRs and can modulate their responses to a ligand both positively and negatively. To do this, the investigators used a mouse model where 5-HT2A agonists cause a head twitch response to compare the action of serotonin with the synthetic hallucinogenic agonist, 2,5-dimethoxy-4-iodoamphetamine (DOI) in β-arrestin-2 knockout mice.

The core finding of the paper is that βARR2 knockout abolished the head twitch response to serotonin, but not to DOI, which surprisingly produced a similar head twitch in both wild-type and knockout mice. “These findings suggest that β-arrestin-2 mediates 5-HT-induced head twitches, whereas DOI produces this behavior in a β-arrestin-independent manner,” the authors write.

How can this happen? β-arrestins promote GPCR internalization, and the researchers found that the knockout mice show more receptors on the cell surface than in the cytoplasm relative to normal mice. In knockout mouse embryo fibroblasts, serotonin did not induce internalization of a transfected 5-HT2A receptor, while DOI did. Schmid and colleagues also looked at activation of the map kinase ERK, which is mediated by β-arrestin. Serotonin robustly activated ERK in wild-type cells, but to a much lower extent in knockouts. DOI activates EFK far less in wt cells, and the activation was barely affected by β-arrestin knockout. DOI activation of ERK went through phospholipase C rather than β-arrestin. These results held up in vivo, where the investigators showed that serotonin did not induce ERK activation in the frontal cortex of knockout mice, while DOI activated ERK in both wild-type and β-arrestin lacking mice.

“Schmid et al. present a compelling set of observations that can only be explicated by the emerging concept of functional selectivity,” write Atheir Abbas of Case Western Reserve University School of Medicine, Cleveland, Ohio, and Bryan Roth, University of North Carolina, Chapel Hill, in an accompanying perspective. The work exemplifies the emerging concept that cellular context is critical to determine the action of drugs at GPCRs, which are themselves conformational assemblages of receptor proteins and associated signaling molecules. “Given that ligands apparently induce differential patterns of signal transduction, it should be possible to design drugs that stabilize these unique conformational entities,” they write.

With a multiplicity of signaling events triggered by a single GPCR, drug developers might be able to pick and choose ligands that activate desired pathways while leaving others alone. “One can envision a scenario whereby drugs are identified via differential actions at a panel of functional readouts, yielding compounds with enhanced therapeutic efficiency and fewer side effects,” Abbas and Roth write.

The β-arrestins themselves can be drug targets, as shown in a recent study. That work demonstrated that lithium acts to disrupt a β-arrestin-2 signaling complex involving the Akt kinase (see SRF related news story). In the case of 5-HT2AR, Schmid and colleagues speculate that the receptor-arrestin association may set the tone for serotonin sensitivity. Therefore, modulating that interaction might be used to tweak the receptor response to endogenous serotonin or to pharmacological agents.

In the schizophrenia arena, clozapine and several other antipsychotics are 5-HT2AR antagonists, which leads Schmid et al. to conclude that their work “could have major implications in drug development for treating neuropsychiatric disorders such as depression and schizophrenia.” They note that clozapine induces 5-HT2AR internalization but not ERK activation in several systems, and speculate that clozapine may act by preventing downstream ERK1/2 activation, without preventing receptor internalization. The role of internalization versus ERK activation in determining the actions of clozapine remains to be determined.

Separating serotonergic and dopaminergic effects
The second paper directly addresses the action of antipsychotic drugs at 5-HT2A receptors, and proposes a link between that activity and improved cognitive symptoms in schizophrenia. Inhibition of 5-HT2A/2C receptors is thought to improve cognitive function, but the extent to which atypical antipsychotic drugs improve cognitive symptoms in patients is a matter of some debate (see SRF related news story). In their PNAS paper, first author Vera Bubenikova-Valesova of the Prague Psychiatric Center and colleagues studied the effects of three compounds in a rat model of memory impairment that they have developed to mimic the cognitive defects seen in schizophrenia. The model involves administration of the NMDA receptor antagonist MK-801 (dizocilpine), which produces psychosis and is proposed to be a model for schizophrenia, and testing of the rats in what the investigators call the active allothetic place avoidance task. The task puts rats on a rotating platform in which one quadrant delivers a mild electric shock. By using visual cues in the room, and ignoring irrelevant cues within the test arena, the rats learn to locate and avoid the shock area. The investigators believe performance in this test, with its competing stimuli, depends on a type of information processing disturbed in schizophrenia.

As previously shown by the same group, MK-801 treatment decreased the rats’ ability to avoid the shock part of the platform. The 5-HT2A/2C antagonist ritanserin blocked MK-801-induced cognitive impairment, while the dopamine D2 antagonist haloperidol did not. A mixed antagonist that hits both receptors—the atypical antipsychotic risperidone—also improved the rats’ cognitive performance. “Our results show that treatment with 5-HT2A/2C receptor antagonists, regardless of their activity at the D2 receptor, blocked the cognitive impairment induced by MK-810,” the authors conclude. Interestingly, in control rats (not treated with MK-801) the D2 blocker alone resulted in cognitive impairment, in agreement with previous studies. They did not see this effect with ritanserin, suggesting a specific effect of D2 receptor antagonism.

The results indicate that risperidone, an antipsychotic with high affinity for both the 5-HT2A/2C and D2 dopamine receptors, can give measurable cognitive improvement in this rat model of a schizophrenia cognitive deficit. The results agree with the idea that second-generation antipsychotics like clozapine treat a wider range of symptoms than specific D2 blockers like haloperidol because they affect the serotonergic system as well, although this has yet to be confirmed as a clinical benefit for any drug except clozapine (see SRF related news story). The model may be useful for testing the cognitive effects of other antipsychotic medications, the authors propose.—Pat McCaffrey.

References:
Schmid CL, Raehal KM, Bohn LM. Agonist-directed signaling of the serotonin 2A receptor depends on {beta}-arrestin-2 interactions in vivo. Proc Natl Acad Sci U S A. 2008 Jan 22;105(3):1079-84. Epub 2008 Jan 14. Abstract

Bubenikova-Valesova V, Stuchlik A, Svoboda J, Bures J, Vales K. Risperidone and ritanserin but not haloperidol block effect of dizocilpine on the active allothetic place avoidance task. Proc Natl Acad Sci U S A. 2008 Jan 22;105(3):1061-6. Epub 2008 Jan 14. Abstract

Abbas A, Roth BL. Arresting serotonin. Proc Natl Acad Sci U S A. 2008 Jan 22;105(3):831-2. Epub 2008 Jan 14. No abstract available. Abstract

Comments on News and Primary Papers
Comment by:  Patricia Estani
Submitted 23 February 2008
Posted 26 February 2008
  I recommend the Primary PapersComment by:  Atheir Abbas
Submitted 25 February 2008
Posted 27 February 2008
  I recommend the Primary Papers

Implicit in the findings of Schmid et al. is the idea that the relationship among ligand, receptor signaling, and cellular context is an extremely complex one that will take a great deal more work to tease out. Thus, Dr. Bryan Roth has proposed on a number of occasions (see, for example, Gray and Roth, 2007; Abbas and Roth, 2005) that novel approaches for drug discovery may prove more effective in producing schizophrenia drugs that have greater therapeutic efficacy with lower side effect liability. Since it will likely be many years before the field has a detailed understanding of the "nitty-gritty" of the receptor signaling and trafficking relevant to schizophrenia and its treatment, we have suggested a number of approaches that are less reliant on such information.

For example, approaches based on screening for drugs that either mimic the gene expression profiles of gold standard drugs such as clozapine or normalize schizophrenia-associated changes in gene expression are being explored. Another approach is behavior-based screening, in which targeted screens are performed with drugs to find those that have efficacy in animal disease models. A further related approach, exemplified by Psychogenics' Smartcube(TM) (the associated database is called Smartbase[TM]) involves injecting drugs and monitoring the resulting behavior using computer-based machine learning to generate a multidimensional behavioral signature for gold standard drugs. Drugs can then be screened to look for those that mimic gold standard drugs in terms of their signatures. Though Psychogenics does not appear to have done much (at least publicly) with this approach, it represents the sort of innovative thinking that may prove fruitful in future behavior-based drug discovery efforts since it is not dependent on knowing anything about the mechanism. In the end, at least in the near future, we believe such approaches may prove extremely useful in drug discovery efforts since they do not rely on extensive mechanistic knowledge of the processes underlying schizophrenia.

References:

Gray JA, Roth BL. The pipeline and future of drug development in schizophrenia. Mol Psychiatry. 2007 Oct ;12(10):904-22. Abstract

Abbas A, Roth B. Progress towards better understanding and treatment of major psychiatric illnesses. Drug Discov Today. 2005 Jul 15;10(14):960-2. Abstract

View all comments by Atheir Abbas

Comments on Related News


Related News: CATIE Comes To Surprising Conclusions

Comment by:  Daniel Weinberger, SRF Advisor
Submitted 18 October 2005
Posted 18 October 2005

The Lieberman et al. CATIE study is a landmark large-scale clinical trial of antipsychotic drug therapy and will generate considerable discussion in the coming months. It offers important insights about real-world treatment of individuals with the diagnosis of schizophrenia, in the sense of typical practices in clinics around the country and the clinical experience of many practitioners. It probably comes as no surprise that the response to available antipsychotic agents is suboptimal and that differences between drugs are not dramatic in many cases.

One of the questions that comes to my mind about the results is whether and to what degree they are generalizable. Do the results of this study accurately characterize the effects of these drugs across the spectrum of patients with chronic schizophrenia who are treated with them? In other words, are the patients in the CATIE trial representative of the patients with chronic schizophrenia who are in need of these medications? I believe there are several indicators to suggest that they may not be. First, of the subjects in this trial, most of whom (75 percent) were male, 40 percent had been or were married. Second, the mean age at first antipsychotic treatment was 26 years. Third, 30 percent of the subjects were on no medication when they entered the trial. These are all somewhat atypical characteristics in my experience, especially for a predominantly male sample.

