Role of Nitric Oxide and Alcohol on Gonadotropin Release in Vitro and in Vivoa
Authors: RETTORI V. , McCANN S. M. 
Published: February 2006

Abstract: Alcohol suppresses reproduction in humans, monkeys, and small rodents by suppressing release of luteinizing hormone (LH). The major action is on the hypothalamus to decrease release of LH‐releasing hormone (LHRH). The release of LHRH is controlled by nitric oxide (NO) as determined by in vivo and in vitro experiments. The hypothesized pathway is via norepinephrine (NE)‐induced release of NO from NOergic neurons, which activates LHRH release. We have evaluated details of this process in male rats by incubating medial basal hypothalamic (MBH) explants in vitro and examining the release of NO and metabolites generated by NO that control LHRH release. NE increased release of NO as measured by determining the content of the enzyme at the end of the experiment (30 min) by adding [14C]arginine to the homogenate and measuring its conversion to [14C]citrulline since this is formed in equimolar quantities with NO by NO synthase (NOS). Because this increase in content, presumably caused by activation of the enzyme by NE, was blocked by the α1 receptor blocker prazosin, it appears that α1 receptors activate NOS by increasing intracellular free calcium in the NOergic neurons, which combines with calmodulin to activate NOS. The release of LHRH induced by nitroprusside (NP), a donor of NO, is accompanied by an increase in cyclic guanosine monophosphate (cGMP) in the medium supporting the activation of guanylate cyclase by NO. This activation is important in releasing LHRH since addition of 8‐monobutyryl cGMP also released the peptide. Ethanol had no effect on the content of NOS or on the increase in content induced by NE, indicating that it did not act on NOS. Earlier experiments indicated that prostaglandin E2 (PGE2) was important in releasing LHRH. PGE2 is produced by activation of cyclooxygenase by NO since this occurred following addition of the NO donor, NP. Not only does NP increase PGE2 release, but it also increases the conversion of [14C]arachidonic acid to its metabolites, particularly PGE2, by activating cyclooxygenase. NP also activated lipoxygenase as indicated by increased release of leukotrienes, which also stimulate LHRH release. Ethanol acts at this step, because it completely blocked the release of PGE2, leukotrienes, and LHRH induced by NP. Therefore, the results support the theory that NE acts to stimulate NO release from NOergic neurons. This NO diffuses to the LHRH terminals, where it activates guanylate cyclase, leading to an increase in cGMP. At the same time, it also activates cyclooxygenase and lipoxygenase. The increase in cGMP increases intracellular free calcium, required for activation of phospholipase A2. Phospholipase A2 converts membrane phospholipids into arachidonic acid, the substrate for conversion by the activated cyclooxygenase and lipoxygenase to PGE2 and leukotrienes that activate the release of LHRH. Because alcohol inhibits conversion of labeled arachidonic acid to PGE2 and leukotrienes, it must act either directly to inhibit cyclooxygenase and lipoxygenase or by some other mechanism which, in turn, inhibits the enzyme. We initially believed that the action of alcohol was exerted directly on the LHRH terminals; however, our recent experiments indicate that alcohol suppresses LHRH release, at least in part, by stimulating β‐endorphinergic neurons that inhibit the release of NE, which drives the NOergic release of LHRH.