Chromatin remodeling has been implicated in the neurochemical mechanisms by which antipsychotic drugs achieve their clinical efficacy8,47
. Although these findings provide compelling evidence for the involvement of epigenetic processes in mediating the molecular events associated with the treatment of schizophrenia, they do not exclude the possibility of compensatory mechanisms that may emerge in response to chronic antipsychotic drug exposure and ultimately restrain their therapeutic effects. Compensatory mechanisms are seen as negative correlations between processes that are pivotal for system function48
. Within an individual neuron or individual animal, alterations in one parameter may produce minor changes in the state of the system if there are mechanisms that cause adjustments in another parameter to compensate for the first change. Here, we demonstrate that chronic treatment with atypical antipsychotic drugs dramatically decreases the density of 5HT2A binding sites in mouse frontal cortex, which leads to repressive histone modifications at the promoter region of the mGlu2
gene. Furthermore, our data reveal a critical role for HDAC2 in mediating these 5HT2A-dependent repressive epigenetic modifications at the mGlu2
promoter in mouse and human frontal cortex. We show that chronic administration of atypical antipsychotic drugs selectively up-regulate HDAC2 expression and binding to the mGlu2
promoter, an effect that is associated with the regulation of HDAC2
promoter transcriptional function by activation of the 5HT2A receptor. We then demonstrate that such over-expression of HDAC2 in frontal cortex is sufficient to down-regulate mGlu2
expression and its physiological inhibitory effects, which exacerbates schizophrenia-like behavior. Activation of mGlu2 is well known to repress cellular, electrophysiological and behavioral responses that require the 5HT2A receptor20
. Similarly, drugs that activate the mGlu2 receptor have potential for the treatment of schizophrenia, whereas 5HT2A agonists, such as hallucinogenic compounds, result in the opposite effect. Taken together, these data suggest that the decreased density of 5HT2A binding sites by chronic treatment with atypical antipsychotic drugs results in a compensatory mechanism of repressive chromatin structure at the promoter region of the mGlu2
gene, with consequently less inhibitory effects of the mGlu2 receptor on 5HT2A-regulated pathways and behaviors.
We also observed that adjunctive SAHA abolishes the repressive histone modifications induced at the mGlu2
promoter by chronic atypical antipsychotics, which augments the behavioral effects induced by atypical and glutamate antipsychotics. Overall, our findings support the hypothesis that compensatory epigenetic events at the mGlu2
promoter may be responsible for the high incidence of patients that do not benefit from conventional therapy with atypical antipsychotic drugs, and provide a biochemical explanation for the clinical association of pharmacological inhibition of HDACs with improved schizophrenia treatment (Supplementary Fig. 8
Previous studies from our laboratory have shown that 5HT2A and mGlu2 interact through specific transmembrane domains to form a G protein-coupled receptor (GPCR) heterocomplex in mouse and human frontal cortex25
. The signaling properties of this receptor heterocomplex have been proposed to be necessary for therapeutic-like effects of atypical antipsychotic drugs23
. We show here that chronic atypical antipsychotic drugs induce 5HT2A-dependent repressive histone modifications at the mGlu2
promoter in frontal cortex, which is consistent with previous reports describing effects of long-term treatment with 5HT2A agonists on behavioral responses that require the mGlu2 receptor36,37
. Further work will be directed to assess the relative contribution of these epigenetic factors to the modulation of expression of 5HT2A and mGlu2 as a GPCR heterocomplex that may balance the response to ligand inputs in cortical pyramidal neurons.
Clozapine is the only antipsychotic drug with proven superior efficacy in treatment-resistant schizophrenic patients4
. We found that both clozapine and risperidone, but not haloperidol, decreased the density of 5HT2A binding sites in mouse frontal cortex. Similar findings on 5HT2A receptor binding have been reported in postmortem human brain of treated schizophrenic subjects25,49
. These results suggest that the 5HT2A receptor is involved in treating the psychotic symptoms of schizophrenia, and that down-regulation of 5HT2A receptor binding sites by chronic atypical antipsychotic drugs may be one of the mechanisms underlying their therapeutic effects. Remarkably, only chronic clozapine induces changes in histone acetylation at the 5HT2A
promoter that correlate with its transcriptional repression. Although further investigation is needed to understand the mechanisms and consequences of this interesting finding, it is tempting to speculate that these histone modifications at the promoter of the 5HT2A
gene may account for the enhanced antipsychotic properties of clozapine. Further work is also needed to determine the molecular mechanisms responsible for down-regulation of 5HT2A
mRNA expression in subcortical regions by chronic treatment with clozapine.
A notable finding of the current study is that chronic treatment with clozapine up-regulates HDAC2 in mouse frontal cortex. In concordance with the effects of clozapine in murine models, the expression of HDAC2 is increased in postmortem frontal cortex of schizophrenic subjects treated with atypical antipsychotic drugs. Since no alteration is detected in untreated schizophrenic subjects, these results suggest that dysregulation of HDAC2 expression represents a consequence of antipsychotic drug medication, and not a biochemical marker of schizophrenia in postmortem human brain. This is an intriguing observation because viral-mediated over-expression of HDAC2 in mouse frontal cortex induces behavioral alterations that replicate psychotic symptoms and cognitive impairments in schizophrenia patients. Notably, clinical studies indicate that deficits in attention and memory function, cognitive impairments that represent core features of schizophrenia1,2
, are exacerbated by treatment with atypical antipsychotic drugs50
. Together, these findings suggest that the 5HT2A-dependent up-regulation of HDAC2 and repressive histone modifications at the mGlu2
promoter might be related to the negative modulation of cognitive processes by chronic treatment with atypical antipsychotic drugs.
The ultimate goal of understanding the pathophysiology of schizophrenia is to develop therapeutic strategies that improve or restore normal brain activity and, ultimately, the associated deficits in sensory processing, perception and memory function. The functions of HDAC2 described here involve important implications for the molecular basis of the currently limited response to treatment with atypical antipsychotics. We propose that atypical antipsychotic drugs induce a selective up-regulation of HDAC2 in frontal cortex of individuals with schizophrenia, which alters the chromatin state of the mGlu2 promoter and thereby limits the therapeutic effects of these agents. Identification of the epigenetic mechanisms through which administration of HDAC inhibitors potentiates the biochemical and behavioral responses to atypical antipsychotics will help in discovering more effective treatments to improve the clinical efficacy of the currently available antipsychotic medications. Specifically, our findings encourage the development and testing of HDAC2-selective inhibitors for schizophrenia.