Evidence that gonadotropin-releasing hormone stimulates gene expression and levels of active nitric oxide synthase type I in pituitary gonadotrophs, a process altered by desensitization and, indirectly, by gonadal steroids

• Published in: Endocrinology (Philadelphia) 1998-04, Vol.139 (4), p.2163-2170
• Main Authors: Garrel, G, Lerrant, Y, Siriostis, C, Bérault, A, Magre, S, Bouchaud, C, Counis, R
• Format: Article
• Language: English
• Subjects: Animals; Blotting, Western; Drug Tolerance; Estradiol - pharmacology; Gene Expression - drug effects; Gonadotropin-Releasing Hormone - agonists; Gonadotropin-Releasing Hormone - antagonists & inhibitors; Gonadotropin-Releasing Hormone - pharmacology; Histocytochemistry; Isoenzymes - genetics; Isoenzymes - metabolism; Kinetics; Male; NADPH Dehydrogenase - analysis; Nitric Oxide Synthase - genetics; Nitric Oxide Synthase - metabolism; Orchiectomy; Pituitary Gland, Anterior - enzymology; Rats; Rats, Wistar; RNA, Messenger - metabolism; Testosterone - pharmacology; Triptorelin Pamoate - pharmacology
• ISSN: 0013-7227
• DOI: 10.1210/en.139.4.2163

To determine the site and mechanism of action of gonadal steroids on pituitary nitric oxide synthase type I (NOS I), present in both gonadotrophs and folliculo-stellate cells, the effects of castration and steroids were examined in male rats, in the presence of a GnRH antagonist (Antarelix). Western analysis showed a rapid and substantial increase with time, after orchidectomy, of NOS I protein, the concentration doubling in 24 h and reaching a maximal 4- to 5-fold increase after 3-7 days, followed by a progressive decline after 2 weeks. Testosterone or estradiol replacement, or administration of GnRH antagonist, totally abolished the effects of castration, demonstrating a mediation of the steroid effects via GnRH. In noncastrated rats, steroids and the GnRH antagonist also caused a reduction in the levels of NOS I (by 50-60%), consistent with inhibition of endogenous GnRH stimulation. In marked contrast, administration of a potent GnRH agonist (Triptorelin) to intact rats increased the levels of NOS I. A time-course study with a long-lasting formulation showed that rise in NOS I developed rapidly after a lag of approximately 5 h, with a 2-fold increase detectable after 8 h and a maximal 4.5-fold after 48 h. The level declined afterwards in a manner consistent with homologous desensitization that may occur in the continuous presence of GnRH; however, the profile was different and delayed compared with those of gonadotropin release. As observed for NOS I protein, NOS I messenger RNA concentration was increased by castration or GnRH agonist and reduced by steroids or GnRH antagonist. Taken together, these data demonstrate that steroids indirectly regulate NOS I messenger RNA and protein levels, through the hypothalamic modulation of GnRH, which represents the primary regulator of NOS I. No effect of steroids on NOS I was seen in the posterior lobe. NADPH-diaphorase histochemistry coupled to immuno-identification of the cells revealed that the treatments affecting the concentration of NOS I concomitantly altered the activity but exclusively in gonadotrophs and not in folliculo-stellate cells (which do not respond to GnRH), reinforcing the idea that GnRH played a major regulatory role. Expression in gonadotrophs of a GnRH-dependent NOS I and the ensuing production of nitric oxide represents a potentially novel signaling pathway for the neuropeptide in the anterior pituitary, consistent with the previously reported GnRH-induced cGMP production, the role of whic