Ovarian Modulation of Immunoreactive Gonadotropins- Releasing Hormone (Gn-RH) in the Rat Brain: Evidence for a Differential Effect on the Anterior and Mid-hypothalamus

Abstract

Gonadotropins-releasing hormone (Gn-RH) in selected regions of the female rat brain was measured by radioimmunoassay. Detectable immunoreactive Gn-RH was found in the anterior hypothalamic-septal region and in the mid-hypothalamic (arcuate-median eminence) region. Gn-RH was several times higher in the middle region than in the anterior region. Gn-RH was undetectable in the posterior hypothalamic region, frontal cerebral cortex and pineal glands, as well as in random blood samples, and low to undetectable in anterior pituitary glands. Gn-RH activity varied during the estrous cycle and after castration. In the mid-hypothalamic region, Gn-RH content was lowest throughout diestrus and in late morning and early afternoon of proestrus, and highest early in the morning of proestrus and during estrus. A significant decrease at mid-day was only found on the day of proestrus, a few hours prior to the critical period for LH release. In the anterior hypothalamic region, low Gn-RH activity was found from 1200 h of estrus to 1200 h of diestrus-2. A comparatively higher activity was seen at 1700 h of diestrus-2 and also from 1400 h of proestrus to 0800 h of estrus. Twenty-one days after ovariectomy, Gn-RH in the mid-hypothalamic region was significantly lower than the lowest values seen during the estrous cycle, while Gn-RH in the anterior hypothalamic region remained between low and high values seen during the cycle, being significantly higher than the low values. The changes observed during the estrous cycle and after castration suggest that gonadal steroids play a direct role in the control of hypothalamic Gn-RH. These data also demonstrate that Gn-RH varies in a different way in the anterior and mid-hypothalamic regions.

Timing of sexual receptivity and the release of gonadotrophins during puberty in female rats

“…These findings are similar to those reported for adult pro-oestrous rats (Nequin, Alvarez & Schwartz, 1975 (Daane & Parlow, 1971 ;Butcher, Collins & Fugo, 1974;Araki, Ferin, Zimmerman & Vande-Wiele, 1975 ;Nequin et al, 1975 ;Chiappa & Fink, 1977). LHRH content in the hypothalamus after LH release during the first oestrus was still low compared with that in adult rats.…”

Changes in hypothalamic LH-RH content and blood levels of LH-RH, gonadotropin and estradiol during the preovulatory stage of rat estrous cycle.

“…Immunoreactive LH-RH is present in the Middle hypothalamus several times higher than in the anterior (Araki et al, 1975;Saito et al, 1977) and changes in the hypothalamic LH-RH content were demonstrated during the rat estrous cycle by several authors (Araki et al, 1975;Asai and Wakabayashi, 1975;Saito et al, 1977;Kalra and Kalra, 1977). In agreement with these authors, we found a reduction in hypothalamic LH-RH content prior to the critical period on proestrus.…”

“…Ramirez and Sawyer (1965) first reported the fluctuations in hypothalamic luteinizing hormone-releasing factor during the rat estrous cycle even before the LH-RH was identified by Matsuo et al (1971). With the availability of new sensitive radioimmunoassay methods for estimation of LH-RH, a number of investigators have revealed the cyclic changes in the amount of hypothalamic LH-RH during the cyclic changes in the amount of hypothalamic LH-RH during the cycle (Araki et al, 1975;Asai and Wakabayashi, 1975;Saito et al, 1977;Kalra and Kalra, 1977). The data among these studies are consistent with the demonstration of rapid changes in hypothalamic LH-RH content in association with the proestrous surge of gonadotropin.…”

Comparison of the effect of electrochemical stimulation of the medial preoptic area and the hypothalamic arcuate nucleus upon LH release in ovariectomized and proestrous rats.

“…Furthermore, short-loop feedback action by the elevated serum LH on its secretion in OVX rats may influence on the MPO sensitivity to electrochemical stimulation (Molitch et al, 1976). Another possibility is that there is less readily releasable LH-RH store in the hypothalamus in OVX rats as indicated by lower hypothalamic content of LH-RH in OVX rats than that in intact rats (Araki et al, 1975). But from our study, the mediobasal hypothalamic region seemed to have LH-RH store enough to activate LH release in OVX rats in response to electrochemical stimulation.…”