Muscarinic M1 receptors stimulate a nonadrenergic noncholinergic inhibitory pathway in the isolated rat duodenum

We examined the effect of the muscarinic agonist McN-A-343 (4-m-chlorophenylcarbamoyloxy-2-butynyl trimethyl ammonium) on in vitro preparations of rat small intestine. McN-A-343 (0.1-10 microM) induced a concentration-dependent relaxation of duodenum, jejunum and ileum. This effect was due to activation of muscarinic receptors of the M1 subtype, inasmuch as it was antagonized by atropine (pA2, 8.93) and by the selective M1 antagonists pirenzepine and dicyclomine with high affinity (pA2, 8.09 and 8.14, respectively). Tetrodotoxin, but not hexamethonium, abolished McN-A-343 relaxation indicating involvement of neural pathways and absence of nicotinic transmission. Moreover, in the presence of apamin (0.1 microM) McN-A-343 induced a contractile response. Unlike McN-A-343, acetylcholine contracted the rat duodenum; its concentration-response curve was significantly potentiated by tetrodotoxin, suggesting stimulation of the McN-A-343 sensitive receptor by acetylcholine (P less than .01). Prior treatment of animals with reserpine reduced the potency of McN-A-343 only by 2.5-fold. The gamma-aminobutyric acid (GABA) antagonist bicuculline inhibited McN-A-343 with an affinity comparable to that found for GABA(A) receptors (pA2, 5.52), indicating involvement of GABAergic fibers. ATP is also likely to play a role, as desensitization experiments showed that alpha-beta-methylene ATP induced a comparable decrease of both McN-A-343 and its own response. In contrast, desensitization induced by GABA occurred to a minor extent. Thus, in the rat duodenum, activation of an M1 muscarinic receptor induces relaxation by releasing GABA, and possibly ATP, from myenteric neurons.