The Effects of Serotonin on the Motility of Small and Large Intestine of Guinea Pigs in Fasting


Abstract: This study was carried out in vitro to observe the effects of serotonin (5-HT), identified as a transmitter candidate for the regulation of intestinal motility, on the motility of the small and large intestines of fed and fasted guinea pigs. In the study, 48 adult guinea pigs weighing 300-450 g were divided equally into control (fed) and experimental (fasted for 96 h) groups. Half of the animals in both groups were used for taking small intestinal segments, the mid jejunum and distal ileum, while the rest of the animals were used to take large intestinal segments, as well as proximal and distal colons. The isometric contractions of tissues induced by various agonists alone and in the presence of various antagonists were recorded. At first, bethanechol (10^{-3} M) and 5-HT (5 x 10^{-5} M), and then physostigmine (10^{-5} M) were added to the solution and after getting the maximum response in the amplitudes of the tissues, serotonin (5 x 10^{-5} M) was used again. After that, 5-HT was added to the solution in the presence of L-NNA (NG-nitro-L-arginine, 10^{-5} M), hexamethonium (10^{-5} M), verapamil (3 x 10^{-5} M) and atropine (5 x 10^{-5} M), and finally Ca^{++}-free Tyrode solution was applied to the tissues. We found that muscarinic receptors and Ca^{++} play a major role in serotonin-induced contractions in both the fed and fasted small and large intestines of guinea pigs. In addition, the increases in 5-HT-induced contractions after inhibiting NO (nitric oxide) synthesis with L-NNA and blocking the nicotinic ganglions with hexamethonium were interpreted as NO and nicotonic synapses that also play a role in these contractions.

Keywords: Fasting, serotonin, guinea pig, small and large intestine, motility

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