GONADOTROPHS BUT NOT SOMATOTROPHS CARRY GONADOTROPIN-RELEASING-HORMONE RECEPTORS - RECEPTOR LOCALIZATION, INTRACELLULAR CALCIUM, AND GONADOTROPIN AND GH RELEASE

Gonadotrophs are the primary target cells for GnRH in the pituitary. H owever, during a limited period of neonatal life in the rat, lactotrop hs 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 tro ut), GnRH is a potent GH secretagogue. In studying hypophysiotrophic a ctions of the two forms of GnRH present in the African catfish (Claria s gariepinus), chicken GnRH-II ([His(5), Trp(7), Tyr(8)] GnRH; cGnRH-I I) and catfish GnRH ([His(5), Asn(8)]GnRH; cfGnRH), we have investigat ed 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)-Ne t]GnRH (sGnRHa), a salmon GnRH analogue with high affinity for the GnR H receptor. Following fixation and immunohistochemistry using antisera against catfish LH and GH, I-125-labelled sGnRHa was localised autora diographically and silver grains were quantified on gonadotrophs and s omatotrophs. 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 loca lisation of functional GnRH receptors, the effects of cfGnRH and cGnRH -II on the cytosolic free calcium concentration ([Ca2+](i)) were exami ned in Fura-2-loaded somatotrophs and gonadotrophs. GnRH-induced incre ases in [Ca2+](i) appeared to be confined to gonadotrophs, in which bo th 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 exp eriments demonstrated that GnRH treatments which markedly elevated pla sma LH levels had no effect on plasma GH levels, while a dopamine agon ist (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 t hat GnRHs cannot be considered as GH secretagogues in teleosts in gene ral.