The regulation of urea excretion in a batrachoidid fish

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The Gulf toadfish, Opsanus beta, is an unusual fish that excretes urea as its primary nitrogen waste product in distinct pulses, measured by a sudden increase in the appearance of urea in the surrounding water. A urea transporter has been cloned from toadfish gill (tUT) that is similar in amino acid sequence to mammalian urea transporters and the pulse of urea is believed to be due to the periodic activation of tUT, allowing for the movement of urea from the blood, across the gill, into the water. How tUT is turned on and off is not well understood, however, but research to this point has determined that the stress hormone, cortisol, and the neurotransmitter, serotonin (5-HT), are involved. High levels of cortisol appear to prevent tUT activation while 5-HT stimulates tUT function. The action of 5-HT appears to be mediated by a serotonin receptor subtype with pharmacological characteristics similar to mammalian serotonin type 2A (5-HT2A) receptors. This project aims to further examine the regulation of tUT by investigating how cortisol and 5-HT control tUT function and determine whether the actions of cortisol and 5-HT influence each other. Molecular techniques and in vitro tissue preparations will be used in parallel with whole animal studies to test the hypothesis that tUT function is controlled directly by 5-HT and indirectly by cortisol. These studies will also test the hypothesis that toadfish 5-HT2A receptors in the gill are involved in tUT activation and these receptors are not able to activate tUT when cortisol levels are high. The findings of this project will expand our understanding of nitrogen waste metabolism and excretion in fish and terrestrial organisms. In addition, results of this research will provide insight into the interactions between 5HT and cortisol, signals that are central to several human disorders. University of Miami has a diverse student body and this project will provide research experiences for students who are underrepresented in science and engineering.