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

In the field of the psychopharmacology of schizophrenia, a lot of research work has been done on dopaminergic systems. Thus, this research news is excellent news because it explores an alternative neurotransmission system in schizophrenia, the glutamatergic system. Since the work of Dr. Bita Moghaddam in 1998, published in Science, a lot of research studies have turned to the important role of glutamate in schizophrenia. More studies are needed to focus on the exact role of this neurotransmitter.

View all comments by Patricia Estani

The pioneering research over the past decade on group II metabotropic glutamate receptor (mGluR) agonists from the Lilly Labs and Bita Moghaddam's research group has provided a strong foundation for the view that activation of these receptors reduces schizophrenia-like behaviors in the PCP and amphetamine models. These phase 2 clinical trials bring mGluR agonists one step closer to clinical use as therapy or co-therapy.

These same data provide the foundation for current and future research aimed at increasing the concentration of the peptide transmitter, N-acetylaspartylglutamate (NAAG) in the synaptic cleft by systemic administration of NAAG peptidase inhibitors. NAAG is the third most prevalent transmitter in the mammalian nervous system and a selective group II mGluR agonist with preference for mGluR3 (Neale et al., 2005). Our research group demonstrated that a NAAG peptidase inhibitor substantially reduces positive and negative behaviors induced in PCP models of schizophrenia (Olszewski et al., 2007;   Read more

The pioneering research over the past decade on group II metabotropic glutamate receptor (mGluR) agonists from the Lilly Labs and Bita Moghaddam's research group has provided a strong foundation for the view that activation of these receptors reduces schizophrenia-like behaviors in the PCP and amphetamine models. These phase 2 clinical trials bring mGluR agonists one step closer to clinical use as therapy or co-therapy.

These same data provide the foundation for current and future research aimed at increasing the concentration of the peptide transmitter, N-acetylaspartylglutamate (NAAG) in the synaptic cleft by systemic administration of NAAG peptidase inhibitors. NAAG is the third most prevalent transmitter in the mammalian nervous system and a selective group II mGluR agonist with preference for mGluR3 (Neale et al., 2005). Our research group demonstrated that a NAAG peptidase inhibitor substantially reduces positive and negative behaviors induced in PCP models of schizophrenia (Olszewski et al., 2007; Olszewski et al., 2004). These NAAG peptidase inhibition studies parallel the preclinical studies from Lilly and Bita Moghaddam on mGluR agonists in animal models of schizophrenia. Since NAAG is an endogenous transmitter, it can be argued that elevating its levels following synaptic release by reducing the rate of its inactivation (analogous to SSRI and serotonin) is a more physiologic means of activating the mGluR, and thus this may be better tolerated with fewer side effects than the continuous receptor activation that is obtained via systemic administration of a receptor agonist.

These phase 2 clinical trials clearly brighten the prospects for both lines of new drug development for treatment of schizophrenia.

References:

Neale JH, Olszewski RT, Gehl LM, Wroblewska B, Bzdega T. The neurotransmitter N-acetylaspartylglutamate in models of pain, ALS, diabetic neuropathy, CNS injury and schizophrenia. Trends Pharmacol Sci. 2005 Sep;26(9):477-84. Review. Abstract

Olszewski RT, Wegorzewska MM, Monteiro AC, Krolikowski K, Zhou J, Kozikowski AP, Long K, Mastropaolo J, Deutsch S, Neale JH. PCP and MK-801 induced behaviors reduced by NAAG peptidase inhibition via metabotropic glutamate receptors. E-pub in advance of print, Biological Psychiatry, 2007.

Olszewski RT, Bukhari N, Zhou J, Kozikowski AP, Wroblewski JT, Shamimi-Noori S, Wroblewska B, Bzdega T, Vicini S, Barton FB, Neale JH. NAAG peptidase inhibition reduces locomotor activity and some stereotypes in the PCP model of schizophrenia via group II mGluR. J Neurochem. 2004 May;89(4):876-85. Abstract

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.

