γ-aminobutyric acid improves gut-brain axis function and behavioral adaptation in broilers under low-protein diet

Low crude protein (CP) diets can improve feed efficiency and reduce ammonia emissions, but negative impacts on broiler productivity can occur. This study aimed to evaluate the effects of γ-aminobutyric acid (GABA) supplementation in low CP diets on productivity, enzyme activity, neuropeptidergic gene expression, and behavior of broilers. A total of 360 male Ross 308 broilers were used in a factorial design experiment with two CP levels (19.5 and 18.0%) and three GABA levels (0, 100, and 200 mg/kg). Dietary CP and GABA levels did not significantly affect broiler productivity (P>0.05). Reducing CP in the diet increased amylase and decreased protease activities (P<0.05). GABA supplementation had no significant effect on enzyme activity (P>0.05). The expression of orexigenic genes (AgRP, NPY, ghrelin) was significantly downregulated, while anorexigenic genes (CCK, PYY, GLP-1, GLP-2) were upregulated in the 18.0% CP group (P<0.05). In contrast, dietary supplementation of GABA significantly increased the expression of orexigenic genes and decreased the expression of anorexigenic genes (P<0.05). The expression of the pro-inflammatory gene TNF-α was increased, while the anti-inflammatory gene IL-10 was decreased in the 18.0% CP group (P<0.05). GABA supplementation reversed this effect by downregulating TNF-α and upregulating IL-10 expression (P<0.05). Similarly, the expression of tight junction genes (ZO-1 and JAM2) was decreased in broiler fed 18.0% CP, but significantly upregulated following GABA supplementation (P<0.05). A significant CP×GABA interaction was observed for orexigenic, anorexigenic, inflammatory, and tight junction gene expression (P<0.05). A 1.5% reduction in CP had no significant effect on sitting or open-wings (P>0.05). However, it increased walking, resting, stretching, and preening, while decreasing feeding, drinking, and pecking (P<0.05). The GABA supplementation enhanced feeding and drinking and reduced pecking, walking, sitting, resting, stretching, preening, and open-wings (P<0.05), with a significant CP×GABA interaction effecting overall broiler behavior (P<0.05). A 1.5% reduction in dietary crude protein had no adverse effect on broiler production but resulted in the downregulation of tight junction and anti-inflammatory genes expression. Conversely, dietary supplementation of GABA in low-protein diets upregulated orexigenic gene expression, promoted homeostatic regulation, improved immune and gut barrier functions, and modulated broiler behavior.