First, Zhang and colleagues examine multiple polymorphisms in the D2 receptor gene and find that none of the "standard" ones that have been linked to clinical characteristics actually affected D2 receptor density in prefrontal cortex or striatum. However, they find that two other, previously unstudied polymorphisms altered the relative expression of short versus long isoforms of the D2 receptor, likely reflecting presynaptic and postsynaptic D2 receptors, respectively. These findings could provide a basis for understanding several perplexing effects in the literature, such as opposing effects of D2 receptor drugs on cognition in individuals with low and high working memory ability, who are shown here to have differential pre- versus postsynaptic D2 receptor function.

Further, the presynaptic receptor is thought to regulate phasic dopamine signaling via its autoreceptor functions (in addition to controlling glutamate release in corticostriatal terminals via the heteroreceptors alluded to in the article). Thus, based on current evidence, it is expected that these...  Read more

First, Zhang and colleagues examine multiple polymorphisms in the D2 receptor gene and find that none of the "standard" ones that have been linked to clinical characteristics actually affected D2 receptor density in prefrontal cortex or striatum. However, they find that two other, previously unstudied polymorphisms altered the relative expression of short versus long isoforms of the D2 receptor, likely reflecting presynaptic and postsynaptic D2 receptors, respectively. These findings could provide a basis for understanding several perplexing effects in the literature, such as opposing effects of D2 receptor drugs on cognition in individuals with low and high working memory ability, who are shown here to have differential pre- versus postsynaptic D2 receptor function.

Further, the presynaptic receptor is thought to regulate phasic dopamine signaling via its autoreceptor functions (in addition to controlling glutamate release in corticostriatal terminals via the heteroreceptors alluded to in the article). Thus, based on current evidence, it is expected that these polymorphisms should affect not only working memory, but also positive and negative reinforcement learning processes thought to depend on phasic dopamine signaling, and which are clearly implicated in the development of addictive habits.

Finally, these polymorphisms are in linkage disequilibrium with some of the other standard D2 SNPs that have been associated with alcohol abuse, schizophrenia, and sensitivity to reinforcement, and therefore may provide a more direct mechanistic explanation for these prior associations.

This fascinating paper contributes to our mechanistic understanding of a fundamental nonlinearity governing the response of prefrontal neurons during working memory to dopaminergic stimulation: the “inverted U” response curve (Goldman-Rakic et al., 2000), which proposes that an optimum range of dopaminergic stimulation exists, and that either too little or too much dopamine impairs tuning, or the relationship between task-relevant (“signal”) and task-irrelevant (“noise”) firing of these neurons. On the level of behavior, this is predicted to result in impaired working memory performance outside the optimum middle range, and this has been confirmed in a variety of species. This is a topic of high relevance for schizophrenia where prefrontal dysfunction and related cognitive deficits, and dopaminergic dysregulation, have long been in the center of research interest (Weinberger et al., 2001), and may be linked (  Read more

This fascinating paper contributes to our mechanistic understanding of a fundamental nonlinearity governing the response of prefrontal neurons during working memory to dopaminergic stimulation: the “inverted U” response curve (Goldman-Rakic et al., 2000), which proposes that an optimum range of dopaminergic stimulation exists, and that either too little or too much dopamine impairs tuning, or the relationship between task-relevant (“signal”) and task-irrelevant (“noise”) firing of these neurons. On the level of behavior, this is predicted to result in impaired working memory performance outside the optimum middle range, and this has been confirmed in a variety of species. This is a topic of high relevance for schizophrenia where prefrontal dysfunction and related cognitive deficits, and dopaminergic dysregulation, have long been in the center of research interest (Weinberger et al., 2001), and may be linked (Meyer-Lindenberg et al., 2002). In particular, evidence for abnormally decreased dopamine levels in prefrontal cortex would predict that patients with schizophrenia are positioned to the left of the optimum. This line of thought has recently received impetus from genetic studies on COMT, the major enzyme catabolizing dopamine in prefrontal cortex (Tunbridge et al., 2004). Neuroimaging studies have shown that genetic variants with high COMT activity are positioned to the left, those with lower activity nearer the optimum of the inverted U curve, and that this position predicts nonlinear response to amphetamine stimulation (Mattay et al., 2003), as well as interactions between dopamine synthesis and prefrontal response (Meyer-Lindenberg et al., 2005). Variants with sub- (Egan et al., 2001; Nicodemus et al., 2007) or superoptimal (Gothelf et al., 2005) stimulation were associated with schizophrenia risk. Task-related and task-unrelated prefrontal function reacted in opposite ways to genetic variation in dopamine synthesis, suggesting a tuning mechanism (Meyer-Lindenberg et al., 2005). Recently, interacting genetic variants in COMT have also been found to affect prefrontal cortex function in an inverted U fashion (Meyer-Lindenberg et al., 2006).

