Reproductive Function in Female Mice Lacking the Gene for Endothelial Nitric Oxide Synthase

Nitric oxide (NO) acts as a neuronal messenger in both the central and peripheral nervous systems and has been implicated in reproductive physiology and behavior. Pharmacological inhibition of nitric oxide synthase (NOS) with the nonspecific NOS inhibitor, l-NG-nitro-Arg-methyl ester (l-NAME), induced deficits in both the number of ovarian rupture sites and the number of oocytes recovered in the oviducts of mice. Female neuronal NOS knockout (nNOS−/−) mice have normal numbers of rupture sites, but reduced numbers of oocytes recovered following systemic injections of gonadotropins, suggesting that NO produced by nNOS accounts, in part, for deficits in ovulatory efficiency observed after l-NAME administration. Additionally, endothelial NOS knockout (eNOS−/−) mice have reduced numbers of ovulated oocytes after superovulation. Because endothelial NOS has been identified in ovarian follicles, and because of the noted reduced breeding efficiency of eNOS−/− mice, the present study sought to determine the role of NO from eNOS in mediating the number of rupture sites present after ovulation. Estrous cycle length and variability were consistently reduced in eNOS−/− females. The number of rupture sites was normal in eNOS−/− mice under natural conditions and after administration of exogenous GnRH. After exogenous gonadotropin administration, eNOS−/− females displayed a significant reduction in the number of ovarian rupture sites. Female eNOS−/− mice also produced fewer pups/litter compared to WT mice. These data suggest that NO from endothelial sources might play a role in mediating rodent ovulation and may be involved in regulation of the timing of the estrous cycle.