In the NIMH schizophrenia genetic study that I direct, we have extensively evaluated over 600 subjects with schizophrenia from around the country. In our sample, the mean age at first antipsychotic treatment is 21 and the ever-married rate is 15 percent, and our sample is one-third female. Moreover, less than 10 percent of our sample is unmedicated at the time that they are evaluated. The finding that a mean dose of 20 mg of perphenazine was as effective as other medications also is somewhat surprising in my experience, as having used this drug for many years, I have rarely seen chronic, actively symptomatic patients respond well without dosing around 32 milligrams and above. Is it possible that the CATIE trial inadvertently enrolled patients more in the schizophrenia spectrum end of the distribution of patients receiving these drugs who may tend not to show as clear benefit? Or maybe the size and breadth of the CATIE trial obscured the signal from the more classic patient with schizophrenia for whom antipsychotic treatment is essential.

It will be interesting to see whether other academic schizophrenia centers concur with the demographics of my experience as noted above or those of CATIE. Multicenter studies—and CATIE involved 57 centers each contributing relatively small samples over a 2-year period—are susceptible to dilution effects and to the possibility that the sample is clinically "noisy." It will be interesting to see, when data analyses from the next stages appear, whether differences are found in the results from different centers who participated in the trial. Will CATIE have told the story of how these drugs work in patients who receive them, or will it have failed to identify the signal from the noise?

View all comments by Daniel Weinberger

Related News: CATIE Comes To Surprising Conclusions

Comment by:  Scott Hemby
Submitted 19 October 2005
Posted 19 October 2005
  I recommend the Primary Papers

Related News: CATIE Comes To Surprising Conclusions

Comment by:  David Lewis, SRF Advisor
Submitted 19 October 2005
Posted 19 October 2005
  I recommend the Primary Papers

Related News: CATIE Comes To Surprising Conclusions

Comment by:  Max Schubert
Submitted 19 October 2005
Posted 19 October 2005
  I recommend the Primary Papers

I also have not seen the response at that dose of perphenazine and even the atypical antipsychotics in chronic schizophrenics. In fact, the only medication that seemed to have an adequate "real-life" dose was olanzapine.

View all comments by Max Schubert

Related News: CATIE Comes To Surprising Conclusions

Comment by:  Iulian Iancu
Submitted 20 October 2005
Posted 20 October 2005
  I recommend the Primary Papers

It seems that the doses used are not equivalent, and the researchers have used somewhat lower doses of perphenazine and risperidone (in favor of olanzapine). Thus, it is obvious that perphenazine and risperidone have showed smaller efficacy.

View all comments by Iulian Iancu

Related News: CATIE Comes To Surprising Conclusions

Comment by:  Xiang Zhang
Submitted 20 October 2005
Posted 21 October 2005
  I recommend the Primary Papers

There is evidence that the Chinese traditional medicines may be an alternative approach in the treatment of schizophrenia. Our recent studies indicate that the extraction of gingko biloba may increase the effectiveness of antipsychotic drugs, but reduce their side effects. This finding may provide a new clue to develop a novel therapeutic drug for treatment of schizophrenia.

References:
1. Zhang XY, Zhou DF, Zhang PY, Wu GY, Su JM, Cao LY. A double-blind, placebo-controlled trial of extract of Ginkgo biloba added to haloperidol in treatment-resistant patients with schizophrenia. Journal of Clinical Psychiatry. 2001; 62(11):878-83. Abstract

2. Zhang XY, Zhou DF, Su JM, Zhang PY. The effect of extract of ginkgo biloba added to haloperidol on superoxide dismutase in inpatients with chronic schizophrenia. Journal of Clinical Psychopharmacology 2001;21(1):85-88. Abstract

View all comments by Xiang Zhang

Related News: CATIE Comes To Surprising Conclusions

Comment by:  Alonso Montoya
Submitted 21 October 2005
Posted 21 October 2005
  I recommend the Primary Papers

Related News: CATIE Comes To Surprising Conclusions

Comment by:  Alexander Miller
Submitted 21 October 2005
Posted 21 October 2005
  I recommend the Primary Papers

Related News: CATIE Comes To Surprising Conclusions

Comment by:  Marvin Swartz
Submitted 26 October 2005
Posted 26 October 2005

Reply to Dr. Weinberger's questions about the generalizability of the CATIE sample, by Marvin Swartz, for the CATIE investigators
As CATIE investigators, we have been mindful of concerns about the generalizability of the CATIE sample. In response to a similar concern, our colleague Jeffrey Swanson at Duke compared CATIE participants to a quasi-random sample of 1,413 patients enrolled in the Schizophrenia Care and Assessment Program (SCAP), an observational, non-interventional study of schizophrenia treatment in usual care settings in the United States. The two samples were similar in demographic characteristics, e.g., gender (70 percent male in SCAP, 74 percent male in CATIE), age (mean age = 43 years in SCAP, mean age = 41 years in CATIE), and education (36 percent of SCAP participants had a high school education and 28 percent attended college; in CATIE these percentages were 35 percent and 39 percent, respectively). The CATIE study had a lower proportion of participants from racial minority backgrounds (40 percent vs. 54 percent). The samples also resembled each other in clinical characteristics. Nearly one-third of the patients in both studies had recently been hospitalized. The CATIE sample had slightly higher average scores on psychotic symptom severity than the SCAP patients (mean PANSS total score = 75 vs. 71), and also slightly higher scores on functioning and quality of life (mean Heinrichs-Carpenter QLS score = 63 vs. 57) (Haya Ascher-Svanum, Ph.D., Senior Research Scientist, Eli Lilly and Company; personal communication). These similarities provide some confidence that CATIE’s RCT design did not result in a biased selection of patients.

Thanks for your comments on the CATIE study.

View all comments by Marvin Swartz

Related News: CATIE Comes To Surprising Conclusions

Comment by:  William Carpenter, SRF Advisor (Disclosure)
Submitted 26 October 2005
Posted 26 October 2005

The antipsychotic drugs mainly treat psychosis (in contrast to cognition impairments and primary negative symptoms). In the CATIE study, the drugs tested share the same mechanism of action (D2 antagonism). Clozapine aside, the second-generation drugs (SGA) have not established superior efficacy over first-generation drugs (FGA). The FDA has granted no such claim, and the Cochrane reviews do not support superior antipsychotic efficacy. The appearance of superiority, including the terrific organization of data in the Davis meta-analyses, may be extensively based on last observation carried forward, excessive dose of the FGA, failure to pretreat with anti-parkinsonian drugs, sponsor bias, and a number of other methodological problems including the fact that most study subjects are doing poorly on FGA when recruited into comparative studies. "Atypical antipsychotic" means only low extrapyramidal symptoms at therapeutic dosing. In this regard, the CATIE findings are not surprising, but simply point to the considerable shortfall in effectiveness associated with current treatments. The drugs will vary considerably along side effect liabilities, and matching patient to side effect profile is the key to individualizing drug choice at the moment.

As to time on drug, there was not a long-acting depot arm to the study, and this method should probably be considered in substantially more patients than is the practice in the U.S. Olanzapine did a little better on the time on drug measure, and risperidone was second. This may relate to the fact that these were the two most common drugs used at study onset, so more patients with known tolerability to these drugs began the trial. In any case, concern with weight and the metabolic syndrome will drastically cut the time on drug for olanzapine in current practice.

It is almost impossible to have a level playing field in comparative drug studies, since optimal dosing and individualized dosing parameters are simply little known with most antipsychotic drugs. In this regard, we don't know if quetiapine and ziprasidone would have done better at higher dose; or if risperidone being yoked to olanzapine led to suboptimal dosing in many cases. In Rosenheck's JAMA report, he observed that pretreatment with an anti-parkinsonian drug led to similar effectiveness comparing olanzapine with haloperidol. Would perphenazine have been even better with anti-cholinergic pretreatment?

In my view, this is a critically important study in that it reasonably represents an effectiveness study in typical settings [probably more representative than the Weinberger data set (see Weinberger commentary)] without sponsor bias. As such, it has succeeded in calling public attention to the relative lack of progress associated with "me-too" dopamine blocking antipsychotic drugs. This conclusion is reinforced by the U.K. study reported by Peter Jones at the ICOSR where SGA did not beat FGA on the primary endpoint (quality of life) or on many secondary measures. Another head-on comparison study with public support.

My hope is that industry will devote discovery resources to the challenging problems of novel treatments with new molecular targets addressing problems with impaired cognition and primary negative psychopathology. Refining antipsychotic drugs has not advanced therapeutics much since the introduction of chlorpromazine. Reducing the neuroleptic adverse effects of FGA is a real advance, especially considering the excessive dosing. But significant new liabilities are associated with some of the SGA. We now need to meet the efficacy challenge for the components of schizophrenia that mainly cause poor functional outcomes.

View all comments by William Carpenter

Related News: CATIE Comes To Surprising Conclusions

Comment by:  Daniel Weinberger, SRF Advisor
Submitted 29 October 2005
Posted 30 October 2005

Dr. Swarz's comment providing data from the SCAP study is helpful in confirming that CATIE patients are similar in many phenomenological respects to other patients in schizophrenia treatment programs. Indeed, in terms of PANSS ratings, sex ratios, age at enrollment in the study, and history of recent hospitalizations, CATIE patients are not substantially different from patients we see at the NIH in Bethesda, Maryland and we saw when our program was located at St. Elizabeths Hospital in Washington, D.C. In my comment, I asked specifically about three CATIE characteristics that seemed atypical to me: age at first antipsychotic treatment (26), precentage of patients who were or had been married (40%), and percentage of patients who were unmedicated at the time they volunteered for the study (30%). It would enlighten this discussion if Dr. Swarz would report these data from the SCAP study.

View all comments by Daniel Weinberger

Related News: CATIE Comes To Surprising Conclusions

Comment by:  Robert McClure (Disclosure)
Submitted 31 October 2005
Posted 1 November 2005
  I recommend the Primary Papers

It would be interesting to learn from Dr. Swartz and the CATIE investigators (a) the age at first antipsychotic treatment, (b) the percentage of patients who were or had been married, and (c) the percentage of patients who were unmedicated at the time they volunteered for the study in the SCAP sample. I suspect these three variables, if available, will more closely resemble those of the CATIE trial sample than the CBDB sibling study sample.

