Effects of Systemic Blockade of Nitric Oxide Synthases on Pulsatile LH, Prolactin, and GH Secretion in Adult Male Rats

Abstract

Background: Nitric oxide (NO) has emerged as an important neurotransmitter involved in the control of the neuroendocrine function. NO acts at hypothalamic, pituitary, and gonadal levels. Previous data from our laboratory showed that blockade of NO generation, after systemic administration of a NO synthase inhibitor (Nω-nitro-arginine methyl ester, NAME), increased the luteinizing hormone (LH) secretion in intact and ovariectomized females, whereas a blockade of spontaneous and steroid-induced LH and prolactin surges after NO synthase inhibition has been also described. Methods and Results: Adult male rats were implanted with chronic intra-auricular cannulae and 5 days later sampled at 15-min intervals during 6 h (10.00–16.00 h). Administration of NAME (40 mg/kg at 08.00 and 13.00 h) stimulated significantly (p ≤ 0.01) the LH secretion, increasing LH pulse amplitude (0.58 ± 0.14 vs. 0.08 ± 0.01 ng/ml in controls), mean LH levels (0.64 ± 0.15 vs. 0.15 ± 0.03 ng/ml in controls), and area under curve (239 ± 56 vs. 57 ± 13 in controls). This effect was blocked by coadministration of sodium nitroprusside (SNP), a NO donor (0.5 mg/kg). The action of NAME was observed 3 h after administration, in contrast to the earlier response detected in female rats, and it appeared selective for LH, as prolactin and growth hormone secretion remained unchanged. Further analysis was carried out to determine whether the effect of NAME on the LH secretion was indirect and mediated by changes in testosterone release. To this end, adult male rats were decapitated 2 h after administration of NAME (40 mg/kg), SNP (0.5 mg/kg), or L-nitro-arginine methyl ester (L-AME), a substrate for NOS (1 g/kg). The serum testosterone concentrations were unchanged after NAME administration, but inhibited by SNP and L-AME. Finally, the effect of NAME and SNP on in vitro testosterone secretion was analyzed. NAME (10 mM) did not affect basal testosterone production, but inhibited the human chorionic gonadotropin stimulated testosterone secretion. Conclusions: These data strongly suggest that the stimulatory effect of NAME on LH secretion is not due to an inhibition of testosterone release and is exerted at the hypothalamic-pituitary level.

Effect of nitric oxide on prolactin secretion and hypothalamic biogenic amine contents

“…If the period of blood collection will be modify to begin later, the long term effect of L-NAME may be masked as another endogenous peak of the hormone would appear at this time around the clock (Esquifino et al, 1998;Lafuente et al, 1999). Also the discrepancies on the effect of L-NAME on episodic secretion of prolactin with the work of Pinilla et al (2001) may be explained considering the frequency of sampling in both works (15 min in Pinilla's compared to 7 min in this work) and the mean half life of the hormone (over 15 minutes), so that to find the prolactin peaks sampling times lower than 15 minutes has to be employed.…”

“…administered with N N -nitro-L-arginine methyl ester (L-NAME), a NO synthase inhibitor, at a dose of 10 mg/Kg in saline, 60 minutes before the beginning of the bleeding period. The doses and timing for L-NAME administration were selected according to previous data from the literature (Matton et al, 1997;Yen and Pan, 1999;Pinilla et al, 2001). One hour after the beginning of the intravenous infusion of saline, and 15 min after the administration of 300 IU of heparin, rats were continuously bled through a peristaltic pump at a flow rate of 50 AL, every 7 min.…”

“…On the other hand blockade of NO synthesis by the administration of N-omega-nitro-arginine methyl ester (L-NAME), inhibits the preovulatory peak of prolactin (Pinilla et al, 2001) or attenuates the inhibitory effect of dopamine on prolactin secretion (Yen and Pan, 1999;González and Aguilar, 1999). In humans whereas L-NAME was not able to modify either the basal or TRH-stimulated prolactin secretion it enhanced the stimulatory effect of vasoactive intestinal peptide .…”

Ghrelin-induced growth hormone release from goldfish pituitary cells is nitric oxide dependent

“…Although NO is reported not to mediate direct pituitary action of a 3-h treatment of GRLN on GH release in adult rats [36], long-term (10 days) in vivo administration of GRLN induces increases in GH release in prepubertal rats, a result which is not seen with the administration of NO inhibitors [35]. On the other hand, the participation of NO in the direct action of GRLN on GH is clearly demonstrated in porcine systems [38] and, as discussed in the introduction, the involvement of NOS/NO in GRLN actions in many other physiological functions including appetite control [31], immune functions [40], and cardiac performance [32,43] is well-established.…”

Evaluation of Hydromethanolic Leaf Extract of Bryophyllum Pinnatum on Reproductive Functions of Male Wistar Rats.

“…Aspartic acid stimulates secretion of gonadotropin -releasing hormone (GnRH), FSH and LH. Aspartic acid is also converted to nitric oxide, which is one of the most important factors controlling the release of FSH and LH [Pinilla, et al, 2001]. These mechanisms may have contributed to override a negative feedback action of TET on the secretion of FSH and LH.…”