Ultrastructural study of the presence of vasoactive intestinal polypeptide and serotonin in mucosal nerve fibres and endocrine cells of the intestine of goldfish (Carassius auratus) and tilapia (Oreochromis mossambicus)

Abstract.

In order to establish a possible role of vasoactive intestinal polypeptide (VIP) and serotonin as (neuro)transmitters involved in the regulation of fish intestinal epithelium, we studied the presence of VIP and serotonin at the ultrastructural level in the intestinal mucosa of tilapia and goldfish. A low percentage of varicosities near the basal membrane of the tilapia intestinal epithelium was found to label for VIP or for serotonin, whereas in the goldfish, this percentage was much higher. The varicosities usually contained large granular and small clear vesicles. Immunogold labeling indicated that serotonin and VIP were localized in the large granular vesicles. Unlabeled large granular vesicles and small clear vesicles were usually also present in varicosities with serotonin- or VIP-labeled vesicles. In the goldfish, the serotonin-labeled varicosities were close to the epithelial cells, and direct contacts between serotonin-labeled nerve fibres and epithelial cells could sometimes be visualized. However, synaptic membrane specializations were never observed. In tilapia, the distance between the VIP- or serotonin-labeled varicosities and the epithelial cells was large (more than 2 μm).