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...  Read more

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

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...  Read more

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

The article by Homayoun and Moghaddam is another in an excellent series of articles investigating effects of metabotropic agents on brain function relevant to schizophrenia. As opposed to previous studies by this group that targeted rodent medial prefrontal cortex, which is used as a model of dorsolateral prefrontal cortex in humans, this study targets orbitofrontal cortex. The main finding of this study, like prior studies by this group, is that effects of the NMDA antagonist MK-801 can be reversed by the LY354740, a selective metabotropic group 2/3 agonist. LY354740 has previously been shown to reverse ketamine effects in humans (Krystal et al., 2005) and to be effective in treatment of generalized anxiety disorder in humans (Dunayevich et al., 2008). It is pharmacologically related to LY2130023 (Rorick-Kehn et al., 2007), a compound that has shown efficacy in treatment of schizophrenia (Patil...  Read more

The article by Homayoun and Moghaddam is another in an excellent series of articles investigating effects of metabotropic agents on brain function relevant to schizophrenia. As opposed to previous studies by this group that targeted rodent medial prefrontal cortex, which is used as a model of dorsolateral prefrontal cortex in humans, this study targets orbitofrontal cortex. The main finding of this study, like prior studies by this group, is that effects of the NMDA antagonist MK-801 can be reversed by the LY354740, a selective metabotropic group 2/3 agonist. LY354740 has previously been shown to reverse ketamine effects in humans (Krystal et al., 2005) and to be effective in treatment of generalized anxiety disorder in humans (Dunayevich et al., 2008). It is pharmacologically related to LY2130023 (Rorick-Kehn et al., 2007), a compound that has shown efficacy in treatment of schizophrenia (Patil et al., 2007).

In addition, the study builds upon prior studies of mGluR5 agonists (e.g., Darrah et al., 2008) to show that CDPPB, a novel modulator of mGluR5 receptors, also reverses acute effects of MK-801. mGluR5 receptors interact closely with NMDA receptors. It has been known for a long time that mGluR5 antagonists induce symptoms similar to those of NMDA antagonists, suggesting a potential role for agents that can stimulate mGluR5 activity. However, mGluR5 receptors are prone to downregulation following application of agonists, so the evaluation of mGluR5 receptors as a therapeutic target in schizophrenia has had to await development of high-affinity, CNS penetrant mGluR5 modulators that do not cause desensitization. The similar effects of an mGluR2/3 agonist and an mGluR5 modulator suggest that multiple approaches may be taken to normalize NMDA function in schizophrenia, including modulation of both presynaptic glutamate and postsynaptic NMDA function. mGluR5 receptors are active also in visual cortex (Sarihi et al., 2008), and so would potentially reverse effects of NMDA antagonists on sensory, as well as frontal deficits associated with schizophrenia.

In our own research studies, we have found that structural white matter alterations in orbitofrontal cortex correlate with ability to identify emotion (Leitman et al., 2007), attesting to the importance of this brain region to cognitive dysfunction in schizophrenia. Structural change in this region also correlates with aggression (Hoptman et al., 2005), which is an important issue determining clinical outcome in individuals with schizophrenia. Our findings thus support the concept that glutamatergic neurotransmission within orbitofrontal cortex may play as important a role in schizophrenia as dysfunction within dorsolateral prefrontal cortex, and deserves to be studied with equal fervor.

Despite the tremendous value of the study, every silver lining must have its cloud. In this case, the caveat relates to the finding that effects of MK-801 in this model were also reversed by haloperidol and clozapine. On the one hand, it is good news, as it suggests that metabotropic compounds may be as effective as antipsychotics in treating the well-known dopaminergic dysregulation associated with schizophrenia. In the one published clinical trial of LY2130023 (Patil et al., 2007), the compound proved almost as effective as olanzapine despite use of what may not have been an optimized dose.

On the other hand, however, it suggests that the orbitofrontal model, like the prior dorsolateral model, does not yet capture the aspects of schizophrenia that respond poorly to antipsychotics, such as primary negative symptoms and cognitive dysfunction. It is important to develop compounds that are as good as antipsychotics in treating positive symptoms, but without the well-known side metabolic and motor side effects. However, it is even more important to develop treatments that target aspects of schizophrenia that remain unresponsive to current therapeutic approaches. To date, no clinical data are available regarding effects of either mGlu2/3 agonists or mGlu5 modulators on neurocognition in humans. The ultimate challenge may be to show that metabotropic modulators can reverse effects of NMDA antagonists in models where antipsychotics such as haloperidol or clozapine prove ineffective. Another critical issue is whether these compounds will be effective during longer-term treatment (Imre et al., 2006). To do so, longer-term treatment studies are required. Nevertheless, these data provide further hope to the development of non-dopaminergic treatment approaches in schizophrenia.

