Specific antagonism of enteric neural serotonin receptors by dipeptides of 5-hydroxytryptophan: evidence that serotonin is a mediator of slow synaptic excitation in the myenteric plexus.

Research on the role of serotonin (5-hydroxytryptamine, 5-HT) in the function of the enteric nervous system has been impeded by the lack of specific inhibitors of the enteric neural actions of 5-HT. Saturable, reversible, high affinity enteric binding sites for 3H-5-HT have recently been characterized and radioautographically located. Affinity for the 3H-5-HT binding site requires an indole ring substituted with a free hydroxyl group. These 3H-5-HT binding sites have been proposed to be enteric neural 5-HT receptors. This hypothesis was tested in the current study by comparing the ability of compounds to inhibit the binding of 3H-5-HT with their electrophysiologically determined actions on myenteric neurons. 5-Methoxytryptamine did not inhibit the binding of 3H-5-HT to enteric membranes and neither mimicked nor antagonized the effects of 5-HT on the membrane potential of myenteric neurons. Two dipeptides of 5-hydroxytryptophan, N-acetyl- and N-hexanoyl-5-hydroxytryptophyl-5-hydroxytryptophan amide (5-HTP-DP and N-hex-5-HTP-DP) inhibited the binding of 3H-5-HT (K1 = 0.25 microM for 5-HTP-DP and 1.19 microM for N-hex-5-HTP-DP). 5-HTP-DP applied by pressure microejection or superfusion (10 microM) antagonized the slow postsynaptic depolarization of myenteric neurons evoked by microejection of 5-HT. 5-HTP-DP also blocked the 5-HT-induced presynaptic reduction in amplitude of nicotinic fast synaptic potentials; however, 5-HTP-DP itself did not affect these responses. Moreover, 5-HTP-DP also failed to affect responses of myenteric neurons to microejected substance P, their muscarinic response to acetylcholine, or antidromic action potentials. In contrast, both dipeptides blocked the slow synaptic potentials seen in type II/AH neurons following stimulation of fiber tracts in interganglionic connectives. These data support the hypotheses that enteric 3H-5-HT binding sites are enteric neural 5-HT receptors, that dipeptides of 5-hydroxytryptophan are specific antagonists at these receptors, and that 5-HT is one of the mediators of slow synaptic potentials in the myenteric plexus.