Expression and localization of orexin A and orexin B and their receptors OX1R and OX2R in the Gastroenteropancreatic (GEP) system of Sea Bass (Dicentrarcus labrax) and goldfish (Carassius auratus).

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.