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

Abstract

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 (Oncorhynchus mykiss) 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.

Keywords: alevins; feeding behavior; first feeding; plant ingredients; tongue sensing system.

Figure 1

Husbandry parameters of trout fed a commercial-like (C) or plant-based (V) diet from first feeding to 1 month. Comparison of body weight gain (mg per periode of 10 days), survival rate (%), feed intake (g feed/kg^0.8 d⁻¹), and feed efficiency (g of BW/g of feed) for trout fed the C or V diet from 0–10, 11−20, 21–30 days after first feeding. An asterisk indicates a significant difference between the dietary treatments as determined by Welch Two Sample t-test (p < 0.05). Results are expressed in g as means ± SEM (n = 5 tanks).

Figure 2

Levels of mRNA encoding genes of fatty acid receptors in the tongue tissue of trout fed C or V diet from first feeding to 5 days and 30 days. Relative gene expression measured by RT-PCR of selected fatty acid receptors (ffar1, ffar2a2, ffar2b2a, ffar2b2b2, cd36, gpr84, fatp1, fatp4a) genes of trout fed from first feeding to 5 days (top panel) and 30 days (bottom panel) the C or V diet before feeding, 20 min and 6 h after meal. Data are expressed as group mean ± SEM; fold change 2^–ΔΔCt versus before meal for all genes. Two-way ANOVA following by one-way ANOVA when the interaction is significant, Tukey's post hoc test when appropriate; different letters indicate a significant difference (p  < 0.05) between diet and time (n = 8).

Figure 3

Levels of mRNA encoding genes of amino acid receptors in the tongue tissue of trout fed C or V diet from first feeding to 5 days and 30 days. Relative gene expression measured by RT-PCR of selected amino acid receptors (gprc6a, lpar5, t1r1-1, t1r1-2, t1r2-a, and t1r3) genes of trout fed from first feeding to 5 days (top panel) and 30 days (bottom panel) the C or V diet before feeding, 20 min and 6 h after meal. Data are expressed as group mean ± SEM; fold change 2^–ΔΔCt versus before meal for all genes. Two-way ANOVA following by one-way ANOVA when the interaction is significant, Tukey's post hoc test when appropriate; different letters indicate a significant difference (p  < 0.05) between diet and time (n = 8).

Figure 4

Levels of mRNA encoding genes of flavors receptors in the tongue tissue of trout fed C or V diet from first feeding to 5 days and 30 days. Relative gene expression measured by RT-PCR of selected flavors (tas2r4, otop1, asic1a, gmr4a) genes of trout fed from first feeding to 5 days (top panel) and 30 days (bottom panel) the C or V diet before feeding, 20 min and 6 h after meal. Data are expressed as group mean ± SEM; fold change 2^–ΔΔCt versus before meal for all genes. Two-way ANOVA following by one-way ANOVA when the interaction is significant, Tukey's post hoc test when appropriate; different letters indicate a significant difference (p < 0.05) between diet and time (n = 8).

Figure 5

Levels of mRNA encoding genes of calcium-signaling pathways in the tongue tissue of trout fed C or V diet from first feeding to 5 days and 30 days. Relative gene expression measured by RT-PCR of selected calcium-signaling (cpla2, plcb4, itpr3, calhm1, orai1a, orai1b, stim1a, stim1b) genes of trout fed from first feeding to 5 days (top panel) and 30 days (bottom panel) the C or V diet before feeding, 20 min and 6 h after meal. Data are expressed as group mean ± SEM; fold change 2^–ΔΔCt versus before meal for all genes. Two-way ANOVA following by one-way ANOVA when the interaction is significant, Tukey's post hoc test when appropriate; different letters indicate a significant difference (p < 0.05) between diet and time (n = 8).

Figure 6

Indolamine metabolites turnover in tongue tissue of trout fed C or V diet from first feeding to 5 days and 30 days. Tongue concentration of indolamines (5-HT, 5-HIAA and turnover ratio 5-HT/5-HIAA) of trout fed from first feeding to 5 days (top panel) and 30 days (bottom panel) the C or V diet before feeding, 20 min and 6 h after meal. Data are shown as means ± SEM; fold change versus before meal (T0). Two-way ANOVA following by one-way ANOVA when the interaction is significant, Tukey's post hoc test when appropriate; different letters indicate a significant difference (p  < 0.05) between diet and time (n = 10).

Figure 7

Catecholamine metabolites turnover in tongue tissue of trout fed C or V diet from first feeding to 5 days and 30 days. Tongue concentration of catecholamine (L-DOPA, HVA, and turnover ratio HVA/L-DOPA) of trout fed from first feeding to 5 days (top panel) and 30 days (bottom panel) the C or V diet before feeding, 20 min, and 6 h after meal. Data are shown as means ± SEM; fold change versus before meal (T0). Two-way ANOVA following by one-way ANOVA when the interaction is significant, Tukey's post hoc test when appropriate; different letters indicate a significant difference (p  < 0.05) between diet and time (n = 10).

Figure 8

Levels of mRNA encoding genes of food intake markers in the brain tissue of trout fed C or V diet from first feeding to 5 days and 30 days. Relative gene expression measured by RT-PCR of selected food intake markers (npya, agpr1, pomca, pomcb, and cartpt) genes in brain tissue of trout fed from first feeding to 5 days (top panel) and 30 days (bottom panel) the C or V diet before feeding, 20 min, and 6 h after meal. Data are expressed as group mean ± SEM; fold change 2^–ΔΔCt versus before meal for all genes. Two-way ANOVA following by one-way ANOVA when the interaction is significant, Tukey's post hoc test when appropriate; different letters indicate a significant difference (p  < 0.05) between diet and time (n = 8).

Figure 9

Levels of mRNA encoding genes of intestinal hormones in the gut tissue of trout fed C or V diet from first feeding to 5 days and 30 days. Relative gene expression measured by RT-PCR of selected intestinal hormones markers (cck, pyy, lepa1, and ghrl) genes in gut tissue of trout fed from first feeding to 5 days (top panel) and 30 days (bottom panel) the C or V diet before feeding, 20 min, and 6 h after meal. Data are expressed as group mean ± SEM; fold change 2^–ΔΔCt versus before meal for all genes. Two-way ANOVA following by one-way ANOVA when the interaction is significant, Tukey's post hoc test when appropriate; different letters indicate a significant difference (p  < 0.05) between diet and time (n = 8).