Regulation of prolactin and changes in prolactin and growth hormone in osmoregulation, metabolism, and reproduction in the tilapia, Oreochromis mossambicus

Abstract

These studies addressed the regulation of the prolactins (PRL) and growth hormone (GH) in the tilapia, Oreochromis mossambicus. Regulation of the PRL cell was investigated in the context of its potential roles in osmoregulation, metabolism, and reproduction. The in vitro release of the tPRLs (tPRL1 77 and tPRL1 SS) was measured as well as changes in serum concentrations and pituitary content of the PRLs and GH. These studies provide evidence that it is the osmotic gradient across the cell membrane that leads to an increase in osmotic pressure, and not osmolality per se, which accounts for the osmoreceptivity of the tilapia PRL cell. Prolactin release from pituitary tissues (rostral pars distalis; RPD) was inhibited when RPD were incubated in medium made hyperosmotic by the addition of NaCl or the membrane-impermeant molecule, mannitol; but not by the addition of the permeant molecules, urea or ethanol. Injections of a gonadotropin-releasing hormone (GnRH) analog elevated serum concentrations of the tPRLs. Furthermore, native forms of GnRH stimulated PRL release in vitro with the following order of potency: chicken-GnRH-II > salmon-GnRH > seabream-GnRH. Prolactin release was accompanied by an increase in intracellular free ca2+ , suggesting Ca2+ operates as a second messenger in mediating the effects of GnRH on PRL release. Estradiol-17β and testosterone potentiated the effects of GnRH on PRL release in vitro. Serum concentrations of the tPRLs were elevated in response to fasting, followed by increases in serum concentrations and pituitary content of GH. Serum concentrations and pituitary content of GH and serum concentrations of PRL were highest late in the brooding phase of the reproductive cycle. Reduced food intake during brooding may contribute to changes in PRL and GH serum concentrations and pituitary content. Nevertheless, patterns of changes in serum and pituitary levels of the tPRLs and GH observed during the reproductive cycle had characteristics that were both similar to and distinct from patterns observed during fasting, suggesting the hormones may have actions in both metabolism and reproduction. In addition, salinity altered the patterns of changes in serum and pituitary levels of the tPRLs, but not GH, observed during the reproductive cycle.