Analysis of Neurogenic Contractions Induced by ML-1035 and Other Benzamides in the Guinea-pig Non-stimulated Isolated Ileum

Abstract 4-Amino-5-chloro-substituted benzamides have been shown to increase gastric motility in-vivo and enhance field-stimulated and peristaltic contractions in-vitro. The present experiments examined the contractile response to a series of benzamides in the guinea-pig non-stimulated ileum. Four benzamides elicited contractions in the isolated ileum which were expressed as a percentage of the contraction induced by 1 μm acetylcholine (% acetylcholine response = 12 ± 2, 19 ± 3, 26 ± 2, 51 ± 3, n= 13, 8, 17, and 21, with EC50 values of 0·85, 1·8, 5·7, and 14·2 μm for cisapride, zacopride, metocloprarmde, and ML-1035 (4-amino-5-chloro-2-((2-methylsulphinyl)-ethoxy)-N-(2-(diethylamino)-ethyl)-benzamide hydrochloride), respectively). ML-1035 contractions were completely blocked by atropine and tetrodotoxin, while ganglionic blockade with hexamethonium was ineffective. Metoclopramide has been reported to sensitize postjunctional muscarinic receptors, however, ML-1035 did not enhance acetylcholine-induced contractions. Tropisetron (ICS 205–930, 1 μm), caused a parallel rightward shift in the concentration-response curve for both ML-1035 and zacopride (EC50 = 14·2 ± 1·3 and 1·8 ± 0·8 μm in the absence, and 26 ± 2·7 and 6·9 ± 2·3 μm in the presence of tropisetron for ML-1035 and zacopride, respectively) with apparent pKB values of 5·9 and 6·0 for the respective compounds. 5-Hydroxytryptaminergic receptor desensitization by 2-methyl-5-hydroxytryptamine (5-HT3) and 5-methoxytryptamine (5-HT4), attenuated the response to ML-1035. We also examined the effect of the benzamides on [3H]acetylcholine release from longitudinal muscle myenteric plexus preparations; however, these compounds had little effect on basal [3H]acetylcholine release. Thus, the pharmacological data indicate that the benzamides can elicit neurogenic contractions in the non-stimulated ileum by activating postganglionic, cholinergic neurons which is independent of an effect on smooth muscle.