Sense of smell is mediated by diverse families of olfactory sensing receptors, conveying important dietary information, fundamental for growth and survival. The aim of this study was to elucidate the role of the sensory olfactory pathways in the regulation of feeding behavior of carnivorous rainbow trout (RT, Oncorhynchus mykiss), from first feeding until 8 months. Compared to a commercial diet, RT fed with a total plant-based diet showed drastically altered growth performance associated with feed intake from an early stage. Exhaustive examination of an RT genome database identified three vomeronasal type 1 receptor-like (ORA), 10 vomeronasal type 2 receptor-like (OLFC) and 14 main olfactory receptor (MOR) genes, all highly expressed in sensory organs, indicating their potential functionality. Gene expression after feeding demonstrated the importance in olfactory sensing perception of some OLFC (olfcg6) and MOR (mor103, -107, -112, -113, -133) receptor family genes in RT. The gene ora1a showed evidence of involvement in olfactory sensing perception for fish fed with a commercial-like diet, while ora5b, mor118, mor124 and olfch1 showed evidence of involvement in fish fed with a plant-based diet. Results indicated an impact of a plant-based diet on the regulation of olfactory sensing pathways as well as influence on monoaminergic neurotransmission in brain areas related to olfactory-driven behaviors. The overall findings suggest that feeding behavior is mediated through olfactory sensing detection and olfactory-driven behavior pathways in RT.

Taste perception plays an important role in an animal’s detection of nutrients, conveying key dietary information, fundamental for its growth and survival. Because alternative terrestrial ingredients are known to affect the feeding of rainbow trout (RT, Oncorhynchus mikyss), we aimed to assess the importance of taste receptors in detection. Using self-feeders, we examined the feeding behavior (30 days of a feeding trial followed by 10 days of a preference trial) of RT fed with a commercial diet (C), vegetable diets supplemented with linseed oil (V1) or algal oil (V2). During the feeding trial those fed V2 decreased their food intake. The preference trial revealed that fish preferred V2 v. C and V1 v. V2 for fish which had consumed V1 and C during their feeding trial. Mechanistically, taste receptors were mainly expressed in taste organs and regulated by diet, which indicated the function of the taste receptors. Some taste receptors for fatty acids (such as the ffar receptor) and amino acids (such as the tasr receptor) were highly expressed in the RT tongue. While ffar2a transcripts were upregulated by vegetal diets in the tongue, ffar1 and ffar4, known for important roles in mammals, were very low expressed and not found in the RT genome, respectively. Overall findings show that RT displayed the fundamental mechanisms for oro-gustatory perception of nutrients related to different diet composition.

Since 20 years of research, free fatty acids receptors (FFARs) have received considerable attention in mammals. To date, four FFARs (FFAR1, FFAR2, FFAR3 and FFAR4) are especially studied owing to their physiological importance in various biological processes. This ubiquitist group of G protein-coupled receptors (GPCRs) are majors reports in the key physiological functions such as the regulation of energy balance, metabolism or fatty acid sensing. However, up till date, even some studies were interested in their potential involvement in fatty acid metabolism, no genome investigation of these FFARs have been carried out in teleost fish. Through genome mining and phylogenetic analysis, we identified and characterised 7 coding sequences for ffar2 in rainbow trout whereas no ffar3 nor ffar4 gene have been found. This larger repertoire of ffar2 genes in rainbow trout results from successive additional whole-genome duplications which occurred in early teleosts and salmonids, respectively. A syntenic analysis was used to assign a new nomenclature to the salmonid ffar2 and showed a clear conservation of genomic organisation, further supporting the identity of these genes as ffar2. RT-qPCR was then used to examine, firstly during ontogenesis and secondly on feeding response the expression pattern of ffar1 and ffar2 genes in proximal gut and brain of all trout ffar genes. Overall, this study presents a comprehensive overview of the ffar family in rainbow trout.