Activation of submucosal 5-HT3 receptors elicits a somatostatin-dependent inhibition of ion secretion in rat colon

Background and purpose: 5-Hydroxytryptamine (5-HT) is a key regulator of the gastrointestinal system and we have shown that submucosal neuronal 5-HT3 receptors exerted a novel inhibitory effect on colonic ion transport. The aim of the present study was to investigate the precise mechanism(s) underlying this inhibitory effect. Experimental approach: Mucosa/submucosa or mucosa-only preparations from rat distal colon were mounted in Ussing chambers for measurement of short-circuit current (I-sc) as an indicator of ion secretion. Somatostatin release was determined with radioimmunoassay. Intracellular cAMP content was measured with enzyme-linked immunoadsorbent assay (elisa). Immunohistochemical techniques were used to study the expression of 5-HT3 receptors, somatostatin and somatostatin receptors in colonic tissue. Key results: In rat distal colonic mucosa/submucosa preparations, pretreatment with 5-HT3 receptor antagonists enhanced 5-HT-induced increases in I-sc. However, in mucosa-only preparations without retained neural elements, pretreatment with 5-HT3 receptor antagonists inhibited 5-HT-induced Delta I-sc. Pretreatment with a somatostatin-2 (sst(2)) receptor antagonist in mucosa/submucosa preparations augmented 5-HT-induced Delta I-sc. Combination of sst(2) and 5-HT3 receptor antagonists did not cause further enhancement of 5-HT-induced Delta I-sc. Moreover, both sst(2) and 5-HT3 receptor antagonists enhanced 5-HT-induced increase in intracellular cAMP concentration in the mucosa/submucosa preparations. 5-HT released somatostatin from rat colonic mucosa/submucosa preparations, an effect prevented by pretreatment with 5-HT3 receptor antagonists. Immunohistochemical staining demonstrated the presence of 5-HT3 receptors on submucosal somatostatin neurons and of sst(2) receptors on colonic mucosa. Conclusion and implications: Activation of neuronal 5-HT3 receptors in the submucosal plexus of rat colon suppressed 5-HT-induced ion secretion by releasing somatostatin from submucosal neurons.