In vitro Secretion of Gonadotropin-Releasing Hormone (GnRH) and [Hydroxyproline9]GnRH from the Rat Hypothalamus Exhibits a Differential Sensitivity to Castration and Second Messengers

The decapeptide [hydroxyproline 9 ]GnRH (HypGnRH) has been characterized as an endogenous posttranslational product of the gonadotropin-releasing hormone (GnRH) precursor in a wide range of mammalian brains. Despite consistent biological effects, its secretion by the hypothalamus remains hypothetical. We report here in vitro secretion of HypGnRH and GnRH by the hypothalamus from intact and castrated male rats and provide evidence that they are differentially regulated. Both peptides were identified by two anti-GnRH antibodies of different specificities after separation under two high-performance liquid chromatography conditions. Calcium dependency of HypGnRH release was demonstrated under stimulation with KCl in the absence or presence of Ca 2+ , as well as with Bay K 8644, veratridine, methoxyverapamil, or tetrodotoxin. Activation of signaling pathways involving adenylate cyclase and protein kinases A and C (PKC) induced HypGnRH release. Expression of data as percentage of release over tissue stores revealed a two- to threefold higher release of HypGnRH than of GnRH under the different modes of stimulation used, except under PKC activation which triggered a comparable recruitment of both peptides. Castration selectively affected PKC-coupled GnRH secretion which showed a twofold lesser release than in intact rats, while the HypGnRH release was unaffected. We conclude that HypGnRH and GnRH are not secreted from the hypothalamus according to the same mechanisms.