Dr. Weinberger has suggested that the CATIE trial inadvertently enrolled patients more in the schizophrenia spectrum end of the distribution, or maybe the size and breadth of the CATIE trial obscured the signal from the more classic patient with schizophrenia, so the results may not be generalizable. I suspect that differences in criteria for recruitment and retention between the CBDB sibling study and the CATIE study explain the differences among the demographic variables of the samples.

The clinical characteristics of the CBDB sibling study sample are what one would expect in a study whose purpose is to find associations between genetic variation and neuroimaging/neuropsychological phenotypes, among affected and unaffected family members. The usual patient included in the CBDB sample probably: had onset of active symptoms in late adolescence or early adulthood (i.e., high school or college age, before many people marry); was started on medications earlier in life; and had more intact nuclear families (parents, siblings, etc.) than the usual CATIE subject. Patients with later onset of illness or milder symptoms (who are more likely to be or have been married) and who did not start on medications once psychotic symptoms occurred, were less compliant with their medications, and/or had fewer intact family relationships were unlikely to successfully travel to Bethesda and complete two full days of research testing. The CATIE recruitment strategy did not exclude the unusual patient with treatment of symptoms later in adulthood, require intact nuclear family, or require compliance with medications at time of study entry.

The CBDB sample better represents a "textbook case" of schizophrenia. Many patients who do meet DSM-IV criteria for schizophrenia may not be good candidates for a genetics study, but may still have schizophrenia and are appropriate candidates for a large clinical study. This would suggest that the findings can be generalized to other groups of patients with the illness, though perhaps not the "classic" cases of schizophrenia gathered in the CBDB study.

View all comments by Robert McClure

Related News: CATIE Comes To Surprising Conclusions

Comment by:  Captain Johann Samuhanand
Submitted 7 November 2005
Posted 7 November 2005

Is there any published evidence that gingko biloba could be useful in containing the side effects of clozapine and other atypicals, or are there studies in progress?

View all comments by Captain Johann Samuhanand

Related News: CATIE Comes To Surprising Conclusions

Comment by:  Xiang Zhang
Submitted 8 November 2005
Posted 9 November 2005
  I recommend the Primary Papers

Reply to comment by Johann Samuhanand
To our best knowledge, there is no published evidence that gingko biloba could be useful in reducing the side effects of clozapine and other atypicals. However, using the same group of patients with schizophrenia as we reported previously (Zhang et al., 2001), our recent study has shown that chronic patients with schizophrenia demonstrated significantly lower CD3+, CD4+, and IL-2 secreting cells, together with CD4/CD8 ratio, than did healthy controls at baseline. After a 12-week treatment, EGb added to haloperidol treatment increased the initially low peripheral CD3+, CD4+, and IL-2 secreting cells, together with CD4/CD8 ratio. There was only a significant increase in CD4+ cells in the placebo plus haloperidol group. These findings suggest that ginkgo biloba may improve the decreased peripheral immune functions in schizophrenia (Zhang et al., 2006).

As we have known, although clozapine is superior over the other drugs in terms of efficacy, it can severely deplete white blood cells, leading to limitations on its use. If gingko biloba may indeed produce beneficial effects on the immune system in schizophrenia, there is a possibility that ginkgo biloba may be useful in reducing the side effects of clozapine, at least in regard to immune function.

On the other hand, a limitation of the design of our previous study (Zhang et al., 2001) is the use of haloperidol as the antipsychotic treatment at a time when atypical antipsychotic drugs are the standard of care. Therefore, a further study is warranted to investigate whether ginkgo biloba shows similar benefits in augmenting the atypical antipsychotics, which already have the capacity to improve the positive and negative symptoms and have better profiles in terms of extrapyramidal side effects.

References:
Zhang XY, Zhou DF, Zhang PY, Wu GY, Su JM, Cao LY. A double-blind, placebo-controlled trial of extract of Ginkgo biloba added to haloperidol in treatment-resistant patients with schizophrenia. Journal of Clinical Psychiatry. 2001; 62(11):878-83. Abstract

Zhang XY, Zhou DF, Cao LY, Wu GY. The effects of Ginkgo biloba extract added to haloperidol on peripheral T-cell subsets in drug-free schizophrenia: a double-blind, placebo-controlled trial. Psychopharmacology 2006 (in press)

View all comments by Xiang Zhang

Related News: CATIE Comes To Surprising Conclusions

Comment by:  Patricia Estani
Submitted 25 November 2005
Posted 25 November 2005
  I recommend the Primary Papers

I recommend this clear and well-written paper for students to understand the basis of the CATIE studies.

I agree with Dr. Weinberger about the variables that could obscure the results in the target population or the schizophrenic population. His remarks about the control conditions or the dissection of the variables in the study are important. The difference between typical and atypical drugs is clear in these data.

New drugs, diferent from the typical and atypical drugs, based on new genetics research and new genetic routes must be developed in order to achieve new successes in the treatment of schizophrenia.

I think that atypical antipsychotics do not mean only low extrapyramidal symptoms at therapeutic doses. Several studies have demonstrated that atypical drugs(especially olanzapine) are better than typical drugs in important characteristics such as cognitive functioning.

View all comments by Patricia Estani

Related News: CATIE Comes To Surprising Conclusions

Comment by:  Mike Irwin
Submitted 29 November 2005
Posted 29 November 2005
  I recommend the Primary Papers

Related News: CATIE Comes To Surprising Conclusions

Comment by:  Patricia Estani
Submitted 13 December 2005
Posted 13 December 2005
  I recommend the Primary Papers

The most important current development of new antipsychotic drugs is focused on two mechanisms, the α7-nicotinic receptor agonists that are good new candidates for the management of the disease (Martin et al., 2004) and, most recently (and I think probably the closest to development), is the one that focuses on glutamatergic neurotransmission (Coyle and Tsai, 2004).

On the other hand, I think that behavioral and cognitive therapy, as well as family support and family management given by a professional in this area of health, are important to ensure an excellent result in schizophrenic patients.

References:
Martin LF, Kem WR, Freedman R. Alpha-7 nicotinic receptor agonists: potential new candidates for the treatment of schizophrenia. Psychopharmacology (Berl). 2004 Jun ;174(1):54-64. Abstract

Coyle JT, Tsai G. The NMDA receptor glycine modulatory site: a therapeutic target for improving cognition and reducing negative symptoms in schizophrenia. Psychopharmacology (Berl). 2004 Jun ;174(1):32-8. Abstract

View all comments by Patricia Estani

Related News: CATIE Comes To Surprising Conclusions

Comment by:  Robert Fisher
Submitted 24 December 2005
Posted 28 December 2005
  I recommend the Primary Papers

[Disclosure: R. Fisher was Study Coordinator, Recruiter, and Diagnostician for the Byerly Group at UT Southwestern CATIE site, the second-largest enrollment site in the study.]

The CATIE study is likely the best designed and implemented research project ever conducted regarding schizophrenia and relevant psychopharmacology. The extensively collected data will have an enormous heuristic value in the study and evaluation of this disorder in all aspects of schizophreinia. I found Drs. Lieberman and McEvoy to be true professionals in this study design.

View all comments by Robert Fisher

Related News: Antipsychotics and Cognition: Practice Makes Perfect Confounder

Comment by:  Richard Keefe
Submitted 12 October 2007
Posted 12 October 2007

As stated in the CATIE and CAFÉ neurocognition manuscripts, it is possible that the small improvements in neurocognitive performance following randomization to one of the antipsychotic treatments in these studies are due solely to practice effects or expectation biases. This statement is affirmed by the excellent recent study by Goldberg et al. in which improvements in cognitive performance were almost identical in magnitude to the practice effects found in healthy controls. While these data may be perhaps disappointing to the hope that second-generation medications improve cognition, they may also suggest that cognitive performance is less recalcitrant to change than previously expected.

In the context of a double-blind study design, the degree of cognitive enhancement observed for each treatment group is a function of three major variables: treatment effect, placebo effect, and practice effect. In studies of antipsychotic medications without a placebo control group, practice and placebo effects in schizophrenia cannot be disentangled from treatment effects. They also cannot be disentangled from each other. Recent data from a double-blind study comparing the effects of donepezil hydrochloride and placebo in a highly refined sample of 226 patients with schizophrenia stabilized while taking second-generation antipsychotics suggested that patients taking placebo had neurocognitive effect size improvements (0.22 SD after being tested twice over 6 weeks; 0.45 SD after the third assessment at 12 weeks) on the same test battery used in the CATIE and CAFÉ studies, suggesting a practice or placebo effect (Keefe et al., Neuropsychopharmacology, in press) consistent with the improvements reported in the CATIE and CAFÉ treatment studies. These cognitive improvements are in contrast to test-retest data collected in patients with schizophrenia tested with the MATRICS Consensus Cognitive Battery (MCCB; Nuechterlein et al., in press) and the Brief Assessment of Cognition in Schizophrenia (BACS; Keefe et al., 2004), which showed very little practice effects. The contrast of the data from these test-retest studies that did not involve the initiation of new treatments with cognitive improvements following the initiation of antipsychotic treatment or placebo suggests that attribution biases beyond simple practice effects may be at work.

Test-retest data from patients tested twice within a briefer period than the test interval in the four treatment studies discussed above suggest that schizophrenia patients demonstrate relatively small improvements in executive functions (Keefe et al., 2004; Nuechterlein et al., in press) and the WAIS digit-symbol test (Nuechterlein et al., in press), and medium improvements on tests of verbal memory only when identical versions are repeated (Hawkins and Wexler, 1999; Keefe et al., 2004) but not on tests of verbal fluency (Keefe et al., 2004; Nuechterlein et al., in press). In the donepezil/placebo study, patients who received placebo improved substantially across several cognitive domains. Although not tested directly, this series of results suggests that the magnitude of placebo effects in cognitive enhancement trials may exceed the reported size of practice-related improvements in studies of schizophrenia patients tested twice without the prospect of the initiation of a cognitive intervention.

