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

In African catfish, two gonadotropin-releasing hormone (GnRH) peptides have been identified: chicken GnRH (cGnRH)-II and catfish GnRH (cfGnRH). The Gn RH receptors on pituitary cells producing gonadotropic hormone signal throu gh inositol phosphate (IP) elevation followed by increases in intracellular calcium concentration ([Ca2+](i)). In primary pituitary cell cultures of m ale African catfish, both cGnRH-II and cfGnRH dose dependently elevated IP accumulation, [Ca2+](i), and the release of the luteinizing hormone (LH)-li ke gonadotropin. In all cases, cGnRH-II was more potent than cfGnRH. The Gn RH-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 fors kolin was able to stimulate LH secretion. Incubating these fragments with a ntibodies against cfGnRH abolished the forskolin-induced LH release but did not compromise the forskolin-induced cAMP elevation. This suggests that cf GnRH-containing nerve terminals are present in pituitary tissue fragments a nd release cfGnRH via cAMP signaling on GnRH stimulation, whereas the GnRH receptors on gonadotrophs use IP/[Ca2+](i) to stimulate the release of LH.