Distribution˚˚ of˚˚ Serotonin-immunoreactive˚˚ Enterochromaffin˚˚ Cells˚˚ in˚˚ the˚˚ Gastrointestinal˚˚ Tract˚˚ of˚˚ the˚˚ Least˚˚ Shrew˚˚ (Cryptotis˚˚ parva) ˚ Distribución˚˚ de˚˚ las˚˚ Células˚˚ Enterocromafines˚˚ Serotonina-inmunorreactiva del˚˚ Tracto˚˚ Gastrointestinal˚˚ de˚˚ la˚˚ Musaraña˚˚ Enana˚˚ (Cryptotis˚˚ parva)

SUMMARY: Serotonin is an important neurotransmitter in the central (CNS) and peripheral (PNS) nervous systems. It is involved in a variety of physiological processes both in the gut and in the CNS. The present study examined the distribution of serotonin containing enterochromaffin cells in the gastrointestinal tract (GIT) of a vomit competent species, the least shrew. These cells were easily recognized by their globular granules stained with the HE and the second were basally located, facing the lamina propria of the mucosa. Their secretory granules were not distinct in shape, and are most probably paracrine in their mode of secretions. ˚ between the intestine and the brain. Serotonin produces its diverse effects via seven different families of serotonergic receptors (5-HT1 - 5-HT7), including multiple subtypes within the 5-HT1 (5-HT1A, 5-HT1B, 5-HT1D , 5-HT1E , 5-HT1F and 5-HT1P), 5-HT2 (5-HT2A, 5-HT2B and 5- HT2C), 5-HT3 (5-HT3A, 5-HT3B and 5-HT3C), and 5- HT5 (5-HT5A and 5-HT5B) families. Indeed, serotonin is involved in the mediation of both immediate and delayed phases of chemotherapy-induced vomiting as well as regulating the GIT motor activity and its secretory function via the enteric nervous system (ENS) (Darmani & Ray). Moreover, antagonists of serotonin 5-HT3 receptors attenuate vomiting induced by chemotherapeutics in patients with cancer (Darmani & Ray). At least 5 different 5-HT3 *