The distribution of 5-hydroxytryptamine in the gastrointestinal tract of reptiles, birds and a prototherian mammal. An immunohistochemical study.

The distribution of 5-hydroxytryptamine in the gut of several species of birds and reptiles, and of a prototherian mammal, the platypus, was studied using a monoclonal antibody. 5-Hydroxytryptamine-like immunoreactivity was found in enterochromaffin cells and, in birds, in thrombocytes. Immunoreactivity was not found in enteric neurons fixed immediately after dissection. A detailed study was made on one avian species, the budgerigar. Following incubation of intestine in physiological solution, immunoreactivity was found in nerve fibres in the gut wall that was more marked after incubation with the monoamine oxidase inhibitor pargyline. These fibres took up exogenous 5-hydroxytryptamine. Similar fibres were found in the intestinal nerves and in perivascular plexuses on mesenteric arteries. Both the uptake of 5-hydroxytryptamine and the appearance of neuronal immunoreactivity after incubation were inhibited by the amine uptake inhibitors desmethylimipramine or fluoxetine. Fibres taking up 5-hydroxytryptamine were damaged by pretreatment with 6-hydroxydopamine. It was concluded that the fibres showing immunoreactivity after incubation were adrenergic fibres that had taken up 5-hydroxytryptamine released in vitro from enterochromaffin cells or thrombocytes. These, and more limited observations made on the other species, suggest that birds, reptiles and prototherian mammals lack enteric neurons that use 5-hydroxytryptamine as a transmitter substance.