Abstract
This study aimed to characterize amino acid sensing systems in the gastrointestinal tract (GIT) of the carnivorous fish model species rainbow trout. We observed that the trout GIT expresses mRNAs encoding some amino acid receptors described in mammals [calcium-sensing receptor (CaSR), G protein-coupled receptor family C group 6 member A (GPRC6A), and taste receptors type 1 members 1 and 2 (T1r1, T1r2)], while others [taste receptor type 1 member 3 (T1r3) and metabotropic glutamate receptors 1 and 4 (mGlur1, mGlur4)] could not be found. Then, we characterized the response of such receptors, as well as that of intracellular signaling mechanisms, to the intragastric administration of l-leucine, l-valine, l-proline or l-glutamate. Results demonstrated that casr, gprc6a, tas1r1 and tas1r2 mRNAs are modulated by amino acids in the stomach and proximal intestine, with important differences with respect to mammals. Likewise, gut amino acid receptors triggered signaling pathways likely mediated, at least partly, by phospholipase C β3 and β4. Finally, the luminal presence of amino acids led to important changes in ghrelin, cholecystokinin, peptide YY and proglucagon mRNAs and/or protein levels. Present results offer the first set of evidence in favor of the existence of amino acid sensing mechanisms within the fish GIT.
Highlights
This study aimed to characterize amino acid sensing systems in the gastrointestinal tract (GIT) of the carnivorous fish model species rainbow trout
The present study offered the first report on the putative functioning of amino acid sensing mechanisms in the GIT of an important aquaculture fish model, the rainbow trout
We demonstrated that the rainbow trout GIT expresses mRNAs encoding key amino acid receptors described in m ammals[10], including CasR, Gprc6a, T1r1 and T1r2
Summary
This study aimed to characterize amino acid sensing systems in the gastrointestinal tract (GIT) of the carnivorous fish model species rainbow trout. EECs respond to the sensing of nutrients with the secretion of different hormones, mainly ghrelin (GHRL), cholecystokinin (CCK), peptide tyrosine-tyrosine (PYY) and glucagon-like peptide-1 (GLP-1) These hormones, apart from acting locally modulating digestion and gut motility, can enter into the lamina propria and bind to their respective receptors on the vagal afferent nerve, sending information on energy status to the central nervous system (CNS)[2,4]. Immunoreactive cells expressing taste-signalling G protein subunits, Gαtran and Gαgust, have been detected in stomach and intestine of common sole (Solea solea)[17] and seabass (Dicentrarchus labrax)[18] These studies reported that the density of these molecules was modulated by diet composition in sole, and that Gαtran-immunoreactive gastric cells in the sea bass co-expressed G HRL18, which is consistent with their likely participation in nutrient sensing mechanisms
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