Evidence for M₂ and M₃ Muscarinic Receptor Involvement in Cholinergic Excitatory Junction Potentials Through Synergistic Activation of Cation Channels in the Longitudinal Muscle of Mouse Ileum

Abstract

Cholinergic nerve–mediated excitatory junction potentials (EJPs) in the longitudinal muscle of mouse ileum were characterized by using M₂ or M₃ muscarinic receptor–knockout (KO) mice and 1-[β-[3-(4-methoxyphenyl) propoxy]-4-methoxyphenethyl]-1H-imidazole hydrochloride (SK&F 96365) and pertussis toxin (PTX). EJPs evoked by electrical field stimulation (EFS) in wild-type preparations, initially determined to be cholinergic in origin using tetrodotoxin, atropine, and eserine, were profoundly depressed after SK&F 96365 treatment known to block muscarinic receptor–operated cation channels. A similar depression of the EJPs was also observed by PTX treatment, which is predicted to disrupt M₂-mediated pathways linked to cation channel activation. In M₂-KO mouse preparations, cholinergic EJPs were evoked by EFS with their relative amplitude of 20% – 30% to the wild-type EJP and strongly inhibited by SK&F 96365. No cholinergic EJP was seen in M₃-KO as well as M₂/M₃ double-KO preparations. The results suggest that the wild-type cholinergic EJP is not a simple mixture of M₂ and M₃ responses, but due to synergistic activation of cation channels by both M₂ and M₃ receptors in the murine ileal longitudinal muscle.