ULTRASTRUCTURAL-STUDY OF THE PRESENCE OF VASOACTIVE INTESTINAL POLYPEPTIDE AND SEROTONIN IN MUCOSAL NERVE-FIBERS AND ENDOCRINE-CELLS OF THEINTESTINE OF GOLDFISH (CARASSIUS-AURATUS) AND TILAPIA (OREOCHROMIS-MOSSAMBICUS)

In order to establish a possible role of vasoactive intestinal polypep tide (VIP) and serotonin as (neuro)transmitters involved in the regula tion of fish intestinal epithelium, we studied the presence of VIP and serotonin at the ultrastructural level in the intestinal mucosa of ti lapia and goldfish. A low percentage of varicosities near the basal me mbrane of the tilapia intestinal epithelium was found to label for VIP or for serotonin, whereas in the goldfish, this percentage was much h igher. The varicosities usually contained large granular and small cle ar vesicles. Immunogold labeling indicated that serotonin and VIP were localized in the large granular vesicles. Unlabeled large granular ve sicles and small clear vesicles were usually also present in varicosit ies with serotonin- or VIP-labeled vesicles. In the goldfish, the sero tonin-labeled varicosities were close to the epithelial cells, and dir ect contacts between serotonin-labeled nerve fibres and epithelial cel ls could sometimes be visualized. However, synaptic membrane specializ ations were never observed. In tilapia, the distance between the VIP- or serotonin-labeled varicosities and the epithelial cells was large ( more than 2 mu m).