GnRH and gonadotropin release is decreased in chronic nitric oxide deficiency

Nitric oxide synthetase (NOS), the conversion enzyme for nitric oxide (NO) is localized in the anterior pituitary of female rats, particularly in gona dotrophs and folliculo-stellate cells, suggesting that NO regulates the rel ease of luteinizing hormone (LH) and follicle stimulating hormone (FSH) fro m the anterior pituitary. The focus of this study was to determine the effe ct of chronic NO deficiency on the subsequent pituitary release of LH and F SH in vitro and the hypothalamic immunoexpression of GnRH in vivo. NO defic iency was induced by adding the NOS inhibitor, N-nitro-L-arginine (L-NNA, 0 .6g/L) to the drinking water of female Wistar rats. After 8 weeks, the anim als were euthanized, the pituitaries were removed, and they were incubated in vitro. Pituitaries were perfused for 4 hr in the presence of pulsatile g onadotropin release hormone (GnRH, 500 ng/pulse) every 30 min. S-Nitroso-L- acetyl penicillamine (SNAP, an NO donor, 0.1 mM) or L-nitro-argine methyl e ster (L-NAME, a NOS inhibitor, 0.1 mM) was added to the media and perfusate samples were collected at 10-min intervals. LH and FSH levels in the perfu sate were measured by double antibody radioimmunoassays. Pituitaries from t he NO-deficient rats had a significantly smaller GnRH-stimulated release of LH and FSH compared with proestrous control rats. The addition of S-NAP to the perfusate resulted in decreased LH and FSH secretion in the control gr oup, but increased LH secretion in the NO-deficient group. The addition of L-NAME to the perfusate suppressed LH secretion from control pituitaries, b ut not in pituitaries from NO-deficient animals. Immunohistochemistry of br ain slices demonstrated that NO-deficient rats had a large qualitative decr ease of GnRH in the median eminence compared with their controls. This decr ease was particularly evident in the external capillary plexus of the media n eminence. We concluded that chronic NO deficiency is associated with a de creased GnRH in neurosecretory terminals in the external capillary layer of the median eminence, accompanied by a decrease in LH and FSH release from the pituitaries.