The current status of PET scanning with respect to schizophrenia

Abstract

PET scan studies of regional brain energy metabolism in schizophrenia have hitherto not been consistent in demonstrating any specific perturbation in heterogenous groups of patients. In some studies there was a tendency to reduced metabolic values in several regions in chronic patients. The variance of metabolic rates also tended to be greater in the group of schizophrenic subjects, but rates for most patients overlapped with those of the controls. Studies of regional brain energy metabolism also failed to disclose consistent effects of clinical antipsychotic drug treatment in schizophrenic patients. PET measurements of dopamine receptor functions in the major basal ganglia using different radioligands for D2 dopamine receptors also gave inconsistent results. One group reporting elevated densities of D2 dopamine receptors in the major basal ganglia of drug-naive schizophrenic patients could not be confirmed. PET measurements of dopamine receptor binding demonstrated profound and selective effects of clinical antipsychotic drug treatment on D2 and D1 dopamine receptor occupancy in schizophrenic patients. All chemically different categories of antipsychotic drugs induced a substantial occupancy of D2 dopamine receptors in relation to clinical treatment. This effect has been shown to be dose dependent and fully reversible. It appears much earlier than the antipsychotic effect and it is also present in neuroleptic-resistant patients. Accordingly, neuroleptic resistance is not related to individual pharmacodynamic or pharmacokinetic factors. Drug resistance is in all probability related to heterogeneity of biologic factors causing schizophrenia. Some, but not all, of the antipsychotic drugs also induce a significant D1 dopamine receptor occupancy. This effect was most marked for the unconventional drug clozapine, which showed about the same degree of D1 and D2 dopamine receptor blockade when given in conventional clinical doses. Further refinements of the resolution of PET scan instruments in the years to come and the development and application of new tracers will supply powerful tools for the further search for fundamental alterations of brain function in schizophrenia.