Neurophysiological and Metabolic Adaptations to Increased Ammonia and Oxygen Starvation in Fishes.

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The overarching goal of my research is to determine how aquatic vertebrates cope with environmental stressors such as build-ups of internal ammonia and O2 starvation. Ammonia is a metabolic waste product that arises from the breakdown of excess amino acids, but at elevated concentrations it is neurotoxic. For this reason ammonia needs to be excreted or converted to less toxic nitrogenous waste (N-waste) products such as urea or uric acid, which are produced by terrestrial vertebrates. Due to its high water solubility, ammonia is the primary N-waste product of fishes because it can be excreted with relative ease across the gills to the water. However, fishes frequently have to cope with fluctuations in internal ammonia following feeding, exercise or exposure to high external ammonia. In mammals, an impaired ability to detoxify ammonia to urea (e.g. due to liver disease) leads to highly elevated ammonia in the body. This then results in "excitotoxicity" characterized by hyper-activation of the central nervous system (CNS), reactive oxygen species generation and increased brain water content leading to potentially fatal brain swelling. In contrast, we demonstrated that that brain swelling is readily tolerated by crucian carp and goldfish when exposed to ammonia and low environmental O2. This observation suggests that these fishes possess novel physiological mechanisms to protect the CNS from ammonia. Another notable finding of my NSERC Discovery work was that tolerance to elevated ammonia was linked to an ability to survive prolonged O2 starvation. Over the next 5 years, my students will test the hypothesis that this anoxia-ammonia cross-tolerance is due to the ability of these fishes to prevent excitotoxicity, high tolerance to oxidative stress, and resilience to brain swelling. My specific research objectives will be to: (I) Characterize the neurophysiological adaptations that protect the goldfish CNS from excitotoxic cell death during exposure to hypoxia or anoxia; (II) Contrast the mechanism(s) of ammonia toxicity and tolerance in the CNS of ammonia sensitive rainbow trout (Oncorhynchus mykiss) to those of the ammonia-tolerant goldfish; (III) Identify the link(s) between oxidative stress and the development of brain swelling in goldfish subjected to anoxia, hypoxia or ammonia exposure; (IV) Determine how elevated ammonia and feeding affect antioxidant capacity and neurophysiological processes in the Pacific hagfish and sea lamprey, two jawless fish species able to withstand ammonia and low O2. I will use an integrative research to address these objectives including whole animal models, isolated mitochondria, cultured brain slice models, molecular techniques, immunohistochemistry, and electrophysiology. This work will ultimately improve our understanding of the physiological adaptations used by fishes to cope with increased internal ammonia and O2 starvation, and identify the underlying mechanisms of anoxia-ammonia cross tolerance.