A 5-HT3 Receptor Agonist Induces Neurally Mediated Chloride Transport in Rat Distal Colon

Having previously demonstrated that serotonin (5HT)-induced chloride secretion in rat distal colon is mediated at both neural and nonneural receptors, we isolated the neural component of this response by adding the selective 5-HT3 receptor agonist, 2-methyl-5-hydroxytryptamine (2Me5HT), to in vitro sheets of rat distal colon with intact neural plexuses. Rats were sacrificed, and the distal colon excised, opened, cut into sections and mounted, all layers intact, as flat sheets in Ussing chambers under short-circuited conditions. 2Me5HT induced a prompt, significant (P < 0.01), concentration-dependent rise in short-circuit current (Isc; EC50 6.2 μM); 50 μM 2Me5HT decreased both net sodium and chloride absorption (-0.1 ± 0.5 and -2.1 ± 0.8 μEq/cm2 · hr, respectively); the difference (2.0 ± 0.8 μEq/cm2 · hr) in these changes was not statistically different from the rise in Isc (1.5 ± 0.3 μEq/cm2 · hr). Since the only significant change inunidirectional flux was the rise in electrogenic CI- secretion (P < 0.01), the ΔIsc induced by 2Me5HT may be used as a measure of electrogenic chloride secretion induced by the agonist. The rise in Isc induced by 2Me5HT was abolished by both 0.2 μM tetrodotoxin and 0.1 μM ICS 205-930 (a 5-HT3 antagonist) but was not inhibited by 1.0 M atropine, 100 μM hexamethonium, 10 μM phentolamine, 10 μM propranolol, 10 μM 5-HTP-DP (a 5-HT1p antagonist), or 0.1 μM ketanserin (a 5-HT2 antagonist). These results indicate that 2-methyl-5-HT is a highly selective agonist for neurally based 5-HT3 receptors in this model. Since 2Me5HT-stimulated secretion is inhibited by neither adrenergic nor cholinergic antagonists, it is likely that the secretomotor neurotransmitter for this transport change is a neuropeptide.