[3H]CLOZAPINE IS NOT A SUITABLE RADIOLIGAND FOR THE LABELING OF D-4 DOPAMINE-RECEPTORS IN POSTMORTEM HUMAN BRAIN

Clozapine displays nanomolar affinity for cloned human D-4-type dopami ne receptors expressed in tissue culture cells. Therefore, [H-3]clozap ine has been introduced as a radioligand for the labelling of the D-4 dopamine receptors. We found that, in membranes of postmortem human st riatum, amygdala, frontal cortex and substantia nigra, neither dopamin e nor the dopamine agonist apomorphine displaced the binding of [H-3]c lozapine (5-20 nM). [H-3]Clozapine competition curves with clozapine a nd loxapine revealed the presence of two binding sites. The high-affin ity site was displaced by atropine and pirenzepine with nanomolar affi nity. The low-affinity site did not correspond to a serotonin, adrener gic, gamma-amino butyric acid, N-methyl-D-aspartate, benzodiazepine, h istamine, sigma, imidazoline receptor-binding site, or catecholamine u ptake site. Our results suggest that [3H]clozapine in post-mortem huma n brain binds to M(1)-type muscarinic cholinergic receptors and to a l ow-affinity binding site but is not a suitable radioligand for investi gating the D-4 dopamine receptors.