References:

Darrah JM, Stefani MR, Moghaddam B. Interaction of N-methyl-D-aspartate and group 5 metabotropic glutamate receptors on behavioral flexibility using a novel operant set-shift paradigm. Behav Pharmacol. 2008 May 1;19(3):225-34. Abstract

Dunayevich E, Erickson J, Levine L, Landbloom R, Schoepp DD, Tollefson GD. Efficacy and tolerability of an mGlu2/3 agonist in the treatment of generalized anxiety disorder. Neuropsychopharmacology. 2008 Jun 1;33(7):1603-10. Abstract

Hoptman MJ, Volavka J, Weiss EM, Czobor P, Szeszko PR, Gerig G, Chakos M, Blocher J, Citrome LL, Lindenmayer JP, Sheitman B, Lieberman JA, Bilder RM. Quantitative MRI measures of orbitofrontal cortex in patients with chronic schizophrenia or schizoaffective disorder. Psychiatry Res. 2005 Nov 30;140(2):133-45. Abstract

Imre G, Fokkema DS, Ter Horst GJ. Subchronic administration of LY354740 does not modify ketamine-evoked behavior and neuronal activity in rats. Eur J Pharmacol. 2006 Aug 21;544(1-3):77-81. Abstract

Krystal JH, Abi-Saab W, Perry E, D'Souza DC, Liu N, Gueorguieva R, McDougall L, Hunsberger T, Belger A, Levine L, Breier A. Preliminary evidence of attenuation of the disruptive effects of the NMDA glutamate receptor antagonist, ketamine, on working memory by pretreatment with the group II metabotropic glutamate receptor agonist, LY354740, in healthy human subjects. Psychopharmacology (Berl). 2005 Apr 1;179(1):303-9. Abstract

Leitman DI, Hoptman MJ, Foxe JJ, Saccente E, Wylie GR, Nierenberg J, Jalbrzikowski M, Lim KO, Javitt DC. The neural substrates of impaired prosodic detection in schizophrenia and its sensorial antecedents. Am J Psychiatry. 2007 Mar 1;164(3):474-82. Abstract

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

Rorick-Kehn LM, Johnson BG, Burkey JL, Wright RA, Calligaro DO, Marek GJ, Nisenbaum ES, Catlow JT, Kingston AE, Giera DD, Herin MF, Monn JA, McKinzie DL, Schoepp DD. Pharmacological and pharmacokinetic properties of a structurally novel, potent, and selective metabotropic glutamate 2/3 receptor agonist: in vitro characterization of agonist (-)-(1R,4S,5S,6S)-4-amino-2-sulfonylbicyclo[3.1.0]-hexane-4,6-dicarboxylic acid (LY404039). J Pharmacol Exp Ther. 2007 Apr 1;321(1):308-17. Abstract

Sarihi A, Jiang B, Komaki A, Sohya K, Yanagawa Y, Tsumoto T. Metabotropic glutamate receptor type 5-dependent long-term potentiation of excitatory synapses on fast-spiking GABAergic neurons in mouse visual cortex. J Neurosci. 2008 Jan 30;28(5):1224-35. Abstract

Homayoun and Moghaddam (PNAS) present important new data concerning the glutamatergic system and psychosis. They suggest the orbital frontal cortex (OFC) is particularly important in the pathophysiology of schizophrenia. They show that treatment with an NMDA receptor (NMDAR) antagonist induces OFC pyramidal neuron hyperactivity (secondary to GABA interneuron hypoactivity). This was reversed with haloperidol, clozapine, and a selective mGlu2/3 agonist, LY354740. This brief essay emphasizes how their findings support hypotheses of a common pathway in the biology of psychotic disorders. This group’s work (Adams et al., 2001; Moghaddam and Adams, 1998) contributes to an extensive body of research on the biology of psychosis. Human research shows that extensive frontal cortical systems and diverse molecular interactions may converge to form a common pathway to produce psychosis.

In their formulations of schizophrenia, Olney (Olney and Farber,...  Read more

Homayoun and Moghaddam (PNAS) present important new data concerning the glutamatergic system and psychosis. They suggest the orbital frontal cortex (OFC) is particularly important in the pathophysiology of schizophrenia. They show that treatment with an NMDA receptor (NMDAR) antagonist induces OFC pyramidal neuron hyperactivity (secondary to GABA interneuron hypoactivity). This was reversed with haloperidol, clozapine, and a selective mGlu2/3 agonist, LY354740. This brief essay emphasizes how their findings support hypotheses of a common pathway in the biology of psychotic disorders. This group’s work (Adams et al., 2001; Moghaddam and Adams, 1998) contributes to an extensive body of research on the biology of psychosis. Human research shows that extensive frontal cortical systems and diverse molecular interactions may converge to form a common pathway to produce psychosis.

