Release of endogenous 5-hydroxytryptamine from resting and stimulated enteric neurons

Physiological and biochemical evidence has indicated that there may be serotoninergic neurons in the enteric nervous system. A critical step in the identification of a neurotransmitter is the demonstration of the release of the substance upon nerve stimulation. We now report the release of endogenous 5-hydroxytryptamine from enteric neurons. Segments of guinea-pig small intestine were everted and perfused in vitro through the newly created serosal lumen. Tests with [³H]5-hydroxytryptamine revealed the existence of a tissue barrier preventing diffusion of mucosal (enteroendocrine cell) 5-hydroxytryptamine into the perfusate; thus, all 5-hydroxytryptamine in the perfusate was of neural origin. The gut was stimulated electrically. 5-Hydroxytryptamine in the perfusate and in the myenteric plexus was assayed by a specific radioenzymatic method. 5-Hydroxytryptamine was present in the myenteric plexus; it was released into the perfusate spontaneously and the release was enhanced by electrical stimulation. The stimulated, but not the spontaneous, release of the amine was Ca²⁺-dependent. Comparison with the release of newly taken up [³H]5-hydroxytryptamine showed that the specific radioactivity of electrically released 5-hydroxytryptamine was higher than that of either the spontaneously released or tissue amine. Stimulation also increased the release of 5-hydroxytryptamine more than that of its metabolites in the perfusate.These results indicate that 5-hydroxytryptamine is an endogeneous constituent of the enteric nervous system, that it is released by electrical field stimulation of enteric nerves, and that newly taken up 5-hydroxytryptamine is released preferentially by these neurons.