| Neuroendocrine regulation of vertebrate growth and reproduction is a multi-factorial process and includes growth hormone (GH) and luteinizing hormone (LH) release. In goldfish, the orexigenic peptide ghrelin (gGRLN 19) stimulates GH and LH release, making it a potential link between energy balance, growth, and reproduction. However, how gGRLN19 interacts with other neuroendocrine regulators of GH and LH release and its mechanisms of actions are unknown. To elucidate these aspects, I cloned a partial sequence of the gGRLN receptor (gGHS-R) and demonstrated the presence of gGHS-R mRNA, together with that for gGRLN, in goldfish brain and pituitary tissues using polymerase chain reactions. Preliminary Western blot results supported the presence of gGHS-R1-like proteins in goldfish pituitary extracts. Using goldfish pituitary cell primary cultures in cell column perifusion hormone release and fura-2, AM Ca2+-imaging studies, gGRLN19-induced GH and LH release were found to involve increases in intracellular free Ca 2+ levels via extracellular Ca2+ entry through L-type voltage-sensitive Ca2+ channels, protein kinase C, and nitric oxide. However, protein kinase A only participated in gGRLN19-induced LH release, while its activation potentiated gGRLN19-elicited GH release. Utilization of nitric oxide synthase (NOS) isoforms differed, with nNOS and iNOS participating in GH and LH responses, respectively. Pretreatment with pituitary adenylate cyclase-activating polypeptide (PACAP) potentiated gGRLN19-induced GH, but not LH, release while pretreatment with gGRLN19 followed by PACAP did not enhance hormone secretion. gGRLN 19 increased GH release in the presence of goldfish GR-releasing hormone (gGHRH) and dopamine (DA), but potentiated GH responses were not observed. Both basal LH release and the LH responses to gGRLN19 were inhibited by gGHRH and DA. On the other hand, gGRLN19 failed to further elevate GH and LH secretion in the presence of salmon gonadotropin-releasing hormone (sGnRH) and chicken (c)GnRH-II.;Overall, gGRLN19 induces both GH and LH release through gGHS-R mediated, cell-selective signalling pathways and exhibits complex interactions with other endogenous regulators of pituitary hormone release in a cell type-, ligand-, and treatment sequence-specifie manner. Results support the idea that gGRLN19 regulates growth and reproduction, identify gGHRH as a novel LH release inhibitor, and further our understanding of neuroendocrine regulation of these processes. |