In their formulations of schizophrenia, Olney (Olney and Farber, 1995), Farber (Farber et al., 2002), and Tamminga (Tamminga et al., 1987) suggested a prominent role for disturbed glutamatergic neurotransmission. Human neurometabolic imaging studies using the NMDAR antagonist ketamine subsequently demonstrated marked brain metabolic hyperactivity. Using blood flow and glucose utilization as surrogate markers of neural activity investigators characterized the brain response to intravenous ketamine administration (Breier et al., 1997; Holcomb et al., 2005; Lahti et al., 1995; Vollenweider et al., 1997). Frontal and anterior cingulate (rostral component) regions of healthy volunteers and schizophrenic participants became hypermetabolic. But it is important to note that hypermetabolic response patterns are also generated in other human, psychotogenic drug models of psychosis. These include high dose amphetamine (Vollenweider et al., 1998), psilocybin (Gouzoulis-Mayfrank et al., 1999; Vollenweider et al., 1997), and cannabis (Mathew et al., 1989; O'Leary et al., 2007).

There is now compelling evidence to directly link cortical metabolic patterns to cortical glutamate/glutamine dynamics (Rothman et al., 1999). Rowland and colleagues’ magnetic resonance spectroscopy (MRS) study of ketamine given to healthy volunteers demonstrated a significant elevation in rostral anterior cingulate glutamine, a putative marker of increased glutamate release (Rowland et al., 2005). It seems reasonable to interpret Theberge and colleagues’ MRS study of never treated schizophrenia (Theberge et al., 2002) as a chemical confirmation of Soyka’s neurometabolic study, also of unmedicated schizophrenic patients (Soyka et al., 2005). Theberge found elevated glutamine in the anterior cingulate. Soyka found elevated glucose utilization in the frontal cortex. These studies, taken together, implicate increased glutamate release as a common mechanism in the pathology of early schizophrenia. Psychosis may arise from NMDA receptor antagonism (ketamine and PCP), stimulation of the 5-HT 2A-mGluR2 complex (psilocybin), or direct stimulation of the CB1 receptor on GABA interneurons (Katona and Freund, 2008). In each instance the consequence is an acute and robust glutamate release caused by disinhibition of pyramidal neurons.

Though Homayoun and Moghaddam have provided an elegant description of this phenomenon in the OFC, it is likely to be equally important in the medial and dorsolateral prefrontal cortex, as well as the anterior cingulate cortex. But the methodology and theory of this paper should help clinical investigators. The thoughtful study of metabotropic glutamatergic receptors and their clinical application (Patil et al., 2007) will go far to illuminate the subtle pathophysiology of psychosis.

References:

1. 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

2. Breier A, Malhotra AK, Pinals DA, Weisenfeld NI, Pickar D: Association of ketamine-induced psychosis with focal activation of the prefrontal cortex in healthy volunteers. Am. J. Psychiatry 1997; 154:805-811. Abstract

3. Farber NB, Kim SH, Dikranian K, Jiang XP, Heinkel C: Receptor mechanisms and circuitry underlying NMDA antagonist neurotoxicity. Mol. Psychiatry 2002; 7:32-43. Abstract

4. Gonzalez-Maeso J, Ang RL, Yuen T, Chan P, Weisstaub NV, Lopez-Gimenez 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; 452:93-97. Abstract

5. Gouzoulis-Mayfrank E, Schreckenberger M, Sabri O, Arning C, Thelen B, Spitzer M, Kovar KA, Hermle L, Bull U, Sass H: Neurometabolic effects of psilocybin, 3,4-methylenedioxyethylamphetamine (MDE) and d-methamphetamine in healthy volunteers. A double-blind, placebo-controlled PET study with [18F]FDG. Neuropsychopharmacology 1999; 20:565-581. Abstract

6. Holcomb HH, Lahti AC, Medoff DR, Cullen T, Tamminga CA: Effects of noncompetitive NMDA receptor blockade on anterior cingulate cerebral blood flow in volunteers with schizophrenia. Neuropsychopharmacology 2005; 30:2275-2282. Abstract

7. Katona I, Freund TF: Endocannabinoid signaling as a synaptic circuit breaker in neurological disease. Nat. Med. 2008; 14:923-930. Abstract

