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Orexin A and B peptides and the receptor OX2R were studied in sea bass and goldfish gastroenteric tract by immunoblotting combined with densitometric analysis using NIH Image J software and immunohistochemical techniques. These teleost species present a different gut organization and diverse feeding habits. Immunoblotting experiments showed one band of 16 kDa corresponding to prepro-orexin, and one band of 38 kDa corresponding to the OX2R receptor. Immunohistochemical localization of OXA and OXB was observed in the enteric nervous system throughout the gastroenteric tract of both species. OXA and OXB immunoreactive cells were found in the gastric and intestinal regions of sea bass, and were mainly found in the basal region of folds in intestinal bulb, and in the midgut and hindgut of goldfish. The distribution of OX2R was mainly detected in the mucosa of the gastroenteric tract of sea bass and goldfish. This distribution suggests an endocrine action of OXA and OXB in the gastrointestinal tract as well as involvement in the peripheral control of food intake and digestive processes in both species. This study might also serve to determine the productive factors in breeding and as a baseline for future experimental studies on the regulation of the gastroenteric functions in non-mammalian vertebrates.

Orexin A and B are neuroregulatory peptides involved in feeding-associate processes, glucidic and energy metabolism, and in arousal maintenance. Recently mRNAs sequences encoding for prepro-orexin are reported in the gastrointestinal tract of some freshwater, seawater and euryaline fish. Moreover only one winter flounder fish orexin GPCR receptor strictly analogous to the mammalian OXR2 has been characterized and sequenced up today. Since teleost species show different organization of the gut, feeding behaviour and habitat, in this study we employed immunohistochemistry and Western Blot analysis to investigate the distribution of preproorexin, orexin A- and B-like peptides and their receptors in the GEP system of two different species, sea bass and goldfish. In seabass, orexin A- and B-like immunoreactive cells were detected in the epithelium along the mucosal folds or at the upper margin of gastric crypts, and in basal regions of intestinal plicae; in goldfish, thin and elongated orexins-A and B-like immunoreactive cells were observed in the epithelium at the basis of mucosal folds in the pseudogaster and in the mid gut. In both species, some orexin-A and B-like, OXR1 and OXR2-like immunoreactive fibres and neurons were detected in the myenteric plexus and thin fibers in the submucosal plexus and in the circular muscular sheet in gut regions. Many rounded or triangular endocrine cells strongly positive to orexin A- and B-like were observed in the marginal zone of endocrine pancreatic islets. Western blot analysis showed a preproorexin band of 16 kDa and two bands of 53 and 38 kDa respectively for OXR1 and OXR2. The prepro-orexin Western Blot showed a more intense band in cardial stomach, blind sac and, mid and hind gut; the receptors molecular weight correspond to those obtained in avian species. Our results could suggest an involvement of these peptides in the regulation of fish gastrointestinal activities and pancreatic functions.