The Neurophysiological Basis of Ammonia Toxicity and Tolerance in Fishes

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The overarching aim of my research is to determine how fishes produce and excrete nitrogenous wastes such as ammonia, and how they cope with potentially toxic build-ups of this waste product. Ammonia normally arises from the breakdown of excess amino acids, but has neurotoxic effects at high concentrations. Fish are generally much more tolerant to ammonia than mammals, which need to detoxify ammonia to urea to survive. Yet little is known about the underlying mechanisms for these differences. Our recent work suggests that anoxia tolerant fishes, such as goldfish (Carassius auratus) and crucian carp (Carassius carassius), that have evolved neurophysiological mechanisms to withstand O2 starvation, are also ammonia tolerant. Indeed, the cascade of events that characterize anoxia/ischemia in the brain are similar to those that occur with ammonia toxicity. In mammals, excess ammonia causes overactivation of glutamate receptors in the brain, leading to excitotoxicity characterized by the generation of reactive oxygen species (ROS) and water accumulation by astrocyte cells, culminating in potentially fatal brain swelling. Our findings suggest, however, that fish readily tolerate ammonia-induced brain swelling. I propose to use an integrated approach to test the working hypothesis that the greater ammonia tolerance of fishes compared to mammals is related to a greater ability of the brain to resist the neurotoxic effects of ammonia. I also propose to test the related hypothesis that the physiological adaptations that contribute to the anoxia tolerance of the goldfish, also explains their greater ammonia tolerance. Over the next 5 years, the objectives of my research program will be to: (I) Characterize the underlying mechanism(s) of acute ammonia toxicity and tolerance in the central nervous system of ammonia-tolerant goldfish and ammonia-sensitive trout (Oncorhynchus mykiss); (II) Determine if high tolerance to ROS results in greater ammonia tolerance in goldfish compared to trout; (III) Identify the underlying mechanisms that lead to brain swelling in fishes in response to ammonia and other aquatic stressors; (IV) Determine how prolonged sub-lethal ammonia exposure and feeding affect ammonia tolerance in goldfish and trout. The integrative approach I will use in my research will include whole animal models, cultured brain slice models, molecular techniques, immunohistochemistry, and electrophysiology methods. An improved understanding of the neural mechanism(s) of ammonia toxicity and tolerance in fishes will improve our fundamental understanding how vertebrates cope with ammonia, and shed light on the selective pressures that led to the evolution of different mechanisms of ammonia detoxification and handling in the vertebrates. This work will also have practical implications by better explaining how fishes respond to and tolerate build-ups of ammonia in aquatic ecosystems.