The greater improvements in cognition found in the context of a placebo-controlled trial could be due to a variety of psychological factors. When a patient enters into a trial or is treated with a medication that is believed to contribute beneficially to cognitive performance, rater bias and expectation bias can have strong effects on performance. Patients who are told that their cognitive abilities might improve may be able to perform better on the test batteries used in the study simply because their expectations become more optimistic. Second, testers who believe that a patient will have cognitive improvement, or hope for such improvement, could administer the tests in a more hopeful, positive manner, which can help the patient raise his or her expectations for performance and thus engage motivational systems that were previously disengaged (Keefe, 2006). Such expectation bias can also lead to inaccuracies in scoring; since many cognitive tests require the use of judgment to determine final scores, hopeful testers are more likely to give the “benefit of the doubt” to patients after they have entered into a study in which the treatment is potentially cognitively enhancing. Third, this same type of expectation could have an impact on the support that a patient receives in his or her community/living situation. If the people who interact regularly with the patient begin looking for better performance on cognitively related tasks, these expectations could become self-fulfilling in that they may raise the confidence and motivation of the patient to perform well on such tasks, including cognitive testing.

The factors associated with improvement during a placebo-controlled trial are indeed complex, and it is difficult to distinguish practice effects from placebo effects. However, the relatively small clinical improvement in test-retest designs without treatment or placebo intervention suggests that any potential practice effects may at least be potentiated by placebo effects.

The implications for this series of results include a methodological caution and a reason for optimism. Regarding the caution, future trials of cognitive-enhancing compounds might need to be designed in such a way that practice and placebo are reduced. Very few treatment studies of patients with schizophrenia have employed a priori methodological strategies to reduce the magnitude of potential practice effects, such as the use of a placebo run-in period with one or more administrations of the cognitive battery prior to randomization. Regarding the optimism, these studies suggest that schizophrenia cognition (perhaps especially when freed from the dampening effects of large doses of high potency medications such as haloperidol) could be more plastic that had been previously assumed; it is possibly as sensitive to experience-dependent learning in schizophrenia patients as healthy controls, and it may benefit from improved psychological expectations. While this is a methodological nuisance for clinical trial designs, it may also reveal an unexpectedly large potential gain for psychological interventions such as cognitive remediation, cognitive-behavioral therapy, and even encouragement.

References:

Goldberg TE, Goldman RS, Burdick KE, Malhotra AK, Lencz T, Patel RC, Woerner MG, Schooler NR, Kane JM, Robinson DG. Cognitive improvement after treatment with second-generation antipsychotic medications in first-episode schizophrenia: Is it a practice effect? Arch Gen Psychiatry. 2007 Oct;64:1115-1122. Abstract

Hawkins KA, Wexler BE (1999). California Verbal Learning Test practice effects in a schizophrenia sample. Schizophr Res 39: 73-78. Abstract

Keefe RSE. Missing the sweet spot: Disengagement in schizophrenia. Psychiatry, 2006; 3: 36-41.

Keefe RSE, Malhotra AK, Meltzer H, Kane JM, Buchanan RW, Murthy A, Sovel M, Li, C, Goldman R. Efficacy and safety of donepezil in patients with schizophrenia or schizoaffective disorder: Significant placebo/practice effects in a 12-week, randomized, double-blind, placebo-controlled trial. Neuropsychopharmacology, 2007 [Epub ahead of print]. Abstract

Keefe RSE¸ Goldberg TE, Harvey PD, Gold JM, Poe M, Coughenour L. The Brief Assessment of Cognition in Schizophrenia: Reliability, sensitivity, and comparison with a standard neurocognitive battery. Schizophrenia Research, 2004; 68: 283-297. Abstract

Nuechterlein KH, Green MF, Kern RS, Baade LE, Barch D, Cohen J, Essock S, Fenton WS, Frese FJ, Gold JM, Goldberg T, Heaton R, Keefe RSE, Kraemer H, Mesholam-Gately R, Seidman LJ, Stover E, Weinberger D, Young AS, Zalcman S, Marder SR. The MATRICS consensus cognitive battery: Part 1. Test selection, reliability, and validity. The American Journal of Psychiatry (in press).

View all comments by Richard Keefe

Related News: Antipsychotics and Cognition: Practice Makes Perfect Confounder

Comment by:  Narsimha Pinninti (Disclosure)
Submitted 15 October 2007
Posted 15 October 2007
  I recommend the Primary Papers

This article questions the prevailing notion that antipsychotic medication (particularly second-generation antipsychotics) improve cognitive functioning in individuals with schizophrenia. As the authors rightly note, practice effects should be taken into account before attributing improvements to drug effects.

View all comments by Narsimha Pinninti

Related News: Antipsychotics and Cognition: Practice Makes Perfect Confounder

Comment by:  Saurabh Gupta
Submitted 15 October 2007
Posted 15 October 2007
  I recommend the Primary Papers

I propose that future studies should use computational cognitive assessment tools like CANTAB or CogTest, which have at least two advantages. These tools have multiple similar test modules, so on each testing during one study, participants get a similar but not the same test to assess the same cognitive function. Besides, computational assessment also reduces chances of subjective bias on the part of investigator.

References:

Levaux MN, Potvin S, Sepehry AA, Sablier J, Mendrek A, Stip E. Computerized assessment of cognition in schizophrenia: promises and pitfalls of CANTAB. Eur Psychiatry. 2007 Mar;22(2):104-15. Review. Abstract

View all comments by Saurabh Gupta

Related News: Antipsychotics and Cognition: Practice Makes Perfect Confounder

Comment by:  Sebastian Therman
Submitted 17 October 2007
Posted 17 October 2007

One remedy would be repeated practice over time before the actual baseline, sufficient to reach asymptotic ability. Computerized testing of reaction time measures, short-term memory span, etc. would all be quite cheap and easy to implement, for example, as a weekly session.

View all comments by Sebastian Therman

Related News: Antipsychotics and Cognition: Practice Makes Perfect Confounder

Comment by:  Andrei Szoke
Submitted 1 November 2007
Posted 5 November 2007
  I recommend the Primary Papers

We recently completed a meta-analysis on "Longitudinal studies of cognition in schizophrenia" (to be published in the British Journal of Psychiatry) based on 53 studies providing data for 31 cognitive variables. When enough data were available (19 variables from eight cognitive tests), we compared the results of schizophrenic participants to those of normal controls.

Given the differences in methods and the fact that most of the studies included in our meta-analysis reported results of patients being past their first episode (FE), it is surprising how close our results and conclusions are compared to those of Goldberg et al. In our analysis we found that, with two exceptions (semantic verbal fluency and Boston naming test, which were stable), participants with schizophrenia improved their performances. The improvement was statistically significant for 19 variables (out of 29). However, controls also showed improvement in most of the variables due to the practice effect. A significant improvement (definite practice effect) was present for 10 variables, an improvement that did not reach significance (possible practice effect) was present in six more variables, and three variables showed no improvement. When compared with schizophrenic patients, controls showed similar improvement for 11 variables, significantly more improvement for seven variables (six of them from the “definite practice effect” group, one from the “possible practice effect”) and for one variable less improvement (the Stroop interference score). Thus, these results suggest that for most of the cognitive variables, improvement seen in schizophrenic subjects does not exceed improvement due to the practice effect.

It is interesting to mention that in our analysis only two variables improved significantly more when patients had a change in their medication from first-generation antipsychotics (FGAs) to second-generation antipsychotics (SGAs). These variables were time to complete TMT B and the delayed recall of the Visual Reproduction (from the WMS). In the Goldberg et al. study the only two tests that showed more improvement in schizophrenic subjects than in controls were also the TMT and visual reproduction. Although in our study schizophrenic subjects did not improve more than controls, the two results (Goldberg’s and ours) taken together could be an indirect argument for a differential, specific effect of SGAs on those two (visuo-spatial) tasks. The placebo effect—see the comment by Richard Keefe—could explain why improvement in the study by Goldberg et al. was greater than in our meta-analysis. Studies of effects of changing medication in the opposite direction, from SGAs to FGAs, could contribute to validate or invalidate these hypotheses.

Goldberg et al. suggested that there could be a set of task characteristics that could be used to develop tasks resistant to the practice effect. Our own results are less optimistic as they show that phonemic verbal fluency, despite a very similar format, does not share the “practice resistance” with the semantic verbal fluency. However, we think that there is already a wealth of data that could be used to select the best cognitive tests. An alternative solution is the use of scales and questionnaires for evaluating cognition (that are sensible to the placebo effect but not to the practice effect).

References:

Szoke A, Trandafir A, Dupont M-E, Meary A, Schurhoff F, Leboyer M. Longitudinal studies of cognition in schizophrenia. British Journal of Psychiatry (in press).

View all comments by Andrei Szoke

Related News: Antipsychotics and Cognition: Practice Makes Perfect Confounder

Comment by:  Patricia Estani
Submitted 7 November 2007
Posted 8 November 2007
  I recommend the Primary Papers

Related News: 5HT and Glutamate Receptors—Unique Complex Linked to Psychosis

Comment by:  Brian Dean
Submitted 20 March 2008
Posted 20 March 2008

Altered receptor dimerization: a new paradigm in the pathology of schizophrenia
Understanding the pathology of complex diseases such as schizophrenia requires the use of the full arsenal at the disposal of medical research. Such an approach has been used to make an exciting new discovery that suggests that abnormal dimerization between the serotonin 2A receptor (2AR) and the metabotropic glutamate 2 receptor(mGluR2) may underlie some of the symptoms of schizophrenia (González-Maeso et al., 2008).

This discovery is based on an initial finding that 2AR is coexpressed with mGluR2 in layer 5 of the mouse somatosensory cortex (SCx) and that levels of mGluR2 were decreased in the cortex of 2AR-/- mice, suggesting a relationship between the expression of the two genes. This hypothesis was further supported by data showing that expression of mGluR2 was selectively restored in mice where 2AR expression had been re-established in layer 5 of the SCx. From these data, and data from other studies suggesting G protein-coupled receptors (GPCRs) can form heterodimers (Angers et al., 2002), the authors began to test the hypothesis that 2AR and mGluR2 could form heterodimers.

Using human cortical samples and an anti-2AR antibody, the authors showed that they could immunoprecipitate an immunogenic band with a molecular weight that matches a 2AR/mGluR2 receptor dimer complex if an anti-GluR2 antibody was used with Western blotting. Significantly, that heterodimer complex could not be visualized in Western blots using anti-mGluR3 antibody instead of an anti-mGluR2 antibody. This reinforces the notion that it is mGluR2 that dimerizes with 2AR. Finally, a close interaction between the two receptors was demonstrated using fluorescence resonance energy transfer in transfected HEK-293 cells.

