5-hydroxytryptamine-evoked contractile activity of large gut in neonatal albino rats

Serotonin, also known as 5-hydroxytryptamine (5-HT), is a monoamine neurotransmitter. It is abundantly present in the gut of mammals. The extent to which it contributes to the contraction of the neonatal gut requires further investigation. This study aimed to assess the effect of 5-HT on the contractions of the large intestine in newborn albino rats. The colon and rectum samples were collected from neonatal and adult albino rats for analysis. Further, in an organ bath, isometric contractions of these isolated gut segments were recorded, in vitro, using a force transducer and a computerised chart recorder, with and without 5-HT in different groups. The 5-HT-induced contractions were also recorded in gut segments pre-treated with various antagonists. 5-HT (0.01–10 μM) caused a significantly (P < 0.05) greater contractile response (g/g wet tissue) in neonate rats as compared to adults. The response was greater in the rectum as compared to the colon in both neonates and adults. In neonate rats, ondansetron, a 5-HT3 antagonist, could not block the 5-HT-induced large gut contractions, while, in adult rats, it significantly blocked the 5-HT-evoked gut contractility. Methysergide, a 5-HT1/2/5-7 antagonist, blocked the response in both the adult and neonate rectum. The 5-HT-evoked response is mediated through 5-HT3 receptor subtypes in adults but not in neonate colon and rectum, indicating possible changes in the distribution of 5-HT receptors in the colon and rectum during development. Furthermore, atropine (a muscarinic cholinergic blocker) and hexamethonium (a ganglion blocker) could not affect the 5-HT-evoked responses in the neonate or adult rats’ colons or rectum. The effect of 5-HT did not appear to involve cholinergic or enteric ganglionic elements.