[3H]Clozapine is not a suitable radioligand for the labelling of D4 dopamine receptors in postmortem human brain.

Clozapine displays nanomolar affinity for cloned human D4-type dopamine receptors expressed in tissue culture cells. Therefore, [3H]clozapine has been introduced as a radioligand for the labelling of the D4 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 [3H]clozapine (5-20 nM). [3H]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 postmortem human brain binds to M1-type muscarinic cholinergic receptors and to a low-affinity binding site but is not a suitable radioligand for investigating the D4 dopamine receptors.