PROJECTIONS OF 5-HYDROXYTRYPTAMINE-IMMUNOREACTIVE NEURONS IN GUINEA-PIG DISTAL COLON

The presence of 5-hydroxytryptamine in enteric neurons of the guinea-p ig distal colon was demonstrated by immunohistochemistry and the proje ctions of the neurons were determined. 5-Hydroxytryptamine-containing nerve cells were observed in the myenteric plexus but no reactive nerv e cells were found in submucous ganglia. Varicose reactive nerve fibre s were numerous in the ganglia of both the myenteric and submucous ple xuses, but were infrequent in the longitudinal muscle, circular muscle , muscularis mucosae and mucosa. Reactivity also occurred in enterochr omaffin cells. Lesion studies showed that the axons of myenteric neuro ns projected anally to provide innervation to the circular muscle and submucosa and to other more anally located myenteric ganglia. The resu lts suggest that a major population of 5-hydroxytryptamine neurons in the colon is descending interneurons, most of which extend for 10 to 1 5 mm in the myenteric plexus and innervate both 5-hydroxytryptamine an d non-5-hydroxytryptamine neurons.