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The status of serotonin 5-HT 2A receptors (5-HT 2A Rs) in schizophrenia has been controversial. In vivo positron emission tomography neuroimaging and in vitro post-mortem binding studies have reported conflicting results about 5-HT 2A R density. Radiotracers bind different receptor conformations depending on their agonist, antagonist or inverse agonist properties. This study investigates 5-HT 2A R density in the post-mortem prefrontal cortex from subjects with schizophrenia and controls using three radiotracers with a different pharmacological profile. The specific binding parameters of the inverse agonist [ 18 F]altanserin, the agonist [ 3 H]lysergic acid diethylamide (LSD) and the antagonist [ 3 H]MDL100907 to brain cortex membranes from 20 subjects with schizophrenia and 20 individually matched controls were evaluated under similar methodological conditions. Ten schizophrenia subjects were antipsychotic-free at death. Saturation curve analyses were performed by non-linear regression to obtain a maximal density of binding sites (B max ) and the affinity of the respective radiotracers (K d ). In schizophrenia subjects, 5-HT 2A R density was decreased when quantified by [ 18 F]altanserin binding, whereas increased when evaluated by [ 3 H]LSD binding. However, [ 3 H]MDL100907 binding was unaltered. A slight loss of affinity (higher K d ) was observed exclusively in [ 3 H]LSD binding. The findings were more evident in antipsychotic-free subjects than in antipsychotic-treated subjects. In conclusion, a higher proportion of the 5-HT 2A R-active functional conformation, which is rather identified by agonist radiotracers, was observed in schizophrenia patients. A consequent reduction of the inactive 5-HT 2A R conformation, which is preferentially identified by inverse agonist radiotracers, was also obtained. Antagonist radiotracers do not distinguish between molecular conformations of the receptor, and accordingly, the absence of changes was shown. These results are compatible with the proposed increased functional activity of brain cortical 5-HT 2A Rs in schizophrenia.

Serotonin 5-HT 2A receptors (5-HT 2A Rs) have been implicated in schizophrenia. However, postmortem studies on 5-HT 2A Rs expression and functionality in schizophrenia are scarce. The 5-HT 2A R mRNA and immunoreactive protein expression were evaluated in postmortem tissue from dorsolateral prefrontal cortex (DLPFC) of antipsychotic-free (n = 18) and antipsychotic-treated (n = 9) subjects with schizophrenia, and matched controls (n = 27). Functional coupling of 5-HT 2A R to G-proteins was tested by measuring the activation induced by the agonist (±)-2,5-dimethoxy-4-iodoamphetamine hydrochloride ((±)DOI) in antibody-capture [ 35 S]GTPγS scintillation proximity assays (SPA). In antipsychotic-free schizophrenia subjects, 5-HT 2A R mRNA expression and protein immunoreactivity in total homogenates was similar to controls. In contrast, in antipsychotic-treated schizophrenia subjects, lower mRNA expression (60±9% vs controls) and a trend to reduced protein immunoreactivity (86±5% vs antipsychotic-free subjects) just in membrane-enriched fractions was observed. [ 35 S]GTPγS SPA revealed a significant ~6% higher stimulation of G αi1 -protein by (±)DOI in schizophrenia, whereas activation of the canonical G αq/11 -protein pathway by (±)DOI remained unchanged. Expression of G αi1 - and G αq/11 -proteins did not differ between groups. Accordingly, in rats chronically treated with clozapine, but not with haloperidol, a 30-40% reduction was observed in 5-HT 2A R mRNA expression, 5-HT 2A R protein immunoreactivity and [ 3 H]ketanserin binding in brain cortical membranes. Overall, the data suggest a supersensitive 5-HT 2A R signaling through inhibitory G αi1 -proteins in schizophrenia. Together with previous results, a dysfunctional pro-hallucinogenic agonist-sensitive 5-HT 2A R conformation in postmortem DLPFC of subjects with schizophrenia is proposed. Atypical antipsychotic treatment would contribute to counterbalance this 5-HT 2A R supersensitivity by reducing receptor expression., (Copyright © 2019. Published by Elsevier B.V.)

