Metabolic consequences of chronic sublethal ammonia exposure at cellular and subcellular levels in Nile tilapia brain

The objective of this study was to elucidate the mechanism of chronic sublethal ammonia exposure in the brain of Nile tilapia, Oreochromis niloticus, by examining the cytosolic (c) and mitochondrial (m) enzymes related to ammonia detoxification. The experiment was conducted for 70 days on the juveniles of Nile tilapia (18 ± 2.1 g) exposed to the total ammonia nitrogen concentration (TAN) of 5 (LL) or 10 (HH) mg L− 1 in a static water system at 26 °C. The activities of c-alanine aminotransferase (c-ALT), c-aspartate aminotransferase (c-AST), c-malic enzyme (c-ME), m-malic enzyme (m-ME), glutamate dehydrogenase (GDH) in the amination direction, and glutamine synthetase (GSase) in fish exposed to one of these sublethal TAN concentrations were significantly increased. The increase in the enzyme activity was positively related to the level of the tested TAN concentration. However, the activity of m-ALT and glutaminase (GLase) was significantly decreased, and that of m-AST did not show any significant changes. On the other hand, in Nile tilapia exposed to HH, there was a significant increase in the levels of free amino acids (FAAs), such as alanine, glutamine, glycine, histidine, serine, and γ-aminobutyric acid (GABA); however, no alteration in asparagine was observed, and there was a significant decrease in the levels of aspartate and glutamate. The significant increase in FAAs levels in fish exposed to LL was mainly attributable to alanine, glutamine, and GABA, and the importance of these metabolic changes in the cytosolic and mitochondrial ALT, AST, and ME was discussed.