A seminal contribution to the cellular mechanisms of the inverted U curve is the paper by Williams (one of the authors of the current study) and Goldman-Rakic in Nature 1995 (Williams and Goldman-Rakic, 1995). In this work, dopamine D1 receptor antagonists were used and shown to increase prefrontal cell activity in low levels, whereas high levels inhibited firing. This implicated a mechanism related to D1 receptors and suggested that the neurons studied were to the right of the optimum on the inverted U curve, that is, their dopamine stimulation was excessive. The present study, from Amy Arnsten’s lab at Yale, further defines the cellular mechanisms underlying the inverted U curve in recordings from PFC neurons of awake behaving monkeys exposed to various levels of stimulation by a dopamine 1 receptor agonist. A spatial working memory paradigm was used, enabling the determination of the degree to which the neurons were tuned by comparing the firing rate to stimuli in the preferred spatial stimulus direction (“signal”) to the firing rate to nonpreferred stimuli (“noise”). The authors recorded both from neurons that were highly tuned (supposedly receiving optimum stimulation) and neurons that were less tuned. As would be predicted from the model, highly tuned neurons did not improve, or worsened, during stimulation, while weakly tuned neurons became more focused in their activity profile. It is not quite clear to me why the previous paper (Williams and Goldman-Rakic, 1995) found neurons that were predominantly to the right of the optimum, while this work identified neurons using a similar paradigm that were either to the left or near the optimum. Perhaps it is because Williams and Goldman-Rakic (Williams and Goldman-Rakic, 1995) screened neurons for a response to the D1 antagonist first. In both studies, extracellular dopamine was not actually measured, meaning that the state of basal stimulation can only be inferred indirectly from the response to the iontophoresed agonist or antagonist. Importantly, the effect of D1 stimulation was always suppressive; effects on tuning were due to the fact that the reduction in response to the signal and the noise were different in extent, such that for weakly tuned neurons and low levels of D1 stimulation, the noise firing was more suppressed than that of the signal, resulting in increased signal to noise. In a second set of pharmacological experiments, which included validation in a rat working memory model, the authors show that these effects are cAMP, but not PKC-dependent, suggesting a preferential cellular mechanism through Gs-proteins, which might be useful for exploration of more specific drug targets.

This work has interesting implications for our understanding of prefrontal function in schizophrenia. Since dopamine stimulation was found to be almost exclusively suppressive, cortical dopamine depletion in schizophrenia would be predicted to lead to relatively increased, but inefficient (untuned) cortical cognitive response, as has indeed been observed (Callicott et al., 2000). However, it is an open question precisely how cortical physiology assessed by imaging relates to these cellular events. The data by Arnsten suggest that each patch of prefrontal cortex will contain a population of neurons at various states of tuning that will respond differently to drug-induced or cognitively related changes in extracellular dopamine, with some improving, some decreasing their tuning. Depending on whether imaging signals and tasks are more sensitive to overall firing rate, or to specific signal-to-noise properties, the resulting blood flow change might be quite different. Perhaps this contributes to some of the puzzling discrepancies between hypo- and hyperactivation both being observed in comparable tasks and regions of prefrontal cortex in schizophrenia.