As well as being expressed as a full-length transcript, the group II metabotropic glutamate receptor 3 (GRM3, mGlu3) gene is expressed as an mRNA isoform which lacks exon 4 (GRM3Δ4) and which is predicted to encode a protein with a novel C terminus (called mGlu3Δ4). This variant may contribute to the mechanism by which GRM3 acts as a schizophrenia risk gene. However, little is known about the properties or function of mGlu3Δ4. Here, using transiently transfected HEK293T/17 cells, we confirm that GRM3Δ4 cDNA is translated, with mGlu3Δ4 existing as a homodimer as well as a monomer, and localizing primarily to cell membranes including the plasma membrane. Co-immunoprecipitation shows that mGlu3Δ4 interacts with canonical mGlu3. mGlu3Δ4 does not bind the mGlu2/3 antagonist [ 3 H]LY341495, but the presence of mGlu3Δ4 reduces binding of [ 3 H]LY341495 to mGlu3, paralleled by a decrease in the abundance of membrane-associated mGlu3. These experiments indicate that mGlu3Δ4 may negatively modulate mGlu3, and thereby impact on the roles of GRM3/mGlu3 in schizophrenia and as a therapeutic target.

Background: Metabotropic glutamate receptor 3 (mGlu3, mGluR3), encoded by GRM3, is a risk gene for schizophrenia and a therapeutic target. It is unclear whether expression of the receptor is altered in the disorder or related to GRM3 risk genotype. Antibodies used to date to assess mGlu3 in schizophrenia have not been well validated., Objective: To characterise six commercially available anti-mGlu3 antibodies for use in human brain, and then conduct a semi-quantitative study of mGlu3 immunoreactivity in schizophrenia., Methods: Antibodies tested using Grm3 -/- and Grm2 -/- /3 -/- mice and transfected HEK293T/17 cells. Western blotting on membrane protein isolated from superior temporal cortex of 70 patients with schizophrenia and 87 healthy comparison subjects, genotyped for GRM3 SNP rs10234440., Results: One (out of six) anti-mGlu3 antibodies was fully validated, a C-terminal antibody which detected monomeric (~100kDa) and dimeric (~200kDa) mGlu3. A second, N-terminal, antibody detected the 200kDa band but also produced non-specific bands. Using the C-terminal antibody for western blotting in human brain, mGlu3 immunoreactivity was found to decline with age, and was affected by pH and post mortem interval. There were no differences in monomeric or dimeric mGlu3 immunoreactivity in schizophrenia or in relation to GRM3 genotype. The antibody was not suitable for immunohistochemistry., Interpretation: These data highlight the value of knockout mouse tissue for antibody validation, and the need for careful antibody characterisation. The schizophrenia data show that involvement of GRM3 in the disorder and its genetic risk architecture is not reflected in total membrane mGlu3 immunoreactivity in superior temporal cortex., (Copyright © 2016 The Authors. Published by Elsevier B.V. All rights reserved.)

Heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) can form multiprotein complexes (heteromers), which can alter the pharmacology and functions of the constituent receptors. Previous findings demonstrated that the Gq/11-coupled serotonin 5-HT2A receptor and the Gi/o-coupled metabotropic glutamate 2 (mGlu2) receptor-GPCRs that are involved in signaling alterations associated with psychosis-assemble into a heteromeric complex in the mammalian brain. In single-cell experiments with various mutant versions of the mGlu2 receptor, we showed that stimulation of cells expressing mGlu2-5-HT2A heteromers with an mGlu2 agonist led to activation of Gq/11 proteins by the 5-HT2A receptors. For this crosstalk to occur, one of the mGlu2 subunits had to couple to Gi/o proteins, and we determined the relative location of the Gi/o-contacting subunit within the mGlu2 homodimer of the heteromeric complex. Additionally, mGlu2-dependent activation of Gq/11, but not Gi/o, was reduced in the frontal cortex of 5-HT2A knockout mice and was reduced in the frontal cortex of postmortem brains from schizophrenic patients. These findings offer structural insights into this important target in molecular psychiatry., (Copyright © 2016, American Association for the Advancement of Science.)

Histone deacetylases (HDACs) compact chromatin structure and repress gene transcription. In schizophrenia, clinical studies demonstrate that HDAC inhibitors are efficacious when given in combination with atypical antipsychotics. However, the molecular mechanism that integrates a better response to antipsychotics with changes in chromatin structure remains unknown. Here we found that chronic atypical antipsychotics downregulated the transcription of metabotropic glutamate 2 receptor (mGlu2, also known as Grm2), an effect that was associated with decreased histone acetylation at its promoter in mouse and human frontal cortex. This epigenetic change occurred in concert with a serotonin 5-HT2A receptor-dependent upregulation and increased binding of HDAC2 to the mGlu2 promoter. Virally mediated overexpression of HDAC2 in frontal cortex decreased mGlu2 transcription and its electrophysiological properties, thereby increasing psychosis-like behavior. Conversely, HDAC inhibitors prevented the repressive histone modifications induced at the mGlu2 promoter by atypical antipsychotics, and augmented their therapeutic-like effects. These observations support the view of HDAC2 as a promising new target for schizophrenia treatment. [ABSTRACT FROM AUTHOR]