RAT STRIATAL MUSCARINIC RECEPTORS COUPLED TO THE INHIBITION OF ADENYLYL-CYCLASE ACTIVITY - POTENT BLOCK BY THE SELECTIVE M(4) LIGAND MUSCARINIC TOXIN-3 (MT3)

1 In rat striatal membranes, muscarinic toxin 3 (MT3), a selective lig and of the cloned m4 receptor subtype, antagonized the acetylcholine ( ACh) inhibition of forskolin-and dopamine D-1 receptor-stimulated aden ylyl cyclase activities with pA(2) values of 8.09 and 8.15, respective ly. 2 In radioligand binding experiments, MT3 increased the K-d but di d not change the B-max value of [H-3]-N-methylscopolamine (H-3]-NMS) b inding to rat striatal muscarinic receptors. The toxin displaced the m ajor portion of the [H-3]-NMS binding sites with a K-i of 8.0 nM. 3 In rat myocardium, MT3 antagonized the ACh inhibition of adenylyl cyclas e with a K-i value of 860 nM. 4 In rat cerebral cortical membranes pre labelled with [H-3]-myo-inositol, MT3 counteracted the methacholine st imulation of [H-3]-inositol phosphates formation with a K-i value of 1 13 nM. 5 The present study shows that MT3 is a potent antagonist of th e striatal muscarinic receptors coupled to inhibition of adenylyl cycl ase activity. This finding provides strong evidence for the classifica tion of these receptors as pharmacologically equivalent to the m4 gene product (M(4)) On the other hand, the weaker potencies of MT3 in anta gonizing the muscarinic responses in cerebral cortex and in the heart are consistent with the reported lower affinities of the toxin for the cloned m1 and m2 receptor subtypes, respectively.