IDENTIFICATION OF RAT-BRAIN MUSCARINIC M-4 RECEPTORS COUPLED TO CYCLIC-AMP USING THE SELECTIVE ANTAGONIST MUSCARINIC TOXIN-3

In membranes of olfactory tubercle and striatum, the selective muscari nic M-4 receptor antagonist muscarinic toxin 3 completely antagonized the acetylcholine-induced inhibition of forskolin- and dopamine D-1 re ceptor-stimulated cyclic AMP formation with K-i values of 7 and 4 nM, respectively. In olfactory bulb, where acetylcholine stimulated basal adenylyl cyclase activity and inhibited forskolin-stimulated enzyme ac tivity, muscarinic toxin 3 caused a partial antagonism of both acetylc holine effects with high potencies (K-i values = 4-6 nM), In frontal c ortex, muscarinic toxin 3 counteracted the acetylcholine-induced poten tiation of corticotropin-releasing hormone-stimulated cyclic AMP with a K-i of 58 nM, which is close to the toxin affinity for the muscarini c M-1 receptor. In the same brain region, the acetylcholine inhibition of forskolin-stimulated enzyme activity was not affected by muscarini c toxin 3. In microdissected regions of the hippocampus, a significant portion (33-48%) of the acetylcholine inhibition of forskolin-stimula ted adenylyl cyclase activity was blocked by muscarinic toxin 3 with K -i values (6-8 nM) consistent with the involvement of muscarinic M-4 r eceptors. These data show that muscarinic toxin 3 discriminates betwee n adenylyl cyclase-coupled muscarinic receptors and demonstrate the ut ility of the toxin in identifying the relative contribution by the mus carinic M-4 receptor subtype. (C) 1998 Elsevier Science B.V. All right s reserved.