8. Lahti AC, Holcomb HH, Medoff DR, Tamminga CA: Ketamine activates psychosis and alters limbic blood flow in schizophrenia. Neuroreport 1995; 6:869-872. Abstract

9. Mathew RJ, Wilson WH, Tant SR: Acute changes in cerebral blood flow associated with marijuana smoking. Acta Psychiatr. Scand. 1989; 79:118-128. Abstract

10. Moghaddam B, Adams BW: Reversal of phencyclidine effects by a group II metabotropic glutamate receptor agonist in rats. Science 1998; 281:1349-1352. Abstract

11. O'Leary DS, Block RI, Koeppel JA, Schultz SK, Magnotta VA, Ponto LB, Watkins GL, Hichwa RD: Effects of smoking marijuana on focal attention and brain blood flow. Hum. Psychopharmacol. 2007; 22:135-148. Abstract

12. Olney JW, Farber NB: NMDA antagonists as neurotherapeutic drugs, psychotogens, neurotoxins, and research tools for studying schizophrenia. Neuropsychopharmacology 1995; 13:335-345. Abstract

13. 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; 13:1102-1107. Abstract

14. Rothman DL, Sibson NR, Hyder F, Shen J, Behar KL, Shulman RG: In vivo nuclear magnetic resonance spectroscopy studies of the relationship between the glutamate-glutamine neurotransmitter cycle and functional neuroenergetics. Philos. Trans. R. Soc. Lond B Biol. Sci. 1999; 354:1165-1177. Abstract

15. Rowland, L. M., Bustillo, J. R., Mullins, P. G., Jung, R. E., Lenroot, R., Landgraf, E., Barrow, R, Yeo, R, Lauriello, J, and Brooks, W. M. Effects of ketamine on anterior cingulate glutamate metabolism in healthy humans: a 4-T Proton MRS study. Am. J. Psychiatry 162(2), 394-396. 2005. Abstract

16. Soyka M, Koch W, Moller HJ, Ruther T, Tatsch K: Hypermetabolic pattern in frontal cortex and other brain regions in unmedicated schizophrenia patients. Results from a FDG-PET study. Eur. Arch. Psychiatry Clin.Neurosci. 2005; 255:308-312. Abstract

17. Tamminga CA, Tanimoto K, Kuo S, Chase TN, Contreras PC, Rice KC, Jackson AE, O'Donohue TL: PCP-induced alterations in cerebral glucose utilization in rat brain: blockade by metaphit, a PCP-receptor-acylating agent. Synapse 1987; 1:497-504. Abstract

18. Theberge J, Bartha R, Drost DJ, Menon RS, Malla A, Takhar J, Neufeld RW, Rogers J, Pavlosky W, Schaefer B, Densmore M, Al Semaan Y, Williamson PC: Glutamate and glutamine measured with 4.0 T proton MRS in never-treated patients with schizophrenia and healthy volunteers. Am. J. Psychiatry 2002; 159:1944-1946. Abstract

19. Vollenweider FX, Leenders KL, Scharfetter C, Antonini A, Maguire P, Missimer J, Angst J: Metabolic hyperfrontality and psychopathology in the ketamine model of psychosis using positron emission tomography (PET) and [18F]fluorodeoxyglucose (FDG). Eur. Neuropsychopharmacol. 1997; 7:9-24. Abstract

20. Vollenweider FX, Leenders KL, Scharfetter C, Maguire P, Stadelmann O, Angst J: Positron emission tomography and fluorodeoxyglucose studies of metabolic hyperfrontality and psychopathology in the psilocybin model of psychosis. Neuropsychopharmacology 1997; 16:357-372. Abstract

21. Vollenweider FX, Maguire RP, Leenders KL, Mathys K, Angst J: Effects of high amphetamine dose on mood and cerebral glucose metabolism in normal volunteers using positron emission tomography (PET). Psychiatry Res. 1998; 83:149-162. Abstract

The newly published paper by Katherine Roche and Paul Roche reports SNAP-23 expression in neuron dendrites and examines the possible role of this neuronal SNAP-23 protein. To this point, SNAP-23 has traditionally been discussed in reference to vesicle trafficking in epithelial cells (see Rodriguez-Boulan et al., 2005 for review), so it is of interest to determine the function of SNAP-23 in neurons. Suh et al. report that surface NMDA receptor expression and NMDA-mediated currents are inhibited following SNAP-23 knockdown. Further, SNAP-23 knockdown results in a specific decrease in NR2B subunit insertion; previously, the NR2B subunit has been reported to preferentially localize to recycling endosomes compared to NR2A (Lavezzari et al., 2004). Given these findings, it is reasonable to conclude that SNAP-23 may be involved in maintaining NMDA receptor surface expression possibly by binding to NMDA-specific recycling endosomes....  Read more

