Bonavera J J, Sahu A, Kalra P S, Kalra S P
Department of Physiology, University of Florida College of Medicine, Gainesville 32610.
Endocrinology. 1993 Dec;133(6):2481-7. doi: 10.1210/endo.133.6.8243268.
The involvement of excitatory N-methyl-D-aspartate (NMDA) receptors in the hypothalamic control of pituitary LH secretion is well recognized. Recent evidence shows that nitric oxide (NO), a free radical gas, may act as neurotransmitter in the brain, and its efflux is stimulated by activation of NMDA receptors. Studies were undertaken to determine whether NO is involved in the hypothalamic release of LHRH and in the LH surge induced by progesterone (P) in estrogen-primed ovariectomized rats. Rats were ovariectomized and 2 weeks later received estradiol benzoate (30 micrograms sc) at 1000 h. Two days later, P was injected at 1000 h to potentiate the estradiol benzoate-induced LH surge in the afternoon. Serial blood samples were collected at hourly intervals from 1400-1800 h via an intraatrial cannula implanted the day before P injection. Additionally, at various times before onset of the LH surge at 1400 h, the rats were injected sc with one of three inhibitors of NO synthase, the enzyme that generates NO. Control, saline-injected rats showed unambiguous LH surges in the afternoon. However, either a single injection at 1000 h of NG-methyl-L-arginine (20 mg/kg) or three injections at 1000, 1200, and 1400 h of either Nw-nitro-L-arginine methyl ester (NAME, 40 mg/kg) or Nw-nitro-L-arginine (60 mg/kg) to inhibit NO efflux markedly suppressed the P-induced LH surge in the afternoon. To ascertain whether suppression of LH surge was due to blockade of hypothalamic LHRH release, a series of in vitro studies were performed in steroid-primed rats. First we examined the effects of sodium nitroprusside (NPS), a compound that spontaneously generates and releases NO. NPS increased basal and KCl-induced LHRH release in vitro from the medial basal hypothalamus-preoptic area and median eminence fragments. No direct effect of NO at the pituitary level was seen, since NPS did not alter basal or LHRH-induced LH in vitro release from hemipituitaries. In addition, we tested the effects of NAME on NMDA-induced LHRH release in vitro from the median eminence-arcuate nucleus fragments. As expected, NMDA alone (50 mM) induced a significant increase in LHRH release. Addition of NO synthase inhibitor, NAME (1 or 10 mM) to suppress NO efflux, significantly diminished the NMDA-induced LHRH release.(ABSTRACT TRUNCATED AT 400 WORDS)
兴奋性N-甲基-D-天冬氨酸（NMDA）受体参与下丘脑对垂体促黄体激素（LH）分泌的调控，这一点已得到广泛认可。最近有证据表明，一氧化氮（NO）这种自由基气体可能在大脑中充当神经递质，并且其释放受NMDA受体激活的刺激。本研究旨在确定NO是否参与雌激素预处理的去卵巢大鼠下丘脑促性腺激素释放激素（LHRH）的释放以及孕酮（P）诱导的LH峰。大鼠接受去卵巢手术，2周后于上午10点皮下注射苯甲酸雌二醇（30微克）。两天后，于上午10点注射P以增强下午苯甲酸雌二醇诱导的LH峰。在注射P前一天通过植入心房的插管，于14点至18点每隔一小时采集系列血样。此外，在14点LH峰开始前的不同时间，给大鼠皮下注射三种一氧化氮合酶抑制剂之一，该酶可生成NO。注射生理盐水的对照大鼠下午出现明显的LH峰。然而，上午10点单次注射NG-甲基-L-精氨酸（20毫克/千克），或上午10点、12点和14点三次注射Nω-硝基-L-精氨酸甲酯（NAME，40毫克/千克）或Nω-硝基-L-精氨酸（60毫克/千克）以抑制NO释放，均显著抑制了下午P诱导的LH峰。为确定LH峰的抑制是否由于下丘脑LHRH释放受阻，在激素预处理的大鼠中进行了一系列体外研究。首先，我们检测了硝普钠（NPS）的作用，该化合物可自发生成并释放NO。NPS增加了体外内侧基底下丘脑-视前区和正中隆起片段基础及氯化钾诱导的LHRH释放。由于NPS未改变体外半垂体基础或LHRH诱导的LH释放，因此未观察到NO对垂体水平的直接作用。此外，我们检测了NAME对体外正中隆起-弓状核片段NMDA诱导的LHRH释放的影响。正如预期的那样，单独使用NMDA（50毫摩尔）可显著增加LHRH释放。添加一氧化氮合酶抑制剂NAME（1或l0毫摩尔）以抑制NO释放，可显著减少NMDA诱导的LHRH释放。（摘要截短于400字）