References:

1. Goldman-Rakic PS, Muly EC 3rd, Williams GV. D(1) receptors in prefrontal cells and circuits. Brain Res Brain Res Rev. 2000 Mar;31(2-3):295-301. Review. No abstract available. Abstract

2. Weinberger DR, Egan MF, Bertolino A, Callicott JH, Mattay VS, Lipska BK, Berman KF, Goldberg TE. Prefrontal neurons and the genetics of schizophrenia. Biol Psychiatry. 2001 Dec 1;50(11):825-44. Review. Abstract

3. Meyer-Lindenberg A, Miletich RS, Kohn PD, Esposito G, Carson RE, Quarantelli M, Weinberger DR, Berman KF. Reduced prefrontal activity predicts exaggerated striatal dopaminergic function in schizophrenia. Nat Neurosci. 2002 Mar;5(3):267-71. Abstract

4. Tunbridge EM, Bannerman DM, Sharp T, Harrison PJ. Catechol-o-methyltransferase inhibition improves set-shifting performance and elevates stimulated dopamine release in the rat prefrontal cortex. J Neurosci. 2004 Jun 9;24(23):5331-5. Abstract

5. Mattay VS, Goldberg TE, Fera F, Hariri AR, Tessitore A, Egan MF, Kolachana B, Callicott JH, Weinberger DR. Catechol O-methyltransferase val158-met genotype and individual variation in the brain response to amphetamine. Proc Natl Acad Sci U S A. 2003 May 13;100(10):6186-91. Epub 2003 Apr 25. Abstract

6. Meyer-Lindenberg A, Kohn PD, Kolachana B, Kippenhan S, McInerney-Leo A, Nussbaum R, Weinberger DR, Berman KF. Midbrain dopamine and prefrontal function in humans: interaction and modulation by COMT genotype. Nat Neurosci. 2005 May;8(5):594-6. Epub 2005 Apr 10. Abstract

7. Egan MF, Goldberg TE, Kolachana BS, Callicott JH, Mazzanti CM, Straub RE, Goldman D, Weinberger DR. Effect of COMT Val108/158 Met genotype on frontal lobe function and risk for schizophrenia. Proc Natl Acad Sci U S A. 2001 Jun 5;98(12):6917-22. Epub 2001 May 29. Abstract

8. Nicodemus KK, Kolachana BS, Vakkalanka R, Straub RE, Giegling I, Egan MF, Rujescu D, Weinberger DR. Evidence for statistical epistasis between catechol-O-methyltransferase (COMT) and polymorphisms in RGS4, G72 (DAOA), GRM3, and DISC1: influence on risk of schizophrenia. Hum Genet. 2007 Feb;120(6):889-906. Epub 2006 Sep 28. Abstract

9. Gothelf D, Eliez S, Thompson T, Hinard C, Penniman L, Feinstein C, Kwon H, Jin S, Jo B, Antonarakis SE, Morris MA, Reiss AL. COMT genotype predicts longitudinal cognitive decline and psychosis in 22q11.2 deletion syndrome. Nat Neurosci. 2005 Nov;8(11):1500-2. Epub 2005 Oct 23. Abstract

10. Meyer-Lindenberg A, Nichols T, Callicott JH, Ding J, Kolachana B, Buckholtz J, Mattay VS, Egan M, Weinberger DR. Impact of complex genetic variation in COMT on human brain function. Mol Psychiatry. 2006 Sep;11(9):867-77, 797. Epub 2006 Jun 20. Abstract

11. Williams GV, Goldman-Rakic PS. Modulation of memory fields by dopamine D1 receptors in prefrontal cortex. Nature. 1995 Aug 17;376(6541):572-5. Abstract

12. Callicott JH, Bertolino A, Mattay VS, Langheim FJ, Duyn J, Coppola R, Goldberg TE, Weinberger DR. Physiological dysfunction of the dorsolateral prefrontal cortex in schizophrenia revisited. Cereb Cortex. 2000 Nov;10(11):1078-92. Abstract

In this landmark study, Arnsten and colleagues used a full dopamine agonist in awake behaving monkeys to make key points about the inverted U response at the cellular level and how this maps to the behavioral level. There were a number of surprises. The first was that stimulation of the D1 receptor had consistently suppressive effects on neuronal firing during delays in a working memory task. The second was that when responses were optimized, suppressive effects differentially affected non-preferred directional neurons, rather than preferred direction neurons. Thus, it appeared that noise was reduced rather than signal amplified. Too much D1 stimulation resulted in suppression of both classes of neurons.

