Inhibitory and stimulatory interactions between endogenous gonadotropin-releasing hormones in the African catfish (Clarias gariepinus)

In the brain of all vertebrate classes, chicken (c) GnRH-II ([His(5),Trp(7) ,Tyr(8)]GnRH, cGnRH-II) is expressed in the mesencephalon, In addition, at least one other form of GnRH is expressed in the preoptical area/hypothalam us. In the human pituitary stalk and the mouse median eminence, cGnRH-II is present together with mammalian GnRH, Similarly, in the pituitary of sever al teleost fish (e,g., goldfish and eel, but not salmon or trout), a teleos t GnRH is found together with cGnRH-II. These GnRHs are not colocalized in the same cells. Hence, these GnRH peptides may differentially regulate gona dotropin secretion and, in addition, may exert their effects simultaneously . The current study therefore investigated the effects of combinations of t he two forms of GnRH present in the African catfish (Clarias gariepinus) pi tuitary--cGnRH-II and catfish GnRH ([His(5),Asn(8)]GnRH, cfGnRH)--on the cy tosolic free calcium concentration ([Ca2+](i)) in single, Fura-2-loaded cat fish gonadotrophs, as well as their effects on both in vitro and in vivo LH secretion. Both inhibitory and stimulatory effects of combinations of cfGn RH and cGnRH-II on [Ca2+](i) were observed, which were mirrored by their ef fects on both in vitro and in vivo LH secretion. The following pattern beca me apparent. The effect of intermediate or maximal effective cfGnRH doses w as inhibited by the simultaneous presence of subthreshold or borderline eff ective cGnRH-II doses, Conversely, subthreshold or borderline effective con centrations of cfGnRH enhanced the effects of intermediate and maximal conc entrations of cGnRH-II, In addition, combinations; of cfGnRH and cGnRH-II c oncentrations that were equally active when tested separately showed an add itive effect. The observed interactions between the two GnRHs may be of par ticular physiological relevance in the control of seasonal LH levels in the African catfish, as well as in other teleost species. Moreover, the occurr ence of mutual inhibitory and stimulatory interactions between endogenous G nRHs may be a widespread aspect of GnRH action in vertebrates.