Changes in Brain Water Content in the Rainbow Trout (Oncorhynchus mykiss) and in the Goldfish (Carassius auratus) due to High External Ammonia Exposure

In fishes, hyperammonemia may occur following feeding or exposure to abnormally high concentrations of environmental ammonia due to sewage effluents, agricultural run-off and in crowded aquaculture pens. Increased internal ammonia can result in hyperactivity, convulsions, coma and death. In mammals, it is also associated with potentially fatal brain edema, in which the accumulation of intracellular water results in swelling, increased intracranical pressure and herniation leading to death. Recently it was shown that rainbow trout (Oncorhynchus mykiss) and goldfish (Carassius auratus) experience brain swelling following exposure to high external ammonia (HEA). However, the mechanism of ammonia-induced brain swelling in fishes remains unsolved. The goal of this thesis was to determine the underlying mechanisms of ammonia-induced swelling in the fish brain. The specific research objectives were to (i) determine if intracellular glutamine accumulation contributed to brain swelling by increasing intracellular osmolarity; (ii) ascertain what role NMDA receptor over-activation might play in mediating ammonia-induced brain swelling, and (iii) determine if changes in brain water content were accompanied by disturbances to ion balance and transport processes in the brain. Exposure of rainbow trout (1mM NH4Cl) and goldfish (5mM NH4Cl) to 48 h HEA lead to a 12-15% increase in brain water content, 10-fold increase in plasma ammonia [TAmm], and 5-6-fold increase in brain glutamine. Dry brain Na+ content displayed similar trends to brain water content, increasing approximately 45-70 %, while K+ was reduced by approximately 10-24 %. These changes were accompanied by simultaneously, 50 % reductions in rainbow trout brain Na+/K+-ATPase activity after 48 h high external ammonia (HEA). Goldfish were able to withstand higher concentrations of external ammonia compared to the rainbow trout, which was consistent with greater tolerance observed in previous studies. Remarkably, brain water content was restored to control levels following 24 h recovery in ammonia-free well water. Administration of MK801 (dizocilpine) an NMDA receptors antagonist prior to HEA prevented significant brain swelling in the rainbow trout but not in goldfish. Additionally, the administration of MK801 caused a reduction from control levels in brain Na+ content in both species. Lastly, the administration of glutamine synthetase inhibitor, methionine sulfoximine (MSO) to each species followed by 48 h HEA did not prevent significant swelling. In conclusion, HEA resulted in water accumulation in the brain, but this was not due to increased glutamine production. Rather, brain swelling appears to be mediated by NMDA receptors followed by ionic disturbances, resulting in brain swelling

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