The implications of this work are important because it suggests that there is a neurobiological algorithm at work that can reliably produce this unexpected physiological pattern (perhaps as the authors suggest on the basis of baseline activity). It remains to be elucidated whether the D1 receptor effects are mediated by glutamatergic neurons or GABA interneurons, or both....  Read more

In this landmark study, Arnsten and colleagues used a full dopamine agonist in awake behaving monkeys to make key points about the inverted U response at the cellular level and how this maps to the behavioral level. There were a number of surprises. The first was that stimulation of the D1 receptor had consistently suppressive effects on neuronal firing during delays in a working memory task. The second was that when responses were optimized, suppressive effects differentially affected non-preferred directional neurons, rather than preferred direction neurons. Thus, it appeared that noise was reduced rather than signal amplified. Too much D1 stimulation resulted in suppression of both classes of neurons.

The implications of this work are important because it suggests that there is a neurobiological algorithm at work that can reliably produce this unexpected physiological pattern (perhaps as the authors suggest on the basis of baseline activity). It remains to be elucidated whether the D1 receptor effects are mediated by glutamatergic neurons or GABA interneurons, or both. There is another layer of complexity to the story. As Arnsten and colleagues note, possible excitatory influences of D1 stimulation may not have been observed because endogenous dopamine had already triggered this process. It is unclear if D2 receptors in the cortex have a role in shaping or terminating this activity.

Last, it is tempting to speculate about the implications of these findings for other types of tasks that engage prefrontal cortex in humans. What does tuning mean in the context of tasks like the N Back which demands updating, the ID/ED test from the CANTAB, which involves suppression of salient distractors at early set shifting stages, or a task which demands heavy doses of cognitive control like the flanker task, all of which have been shown to be sensitive to manipulations of the dopamine system (Goldberg et al., 2003; Jazbec et al., 2007; Diaz-Asper et al., in press; Blasi et al., 2005)?

A toast to success, or new wine in an old skin?
Patil et al. present a landmark study. It is the kind of study that represents the best of how science should work. It pulls together the numerous strands of schizophrenia research from the last 50 years, from the development of PCP psychosis as a model for schizophrenia in the late 1950s, through the links to glutamate, the discovery of metabotropic receptors, and the seminal discovery in 1998 by Moghaddam and Adams that metabotropic glutamate 2/3 receptor (mGluR2/3) agonists reverse the neurochemical and behavioral effects of PCP in rodents (Moghaddam and Adams, 1998. The story would not be possible without the elegant medicinal chemistry of Eli Lilly, which provided the compounds needed to test the theories; the research support of NIMH and NIDA, who have been consistent supporters of the “PCP theory”; or the hard work of academic investigators, who provided the theories and the platforms for testing. The study is large and the effects robust. Assuming they replicate...  Read more

A toast to success, or new wine in an old skin?
Patil et al. present a landmark study. It is the kind of study that represents the best of how science should work. It pulls together the numerous strands of schizophrenia research from the last 50 years, from the development of PCP psychosis as a model for schizophrenia in the late 1950s, through the links to glutamate, the discovery of metabotropic receptors, and the seminal discovery in 1998 by Moghaddam and Adams that metabotropic glutamate 2/3 receptor (mGluR2/3) agonists reverse the neurochemical and behavioral effects of PCP in rodents (Moghaddam and Adams, 1998. The story would not be possible without the elegant medicinal chemistry of Eli Lilly, which provided the compounds needed to test the theories; the research support of NIMH and NIDA, who have been consistent supporters of the “PCP theory”; or the hard work of academic investigators, who provided the theories and the platforms for testing. The study is large and the effects robust. Assuming they replicate (and there is no reason to suspect that they will not), this compound, and others like it, will represent the first rationally developed drugs for schizophrenia. Patients will benefit, drug companies will benefit, and academic investigators and NIH can feel that they have played their role in new treatment development.

Nevertheless, it is always the prerogative of the academic investigator to ask for more. In this case, we do not yet know if this will be a revolution in the treatment of schizophrenia, or merely a platform shift. What is striking about the study, aside from the effectiveness of LY2140023, is the extremely close parallel in both cross-sectional and temporal pattern of response between it and olanzapine. Both drugs change positive and negative symptoms in roughly equal proportions, despite their different pharmacological targets. Both drugs show approximately equal slopes over a 4-week period. There is no intrinsic reason why symptoms should require 4 or more weeks to resolve, or why negative and positive symptoms should change in roughly the same proportion with two medications from two such different categories, except that evidently they do.

