Estrogen Induces Neurotensin/Neuromedin N Messenger Ribonucleic Acid in a Preoptic Nucleus Essential for the Preovulatory Surge of Luteinizing Hormone in the Rat*

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Abstract
Ovarian steriods act on unidentified neurons to trigger preovulatory secretion of GnRH. In the rat, important steroid target cells reside in the anterior medial preoptic nucleus (AMPN), a sexually dimorphic structure essential for stimulatory effects of ovarian steroids on LH secretion. The AMPN contains neurotensin (NT)-immunoreactive neurons, and immunoneutralization of NT in the preoptic region markedly attenuates steroid-induced LH surges. Using probes derived from the rate gene that encodes NT and neuromedin N (NT/N), we investigated the ability of estrogen to influence NT/N and mRNA levels in the AMPN. Ovariectomized rats were treated for 14 days with sham capsules or capsules that produce supraphysiological serum levels of 17.beta.-estradiol (250 .+-. 20 pg/ml). As determined by in situ hybridization, estradiol markedly altered the distribution of NT/N mRNA in the medial preoptic region, causing a striking increase in NT/N mRNA abundance specifically in the AMPN and adjacent medial preoptic nucleus (MPN). In contrast, estradiol caused no obvious changes in labeling in the lateral septum, diagonal band of Broca, bed nucleus of the stria terminalis, and lateral preoptic area. The distribution of NT/N mRNA in the AMPN of normal male rats closely resembled that in ovariectomized rats, where labeled cells were rarely observed. Microdissection and S1 nuclease protection analysis were used to quantitate the effect of estradiol on NT/N mRNA levels. Supraphysiological estradiol treatment for 14 days caused a 3.4-fold increase (P < 0.0002) in NT/N mRNA levels in the combined AMPH/MPN, whereas levels in the central amygdaloid nucleus remained constant, providing further evidence of regional specificity. Forty-eight hours of estradiol treatment, at concentrations (60 .+-. 1 pg/ml) similar to those observed on the morning of proestrus, caused a 1.8-fold increase (P < 0.001) in NT/N mRNA levels in the AMPN/MPN, indicating that the time course of NT/N mRNA induction by estrogen is compatible with events of the normal estrous cycle. Together with previous findings, our results strongly suggest that NT neurons mediate, directly or indirectly, stimulatory effects of ovarian steroids on GnRH secretion.