Increasing water temperature associated with global warming directly or indirectly influences survival, growth, physiological activity, and immunity of cultured fish, and this environmental change can be a stressor for cold water fish species such as rainbow trout. Gamma-aminobutyric acid (GABA) is one of the free amino acids, classified as a non-essential amino acid, playing an important role as a neurotransmitter inhibitor in the nervous system. In mammals, stress-relieving and sleep-enhancing effects of GABA have been demonstrated, and a growth-promoting effect of this free amino acid has been shown in cultured fishes. Up to date, little is known about the functional role of GABA in rainbow trout. Therefore, the current study was conducted to evaluate effects of graded dietary GABA levels on growth performance and stress responses under acute temperature exposure in this species.
Two hundred twenty-five juveniles with an initial body weight averaging 18.9±0.1 g (mean±SEM) were randomly distributed into each of 15 rectangular tanks (15 fish per tank; N = 3 tanks per treatment). Addition of 0 (G0), 50 (G50), 100 (G100), 150 (G150), and 200 (G200) ppm GABA in a basal diet was made to prepare for the five experimental diets. Following the 8-week feeding trial, the juveniles from each treatment were abruptly exposed to 2-h heat shock at three different temperatures (20, 22, and 24 ?), respectively and recovered at ambient water temperature for 2-h.
Results of the feeding trial showed that the juveniles fed the G200 diet had a significantly higher weight gain, specific growth rate, and feed efficiency in comparison to those fed the G0 diet (P < 0.05), whereas no difference in survival rate, morphological changes (condition factor, hepatosomatic index, and viscerosomatic index) and plasma metabolites (glutamic oxaloacetic transaminase, glutamic pyruvic transaminase, total protein, total cholesterol, and glucose) of the juveniles fed the experimental diets was observed.
Results of the acute temperature exposure showed that the pattern of changes in the plasma metabolites of the juveniles exposed to the different water temperature was not comparable. No significant interactive and main effects of the GABA level and temperature was detected in the juveniles exposed to 22 ?, whereas there was a significant main effect of the GABA level on plasma total cholesterol, triglyceride, total protein, and glucose levels in the juveniles exposed to 20 ?. A significant interactive effect of the GABA level and temperature on plasma total cholesterol and total protein levels in the juveniles exposed to 24 ? was detected, whereas there was a significant main effect of the temperature on plasma triglyceride level in the juveniles exposed to the same temperature.
Expression levels of genes involved in temperature stress responses in various tissues of the juveniles exposed to the different water temperature will be discussed later.