Effect of ovariectomy on the hypothalamic content of immunoreactive gonadotropin-releasing hormone in the female mouse as revealed by quantitative immunocytochemistry and radioimmunoassay

Abstract

Our radioimmunoassay data reveal that ovariectomy for either one or two months resulted in a significant decrease in the GnRH content of the mouse basal hypothalamus, while in mice ovariectomized for two weeks the GnRH levels did not differ significantly from controls. Parallel immunocytochemical analyses gave similar results. Tissue sections from specific regions of the median eminence in intact and ovariectomized mice were immunolabeled for GnRH with the peroxidase-antiperoxidase procedure of Sternberger. Differences in area and average unit density of the reaction product were measured with a Quantiment 720 image analyzer. At two weeks after ovariectomy, a majority of matched sections from intact and ovariectomized animals, taken in each sampled region of the median eminence, showed no visually apparent difference in GnRH immunoreactivity. Quantitative image analysis of these preparations revealed no statistically significant difference in either the area of neural tissue covered by deposit or the average unit density of this material in any given region. In contrast, in mice ovariectomized for either one or two months, immunolabeled GnRH was decreased throughout the cephalo-caudal extent of the median eminence. In each region, a reduction in reaction deposit was observed over most of the area in which GnRH is localized, especially in the medial aspects of this distribution. Image analysis of these preparations revealed a significant reduction of both specimen area immunolabeled for GnRH and the average unit density of reaction product in each region of the median eminence. While the decrease in density was uniform throughout the median eminence, the greatest reduction in area covered by deposit occurred in the infundibular region. The demonstration of a decrease in basal hypothalamic GnRH in the chronically ovariectomized mouse suggests that GnRH neurosecretion is increased in response to the absence of the ovaries and supports the premise that tonic secretion of this neurohormone, in the mouse, may be subject to regulation by the gonads. At two weeks after ovariectomy, it is possible that changes in GnRH secretion occur without a concomitant alteration in hypothalamic hormone content. In addition, the quantitative immunocytochemical data suggest that GnRH neurons responsive to ovariectomy terminate throughout the rostro-caudal extent of the mouse median eminence, especially in the medial sites of localization. A proportionally greater number of these endings appear to be present within the infundibular region of the median eminence, relative to the other regions sampled.