Nitric oxide/cGMP mediation of norepinephrine facilitation of reproductive behavior in female rats
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The purpose of these experiments is to investigate whether nitric oxide (NO) and its downstream effectors interact with norepinephrine to regulate lordosis, an ovarian hormone-dependent reproductive behavior in female rats. We hypothesize that NO/cGMP pathway mediates the facilitatory effects of norepinephrine on lordosis in the preoptic area and the hypothalamus either by increasing norepinephrine release, by mediating the postsynaptic signal transduction of alpha1-adrenoceptors, or by both.;We first tested the hypothesis that NO facilitates norepinephrine release in vitro from brain slices of the hypothalamus and the preoptic area of hormone-primed animals using a superfusion apparatus. Neither basal nor N-methyl-D-aspartate-stimulated norepinephrine release is altered by a NO precursor or NOS inhibitors regardless of hormone priming.;Next, we tested the hypothesis that NO-stimulated cGMP regulates lordosis. Ovariectomized female rats were implanted with a guide cannula aiming at one of the brain ventricles. When a cell-permeable cGMP analog was infused into hormone-primed rats, lordosis behavior was enhanced. Next, a specific inhibitor for NO-activated soluble guanylyl cyclase was infused into the ventricle of hormone-primed animals. This inhibitor reduced lordosis in a dose-dependent manner without affecting general locomotor activities. Because cGMP-dependent protein kinase (PKG) is one of the major targets of cGMP, we also tested the role of PKG in lordosis. A selective PKG inhibitor reduced lordosis in hormone-primed female rats. RU486, a progesterone receptor antagonist, was used to assessed whether the facilitatory effects of cGMP on lordosis are mediated through the progesterone receptor. RU 486 significantly attenuated cGMP-potentiated lordosis, suggesting that the NO/cGMP pathway acts via the progesterone receptor to enhance lordosis behavior.;Furthermore, we examined the hypothesis that the NO/cGMP pathway mediates the potentiating effects of alpha1-adrenoceptor on lordosis behavior. Norepinephrine and phenylephrine, an alpha1-adrenoceptor agonist, stimulated cGMP formation in the hypothalamic slices of female rats treated with both estrogen and progesterone prior to experimentation. No stimulatory effects of norepinephrine on cGMP formation were found in the slices of animals treated with estrogen alone, progesterone alone, or oil vehicle. The alpha 1-adrenoceptor antagonist prazosin blocked the effect of phenylephrine on cGMP production. In fact, the basal content of cGMP was significantly elevated in the hypothalamic slices of animals primed with both hormones. NO synthase inhibitors totally diminished norepinephrine-evoked cGMP production, suggesting that norepinephrine, through alpha1-adrenoceptors, elevates cGMP levels via NO. In addition, a cGMP analog reversed the inhibitory effects of prazosin on lordosis behavior of estrogen- and progesterone-primed female rats.;Taken together, we conclude that NO regulation of lordosis behavior in female rats is not exerted at the level of norepinephrine release from the preoptic area or the hypothalamus; rather, norepinephrine activates the NO/cGMP/PKG pathway to facilitate lordosis only in estrogen- and progesterone-primed animals.
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