Clozapine interaction with the M-2 and M-4 subtypes of muscarinic receptors

Available evidence indicates that the antipsychotic drug clozapine acts as a partial agonist at the muscarinic M-4 and as an antagonist at the M-2 rec eptors. We wondered whether there is indeed a fundamental difference betwee n its action on these two receptor subtypes, and whether it interacts with their classical or allosteric binding sites. In experiments on Chinese hams ter ovary cells stably expressing the M-2 or M-4 receptors, clozapine inhib ited the binding of the specific muscarinic ligand [H-3]N-methylscopolamine to either receptor subtype. The affinity of the high-affinity sites for cl ozapine was diminished by GTP in the way expected for agonists on both the M-2 and the M-4 receptor subtypes. Arunlakshana-Schild plots of data obtain ed in saturation binding experiments with [H-3]N-methylscopolamine at diffe rent concentrations of clozapine were linear with a slope of unity. Clozapi ne did not alter the time course of [H-3]N-methylscopolamine dissociation f rom muscarinic M-2 or M-4 receptors. It inhibited the synthesis of cyclic A MP in cells expressing the M-4 receptor subtype, but did not measurably inh ibit the synthesis of cyclic AMP in cells expressing the M-2 receptor subty pe. We conclude that clozapine has a high affinity for muscarinic M-2 and M -4 receptor subtypes, that it associates with the classical and not with th e allosteric binding site, and that it acts as a partial agonist on both th e M-2 and the M-4 receptor subtype. (C) 1999 Elsevier Science B.V. All righ ts reserved.