Ovarian modulation of immunoreactive gonadotropins-releasing hormone (Gn-RH) in the rat brain: evidence for a differential effect on the anterior and mid-hypothalamus.

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.