The newly published paper by Katherine Roche and Paul Roche reports SNAP-23 expression in neuron dendrites and examines the possible role of this neuronal SNAP-23 protein. To this point, SNAP-23 has traditionally been discussed in reference to vesicle trafficking in epithelial cells (see Rodriguez-Boulan et al., 2005 for review), so it is of interest to determine the function of SNAP-23 in neurons. Suh et al. report that surface NMDA receptor expression and NMDA-mediated currents are inhibited following SNAP-23 knockdown. Further, SNAP-23 knockdown results in a specific decrease in NR2B subunit insertion; previously, the NR2B subunit has been reported to preferentially localize to recycling endosomes compared to NR2A (Lavezzari et al., 2004). Given these findings, it is reasonable to conclude that SNAP-23 may be involved in maintaining NMDA receptor surface expression possibly by binding to NMDA-specific recycling endosomes.

Interestingly, there is recent evidence that PKC-induced NMDA receptor insertion is mediated by another neuronal SNARE protein; postsynaptic SNAP-25 (Lau et al., 2010). It is possible that activity-induced NMDA receptor trafficking is mediated by SNAP-25, while baseline maintenance of NMDA receptor levels relies on SNAP-23. Other evidence to suggest a strictly regulatory role for SNAP-23 in neuronal NMDA insertion is the finding that activity-dependent receptor insertion from early endosomes has previously been reported to be restricted to AMPA-type glutamate receptors (Park et al., 2004). However, it is possible that activity-induced insertion of AMPA receptors occurs via a distinct endosome pool than NMDA receptors; AMPA and NMDA receptor trafficking has been reported to proceed by distinct vesicle trafficking pathways (Jeyifous et al., 2009).

Although SNAP-23 may not be involved in activity-dependent early endosome receptor trafficking, it is possible that SNAP-23 operates in other pathways linked to activity-induced NMDA receptor trafficking. For instance, SNAP-23 may be the SNARE protein by which lipid raft shuttling of NMDA receptors occurs. SNAP-23 has been found to preferentially associate with lipid rafts over SNAP-25 in PC12 cells (Salaün et al., 2005). As well, NMDA receptors have been found to associate with lipid raft associated proteins flotilin-1 and -2 in neurons (Swanwick et al., 2009). Lipid raft trafficking of NMDA receptors to post-synaptic densities has been reported to follow global ischemia (Besshoh et al., 2005), and the possibility remains that under certain circumstances, NMDA trafficking occurs by lipid raft association to SNAP-23.

Taken together, the discovery of post-synaptic SNARE proteins offers several avenues of research to determine their roles and functions in glutamatergic synapse organization. Further, investigating disruption of synaptic receptor organization presents several possibilities for potential etiologies of disorders linked to compromised glutamate signaling like schizophrenia.

References:

Besshoh, S., Bawa, D., Teves, L., Wallace, M.C. and Gurd, J.W. (2005). Increased phosphorylation and redistribution of NMDA receptors between synaptic lipid rafts and post-synaptic densities following transient global ischemia in the rat brain. Journal of Neurochemistry, 93: 186-194. Abstract

Jeyifous, O., Waites, C.L., Specht, C.G., Fujisawa, S., Schubert, M., Lin, E.I., Marshall, J., Aoki, C., de Silva, T., Montgomery, J.M., Garner, C.C. and Green, W.N. (2009). SAP97 and CASK mediate sorting of NMDA receptors through a previously unknown secretory pathway. Nature Neuroscience, 12: 1011-1019. Abstract

Lau, C.G., Takayasu, Y., Rodenas-Ruano, A., Paternain, A.V., Lerma, J., Bennet, M.V.L. and Zukin, R.S. (2010). SNAP-25 is a target of protein kinase C phosphorylation critical to NMDA receptor trafficking. Journal of Neuroscience, 30: 242-254. Abstract

Lavezzari, G., McCallum, J., Dewey, C.M. and Roche, K.W. (2004). Subunit-specific regulation of NMDA receptor endocytosis. Journal of Neuroscience, 24: 6383-6391. Abstract

