[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 dopamine receptors expressed in tissue culture cells. Therefore, [H-3]clozapine has been introduced as a radioligand for the labelling of the D-4 dopamine receptors. We found that, in membranes of postmortem human striatum, amygdala, frontal cortex and substantia nigra, neither dopamine nor the dopamine agonist apomorphine displaced the binding of [H-3]clozapine (5-20 nM). [H-3]Clozapine competition curves with clozapine and loxapine revealed the presence of two binding sites. The high-affinity site was displaced by atropine and pirenzepine with nanomolar affinity. The low-affinity site did not correspond to a serotonin, adrenergic, gamma-amino butyric acid, N-methyl-D-aspartate, benzodiazepine, histamine, sigma, imidazoline receptor-binding site, or catecholamine uptake site. Our results suggest that [3H]clozapine in post-mortem human brain binds to M(1)-type muscarinic cholinergic receptors and to a low-affinity binding site but is not a suitable radioligand for investigating the D-4 dopamine receptors.