Mechanisms of ammonia tolerance in the gulf toadfish (Opsanus beta)

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)

First Committee Member

Patrick J. Walsh, Committee Chair


The gulf toadfish, Opsanus beta, has a greater tolerance than most other fish and mammals to ammonia. The aim of this thesis was to describe mechanisms that allow this ammonia tolerance using biochemical and molecular techniques. Guided by several hypotheses generated from studies in the medical field, namely, ammonia-induced mitochondrial dysfunction and alterations of the brain glutamate-glutamine cycle, we explored several hypotheses. We tested: (1) the effect of ammonia on mitochondrial energy metabolism and oxidative phosphorylation; (2) the effect of ammonia-induced increase in brain glutamine concentration on brain water content; (3) the effect of urea synthesis on the ability to prevent ammonia-induced increase in brain glutamine; and (4) the effect of ammonia treatment on the expression of the recently described putative ammonium channels, the Rh-like proteins. We observed that toadfish brain mitochondria, unlike rat brain mitochondria, did not experience a decrease in the concentration of NADH in the matrix when subjected to ammonia in the medium and therefore were able to maintain a stable energy metabolism. In contrast to mammals, toadfish did not experience an increase in brain water during sub-lethal or supra-lethal exposure to ammonia in the water. Also, over time toadfish were able to decrease the ammonia-induced increase in brain glutamine concentration which is believed to be toxic to brain cells and mitochondria. It was previously known that toadfish have the uncommon ability among other fish to synthesize urea de novo in the liver. This ability is also different from urea synthesis in mammals since the toadfish urea cycle prefers glutamine over ammonia as the nitrogen donor molecule. Toadfish were capable of decreasing the ammonia-induced increase in the concentration of urea throughout the whole body. Perhaps, its urea synthesis and excreting capabilities contributed to the observed decrease in brain glutamine. Finally, ammonia treatment had no effect on the relative expression of RhBG and RhCG mRNA when compared to control treated expression levels. We conclude that the tolerance of the toadfish to ammonia can be attributed to its special mitochondrial physiology and its particular manner of handling brain glutamine.


Biology, Neuroscience; Biology, Physiology

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