PACAP stimulation of maturational gonadotropin secretion in goldfish involves extracellular signal-regulated kinase, but not nitric oxide or guanylate cyclase, signaling

Nitric oxide signaling in ghrelin-induced LH release from goldfish pituitary cells

Among its many known functions, ghrelin has been proposed to participate in the regulation of reproduction; however, its effect on pituitary LH release is controversial, especially in mammals. In the goldfish, ghrelin directly stimulates pituitary LH release via increased entry of calcium through voltage sensitive channels and activation of protein kinase C. Nitric oxide (NO) is an important signaling molecule in many physiological systems including hormone regulation at the level of the pituitary. Goldfish pituitary cells and extracts have previously been reported to express immunoreactivity for inducible and neuronal NO synthase (iNOS and nNOS). In this study, we determined if NO is involved in goldfish ghrelin (gGRLN(19))-induced LH release from primary cultures of dispersed goldfish pituitary cells in column perifusion. Treatment with the NO scavenger PTIO significantly decreased gGRLN(19)-induced LH release and co-treatment with the NO donor SNP and gGRLN(19) did not induce an additive increase in LH release, suggesting that NO is critical to gGRLN(19) stimulation of LH release in goldfish pituitary cells. Further work examined the involvement of the NOS using the NOS isoform-selective inhibitors 1400W, 7-Ni, and AGH. While 1400W (selective for iNOS) and AGH (selective for iNOS and nNOS) abolished gGRLN(19)-induced LH release from goldfish pituitary cells, 7-Ni (selective for nNOS and endothelial NOS) had no significant effect on this stimulation. Our results indicate, for the first time in a teleost species, that gGRLN(19)-induced LH release from pituitary cells is NO-dependent and likely involves iNOS, adding to the understanding of GRLN intracellular signaling in general and specifically to the regulation of LH release from the pituitary.

Nitric oxide and guanylate cyclase signalling are differentially involved in gonadotrophin (LH) release responses to two endogenous GnRHs from goldfish pituitary cells

Nitric oxide synthase (NOS) immunoreactivity is present in goldfish gonadotrophs. The present study investigated whether two native goldfish gonadotrophin-releasing hormones (GnRHs), salmon (s)GnRH and chicken (c)GnRH-II, use NOS/nitric oxide (NO) and soluble guanylate cyclase (sGC)/cyclic (c)GMP/protein kinase G (PKG) signalling to stimulate maturational gonadotrophin [teleost gonadotrophin-II, luteinising hormone (LH)] release. In cell column perifusion experiments with dispersed goldfish pituitary cells, the application of three NOS inhibitors (aminoguanidine hemisulphate, 1400W and 7-nitroindazole) and two NO scavengers [2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO) and rutin hydrate] reduced sGnRH-elicited, but not cGnRH-II-induced, LH increases. The NO donor sodium nitroprusside (SNP) increased NO production in goldfish pituitary cells in static incubation. SNP-stimulated LH release in column perifusion was attenuated by PTIO and the sGC inhibitor 1H-(1,2,4)oxadiazolo[4,3-a]quinoxalin-1-oneon (ODQ), and additive to responses elicited by cGnRH-II, but not sGnRH. ODQ and the PKG inhibitor KT5823 decreased sGnRH- and cGnRH-II-stimulated LH release. Similarly, the LH response to dibutyryl cGMP was reduced by KT5823. These results indicate that, although only sGnRH uses the NOS/NO pathway to stimulate LH release, both GnRHs utilise sGC/PKG to increase LH secretion.