Park, M., Penick, E.C., Edward, J.G., Kauer, J.A. and Ehlers, M.D. (2004). Recycling endosomes supply AMPA receptors for LTP. Science, 305: 1972-1975. Abstract

Rodriguez-Boulan, E., Kreitzer, G. and Müsch, A. (2005) Organization of vesicular trafficking in epithelia. Nature Reviews: Molecular Cell Biology, 6: 233-247. Abstract

Salaün, C., Gould, G.W. and Chamberlain, L.H. (2005). The SNARE proteins SNAP-25 and SNAP-23 display different affinities for lipid rafts in PC12 cells. Journal of Biological Chemistry, 280: 1236-1240. Abstract

Suh, Y.H., Terashima, A., Petralia, R.S., Wenthold, R.J., Isaac, J.T.R., Roche, K.W. and Roche, P.A. (2010). A neuronal role for SNAP-23 in postsynaptic glutamate receptor trafficking. Nat Neurosci. 2010 Mar;13(3):338-43. Abstract

Swanwick, C.C., Shapiro, M.E., Chang, Y.Z. and Wenthold, R.J. (2009). NMDA receptors interact with flotillin-1 and -2, lipid raft-associated proteins. FEBS Letters, 583: 1226-1230. Abstract

Thank you for your summary of the presentations from the New Drug Session at ICOSR 2011 on the Schizophrenia Research Forum. The Forum is a helpful and important resource.

I just wanted to clarify your description of ITI-007’s properties at the D2 site. As a dopamine phosphorylation modulator, ITI-007 acts as a pre-synaptic partial agonist and a post-synaptic antagonist with mesolimbic/mesocortical selectivity (Wennogle et al., 2008). In addition to its antagonism of 5-HT2A receptors and unique interaction with D2 receptors, it has affinity for D1 receptors, consistent with partial agonism linked to downstream increases in NMDA NR2B receptor phosphorylation (Zhu et al., 2008), and it is a serotonin reuptake inhibitor (Wennogle et al., 2008). Unfortunately, the short, 10-minute talk during the ICOSR session wasn’t sufficient time to go into the details of the mechanism and supporting preclinical data.

I did notice that a brief description for the mode of action for ITI-007 is listed as “5-HT2A antagonist + dopamine phosphoprotein modulator” with a role in...  Read more

Thank you for your summary of the presentations from the New Drug Session at ICOSR 2011 on the Schizophrenia Research Forum. The Forum is a helpful and important resource.

I just wanted to clarify your description of ITI-007’s properties at the D2 site. As a dopamine phosphorylation modulator, ITI-007 acts as a pre-synaptic partial agonist and a post-synaptic antagonist with mesolimbic/mesocortical selectivity (Wennogle et al., 2008). In addition to its antagonism of 5-HT2A receptors and unique interaction with D2 receptors, it has affinity for D1 receptors, consistent with partial agonism linked to downstream increases in NMDA NR2B receptor phosphorylation (Zhu et al., 2008), and it is a serotonin reuptake inhibitor (Wennogle et al., 2008). Unfortunately, the short, 10-minute talk during the ICOSR session wasn’t sufficient time to go into the details of the mechanism and supporting preclinical data.

I did notice that a brief description for the mode of action for ITI-007 is listed as “5-HT2A antagonist + dopamine phosphoprotein modulator” with a role in schizophrenia listed as “DA stabilizer + 5hT-T inhibitor” in the Forum’s Drugs in Clinical Trials section. This is a nice, brief way to describe a rather complex mechanism.

References:

Wennogle LP, Snyder GL, Hendrick JP, Vanover KE, Tomesch JT, Li P, O’Callaghan JP, Miller DB, Fienberg AA, Davis RE, Mates S (2008) Unique antipsychotic profile of a novel 5-HT2A receptor antagonist and dopamine receptor protein phosphorylation modulator. Schizophrenia Research 98:Suppl1:15.

Zhu H, Snyder GL, Vanover KE, Rana M, Tsui T, Hendrick JP, Li P, Tomesch J, O’Brien JJ, Guo H, Davis RE, Fienberg AA, Wennogle LP, Mates S (2008) ITI-007: A novel 5-HT2A antagonist and dopamine protein phosphorylation modulator (DPPM) induces a distinct NR2B expression pattern in mouse brain. Program No. 155.14 2008 Neuroscience Meeting Planner. Washington, DC Society for Neuroscience, 2008. Online.

