Orbitofrontal cortex neurons as a common target for classic and glutamatergic antipsychotic drugs

Until recently, all known antipsychotic drugs were thought to block the dopamine D2 receptor. New evidence that agonists of the metabotropic glutamate 2/3 (mGlu2/3) receptors ameliorate psychotic and affective symptoms of schizophrenia suggests that compounds with different molecular targets may act on a common cellular target to treat schizophrenia. We hypothesized that normalizing the activity of neurons in the orbitofrontal cortex (OFC), a region that is increasingly implicated in the pathophysiology of schizophrenia, presents such a target. We disrupted OFC activity in behaving rats with a use-dependent NMDA antagonist to model the NMDA hypofunction state that may occur in schizophrenia. This systemic treatment increased the activity of most pyramidal cells while inhibiting the activity of putative inhibitory GABA interneurons and increasing behavioral stereotypy. A similar pattern of OFC firing disruption was observed after amphetamine, which models a dopamine hyperactivity state in schizophrenia and which produces a pattern of firing disruption different from those of NMDA antagonists in other prefrontal cortex regions. Antipsychotic drugs haloperidol and clozapine, which target monoamine receptors, as well as an mGlu2/3 agonist and an mGlu5 receptor modulator proposed to have antipsychotic efficacy, reversed the impact of NMDA hypofunction on OFC cells and on behavior. A similar pattern of normalization of OFC activity was observed when treatments were given after amphetamine. Thus, proven or putative antipsychotic drugs with different mechanisms of action similarly reduced the impact of NMDA hypofunction and dopamine hyperfunction on OFC neurons, suggesting that these neurons are a candidate target for the therapeutic effects of antipsychotic medications.