The authors then used molecular chimaeras to localize the site on mGluR2 that was a requirement for heterodimerization with 2AR and showed that the transmembrane helices 4 and 5 were required for this interaction. The authors then tested the posit that the interaction between 2AR and mGluR2 served to integrate cross-talk between the serotonergic and glutamatergic pathways in the CNS. To this end they showed that activation of Gαq/11 by 2AR was reduced in cells coexpressing mGluR2 and that this effect was lessened by mGluR2 receptor agonists. Significantly, this activity was dependent on the 4 and 5 transmembrane domain of the mGluR2, the domain required to form heterodimers.

Having demonstrated an impact of receptor dimerization on G protein signaling, the authors then investigated whether dimerization affected either receptor-modulated changes in c-fos, which is a marker of the signal-transduction stimulated by non-hallucinogenic 2AR agonists, or on levels of egr-2, which is induced by hallucinogens such as lysergic acid diethylamide (González-Maeso et al., 2007). The authors showed that stimulating mGluR2 with an mGluR2/3 agonist only affected the ability of hallucinogens to induce egr-2 in mouse SCx, suggesting the 2AR/mGluR2 dimers were involved in modulating hallucinogenic pathways of the CNS. To confirm this finding might have functional consequences. The authors then showed that the mGluR2/3 agonist suppressed the induction of hallucinogen-induced head twitches in the mice. These data supported the notion that receptor heterodimers are active in appropriate pathways in the CNS that have been used to model hallucinogenic effects. To extend this behavioral data, the authors also showed that mGluR2/3 agonist-induced locomotion and vertical activity were attenuated in 2AR-/- mice.

The authors had amassed a large quantity of data to suggest that 2AR/mGluR2 dimers may be important in generating hallucinogenic activity, which raised the possibility that altered levels of such dimers may be altered in the CNS of subjects with schizophrenia. To address this issue, the authors used radioligand binding to show that expression levels of 2AR and mGluR2/3 receptors were increased and decreased, respectively, in the dorsolateral prefrontal cortex (DLPFC) from untreated subjects with schizophrenia. In addition, the authors showed that the level of mGluR2, but not mGluR3, mRNA was decreased in the same CNS regions from the subjects with schizophrenia. These differences were not apparent in the same CNS regions from subjects with schizophrenia who had been treated with antipsychotic drugs. This raised the possibility that antipsychotic drug treatment may affect levels of 2AR/mGluR2 dimerization, and therefore the authors went on to show that clozapine downregulated levels of the mRNA for the two receptors in mouse cortex. The 2AR was critical in this process as clozapine did not downregulate mGluR2 mRNA in 2AR-/- mice. Haloperidol treatment had no effect on the expression of either 2AR or mGluR2. Finally, it was shown that levels of receptor binding to both receptors were reduced with aging.

From this large amount of data, the authors could conclude that they had shown that 2AR/mGluR2 heterodimers are important in hallucinogenic pathways of the CNS, using both cellular and animal models. They also argue that increased expression of 2AR and decreased expression of mGluR2 in the cortex of subjects with schizophrenia predispose these individuals to hallucinations. Presumably, therefore, the reduction in 2AR caused by certain antipsychotic drugs would be a mechanism by which a potential imbalance in heterodimer formation could be reversed to lessen hallucinations. Finally, the authors argue that the propensity for antipsychotic drugs and age to decrease levels of 2AR is why 2AR levels are reported as decreased in the majority of prior studies in schizophrenia (Dean, 2003), which mainly used cohorts made up of either treated or older subjects with schizophrenia.

As is often the case, the proposed link of a clear finding of 2AR/mGluR2 heterodimers in the mammalian cortex to hallucinations and then schizophrenia is dependent on data from the CNS of subjects with the disorder. Like many novel and compelling discoveries, the data from animal and cellular models appear clear-cut. However, there are some issues that leave in doubt the link between changes in receptor dimerization and schizophrenia. In particular, the authors did not demonstrate altered levels of dimerized receptors using the co-immunoprecipitation/Western blot approach; rather, they rely on inferences from the measurement of the two receptors separately using radioligand binding. In addition, the authors have not addressed the fact that the majority of imaging studies, many using young drug naïve subjects, did not find changes in levels of the 2AR in subjects with the disorder (Verhoeff et al., 2000; Lewis et al., 1999; Okubo et al., 2000; Trichard et al., 1998). The argument that findings in postmortem studies showing decreased levels of 2AR were due to studies being completed on treated or older subjects with the disorder is also not supported by neuroimaging studies showing decreased levels of 2AR in subjects with schizophrenia who were younger than 29 years of age (Ngan et al., 2000) or who were at high risk for the disorder (Hurlemann et al., 2005). These later studies suggest that low levels of 2AR may be more apparent earlier in the disease progression.

It is clear that the report of increased levels of 2AR with schizophrenia in the paper reporting the discovery of the 2AR/mGluR2 heterodimers (González-Maeso et al., 2008) is at odds with other postmortem (Dean, 2003) and neuroimaging studies (see above). This raises the possibility that the postmortem findings are in some way unique to the tissue collection used in the study. One difference in the postmortem tissue used in the study is that 85 percent of the subjects with schizophrenia had died by suicide. This would be higher than in most other studies of schizophrenia using postmortem CNS. Significantly, a number of studies have reported an increase in 2AR in the cortex of subjects that had died by suicide (Pandey et al., 2002; Mann et al., 1986; Hrdina et al., 1993; Escribá et al., 2004). This means the increased levels of 2AR reported in the study on heterodimers may be associated with suicide within schizophrenia, rather than schizophrenia per se.

In conclusion, like any novel finding, there are a number of important issues that will need addressing in future testing of the hypothesis that altered 2AR/mGluR2 heterodimerization is involved in the pathology of schizophrenia. However, the idea that changes in receptor heterodimerization could be involved in the pathology of schizophrenia is an exciting new direction arising from what is an excellent broad-based approach to understanding this complex disorder.

References:

González-Maeso J, Ang RL, Yuen T, Chan P, Weisstaub NV, López-Giménez JF, Zhou M, Okawa Y, Callado LF, Milligan G, Gingrich JA, Filizola M, Meana JJ, Sealfon SC. Identification of a serotonin/glutamate receptor complex implicated in psychosis. Nature. 2008 Mar 6;452(7183):93-7. Abstract

Angers S, Salahpour A, Bouvier M. Dimerization: an emerging concept for G protein-coupled receptor ontogeny and function. Annu Rev Pharmacol Toxicol. 2002 Jan 1;42():409-35. Abstract

González-Maeso J, Weisstaub NV, Zhou M, Chan P, Ivic L, Ang R, Lira A, Bradley-Moore M, Ge Y, Zhou Q, Sealfon SC, Gingrich JA. Hallucinogens recruit specific cortical 5-HT(2A) receptor-mediated signaling pathways to affect behavior. Neuron. 2007 Feb 1;53(3):439-52. Abstract

Dean B. The cortical serotonin2A receptor and the pathology of schizophrenia: a likely accomplice. J Neurochem. 2003 Apr 1;85(1):1-13. Abstract

Verhoeff NP, Meyer JH, Kecojevic A, Hussey D, Lewis R, Tauscher J, Zipursky RB, Kapur S. A voxel-by-voxel analysis of [18F]setoperone PET data shows no substantial serotonin 5-HT(2A) receptor changes in schizophrenia. Psychiatry Res. 2000 Oct 30;99(3):123-35. Abstract

Lewis R, Kapur S, Jones C, DaSilva J, Brown GM, Wilson AA, Houle S, Zipursky RB. Serotonin 5-HT2 receptors in schizophrenia: a PET study using [18F]setoperone in neuroleptic-naive patients and normal subjects. Am J Psychiatry. 1999 Jan 1;156(1):72-8. Abstract

Okubo Y, Suhara T, Suzuki K, Kobayashi K, Inoue O, Terasaki O, Someya Y, Sassa T, Sudo Y, Matsushima E, Iyo M, Tateno Y, Toru M. Serotonin 5-HT2 receptors in schizophrenic patients studied by positron emission tomography. Life Sci. 2000 Jan 1;66(25):2455-64. Abstract

Trichard C, Paillère-Martinot ML, Attar-Levy D, Blin J, Feline A, Martinot JL. No serotonin 5-HT2A receptor density abnormality in the cortex of schizophrenic patients studied with PET. Schizophr Res. 1998 May 4;31(1):13-7. Abstract

Ngan ET, Yatham LN, Ruth TJ, Liddle PF. Decreased serotonin 2A receptor densities in neuroleptic-naive patients with schizophrenia: A PET study using [(18)F]setoperone. Am J Psychiatry. 2000 Jun 1;157(6):1016-8. Abstract

Hurlemann R, Boy C, Meyer PT, Scherk H, Wagner M, Herzog H, Coenen HH, Vogeley K, Falkai P, Zilles K, Maier W, Bauer A. Decreased prefrontal 5-HT2A receptor binding in subjects at enhanced risk for schizophrenia. Anat Embryol (Berl). 2005 Dec 1;210(5-6):519-23. Abstract

Pandey GN, Dwivedi Y, Rizavi HS, Ren X, Pandey SC, Pesold C, Roberts RC, Conley RR, Tamminga CA. Higher expression of serotonin 5-HT(2A) receptors in the postmortem brains of teenage suicide victims. Am J Psychiatry. 2002 Mar 1;159(3):419-29. Abstract

Mann JJ, Stanley M, McBride PA, McEwen BS. Increased serotonin2 and beta-adrenergic receptor binding in the frontal cortices of suicide victims. Arch Gen Psychiatry. 1986 Oct 1;43(10):954-9. Abstract

Hrdina PD, Demeter E, Vu TB, Sótónyi P, Palkovits M. 5-HT uptake sites and 5-HT2 receptors in brain of antidepressant-free suicide victims/depressives: increase in 5-HT2 sites in cortex and amygdala. Brain Res. 1993 Jun 18;614(1-2):37-44. Abstract

Escribá PV, Ozaita A, García-Sevilla JA. Increased mRNA expression of alpha2A-adrenoceptors, serotonin receptors and mu-opioid receptors in the brains of suicide victims. Neuropsychopharmacology. 2004 Aug 1;29(8):1512-21. Abstract

