Is 5-HT a mediator in the motor control of the feline pylorus?

Since serotonin (5-HT) is a potent spasmogen of pyloric smooth muscle in vivo, and since it is released on electrical stimulation of extrinsic nerves to the gut, the amine may mediate the excitatory motor responses elicited by vagal or splanchnic stimulation. In this study in cats intra-arterial administration of 5-HT to the stomach evoked a dose-dependent pyloric contraction, which was efficiently antagonized by Ketanserin, a peripheral 5-HT2 receptor antagonist. Such treatment did not affect the vagally or splanchnically induced pyloric motor responses, whereas the blood pressure reaction on splanchnic stimulation was greatly reduced. Therefore, 5-HT does not seem to be essential for the extrinsic neural control of pyloric motility. On the other hand, by means of immunocytochemistry, using a 5-HT antiserum, varicose nerve fibres with tryptamine-like immunofluorescence were demonstrated not only in the myenteric plexus and circular muscle layer but also in vascular nerve terminals of the feline pylorus. The function of these tryptamine-containing nerves is still unclear.