EXOCYTOTIC RELEASE OF VASOACTIVE INTESTINAL POLYPEPTIDE AND SEROTONINFROM MUCOSAL NERVE-FIBERS AND ENDOCRINE-CELLS OF THE INTESTINE OF THEGOLDFISH (CARASSIUS-AURATUS) AND THE TILAPIA (OREOCHROMIS-MOSSAMBICUS) - AN ULTRASTRUCTURAL-STUDY

In earlier studies wt determined the effect, presence and ultrastructu re of vasoactive intestinal polypeptide (VIP) and 5- hydroxytryptamine (5-HT)-containing nerve fibres in the tilapia and goldfish intestinal mucosa. 5-HT-labelled varicosities were found close to the epithelial cells; however, synaptic membrane specializations have never been obs erved. VIP-like immunoreactive nerve fibres appear to be located less frequently close to the goldfish epithelium, as in the tilapia intesti ne, in which the distance between the VIP- or 5-HT-labelled varicositi es and the epithelial cells was also rather large (more than 2 mu m). To establish a possible role of VIP and 5-HT-labelled neurotransmitter s involved in the regulation of fish intestinal epithelium, both elect ron microscopical and immunoelectron microscopical methods were used t o visualize the release of 5-HT and VIP from nerve fibres. We found ex ocytoses from VIP-ergic and serotonergic varicosities in the muscle la yers of both fish. Directly underneath the intestinal epithelium of th e goldfish, it was demonstrated that 5-HT could be released from scarc e varicosities. The release of 5-HT in the tilapia intestinal mucosa c ould only be observed from endocrine cells.