Serotonin interferes with Ca2+ and PKC signaling to reduce gonadotropin-releasing hormone-stimulated GH secretion in goldfish pituitary cells

In goldfish, two endogenous gonadotropin-releasing hormones (GnRH), salmon GnRH (sGnRH) and chicken GnRH-II (cGnRH-II), are thought to stimulate growth hormone (GH) release via protein kinase C (PKC) and subsequent increases in intracellular Ca2+ levels ([Ca2+]i). In contrast, the signaling mechanism for serotonin (5-HT) inhibition of GH secretion is still unknown. In this study, whether 5-HT inhibits GH release by actions at sites along the PKC and Ca2+ signal transduction pathways leading to hormone release were examined in primary cultures of goldfish pituitary cells. Under static incubation and column perifusion conditions, 5-HT reduced basal, as well as sGnRH- and cGnRH-II-stimulated, GH secretion. 5-HT also suppressed GH responses to two PKC activators but had no effect on the GH-releasing action of the Ca2+ ionophore ionomycin. Ca2+-imaging studies with identified somatotropes revealed that 5-HT did not alter basal [Ca2+]i but attenuated the magnitude of the [Ca2+]i responses to the two GnRHs. Prior treatment with 5-HT and cGnRH-II reduced the magnitude of the [Ca2+]i responses induced by depolarizing levels of K+. Similar inhibition, however, was not observed with prior treatment of 5-HT and sGnRH. These results suggest that 5-HT, by direct actions at the somatotrope level, interferes with PKC and Ca2+ signaling pathways to reduce the GH-releasing effect of GnRH. 5-HT action may occur at the level of PKC activation or its downstream signaling events prior to the subsequent rise in [Ca2+]i.. The differential Ca2+ responses by depolarizing doses of K+ is consistent with our previous findings that sGnRH and cGnRH-II are coupled to overlapping and yet distinct Ca2+-dependent mechanisms.