**Background:** In mammals, the gastrointestinal tract (GIT) senses luminal nutrients via specific receptors and transporters on enteroendocrine cells and enterocytes, triggering release of appetite-regulating hormones (ghrelin, CCK, PYY, GLP-1) that communicate with the brain via the gut-brain axis. Fatty acid (FA) sensing in mammals involves G protein-coupled receptors (FFAR1/4 for medium- and long-chain FAs; FFAR2/3 for short-chain FAs; GPR84 for MCFAs; GPR119 for lipid derivatives) and transporters (FAT/CD36, FATP4 for LCFAs; MCT-1 for SCFAs). However, gut nutrient sensing mechanisms remain almost unexplored in fish, despite lipids being the primary energy source in aquaculture and the major aerobic fuel for fish muscle. Rainbow trout, a carnivorous species with high lipid digestion capacity, may exhibit evolutionary differences from omnivorous mammalian models.
**Methods:** Rainbow trout (90 ± 20 g) were fasted for 48 h and intragastrically administered 1 mL/100 g body weight of vehicle (control) or 50 μmol/mL of octanoate (8-carbon saturated FA), oleate (18-carbon monounsaturated FA), α-linolenate (ALA, 18-carbon PUFA), or butyrate (4-carbon saturated FA). GIT samples (stomach, pyloric caeca, proximal/middle/distal intestine) were collected at 20 min and 2 h post-administration. mRNA abundance of FA receptors (ffar1, ffar2b1.1, ffar2b1.2, ffar2b2a, ffar2b2b, ffar2a1b, ffar2a2, gpr84, gpr119), transporters (fat/cd36, fatp4, slc16a1a/b encoding Mct-1), intracellular signaling elements (gnai1, plcβ1, plcβ3, plcβ4, itpr1, itpr3, ac), and gastrointestinal hormones (ghrl, cck, pyy, gcg) were quantified by RT-qPCR. Protein levels of Ghrl, Cck, and Pyy were assessed by Western blot.
**Key Results:** (1) All FA receptors and transporters were expressed along the trout GIT, with transporters showing higher abundance (Ct ≈25–29) than receptors (Ct ≈28–34). Ffar2b2a and ffar2b2b were the most abundant receptor mRNAs. Slc16a1a (Mct-1a) was highly expressed in distal intestine, while slc16a1b was almost exclusively detected in stomach. (2) At 20 min post-administration, octanoate upregulated ffar1, ffar2b1.2, ffar2a1b, ffar2a2, and slc16a1a in middle intestine. Oleate induced ffar2b1.1, ffar2b1.2, ffar2a1b, ffar2a2, gpr84, and gpr119 in proximal intestine. ALA increased ffar1, ffar2b1.2, and gpr119 across multiple regions. Butyrate upregulated gpr84, gpr119, cd36, fatp4, and slc16a1a in proximal/middle intestine. (3) At 2 h, all FAs upregulated cd36 and/or fatp4 in proximal GIT regions. Butyrate increased cd36 in all GIT regions. (4) Intracellular signaling: oleate and ALA increased plcβ1, plcβ3, and itpr1 mRNAs in anterior GIT regions. Butyrate upregulated plcβ1 and itpr1 but downregulated ac mRNAs. Octanoate showed minimal effects on these pathways. (5) Hormone responses: octanoate and oleate increased gcg (GLP-1) at 20 min and pyy/Pyy at 2 h; oleate also increased cck/Cck. ALA increased ghrl and gcg at 20 min and cck and pyy at 2 h. Butyrate increased ghrl but decreased cck and pyy mRNA levels.
**Clinical Implications:** This study establishes the first evidence for functional FA sensing mechanisms in the fish GIT, with important implications for aquaculture nutrition. The findings suggest that rainbow trout lack FFAR4 (a major LCFA sensor in mammals) and that Ffar2 isoforms and Gpr84/119 may compensate. Notably, ALA (an n-3 PUFA) activated FA sensors and hormone release in trout, contrasting with mammals where n-3 PUFAs do not activate FA sensors. Butyrate responses differed markedly from other FAs, suggesting SCFAs may have orexigenic effects in trout (increased ghrelin, decreased CCK/PYY) versus anorexigenic effects in mammals. These species-specific differences may reflect evolutionary divergence between fish and mammals and/or dietary habit differences (carnivore vs. omnivore). Understanding these mechanisms could optimize feed formulations to regulate feed intake and improve growth efficiency in aquaculture.