Induced release of acetylcholine from guinea pig ileum longitudinal muscle-myenteric plexus by anatoxin-a.

Anatoxin-a (ANTX), a nicotinic agonist, has been shown to induce contraction of guinea pig ileum, which was abrogated by the muscarinic antagonist atropine and the nicotinic antagonists tubocurarine and hexamethonium. We showed here that the ganglionic nicotinic antagonist mecamylamine was a better inhibitor of the contraction of ileum induced by ANTX. The sodium channel blocker tetrodotoxin also abolished ANTX-induced contraction. In contrast, alpha-bungarotoxin, the muscle type nicotinic receptor blocker, had no effect on ANTX-induced contraction of guinea pig ileum. Longitudinal muscle-myenteric plexus prepared from guinea pig ileum, labeled with [3H]choline and then incubated with ANTX was shown for the first time to release [3H]acetylcholine (ACh) in a dose-dependent manner. Pretreatment of longitudinal muscle-myenteric plexus with tubocurarine, hexamethonium or mecamylamine blocked ANTX-induced release of [3H]ACh. In contrast, atropine was without effect. Mecamylamine was the most potent antagonist. As observed in ileum contraction, tetrodotoxin completely and potently blocked the release of [3H]ACh induced by ANTX. Neither alpha-bungarotoxin nor the neuromuscular junction blockers conotoxin G1 or M1 could inhibit the [3H]ACh release. Taken together, these results suggested that ANTX activated nicotinic receptors on ganglionic interneurons to trigger a release of ACh, which next stimulated muscarinic receptors and induced ileum contraction.