Effects of a Plant-Based Diet During the First Month of Feeding on Alevin Rainbow Trout () in the Development of Tongue Sensory System Regulating Feeding Behavior.

Taste perception is essential for animals to detect nutrients, providing critical dietary information necessary for growth and survival. Since the early growth performance of alevin rainbow trout () can be affected by food intake influenced by terrestrial ingredients without fish meal and fish oil, our study aimed to evaluate the role of taste receptors in nutrient detection and the associated signaling pathways leading to central nervous system activation in the regulation of feeding behavior. We conducted a nutritional experiment from the first feeding to 30 days, comparing the performance of fish fed a commercial-like diet (C diet: a blend of fish meal, fish oil, and plant ingredients) with those on a totally plant-based diet (V diet). After 5 and 30 days of feeding, fish were fasted for 16 h and then fed either the C or V diet, with sampling conducted at 20 min and 6 h post-meal. We evaluated the expression of nutrient-sensing genes related to fatty acids, amino acids, and sweetness, and taste receptor genes for flavors. Additionally, we examined calcium signaling pathways in the tongue, focusing on indolamine and catecholamine pathways, alongside appetite-regulating neuropeptides in the brain and intestinal hormones in the gut of alevins. Results indicated that fish on the V diet experienced a decrease in body weight gain starting 10 days after feeding to 30 days, along with changes in feed intake during the periods of 0-10 days and 21-30 days after the first meal. In tongue tissue, after 5 days of feeding, fish on the C diet showed a slight upregulation of nutrient taste receptors, but not those related to flavor, along with an upregulation of the calcium signaling pathway. By 30 days, there was a general upregulation of nutrient and flavor taste receptors, although the calcium signaling pathway showed less clear evidence of regulation. A significant dysregulation of the serotonin pathway, along with its degradation, was observed in the tongues of fish fed the V diet at both 5 and 30 days. For the first time in fish, catecholamine quantification levels in the tongue emerged as a potential marker for nutrient detection, with high quantification of L-DOPA after 5 days on the V diet, but much lower after 30 days. This impaired monoamine and catecholamine turnover in the tongue could be linked to a failure in activating the tongue-brain axis, potentially contributing to reduced food intake, as indicated by poorly regulated brain neuropeptides but also intestinal hormones in fish fed the V diet after 30 days. Overall, these findings demonstrate that the V diet disrupts the feeding response at an early stage, underscoring the heightened sensitivity of rainbow trout alevins' tongue sensing systems to novel food sources during critical early development.