Mechanism of Hypoxia Down-Regulation of Reproductive Neuroendocrine Function in an Estuarine Teleost, Atlantic Croaker.

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There is growing concern over the recent dramatic increase in the incidence of seasonal hypoxia (low oxygen levels) in coastal regions worldwide due to increased nitrogen inputs through human activities. The impact of this environmental deterioration on the abundance of marine animals and valuable coastal fishery resources is unknown because basic information is lacking on the long term reproductive effects of hypoxia. Preliminary results suggest the presence of a specific mechanism in the brain of an estuarine fish, Atlantic croaker, to shut down reproduction in response to hypoxia by decreasing hormone secretion. The hypothesis that hypoxia impairs reproduction in croaker through down regulation of tryptophan hydroxylase (TPH), a critical enzyme for synthesis of serotonin, a chemical in the brain that regulates reproductive hormones, will be investigated using a variety of biochemical and molecular techniques. The results are expected to show that hypoxia impairs reproduction by a specific mechanism involving down regulation TPH activity in the brain. Moreover, it is predicted that the decrease in TPH activity is due to a decline in TPH mRNA expression which is a specific response to hypoxia and is not observed with other environmental stressors. This research is likely to have broad impacts on our understanding of reproductive adaptations of organisms to environmental changes, particularly to oxygen-limited conditions such as hypoxia, at high altitudes, and during hibernation, and for the many other physiological functions of serotonin. The demonstration that the down regulation of reproductive and endocrine functions by hypoxia is mediated through a hitherto unrecognized specific adaptation mechanism would suggest a major influence of hypoxia in the evolution and ecology of estuarine organisms such as croaker. However, the results of this study may have the greatest impact in formulating policy decisions, because identifying the precise mechanism by which hypoxia causes reproductive suppression in fish provides a compelling argument for the existence of this hypoxia effect and its ecological importance. Two graduate students will be supported off this project and receive training in all aspects of the research. In addition, a minority undergraduate student will be recruited for a summer research internship to conduct a senior project on hypoxia effects in croaker.