The Lilly results of 11 July 2012 are not surprising, considering that the main ingredient of LY2140023 is LY404039, which is both a glutamate agonist and a weak partial dopamine agonist with only one-hundredth the potency of aripiprazole (Seeman and Guan, 2009; Seeman, 2012a), and considering that closer inspection of the clinical data (Kinon et al., 2011) showed that olanzapine was effective in schizophrenia, while LY2140023 was not (Seeman, 2012b).

References:

Kinon BJ, Zhang L, Millen BA, Osuntokun OO, Williams JE, Kollack-Walker S, Jackson K, Kryzhanovskaya L, Jarkova N, . A multicenter, inpatient, phase 2, double-blind, placebo-controlled dose-ranging study of LY2140023 monohydrate in patients with DSM-IV schizophrenia. J Clin Psychopharmacol . 2011 Jun ; 31(3):349-55. Abstract

Seeman P, Guan HC. Glutamate agonist LY404,039 for treating schizophrenia has affinity for the dopamine D2(High) receptor. Synapse. 2009 Oct ; 63(10):935-9. Abstract

Seeman P. An agonist at glutamate and dopamine D2 receptors, LY404039. Neuropharmacology. 2012a Jul 4. Abstract

Seeman P. Comment on "A multicenter, inpatient, phase 2, double-blind, placebo-controlled dose-ranging study of LY2140023 monohydrate in patients with DSM-IV schizophrenia" by Kinon et al. J Clin Psychopharmacol. 2012b Apr ; 32(2):291-2; author reply 292-293. Abstract

View all comments by Philip Seeman

This is indeed another setback for the schizophrenia patient community, and it underscores the difficulty of translating animal model outcomes to the clinical situation. We have to think about introducing a new technology in schizophrenia drug discovery and development that would combine the best of preclinical animal information, but transplanted into a humanized environment to reverse this string of clinical failures.

One such approach is Quantitative Systems or Network Pharmacology, a computer-based mechanistic disease model of biophysically realistic neuronal networks that combines preclinical neurophysiology with human pathology, and clinical and imaging data (the topic of a recent NIH White Paper). Such an approach can be calibrated with retrospective clinical data, and then used to predict and examine future clinical trials. Applying this quantitative paradigm to the (also much publicized) failure of Dimebon in AD, researchers found that there was a fundamental off-target effect that precluded Dimebon from having cognitive benefits. Further analyses suggested that...  Read more

This is indeed another setback for the schizophrenia patient community, and it underscores the difficulty of translating animal model outcomes to the clinical situation. We have to think about introducing a new technology in schizophrenia drug discovery and development that would combine the best of preclinical animal information, but transplanted into a humanized environment to reverse this string of clinical failures.

One such approach is Quantitative Systems or Network Pharmacology, a computer-based mechanistic disease model of biophysically realistic neuronal networks that combines preclinical neurophysiology with human pathology, and clinical and imaging data (the topic of a recent NIH White Paper). Such an approach can be calibrated with retrospective clinical data, and then used to predict and examine future clinical trials. Applying this quantitative paradigm to the (also much publicized) failure of Dimebon in AD, researchers found that there was a fundamental off-target effect that precluded Dimebon from having cognitive benefits. Further analyses suggested that an imbalance in a common dopaminergic phenotype could increase part of the clinical signal difference as observed in the first (successful) Phase 2 trial.

In the case of schizophrenia, we find that affecting glutamatergic (such as with the mGluR2/R3 agonist) or GABA neurotransmission almost always leads to an inverse U-shaped dose response, because of the intrinsic balance between excitation and inhibition in cortical networks. Using such an approach forces discovery scientists to look beyond the single target and think about the impact on networks and circuits that ultimately drive human behavior and pathology in CNS disorders.

Unlike the traditional, currently used "cartoon"-based qualitative drawings, this approach allows for a quantitative outcome that, in principle, can help define the optimal "sweet spot" of the dose response by looking at the outcome of endophenotypes such as BOLD fMRI.

References:

Athan Spiros, Hugo Geerts. 2012. A quantitative way to estimate clinical off-target effects for human membrane brain targets in CNS Research and Development. Exp Pharmacology, 4; 53-61.

Athan Spiros, Patrick Roberts, Hugo Geerts. (2012) A Quantitative Systems Pharmacology Computer Model for Schizophrenia Efficacy and Extrapyramidal Side Effects, Drug Dev. Res, 73(4): 1098-1109.