View all comments by Brian Dean

Related News: 5HT and Glutamate Receptors—Unique Complex Linked to Psychosis

Comment by:  Gerard J. Marek (Disclosure)
Submitted 21 March 2008
Posted 21 March 2008

Another bicycle trip?
Ever since dopamine was first implicated in the therapeutic effects of antipsychotic drugs by Arvid Carlsson and colleagues over 50 years ago, and then dopamine D2 receptors were implicated in the Parkinsonian side effects and late-evolving movement disorders, an intense search has been underway for antipsychotic drugs that might act through other mechanisms. In parallel with this search, drugs with psychotomimetic effects in healthy volunteers or exacerbating psychosis have also been used to discover new antipsychotic drugs. With an evolving understanding of the neuropharmacology underlying ketamine or PCP, amphetamines, and serotonergic hallucinogens (LSD, mescaline, and psilocybin), glutamatergic, dopaminergic, and serotonergic theories of psychotic pathophysiology have been advanced. Converging evidence points to activation of 5-HT2A receptors as a necessary action in the psychotomimetic effects of the serotonergic “hallucinogens.” The recent description of a proof-of-concept clinical study where a prodrug for a metabotropic glutamate2/3 (mGlu2/3) receptor agonist exerted therapeutic effects in schizophrenic patients may be the most promising report for an elusive antipsychotic medication generally viewed as lacking direct effects on dopamine D2 receptors (Patil et al., 2007). More recently, a report has appeared which raises the possibility that glutamate and serotonin may be involved in the therapeutic effects of mGlu2/3 receptors by virtue of a molecular complex between mGlu2 and 5-HT2A receptors (González-Maeso et al., 2008). Beyond replication of these effects in other laboratories, several fundamental questions have been raised that should be addressed.

First, does this type of interaction occur in the prefrontal cortex, which (through cortico-thalamo-striatal loops) is more closely related to the core symptoms of schizophrenia than the somatosensory cortex? Second, are the therapeutic actions of mGlu2/3 receptors mediated through activation of postsynaptic mGlu2 receptors, rather than presynaptic mGlu2 receptors (Marek et al., 2001)? Third, do other G protein-coupled receptors similarly act through complexes with 5-HT2A receptors?

Further research will be required to address this first question, especially since both mGlu2/3 agonists and NMDA receptor antagonists appear to have more potent or efficacious effects in the prefrontal cortex than the somatosensory cortex under either in vitro or in vivo conditions. The second question will be important to address at a fundamental level, since “simple” intra-cortical processes invoke different levels of analyses than do hypotheses that presynaptic mGlu2 receptors on long-loop afferents may play key roles as therapeutic targets. In fact, previous experiments involving rescue of 5-HT2A receptors in the cortex or thalamus appear to be compromised by confounds. Namely, the cortical rescue of 5-HT2A receptors in the htr2A-/- mice using the Emx1 promoter does not rule out an involvement of afferents to the cortex from a poorly understood region involved in integrating multi-modal associations, the claustrum. 5-HT2A receptor expression was also restored to the claustrum with this rescue strategy (Weisstaub et al., 2006). The thalamic rescue of 5-HT2A receptors, which generally fails to reprise the effects seen in the cortical rescue preparation, may be problematic in that the promoter utilized expresses SERT in thalamocortical projections from primary sensory relay neurons rather than the midline and intralaminar thalamic neurons intimately involved in arousal and stress-related biology (Lebrand et al., 1996; Van der Werf et al., 2002). The relatively dense expression of cortical 5-HT2A and mGlu2 receptor expression in layers I and Va of the prefrontal cortex is an excellent match for the laminar distribution of afferents from the midline and intralaminar thalamic nuclei (Marek et al., 2001). Further work is required to understand the magnitude of the involvement of thalamic afferents from the posterior thalamic nucleus to the somatosensory cortex vs. involvement of the afferents from midline and intralaminar thalamic nuclei throughout the prefrontal cortex. Third, do other Gi/Go-coupled GPCRs form complexes with 5-HT2A receptors? Other, much stronger candidates for such a role than mGlu3 receptors would be μ-opioid receptors and adenosine A1 receptors. The physiology of both μ-opioid receptors and adenosine A1 receptors share a striking degree of similarity with mGlu2 receptors ranging from regulating excitatory synaptic afferents to the prefrontal cortex in slice preparations to in vivo modulation of the three major classes of psychotomimetic drugs.

Both the replication of the exciting basic findings reported by the Gingerich and Sealfon laboratories and answers to these questions above should add another chapter to the story that began in earnest over 60 years ago with a bicycle ride by the Sandoz chemist Albert Hoffman following the ingestion of the twenty-fifth lysergic diethylamide that he had synthesized.

References:

Patil ST, Zhang L, Martenyi F, Lowe SL, Jackson KA, Andreev BV, Avedisova AS, Bardenstein LM, Gurovich IY, Morozova MA, Mosolov SN, Neznanov NG, Reznik AM, Smulevich AB, Tochilov VA, Johnson BG, Monn JA, Schoepp DD. Activation of mGlu2/3 receptors as a new approach to treat schizophrenia: a randomized Phase 2 clinical trial. Nat Med. 2007 Sep 1;13(9):1102-7. Abstract

González-Maeso J, Ang RL, Yuen T, Chan P, Weisstaub NV, López-Giménez JF, Zhou M, Okawa Y, Callado LF, Milligan G, Gingrich JA, Filizola M, Meana JJ, Sealfon SC. Identification of a serotonin/glutamate receptor complex implicated in psychosis. Nature. 2008 Mar 6;452(7183):93-7. Abstract

Marek GJ, Wright RA, Gewirtz JC, Schoepp DD. A major role for thalamocortical afferents in serotonergic hallucinogen receptor function in the rat neocortex. Neuroscience. 2001 Jan 1;105(2):379-92. Abstract

Weisstaub NV, Zhou M, Lira A, Lambe E, González-Maeso J, Hornung JP, Sibille E, Underwood M, Itohara S, Dauer WT, Ansorge MS, Morelli E, Mann JJ, Toth M, Aghajanian G, Sealfon SC, Hen R, Gingrich JA. Cortical 5-HT2A receptor signaling modulates anxiety-like behaviors in mice. Science. 2006 Jul 28;313(5786):536-40. Abstract

Lebrand C, Cases O, Adelbrecht C, Doye A, Alvarez C, El Mestikawy S, Seif I, Gaspar P. Transient uptake and storage of serotonin in developing thalamic neurons. Neuron. 1996 Nov 1;17(5):823-35. Abstract

Van der Werf YD, Witter MP, Groenewegen HJ. The intralaminar and midline nuclei of the thalamus. Anatomical and functional evidence for participation in processes of arousal and awareness. Brain Res Brain Res Rev. 2002 Sep 1;39(2-3):107-40. Abstract

View all comments by Gerard J. Marek

Related News: Sweeping SchizophreniaGene Study Applies New Criteria to Finger Suspects

Comment by:  Stephen J. Glatt
Submitted 17 July 2008
Posted 21 July 2008
  I recommend the Primary Papers

The paper by Allen et al. is a tremendously useful addition to the fields of schizophrenia research, psychiatric genetics, and medical genetics. By efficiently summarizing a tremendous amount of work, Allen et al. have endeavored to provide a "state-of-the-art" summary that most of us, as individuals, struggle to accomplish; they have largely succeeded in their attempt. This manuscript, and the continual availability of the SZGene database, should long serve as invaluable resources for the increasingly complex task of building polygenic models of risk for schizophrenia. Furthermore, these methods, which were initially implemented in the AlzGene database, have clearly generalized quite successfully to SZGene and thus, should be easy enough to scale up to cover many other psychiatric disorders as well. In this way, the contribution to psychiatric genetics, and possibly other disorders outside of psychiatry, is crystalline.

Aside from the database, the contribution of the recent manuscript to the field of schizophrenia research is also tremendous. As pointed out by the authors, several of the significantly associated genes identified by their meta-analyses were never before studied in this manner, so a whole new set of top candidate genes was identified. This work also served to confirm the results of prior meta-analyses from my group and others, which is always reassuring. Application of the HuGENet criteria to grading the detected associations is useful as a heuristic, but it must be kept in mind that that while these criteria reflect a consensus, they also reflect a moving target. One difficulty in implementing grades (especially the "overall" grade) is analogous to difficulties often encountered in meta-analyses when rating the quality of studies, and that is the ambiguity of ratings. Thus, on a seven-point quality scale (or a three-letter-grade scale), a score can be arrived at by a variety of combinations of flaws or strengths, but similar scores may not (often do not) reflect identical strengths and weaknesses of the graded studies. For example, I, for one, am not certain that having a relatively low number of minor alleles reflected in a meta-analytic result (especially if it is a rare variant) is as big a decrement as the pooled OR dropping from significance when the initial study is omitted.

Nevertheless, I reiterate that the use of this heuristic grading system is helpful, but should be taken with a grain of salt. Overall, the paper and its conclusions are a great contribution to this field and warrant mass attention. The ultimate question, not yet addressed here but apparently on the horizon, is how well the emerging GWASs detect these "positive control" associations, or we might say how well these hypothesis-driven results stack up against new candidates to emerge from the high-throughput generation of novel hypotheses....

View all comments by Stephen J. Glatt

Related News: An Arrestin Development: Antipsychotic Drugs Hit Dopamine Signaling in New Way

Comment by:  Zachary Z. FreybergEneko UrizarHolly MooreJeffrey Lieberman (SRF Advisor)Jonathan Javitch
Submitted 30 December 2008
Posted 30 December 2008

Reevaluation of the dopamine D2 receptor in the treatment of schizophrenia: Novel intracellular mechanisms as predictors of antipsychotic efficacy
Since the advent of antipsychotic medications, there have been many speculations about their precise mechanisms of therapeutic action. Although it is apparent that blockade of dopamine D2 receptors (D2R) is crucial to the efficacy of all current antipsychotic medications, it is not clear which signaling events downstream of the D2R must be blocked for the therapeutic actions of antipsychotics and which events, when blocked, lead instead to side effects.

