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Am J Physiol Regul Integr Comp Physiol 278: R1572-R1578, 2000;
0363-6119/00 $5.00
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Vol. 278, Issue 6, R1572-R1578, June 2000

GnRH stimulates LH release directly via inositol phosphate and indirectly via cAMP in African catfish

Frank E. M. Rebers1, Peter T. Bosma2, Wytske van Dijk1, Henk J. T. Goos1, and Rüdiger W. Schulz1

1 Department of Experimental Zoology, Research Group for Comparative Endocrinology, Utrecht University, Padualaan 8, 3584 CH Utrecht; and 2 Department of Medical Oncology, Josephine Nefkens Institute, University Hospital Rotterdam, 3000 DR Rotterdam, The Netherlands

In African catfish, two gonadotropin-releasing hormone (GnRH) peptides have been identified: chicken GnRH (cGnRH)-II and catfish GnRH (cfGnRH). The GnRH receptors on pituitary cells producing gonadotropic hormone signal through inositol phosphate (IP) elevation followed by increases in intracellular calcium concentration ([Ca2+]i). In primary pituitary cell cultures of male African catfish, both cGnRH-II and cfGnRH dose dependently elevated IP accumulation, [Ca2+]i, and the release of the luteinizing hormone (LH)-like gonadotropin. In all cases, cGnRH-II was more potent than cfGnRH. The GnRH-stimulated LH release was not associated with elevated cAMP levels, and forskolin-induced cAMP elevation had no effect on LH release. With the use of pituitary tissue fragments, however, cAMP was elevated by GnRH, and forskolin was able to stimulate LH secretion. Incubating these fragments with antibodies against cfGnRH abolished the forskolin-induced LH release but did not compromise the forskolin-induced cAMP elevation. This suggests that cfGnRH-containing nerve terminals are present in pituitary tissue fragments and release cfGnRH via cAMP signaling on GnRH stimulation, whereas the GnRH receptors on gonadotrophs use IP/[Ca2+]i to stimulate the release of LH.

gonadotropin-releasing hormone neurons; luteinizing hormone; pituitary gonadotrophs; second messengers; hormone secretion


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