Serotonin takes part in the regulation of the gastrointestinal motility, secretion and sensitivity. It produces its effects by binding specific receptors thus being one of the important neurotransmitters and signal molecules in the gut. There are different types of specific serotonin receptors, recognized and cloned into seven groups (from 5HT1 to 5HT7) with various subtype numbers. The aim of this work is to determine the presence and localization of immunohistochemical expression of serotonin

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... and serotonin receptor 3 (5-HTR3) in the developing gastrointestinal tract of rats. 20 rat embryos (16 th-20 th day of gestation) and fragments of the stomach and small intestine of 10 one-day-old rats were used for the study. On paraffin sections immunohistochemical reactions for serotonin (primary antibody rabbit polyclonal antibody, Chemicon, USA) and serotonin receptor 5-HTR3 (primary antibody goat polyclonal antibody, Santa Cruz Biotechnology, USA) were carried out using the ABC. Immunohistochemical expression for serotonin and serotonin receptor 3 (5-HTR3) were not detected in the gastrointestinal tract of rat embryos. In the gastrointestinal tract of the one-day-old rats there was positive reaction for serotonin. Serotonin-producing cells were present in the covering epithelium of the villi in the small intestine. They were small in numbers and separately located. The immunhistochemical reaction for 5-HT3 receptors was positivated in the cytoplasm of a great number of smooth muscle cells in both transverse and longitudinal muscle layer of the stomach wall of the newborn rats. Serotonin production and secretion starts after birth and through its 5-HT3 receptors takes part in the regulation of the morphological and functional maturation of the gastrointestinal tract which starts in the embryonal period but continues postnatally.