The best characterized D2R-mediated signaling pathways involve coupling of the receptor to pertussis toxin-sensitive G proteins of the Gi and Go subfamilies (Sidhu and Niznik, 2000), through which D2R activation results in a decrease in cyclic AMP (cAMP). D2R activation can also have a number of other effects, including enhancement of specific potassium currents, inhibition of L-type calcium currents, mediation of extracellular signal-regulated kinase 1 (ERK1) and potentiation of arachidonic acid release (Beom et al., 2004; Missale et al., 1998; Perez et al., 2006; Hernández-López et al., 2000). There is growing evidence that D2Rs can interact with a number of membrane-bound or intracellular proteins, which may further modulate signaling specificity (reviewed in Terrillon and Bouvier, 2004; Ferré et al., 2007a). In particular, D2R heteromerization may result in a switch from Gi/o coupling to Gs (i.e., through D2R and cannabinoid 1 receptor interaction) (Kearn et al., 2005) or to coupling with Gq (as suggested in D2R and D1R interactions) (Rashid et al., 2007). Moreover, heteromerization between D2R and other receptors such as the adenosine A2A receptor may allow for reciprocal modulation of D2R function (Ferré et al., 2007a; Ferré et al., 2007b). It also has been suggested that calcium signaling mechanisms may modulate D2R’s signaling efficacy; interaction between D2R and calcium-binding protein S100B results in enhanced D2R intracellular signaling (Liu et al., 2008; Stanwood, 2008).

The interaction between D2R and arrestin has received increasing attention. Following D2R activation, D2R signaling is attenuated by recruitment of arrestin 3 to the cell surface where it binds to the receptor (Klewe et al., 2008; Lan et al., 2008a ; Lan et al., 2008b), leading to inactivation and internalization of the D2R. Arrestin 3 also binds Akt—a serine/threonine kinase involved in multiple cellular functions and implicated clinically in schizophrenia (Arguello and Gogos; 2008; Beaulieu et al., 2005; Brazil and Hemmings, 2001; Brazil et al., 2004; Emamian et al., 2004; Kalkman, 2006). Following D2R activation by dopamine, the signaling scaffold formed by arrestin 3, while facilitating receptor desensitization and internalization, also recruits Akt into a complex with the phosphatase PP2A, which dephosphorylates and consequently inactivates Akt (Beaulieu et al., 2007a ). Thus, D2R activation inhibits Akt activity through an arrestin-dependent but G protein-independent pathway (Beaulieu et al., 2007a ; Beaulieu et al., 2007b). Curiously, the mood stabilizer, lithium, has been shown to disrupt the arrestin 3-Akt-PP2A complex, thereby preventing dopamine-induced dephosphorylation of Akt and blocking amphetamine-induced locomotion (Beaulieu et al., 2008). Moreover, amphetamine-induced locomotion is greatly diminished in arrestin 3 knockout mice, suggesting that this pathway is critical to at least some psychostimulant effects (Beaulieu et al., 2005).

Using newly developed BRET (bioluminescent resonance energy transfer) biosensors in assays that measure direct protein-protein interactions within the living cell, recent studies have demonstrated that antipsychotic medications prevent arrestin 3 recruitment by blocking D2R activation (Klewe et al., 2008; Masri et al., 2008). Masri et al. (2008) hypothesized that antipsychotic drugs achieve their therapeutic effect through a common mechanism involving blockade of arrestin-mediated signaling (Masri et al., 2008). Masri et al. (2008) also demonstrated that nearly all antipsychotics tested (including haloperidol, clozapine, olanzapine, desmethylclozapine, chlorpromazine, quetiapine, risperidone and ziprasidone) behave as inverse agonists to decrease constitutive G protein signaling as well as to prevent the agonist quinpirole from inhibiting cAMP synthesis (via D2R-mediated Gi/o signaling). The lone exception, aripiprazole, behaved as a partial agonist instead of as an inverse agonist of the G protein mediated effects. The latter finding is consistent with previous studies highlighting aripiprazole’s ability to differentially modulate various G protein-mediated effector pathways, a property termed “functional selectivity” (Mailman, 2007; Urban et al., 2007). Using the BRET assay, Klewe et al. (2008) and more recently Masri et al. (2008) demonstrated that all antipsychotics, including aripiprazole, block arrestin 3 recruitment. This finding has led Masri et al. (2008) to suggest that blockade of arrestin 3 recruitment to the D2R, and not modulation of G-protein-mediated pathways, is a common and specific property of all current antipsychotics and may be used to predict the antipsychotic efficacy of drugs in development (Masri et al., 2008). This hypothesis remains to be tested and at present appears to lean heavily on the evidence for aripiprazole’s atypical effects on constitutive (non-agonist-dependent) D2R-mediated G-protein signaling. Indeed, the fact that lithium acts to prevent arrestin-mediated signaling in response to amphetamine but is not an effective antipsychotic in monotherapy suggests that antipsychotic action may be more complex than simple blockade of D2R-mediated arrestin signaling. In addition, the ability of antipsychotics, including aripiprazole, to block agonist binding to the D2R and thus activation of the receptor, makes it likely that agonist-induced activity in multiple signaling pathways will also be blocked by these drugs.

Despite the paucity of direct evidence for D2R-arrestin coupling as the mechanism underlying the antipsychotic effects of drugs, the hypothesis remains quite intriguing Given that Akt and its downstream target GSK-3 (glycogen synthase kinase-3) have been implicated in schizophrenia in a number of genetic and postmortem studies, and the Akt/GSK-3 pathway might represent an opening into alternative therapeutics of schizophrenia. Akt is a serine/threonine kinase that may have significant roles in synaptic physiology and neurodegeneration (Brazil et al., 2004). Recruited to the cell surface by binding to phosphatidylinositol 3,4,5 trisphosphate, Akt is activated via phosphorylation of 3-phosphoinoitide-dependent protein kinase 1 (PDK1) and the rictor-mTOR complex (Brazil and Hemmings, 2001; Sarbassov et al., 2005). Once active, Akt phosphorylates GSK-3, thereby inactivating it. Since D2R activation leads to inactivation of Akt, this also results in increased GSK-3 activity (Beaulieu et al., 2004; Lovestone et al., 2007). GSK-3 activity also plays an important role in modulating the dopaminergic response to amphetamine. Amphetamine’s stimulation of DAT-mediated dopamine efflux and subsequent D2R stimulation likely results in Akt inactivation and increased GSK-3 activity. Rats treated with the specific GSK-3 inhibitor, AR-A014418, failed to display amphetamine-induced hyperactivity (Gould et al., 2004). Similarly, heterozygous GSK-3β knockout mice (expressing approximately half of wildtype levels of GSK-3β) displayed significantly reduced levels of locomotor activity following amphetamine treatment (Beaulieu et al., 2004). Additionally, treatment of dopamine transporter (DAT) knockout mice with multiple GSK-3 inhibitory drugs inhibited the ordinarily hyperactive behavior of the non-treated DAT knockout mice (Beaulieu et al., 2004).

In a mouse model, acute and chronic haloperidol treatment was shown to increase levels of active, phosphorylated Akt isoform Akt1 and increased phosphorylation and inactivation of GSK-3β (Emamian et al., 2004). Thus, it was suggested that haloperidol treatment may compensate for the decreased levels of endogenous Akt1 in the frontal cortex of people with schizophrenia (Emamian et al., 2004). Atypical antipsychotics also impact on the regulation of Akt and GSK-3β activities. For example, treatment with clozapine results in increased levels of phosphorylated GSK-3β (Kang et al., 2004; Sutton et al., 2007). Interestingly, however, differences between haloperidol and atypical antipsychotics have emerged in the kinetics of Akt/GSK-3 phosphorylation, the levels of proteins expressed following drug exposure, and the signaling pathways that are preferentially activated (Roh et al., 2007).

The abilities of antipsychotic drugs to activate distinct signaling pathways to mediate their ostensible differential pharmacologic effects would suggest clinical variation in their therapeutic effects. However, meaningful differences in the clinical effects of these compounds have not been clearly or consistently evident. The initial reports of superior efficacy of the so-called second generation or atypical antipsychotics on measures of psychosis (Kane et al., 1988), negative symptoms (Tollefson et al., 1997), cognitive deficits (Keefe et al., 1999), relapse prevention (Csernansky et al., 2002), adherence (Wahlbeck et al., 2001) and illness progression (Lieberman et al., 2005a), have not been borne out by more recent studies (Geddes et al., 2000; Lieberman et al., 2005b; Jones et al., 2006; Leucht et al., 2008). Indeed, the differences between antipsychotic drugs are most evident in the types, frequency and severity of side effects rather than in their therapeutic actions (Leucht et al., 1999; Allison et al., 1999; Henderson et al., 2005). In this regard the emerging pattern of variation in the molecular mechanisms of antipsychotic drugs in the face of their common clinical profiles resembles what was previously observed with the variability in neuroreceptor binding profiles (Kinon and Lieberman, 1996). The marked differences in the affinities and selectivity of the various antipsychotics for the receptors of different neurotransmitters were thought to underlie a rich pattern of clinical variation in the therapeutic actions of this group of drugs (Miyamoto et al., 2005). However, this hypothesis has not been supported by clinical studies (Lieberman, 2006; Lewis and Lieberman, 2008).

Nevertheless, there is reason to be hopeful that through functional selectivity, or other potential actions, the abilities of drugs to engage different signaling pathways will confer novel therapeutic effects that will improve the efficacy of treatments. In this context, the studies of Masri et al. (2008) and Klewe et al. (2008) highlight the plausibility that D2R/arrestin 3 modulation of Akt and GSK-3 activity is an important mechanism underlying psychosis and a potential target for future antipsychotic drugs. Further study of this pathway, including studies designed to reverse the effects of D2R antagonists or partial agonists (antipsychotic drugs) with systematic differential manipulation of the signaling pathways induced by D2R activation, is likely to be a fruitful path toward the development of novel treatments for schizophrenia-related disorders.

