Gonadotrophs but not somatotrophs carry gonadotrophin-releasing hormone receptors: Receptor localisation, intracellular calcium, and gonadotrophin and GH release

Gonadotrophs are the primary target cells for GnRH in the pituitary. However, during a limited period of neonatal life in the rat, lactotrophs and somatotrophs respond to GnRH as well. Also, in the adults of a number of teleost fishes (e.g. carp, goldfish, and tilapia but not trout), GnRH is a potent GH secretagogue. In studying hypophysiotrophic actions of the two forms of GnRH present in the African catfish (Clarias gariepinus), chicken GnRH-II ([His(5), Trp(7), Tyr(8)] GnRH; cGnRH-II) and catfish GnRH ([His(5), Asn(8)]GnRH; cfGnRH), we have investigated the effects of GnRH on catfish gonadotrophs and somatotrophs. GnRH binding was examined by incubating dispersed pituitary cells attached to coverslips with I-125-labelled [D-Arg(6), Trp(7), Leu(8), Pro(9)-Net]GnRH (sGnRHa), a salmon GnRH analogue with high affinity for the GnRH receptor. Following fixation and immunohistochemistry using antisera against catfish LH and GH, I-125-labelled sGnRHa was localised autoradiographically and silver grains were quantified on gonadotrophs and somatotrophs. Specific binding of I-125-labelled sGnRHa was restricted to gonadotrophs. Both cfGnRH and cGnRH-II dose-dependently inhibited I-125-labelled sGnRHa binding to gonadotrophs. To substantiate the localisation of functional GnRH receptors, the effects of cfGnRH and cGnRH-II on the cytosolic free calcium concentration ([Ca2+](i)) were examined in Fura-2-loaded somatotrophs and gonadotrophs. GnRH-induced increases in [Ca2+](i) appeared to be confined to gonadotrophs, in which both endogenous GnRHs caused a single and transient increase in [Ca2+](i). The amplitude of this [Ca2+](i) transient depended on the GnRH dose and correlated well with the GnRHs' effect on LH release. In vivo experiments demonstrated that GnRH treatments which markedly elevated plasma LH levels had no effect on plasma GH levels, while a dopamine agonist (apomorphine) significantly elevated plasma GH levels. We conclude that the two endogenous forms of GnRH in the African catfish are not directly involved in the regulation of the release of GH, suggesting that GnRHs cannot be considered as GH secretagogues in teleosts in general.