Osmoregulation in the Mozambique tilapia, Oreochromis mossambicus: Role of prolactin, growth hormone and cortisol in chloride cell differentiation

The Mozambique tilapia (Oreochromis mossambicus) has the ability to tolerate a wide range of environmental salinities that is mediated, in part, by a capacity for the gill to rapidly shift the direction of active ion transport. This dissertation characterized the endocrine control of ionoregulatory pathways in the gill that underlie acclimation to hypo- and hyperosmotic environments.;Transfer of freshwater (FW)-acclimated tilapia to seawater (SW) resulted in elevations of plasma growth hormone (GH) and branchial GH receptor gene expression concurrent with reduced plasma prolactin (PRL) and PRL receptor expression. These responses were in phase with increased branchial expression of a Na+/K+/2Cl- cotransporter (NKCC), a hallmark of SW-type chloride cells, and decreased expression of a Na+/Cl- cotransporter (NCC) specific to FW-type chloride cells. When the responses of NKCC and NCC to salinity challenges were compared between Mozambique tilapia and the less SW-tolerant Nile tilapia, their differing responses provided further evidence that dynamic expression of these factors is a key aspect of euryhalinity.;During acute stress in both FW and SW, salinity-specific plasma PRL responses to disturbed hydromineral balance were observed that were consistent with PRL's hyperosmoregulatory activities. In SW, branchial NKCC expression increased in stressed fish concurrently with elevated plasma osmolality and diminished gill Na+, K+-ATPase activity, suggesting that ionoregulatory responses are also important facets of the stress response in tilapia.;Lastly, hypophysectomy and replacement therapy experiments revealed that the pituitary is necessary for the recruitment of branchial FW-type chloride cells that express NCC. Replacement therapy with PRL was sufficient to restore the recruitment of NCC-expressing chloride cells in both FW- and SW-environments. Hypophysectomy trials provided additional evidence that the acquisition of SW-type chloride cells can effectively occur without pituitary-mediation in this tilapia species. Collectively, this dissertation describes how adaptive changes in the gill for ionoregulation are part of an integrated response between cues derived from both the endocrine system and direct sensing of the external and/or internal osmotic environment.