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.
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 which remains to be evaluated.
Garrel
G
G
Endocrinologie Cellulaire et Moléculaire de la Reproduction, Université Pierre & Marie Curie, CNRS-URA 1449, Paris, France.
Lerrant
Y
Y
Siriostis
C
C
Bérault
A
A
Magre
S
S
Bouchaud
C
C
Counis
R
R
D000818:Animals; D015153:Blotting, Western; D004361:Drug Tolerance; D004958:Estradiol / Q000494:pharmacology*; D015870:Gene Expression / Q000187:drug effects*; D007987:Gonadotropin-Releasing Hormone / Q000819:agonists / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D006651:Histocytochemistry; D007527:Isoenzymes / Q000235:genetics / Q000378:metabolism; D007700:Kinetics; D008297:Male; D009252:NADPH Dehydrogenase / Q000032:analysis; D019001:Nitric Oxide Synthase / Q000235:genetics* / Q000378:metabolism*; D009919:Orchiectomy; D010903:Pituitary Gland, Anterior / Q000201:enzymology*; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D013739:Testosterone / Q000494:pharmacology*; D017329:Triptorelin Pamoate / Q000494:pharmacology
D007527:Isoenzymes; D012333:RNA, Messenger; D017329:Triptorelin Pamoate; D007987:Gonadotropin-Releasing Hormone; D013739:Testosterone; D004958:Estradiol; D019001:Nitric Oxide Synthase; D009252:NADPH Dehydrogenase