Intragastric capsaicin stimulates colonic motility via a neural reflex in conscious dogs

Capsaicin is known to contract smooth muscles and to have an excitatory action on thin, primary afferent neurons. The aim of this study was to test the hypothesis that intragastric capsaicin may have an excitatory effect on colonic motility.

Mongrel dogs equipped with five strain-gauge force transducers on the colon (C1-C5) were used. In 5 dogs, the effect of intragastric capsaicin (0.05, 0.1, 0.3, and 0.5 mg/kg) on colonic motility in the presence or absence of the cholinergic blockers atropine and hexamethonium was studied. In 4 dogs, the effect of capsaicin (0.5 mg/kg) administration into a vagally innervated or denervated gastric pouch on colonic motility was investigated.

Motility at C2-C3, C1-C3, and C1-C5 was significantly increased by 0.1, 0.3, and 0.5 mg/kg of capsaicin, respectively. The excitatory effect of capsaicin on colonic motility was inhibited by atropine and hexamethonium. Colonic motility at C1-C4 was significantly increased by capsaicin administration into a vagally innervated pouch but was not affected by capsaicin administration into a vagally denervated pouch.

These results indicate that intragastric capsaicin predominantly stimulates motility in the proximal half of the colon through neural pathways, probably through a vagovagal reflex, but sympathetic involvement cannot be excluded.