Glutamate pharmacology and the treatment of schizophrenia: current status and future directions

Glutamate-containing neuronal terminals are ubiquitous in the central nervous system and their functional importance in mental activity is considerable. Therefore, the involvement of this neurotransmitter in the pathology of schizophrenia is being studied. Biochemical evidence has suggested that glutamatergic transmission may be regionally reduced in schizophrenia, although this evidence has never been completely consistent nor fully replicable. More striking has been the behavioral effects in humans of the antiglutamatergic drugs phencyclidine (PCP) and its congener ketamine. By historical report, PCP produces a 'schizophrenia-like' psychosis in normal humans and aggravates the psychosis in schizophrenics. More recently, ketamine has been shown to produce a mild psychotomimetic effect in normal volunteers, which has some schizophrenia-like features. We have studied the effects of ketamine in schizophrenic patients. Here, ketamine intensified each patient's specific underlying psychosis, an effect not blocked by haloperidol. Moreover, ketamine selectively increased cerebral blood flow (CBF) in the anterior cingulate cortex and reduced CBF in hippocampus and lingual gyrus. These data may be pertinent to the subject's psychosis exacerbation, especially because both cingulate and hippocampus have been previously implicated in schizophrenic psychosis. In addition, ketamine produced a distinctive dynamic time-course of regional CBF changes in different anatomic regions, with immediate (5-10 min) changes in cingulate, but somewhat more delayed changes (20-40 min) in the thalamus and cerebellum. Our immediate early gene (IEG) time-course data with c-fos and zif268 in rats following PCP suggest that a single dose of this antiglutamatergic compound can have an effect in some brain areas which lasts beyond 48 h, an effect which is distinct by IEG and by region. Together, these data suggest that glutamate-mediated neurotransmission has a strong influence in schizophrenia, although the specifics of this involvement have yet to be articulated.