There are many things about mGluR2/3 agonists that we do not yet know. The medication used here was administered at a single, fixed dose. It is possible that a higher dose might have been better, and that optimal results have not yet been achieved. The medications were used in parallel. It is possible that combined medication might be more effective than treatment with either class alone. The study was stopped at 4 weeks, with the trend lines still going down. It is possible that longer treatment duration in future studies might lead to even more marked improvement and that the LY and olanzapine lines might separate. No cognitive data are reported. It is possible that marked cognitive improvement will be observed with these compounds when cognition is finally tested, in which case a breakthrough in pharmacotherapy will clearly have been achieved.

If one were to look at the glass as half empty, then the question is why the metabotropic agonist did not beat olanzapine, and why the profiles of response were so similar. If these compounds work, as suggested in the article by modulating mesolimbic dopamine, then it is possible that metabotropic agonists will share the same therapeutic limitations as current antipsychotics—good drugs certainly and without the metabolic side effects of olanzapine, but not “cures.” The recent study with the glycine transport inhibitor sarcosine by Lane and colleagues showed roughly similar overall change in PANSS total (-17.1 pts) to that reported in this study, but larger change in negative symptoms (-5.5 pts), and less change in positive symptoms (-2.3 pts) in a similar type of patient population. Onset of effect in the sarcosine study also appeared somewhat faster. The sarcosine study was smaller (n = 20) and did not include a true placebo group. As with the Lilly study, it was only 4 weeks in duration, and did not include cognitive measures. It also included only two, possibly non-optimized doses. As medications become increasingly available to test a variety of mechanisms, side-by-side comparisons will become increasingly important.

There are also causes for concern and effects to be watched. For example, a side effect signal was observed for affect lability in the LY group, at about the same prevalence rate as weight increase in the olanzapine group. What this means for the mechanism and how this will effect treatment remains to be determined. Since these medications are agonists, there is concern that metabotropic receptors may downregulate over time. Thus, whether treatment effects increase, decrease, or remain constant over the course of long-term treatment will need to be determined. Nevertheless, 50 years since the near-contemporaneous discovery of both PCP and chlorpromazine, it appears that glutamatergic drugs for schizophrenia may finally be on the horizon.

References:

Moghaddam B, Adams BW. Reversal of phencyclidine effects by a group II metabotropic glutamate receptor agonist in rats. Science. 1998 Aug 28;281(5381):1349-52. Abstract

The publication by Iizuka and colleagues is an important advance toward unraveling the basic biology of psychosis in general, and schizophrenia in particular. This is because they have found that a pathway known to be genetically associated with schizophrenia can alter the surface expression of dopamine D2 receptors. D2 continues to be the main target for all antipsychotic drugs (including aripiprazole and even the new Lilly glutamate agonists that have a potent affinity for D2High receptors).

In fact, the authors of this excellent study may do well to go one step further by testing whether the downregulation of dysbindin actually increases the proportion of D2 receptors that are in the high-affinity state, namely D2High. This is because all schizophrenia animal models markedly increase the proportion of D2High receptors by 100 to 900 percent (Seeman et al., 2005; Seeman et al., 2006). This generalization holds for animal models based on brain lesions, sensitization by...  Read more

The publication by Iizuka and colleagues is an important advance toward unraveling the basic biology of psychosis in general, and schizophrenia in particular. This is because they have found that a pathway known to be genetically associated with schizophrenia can alter the surface expression of dopamine D2 receptors. D2 continues to be the main target for all antipsychotic drugs (including aripiprazole and even the new Lilly glutamate agonists that have a potent affinity for D2High receptors).

In fact, the authors of this excellent study may do well to go one step further by testing whether the downregulation of dysbindin actually increases the proportion of D2 receptors that are in the high-affinity state, namely D2High. This is because all schizophrenia animal models markedly increase the proportion of D2High receptors by 100 to 900 percent (Seeman et al., 2005; Seeman et al., 2006). This generalization holds for animal models based on brain lesions, sensitization by amphetamine, phencyclidine, cocaine, caffeine or corticosterone, birth injury, social isolation, and more than 15 gene deletions in pathways for glutamate (NMDA), dopamine, GABA, acetylcholine, and norepinephrine. Although the proportion of D2High receptors invariably increases markedly, the total number of D2 receptors is generally unchanged, slightly reduced, or modestly elevated.

