Gamma-aminobutyric acid (GABA) is an important inhibitory neurotransmitter in the central nervous system of living organisms and has the ability to reduce the magnitude of stress in humans and animals. In this study, we evaluated the supplemental effects of GABA on normal and high water temperature based on growth, blood plasma composition as well as heat shock proteins and GABA-related gene expression in juvenile olive flounder. For this, a 2 × 2 factorial design of experiment was employed to investigate the dietary effects of GABA at 0 mg/kg of diet (GABA0 diet) and 200 mg/kg of diet (GABA200 diet) in water temperatures of 20 ± 1 °C (normal temperature) and 27 ± 1 °C (high temperature) for 28 days. A total of 180 fish with an average initial weight of 40.1 ± 0.4 g (mean ± SD) were distributed into 12 tanks, of which, each tank contained 15 fish based on the 4 dietary treatment groups in triplicate. At the end of the feeding trial, the results demonstrated that both temperature and GABA had significant effects on the growth performance of the fish. However, fish fed the GABA200 diet had a significantly higher final body weight, weight gain and specific growth rate as well as a significantly lower feed conversion ratio than the fish fed the GABA0 diet at the high water temperature. A significant interactive effect of water temperature and GABA was observed on the growth performance of olive flounder based on the two-way analysis of variance. The plasma GABA levels in fish were increased in a dose-dependent manner at normal or high water temperatures, whereas cortisol and glucose levels were decreased in fish fed GABA-supplemented diets under temperature stress. The GABA-related mRNA expression in the brains of the fish such as GABA type A receptor-associated protein (Gabarap), GABA type B receptor 1 (Gabbr1) and glutamate decarboxylase 1 (Gad1) were not significantly affected by GABA-supplemented diets under normal or temperature stressed conditions. On the other hand, the mRNA expression of heat shock proteins (hsp) in the livers of the fish, such as hsp70 and hsp90, were unchanged in fish fed the GABA diets compared to the control diet at the high water temperature. Collectively, the present study showed that dietary supplementation with GABA could enhance growth performance, and improve the feed utilization, plasma biochemical parameters and heat shock proteins and GABA-related gene expression under the stress of high water temperatures in juvenile olive flounder.

Recent research is increasingly shedding light on the important role that microbial metabolites such as γ-aminobutyric acid (GABA) play in the context of nutrition, cognition, immune function, and the modulation of the gut microbiome. Yet, very few trials were conducted to assess the effects of its supplementation on biomarkers of fish health. Therefore, an eight-week feeding trial was devised to evaluate GABA supplementation in juvenile olive flounder, (Paralichthys olivaceus). A total of 630 fish with an average weight of 4.90 ± 0.10 g (±SD) were randomly assigned to one of seven triplicate groups and fed a non-GABA supplemented diet (CON, with 92 mg/kg GABA content), a positive control with 4 g/kg oxytetracycline (OTC), and five other diets supplemented with 50, 100, 150, 200 and 250 mg/kg GABA (corresponding to a total GABA content of 154, 229, 282, 327 and 352 mg/kg, respectively). Growth, blood chemistry, nonspecific immunity, digestive enzyme activity and disease resistance were assessed. The results showed that 100 and 150 mg/kg GABA supplementation consistently yielded significant improvements (p < 0.05) in growth, intestinal amylase, serum lysozyme, and survival against infection with Streptococcus iniae. Based on polynomial analysis, the optimal supplementation level was determined to be 237 mg/kg. These results support GABA as an important functional feed additive in juvenile olive flounder.

An 8-week feeding trial was conducted to evaluate the effects of dietary γ-aminobutyric acid (GABA) on growth performance, immune response, and disease resistance for juvenile whiteleg shrimp, Litopenaeus vannamei. Five isonitrogenous diets were formulated by supplementing 0 (CON), 50 (GABA₅₀), 100 (GABA₁₀₀), and 300 (GABA₃₀₀) mg of GABA and 4 g of oxytetracycline (OTC) per kilogram of diet. A total of 225 juvenile whiteleg shrimp with an initial weight of g were randomly distributed and reared in 15 aquaria as triplicates. After 8 weeks of the feeding trial, weight gain, specific growth rate, feed efficiency, and protein efficiency ratio of shrimp fed GABA₁₀₀ were significantly higher than those of shrimp fed CON, GABA₅₀, and GABA₃₀₀ diets ( ). However, there were no significant differences among shrimp fed CON, GABA₅₀, GABA₃₀₀, and OTC diets ( ). After nine days of challenge test with Vibrio alginolyticus, the average cumulative survival rate of shrimp fed GABA₅₀, GABA₁₀₀, and OTC was significantly higher than that of shrimp fed GABA₃₀₀ and CON diets ( ). These results may suggest that 100 mg dietary GABA supplementation (including endogenous GABA, 175.6 mg/kg diet) per kilogram of diet could be the optimum dietary level to replace antibiotics and improve growth performance and disease resistance in whiteleg shrimp L. vannamei.