Evidence of 5-HT participation in vagal inhibitory pathway to opossum LES

Studies were performed in anesthetized opossums to investigate the nature of vagal-stimulated sphincter relaxation, which is resistant to antagonism by a combination of hexamethonium and atropine. The sphincter pressures were measured with water-filled and continuously perfused catheters anchored in the lower esophageal sphincter. Neither increase in the doses of hexamethonium and atropine nor addition of diphenhydramine further modified the vagal response. However, administration of 5-methoxydimethyltryptamine in the presence of hexamethonium and atropine abolished vagally stimulated sphincter relaxation. In animals pretreated with parachlorophenylalanine, addition of atropine and hexamethonium also abolished vagally stimulated sphincter relaxation. In the experiments in which lower esophageal sphincter relaxation on vagal stimulation was abolished, the local stimulation of intramural neurons still produced normal lower esophageal sphincter relaxation. These studies suggest that 5-hydroxytryptamine may participate in the vagal inhibitory pathway to the lower esophageal sphincter.