Mixed Agonist–Antagonist Properties of Clozapine at Different Human Cloned Muscarinic Receptor Subtypes Expressed in Chinese Hamster Ovary Cells

Abstract We recently reported that clozapine behaves as a partial agonist at the cloned human m4 muscarinic receptor subtype. In the present study, we investigated whether the drug could elicit similar effects at the cloned human m1, m2, and m3 muscarinic receptor subtypes expressed in the Chinese hamster ovary (CHO) cells. Clozapine elicited a concentration-dependent stimulation of [ 3 H]inositol phosphates accumulation in CHO cells expressing either the m1 or the m3 receptor subtype. Moreover, clozapine inhibited forskolin-stimulated cyclic AMP accumulation and enhanced [ 35 S] GTPγS binding to membrane G proteins in CHO cells expressing the m2 receptor. These agonist effects of clozapine were antagonized by atropine. The intrinsic activity of clozapine was lower than that of the full cholinergic agonist carbachol, and, when the compounds were combined, clozapine potently reduced the receptor responses to carbachol. These data indicate that clozapine behaves as a partial agonist at different muscarinic receptor subtypes and may provide new hints for understanding the receptor mechanisms underlying the antipsychotic efficacy of the drug.