ALPHA‐TRANSDUCIN EXPRESSION IN THE GASTROINTESTINAL TRACT OF THE EUROPEAN SEA BASS (DICENTRARCHUS LABRAX)

The gustatory system plays the critical function of distinguishing between nutrients and non‐nutrient, potentially dangerous substances (1) allowing adaptation to different habitats (2). Different tastes are detected by G protein couple receptors and their signalling molecules,including the heterotrimeric G proteins,α‐transducin (Gαtran)and α‐gustducin(Gαgust) (3,4). Taste‐related molecules have been discovered in the gastrointestinal tract of a variety of species from fish to humans. In this study we examined the distribution and peptide content of cells expressing Gαtran‐immunoreactivity in the gastrointestinal(GI)mucosa of the sea bass. Adult European sea bass(Dicentrarchus labrax)were sampled from different tanks and euthanized by an overdose of anaesthetic. The stomach,pyloric caeca and intestine were harvested;the intestine was divided into three regions: cranial, middle and caudal. Tissue sections were processed for immunofluorescence with the following primary antisera: Gαtran, Gαgust, ghrelin(GHR), 5‐hydroxytryptamine (5‐HT), obestatin (OB), somatostatin (SOM), gastrin/cholecystokinin (GAS/CCK), glucagon‐like peptide‐1 (GLP‐1), calcitonin gene‐related peptide (CGRP) and substance P(SP). Each antibody was either used alone for single labelling or in combination with others for double labelling. Gαtran immunoreactivity was observed throughout the sea bass GI tract, but was more abundant in the stomach compared to the intestine with decreasing density of cells from the cranial to the caudal regions. Specificity of Gαtran and Gαgust immunostaining was established by Western blot analysis, which showed immunopositive bands at the expected molecular weight of ~45 and ~40 kDa, respectively, in sea bass gut tissue as well as in positive tissue (e.g. brain and eye). Staining specificity was also demonstrated by immunoblocking with the homologous peptides. Colocalization of Gαtran and Gαgust immunoreactivity was visualized in some cells in the stomach. A subpopulation of Gαtran immunoreacitve cells in the stomach also contained GHR, OB or 5‐HT immunoreactivity. Colocalization of Gαtran immumoreactivity with SOM, GAS/CCK, GLP‐1, SP, or CGRP immunistaining was not observed in any GI tract regions. Our data provide evidence that Gαtran and Gαgust are involved in chemosensory transmission in the sea bass GI enteroendocrine system. It is likely that nutrients activate taste receptors and their signalling molecules to induce the release of peptides from enteroendocrine cells, which in turn activate other cells types directly or via neural reflexes thus initiating a variety of physiological responses controlling GI functions and caloric intake. 1 Sternini C. Am J Physiol Gastrointest Liver Physiol, 2007, 292:G457‐G461. 2 Barreiro‐Iglesias et. all, Brain Behav Evol 2010,75:241‐250. 3 Mazzoni et. all, J Cell Mol Med 2013, Apr;17(4):466‐74. 4 Oike H et. all, J Neurosci, 2007, 27:5584‐5592.