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a University of Utrecht, Faculty of Biology, Department of Experimental Zoology, Research Group Reproductive Endocrinology, 3508 TB Utrecht, The Netherlands
b Catholic University of Nijmegen, Department of Biochemistry, 6500 HB Nijmegen, The Netherlands
In the brain of all vertebrate classes, chicken (c) GnRH-II ([His5,Trp7,Tyr8]GnRH, cGnRH-II) is expressed in the mesencephalon. In addition, at least one other form of GnRH is expressed in the preoptical area/hypothalamus. In the human pituitary stalk and the mouse median eminence, cGnRH-II is present together with mammalian GnRH. Similarly, in the pituitary of several teleost fish (e.g., goldfish and eel, but not salmon or trout), a teleost GnRH is found together with cGnRH-II. These GnRHs are not colocalized in the same cells. Hence, these GnRH peptides may differentially regulate gonadotropin secretion and, in addition, may exert their effects simultaneously. The current study therefore investigated the effects of combinations of the two forms of GnRH present in the African catfish (Clarias gariepinus) pituitary—cGnRH-II and catfish GnRH ([His5,Asn8]GnRH, cfGnRH)—on the cytosolic free calcium concentration ([Ca2+]i) in single, Fura-2-loaded catfish gonadotrophs, as well as their effects on both in vitro and in vivo LH secretion. Both inhibitory and stimulatory effects of combinations of cfGnRH and cGnRH-II on [Ca2+]i were observed, which were mirrored by their effects on both in vitro and in vivo LH secretion. The following pattern became apparent. The effect of intermediate or maximal effective cfGnRH doses was inhibited by the simultaneous presence of subthreshold or borderline effective cGnRH-II doses. Conversely, subthreshold or borderline effective concentrations of cfGnRH enhanced the effects of intermediate and maximal concentrations of cGnRH-II. In addition, combinations of cfGnRH and cGnRH-II concentrations that were equally active when tested separately showed an additive effect. The observed interactions between the two GnRHs may be of particular physiological relevance in the control of seasonal LH levels in the African catfish, as well as in other teleost species. Moreover, the occurrence of mutual inhibitory and stimulatory interactions between endogenous GnRHs may be a widespread aspect of GnRH action in vertebrates.
1 This study was financially supported by the Netherlands Organization for Research (NWO). Preliminary results have been presented (abstract 152) at the 17th Conference of European Comparative Endocrinologist, Córdoba (Spain), 5-10 September 1994.
2 Correspondence: R.W. Schulz, University of Utrecht, Faculty of Biology, Department of Experimental Zoology, Research Group Reproductive Endocrinology, P.O. Box 80058, 3508 TB Utrecht, The Netherlands. FAX: 31 30 2532837; r.w.schulz{at}bio.uu.nl
3 Current address: Department of Medical Oncology, Josephine Nefkens Institute, University Hospital Rotterdam, Rotterdam, The Netherlands.
4 Current address: Central Laboratory of The Netherlands Red Cross Blood Transfusion Service, Experimental Immunohematology, Amsterdam, The Netherlands.
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