Physiological & Cellular Mechanisms of Ammonia Tolerance in the Goldfish (Carassius auratus)

Ammonia may increase in the blood and tissues of fishes following feeding or exposure to high concentrations of environmental ammonia arising from the degradation of organic matter in aquatic environments, sewage effluent, and run-off from landfills. Ammonia may be detoxified using the enzyme glutamine synthetase (GS), which catalyzes the conversion of ammonia to glutamine. It has been suggested, however that build-ups of glutamine within the astrocytes of the brain can cause brain swelling by increasing intracellular osmolarity and water uptake by the cells. Using in vivo and in vitro approaches, the goal of this thesis was to determine if: (i) exposure to high external ammonia (HEA) caused brain swelling in ammonia-sensitive trout (Oncorhynchus mykiss) compared to ammonia-tolerant goldfish (Carassius auratus), and (ii) glutamine accumulation was the underlying cause of brain swelling. Exposure of goldfish and trout to HEA led to an increase in plasma and brain ammonia concentration, and consequently an increase in brain tissue water content in both species, which was consistent with brain swelling. The threshold external ammonia concentration for initiating brain swelling was 5-fold greater in the goldfish than in the trout, which was consistent with the greater ammonia tolerance of goldfish. Brain water content was restored to control levels following recovery in ammonia-free water in both species. Administration of the GS inhibitor methionine sulfoximine (MSO) to each species prior to HEA caused a reduction in GS activity and glutamine accumulation but did not lead to the attenuation of brain swelling. Experiments using trout and goldfish brain cell lines demonstrated that MSO exposure did not increase cell viability, but the osmoprotectant amino acid taurine did increase cell viability of rainbow trout brain cell lines. In conclusion, increased internal ammonia does cause water accumulation in the brain, but this is not the result of increased glutamine production