Acknowledgements: The authors would like to acknowledge the generous support of the Lieber Center for Schizophrenia Research at Columbia University

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Ferré S., Agnati LF., Ciruela F., Lluis C., Woods AS., Fuxe K. and Franco R. (2007a). Neurotransmitter receptor heteromers and their integrative role in 'local modules': the striatal spine module. Brain Res Rev. 55: 55-67. Abstract

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Rashid AJ., So CH., Kong MM., Furtak T., El-Ghundi M., Cheng R., O'Dowd BF. and George SR. (2007). D1-D2 dopamine receptor heterooligomers with unique pharmacology are coupled to rapid activation of Gq/11 in the striatum. Proc Natl Acad Sci USA 104: 654-659. Abstract

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Liu Y., Buck DC. and Neve KA. (2008). Novel interaction of the dopamine D2 receptor and the Ca2+ binding protein S100B: role in D2 receptor function. Mol Pharmacol. 74: 371-378. Abstract

Stanwood GD. (2008). Protein-protein interactions and dopamine D2 receptor signaling: a calcium connection. Mol Pharmacol. 74: 317-319. Abstract

Klewe IV., Nielsen SM., Tarpø L., Urizar E., Dipace C., Javitch JA., Gether U., Egebjerg J. and Christensen KV. (2008). Recruitment of beta-arrestin2 to the dopamine D2 receptor: insights into anti-psychotic and anti-parkinsonian drug receptor signaling. Neuropharmacology 54: 1215-1222. Abstract

Lan H., Teeter MM., Gurevich VV. and Neve KA. (2008a). An intracellular loop2 amino acid residue determines differential binding of arrestin to the dopamine D2 and D3 receptors. Mol Pharmacol. Abstract

Lan H., Liu Y., Bell MI., Gurevich VV. and Neve KA. (2008b). A dopamine D2 receptor mutant capable of G protein-mediated signaling but deficient in arrestin binding. Mol Pharmacol. Abstract

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Related News: Serotonin Receptors Appear Reduced in First-Episode Schizophrenia

Comment by:  Brian Dean
Submitted 18 January 2010
Posted 18 January 2010

Serotonin2A Receptors and Schizophrenia: The Controversies Continue
A new study reporting decreased serotonin2A receptor (HTR2A) density in drug naïve first-episode schizophrenia (Rasmussen et al., 2010) has again raised the issue of the role of that receptor in the pathophysiology of the disorder.

The notion that the HTR2A is involved in pathophysiology and treatment of schizophrenia remains an open issue. The observation that a number of the atypical antipsychotic drugs are high-affinity antagonists at the receptor would suggest that blocking stimulation of the receptor can have some therapeutic benefit (Meltzer, 1995). It is also significant that, overall, available data from postmortem studies would suggest that the HTR2A is decreased in density in the cortex of subjects with schizophrenia (Dean, 2003). This could either be due to altered gene expression per se or an appropriate agonist induced downregulation of the receptor due to chronic overactivation (Rahimian et al., 1995). Recently, it has been suggested that different outcomes from the postmortem studies are due to differing methodologies (Dean et al., 2008); the differences being due to studies using particulate membrane separating a component in the cytosol which regulates receptor binding from the receptor protein. It is becoming clear that the HTR2A is part of protein complexes in the membrane (Gonzalez-Maeso et al., 2008), and, therefore, it could well be that further exploration of the interactions between the HTR2A and its membrane complexes as well as the cytosol are required to make postmortem data on the HTR2A more interpretable.

As with postmortem studies, positron emission tomography studies appear to be giving disparate outcomes regarding the levels of HTR2A in schizophrenia (Table 1). Clearly, in these studies different results cannot be accounted for by separating the receptor from intracellular regulatory proteins or membrane complexes. Moreover, whilst there are differences in the number of subjects in each study, an analysis of cohort sizes (Fisher exact test) shows this measure does not differ significantly across studies (p = 0.09). Finally, the radioligand used would not appear to be influencing outcomes, as studies using [18F]setoperone and [18F]altanserin have both given different outcomes in different studies of levels of HTR2A in subjects with schizophrenia.

One intriguing observation is that, if current studies are ranked by the mean age of the total cohort of subjects studied, then the two studies reporting decreases in the HTR2A in schizophrenia were completed on the youngest of the seven cohorts (Table 1). This raises the possibility that decreases in the HTR2A may be detectable by neuroimaging early in the disease process. If that is the case, could a decrease in cortical HTR2A be a potential prodromal marker to predict the transition from high risk to frank schizophrenia?



References:

Rasmussen, H, et al., (2010) Decreased frontal serotonin2A receptor binding in antipsychotic-naive patients with first-episode schizophrenia, Arch. Gen. Psychiatry 67, 9-16. Abstract

Meltzer, HY, (1995) The role of serotonin in schizophrenia and the place of serotonin-dopamine antagonist antipsychotics, J. Clin. Psychopharmacol. 15, 2S-3S. Abstract

Dean, B, (2003) The cortical serotonin2A receptor and the pathology of schizophrenia: a likely accomplice, J. Neurochem. 85, 1-13. Abstract

Rahimian, R, et al., (1995) Possible role of protein kinase C in regulation of 5-hydroxytryptamine 2A receptors in rat brain, Can. J. Physiol Pharmacol. 73, 1686-1691. Abstract

Dean, B, et al., (2008) Evidence for altered post-receptor modulation of the serotonin 2a receptor in schizophrenia, Schizophr. Res. 104, 185-197. Abstract

Gonzalez-Maeso, J, et al., (2008) Identification of a serotonin/glutamate receptor complex implicated in psychosis, Nature 452, 93-97. Abstract

Ngan, ET, et al., (2000) Decreased serotonin 2A receptor densities in neuroleptic-naive patients with schizophrenia: A PET study using [(18)F]setoperone, Am. J. Psychiatry 157, 1016-1018. Abstract

Okubo, Y, et al., (2000) Serotonin 5-HT2 receptors in schizophrenic patients studied by positron emission tomography, Life Sci. 66, 2455-2464. Abstract

Trichard, C, et al., (1998) No serotonin 5-HT2A receptor density abnormality in the cortex of schizophrenic patients studied with PET, Schizophr. Res. 31, 13-17. Abstract

Erritzoe, D, et al., (2008) Cortical and subcortical 5-HT2A receptor binding in neuroleptic-naive first-episode schizophrenic patients, Neuropsychopharmacology 33, 2435-2441. Abstract

Lewis, R, et al., (1999) Serotonin 5-HT2 receptors in schizophrenia: a PET study using [18F]setoperone in neuroleptic-naive patients and normal subjects, Am. J. Psychiatry 156, 72-78. Abstract

Verhoeff, NP, et al., (2000) A voxel-by-voxel analysis of [18F]setoperone PET data shows no substantial serotonin 5-HT(2A) receptor changes in schizophrenia, Psychiatry Res. 99, 123-135. Abstract

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Related News: Serotonin Receptors Appear Reduced in First-Episode Schizophrenia

Comment by:  Albert Adell
Submitted 18 January 2010
Posted 18 January 2010

The search for biomarkers in complex psychiatric disorders such as schizophrenia has been a crucial goal in clinical research, but remains to be fully accomplished. Serotonin 5-HT2A receptors have been associated with the pathophysiology and pharmacotherapy of schizophrenia for two main reasons: 5-HT2A receptor agonists such as LSD elicit hallucinogenic states in humans and second-generation, atypical antipsychotics are effective 5-HT2A receptor antagonists. However, 5-HT2A receptor agonists usually evoke visual hallucinations, whereas those associated with schizophrenia are commonly auditory (Hollister, 1962). Contradictory changes in 5-HT2A receptor density in cortical areas of the brain have been detected in several postmortem studies. The case-control study of Rasmussen and coworkers (Rasmussen et al., 2010) shows decreased 5-HT2A binding in the frontal cortex of antipsychotic-free, first-episode schizophrenics. Although similar findings were previously observed (Ngan et al., 2000; Erritzoe et al., 2008), the present study used the largest sample of patients to date, which gives further strength and validity to the results.

A question that remains to be answered is, What causes this reduced 5-HT2A binding? Although a hyperactive serotonergic transmission in the prefrontal cortex has been implicated in schizophrenia (Meltzer, 1989), the precise role of cortical serotonin on this effect is not fully understood. It has been shown in an animal model of the illness that antagonists of the NMDA glutamate receptor increased serotonin release in prefrontal cortex (Adams and Moghaddam, 2001; Amargós-Bosch et al., 2006; López-Gil et al., 2007), an effect prevented by atypical but not typical antipsychotic drugs (López-Gil et al., 2007; 2010). If these changes also occur in schizophrenia, the decreased 5-HT2A binding might be a compensatory effect resulting from increased cortical serotonergic transmission. Interestingly, although negative symptoms are usually thought to be associated with impaired serotonergic transmission in schizophrenia (Reynolds, 2004), the present study found a significant negative correlation between 5-HT2A binding in the frontal cortex and positive symptoms in the group of male patients. A study with a larger population is likely needed to draw unequivocal conclusions on that matter. Also, an important point not to be missed is that PET measures were performed, for obvious reasons, in a resting state. Differences in serotonergic changes may thus exist, depending on the pathological phase of the illness.

From a pharmacological point of view, it is worth noting that blockade of 5-HT2A receptors alone does not confer antipsychotic activity. Thus, clinical evaluation of the selective 5-HT2A antagonist, M100907, failed to demonstrate therapeutic efficacy (de Paulis, 2001), despite the high level of 5-HT2A receptor occupancy achieved in frontal cortex (Offord et al., 1999). With the exception of dopamine D2 receptor antagonists (Seeman et al., 1976), no other monoamine receptor antagonist has been shown to possess antipsychotic activity per se. Therefore, there is a clear need for targeting different transmitter receptors in order to achieve better treatment (as well as a lower side effect profile) of schizophrenia. What the study by Rasmussen and coworkers clearly points out is that 5-HT2A receptors in the frontal cortex are involved in the pathophysiology of schizophrenia, and this region plays a role in the pharmacological effects of antipsychotic drugs.

References:

Adams BW, Moghaddam B. Effect of clozapine, haloperidol, or M100907 on phencyclidine-activated glutamate efflux in the prefrontal cortex. Biol Psychiatry. 2001;50:750–757. Abstract

Amargós-Bosch M, López-Gil X, Artigas F, Adell A. Clozapine and olanzapine, but not haloperidol, suppress serotonin efflux in the medial prefrontal cortex elicited by phencyclidine and ketamine. Int J Neuropsychopharmacol. 2006;9:565–573. Abstract

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