This publication for the first time bridges the hitherto wide gap between genetics and the antipsychotic targeting of the main cause of psychotic signs and symptoms, which is excessive D2 activity, presumably that of D2High, the functional component of D2.

References:

Seeman P, Weinshenker D, Quirion R, Srivastava LK, Bhardwaj SK, Grandy DK, Premont RT, Sotnikova TD, Boksa P, El-Ghundi M, O'dowd BF, George SR, Perreault ML, Männistö PT, Robinson S, Palmiter RD, Tallerico T. Dopamine supersensitivity correlates with D2High states, implying many paths to psychosis. Proc Natl Acad Sci U S A. 2005 Mar 1;102(9):3513-8. Epub 2005 Feb 16. Abstract

Seeman P, Schwarz J, Chen JF, Szechtman H, Perreault M, McKnight GS, Roder JC, Quirion R, Boksa P, Srivastava LK, Yanai K, Weinshenker D, Sumiyoshi T. Psychosis pathways converge via D2high dopamine receptors. Synapse. 2006 Sep 15;60(4):319-46. Review. Abstract

The study by Iizuka and colleagues is indeed very interesting. It suggests that one of the most promising risk genes for schizophrenia, the dysbindin gene, may functionally interact with dopamine D2 receptors. The D2 receptor itself is an old candidate in the study of schizophrenia, mostly because until very recently all antipsychotic medication had been directed against D2 receptors. But in addition, PET imaging studies have shown that the density and occupancy of D2 receptors is increased in drug-free and drug-naïve patients with schizophrenia.

How could this increase arise? In a subpopulation of patients it may be due to a polymorphism in the D2 receptor gene, the C957T polymorphism. The C-allele increases mRNA stability and has been found to be associated with schizophrenia, though obviously not all patients carry the C-allele. Iizuka and colleagues found an independent way in which the genetic risk factor dysbindin may upregulate D2 receptor signaling. Because dysbindin is downregulated in the brains of patients with schizophrenia, they used siRNA technology to study...  Read more

The study by Iizuka and colleagues is indeed very interesting. It suggests that one of the most promising risk genes for schizophrenia, the dysbindin gene, may functionally interact with dopamine D2 receptors. The D2 receptor itself is an old candidate in the study of schizophrenia, mostly because until very recently all antipsychotic medication had been directed against D2 receptors. But in addition, PET imaging studies have shown that the density and occupancy of D2 receptors is increased in drug-free and drug-naïve patients with schizophrenia.

How could this increase arise? In a subpopulation of patients it may be due to a polymorphism in the D2 receptor gene, the C957T polymorphism. The C-allele increases mRNA stability and has been found to be associated with schizophrenia, though obviously not all patients carry the C-allele. Iizuka and colleagues found an independent way in which the genetic risk factor dysbindin may upregulate D2 receptor signaling. Because dysbindin is downregulated in the brains of patients with schizophrenia, they used siRNA technology to study the molecular consequences of decreased dysbindin levels in cell culture.

They found that downregulation of dysbindin increases D2 receptor density in the outer cell membrane, suppresses dopamine-induced D2 receptor internalization, and increases D2 receptor signaling. The study is very promising but requires further confirmation.

How specific are the observed effects for D2 receptors? Because dysbindin is involved in both membrane trafficking and degradation of synaptic vesicles, knocking down dysbindin in growing cells may affect many physiological processes, one of them being D2 receptor signaling. Does quinpirole treatment differentially affect GTPgS incorporation in siRNA and control cells? This would be a more immediate way of looking at D2 signaling than measuring CREB phosphorylation. And, of course, the most important question is, What will happen in vivo? Maybe the sandy mouse, which carries a deletion in the dysbindin gene, could be of help here. Using these mice for a similar line of experiments may answer this question.

Iizuka and colleagues found an exciting new functional interaction between two major molecules involved in schizophrenia. I believe that these are the kind of interactions we have to look for if we want to understand complex genetic disorders such as schizophrenia.