EFFECT OF CLOZAPINE AND RISPERIDONE ON 5-HT2 AND D-2-DOPAMINE RECEPTOR-BINDING IN THE POSTMORTEM HUMAN BRAIN - AN AUTORADIOGRAPHIC STUDY

Effects of the antipsychotics risperidone and clozapine on 5-HT2 and D-2-dopamine receptor binding were examined using [H-3]N-methylspiperone ([H-3]NMSP) and in vitro receptor autoradiography on human whole hemisphere cryosections. The 5-HT2 receptor antagonist ketanserin and the D-2-dopamine receptor antagonist raclopride were used as references. [H-3]NMSP binding was observed in caudate nucleus, putamen, and cerebral cortex indicating binding to D-2-dopamine and 5-HT2 receptors. Risperidone and clozapine counteracted the binding to both receptor types. This was in contrast to raclopride, which selectively blocked the D-2-dopamine receptors in the basal ganglia, and ketanserin, which selectively blocked the 5-HT2 receptors in the cerebral cortex. Risperidone (100 nM and 10 mu M) blocked up to 90% of [H-3]NMSP binding to both receptor types, whereas the blocking capacity of clozapine (10 mu M) was lower (approximately 60%). The lack of total blockade of D-2-dopamine receptors is in line with results obtained in with [C-11]raclopride and positron emission tomography studies of clozapine treated human subjects. However, autoradiographic studies of clozapine competition of [H-3]raclopride binding show total displacement of the binding at high clozapine concentrations, thus contradicting the PET results with [C-11]raclopride, as well as the autoradiographic results obtained with [H-3]NMSP. In conclusion it can be stated that pharmacological concentrations of the two drugs clozapine and risperidone block a large proportion of D-2-dopamine receptors and 5-HT2 receptors in the human brain. Moreover, the study shows the usefulness of human whole hemisphere autoradiography for the study of interaction of drugs with different central neurotransmitter receptors.