The Site of Action of Intrahypothalamic Estrogen Implants in Feminine Sexual Behavior: An Autoradiographic Analysis

Abstract

Estrogenic stimulation of the ventromedial hypothalamus is sufficient to prime progesterone-facilitated estrous behavior in ovariectomized rats. To determine precisely the site(s) of estrogenic stimulation and the locus of its priming action on estrous behavior, we used steroid autoradiographic methods to assess the diffusion of [³H]estradiol ([³H]E₂) from behaviorally effective implants diluted 1:300 with cholesterol. Ovariectomized rats received [³H]E₂-cholesterol implants aimed at the ventromedial hypothalamic nucleus (VMN). Females were tested twice for feminine sexual behavior after stereotaxic surgery. They received progesterone on the day of behavioral testing. Animals were killed on the day after the second behavior test, cannulae were removed, and the brains were frozen rapidly and processed for autoradiography. Sections of 6-micron thickness from the preoptic region through the posterior hypothalamus were exposed to the nuclear emulsion-coated slides for varying periods of up to 12 months. Sections exposed for 10 months were analyzed for diffusion of the steroid. To determine background levels of grain reduction, grain counts were taken in the cortex of each brain section that contained a tip in the VMN. To determine the extent of diffusion from a cannula, grain counts were taken ventrally and laterally from each implant tip. Counting began at 0.10 mm-0.30 mm from a tip, and counts were taken at intervals of 0.02 mm in the specified direction. From this procedure, the mean grains per square at each of 12–15 intervals from the implant tip was obtained. These data were used to establish diffusion gradients in ventral and lateral directions. It was then possible to use the derived exponential functions to estimate the extent of the diffusion in both the lateral and ventral directions. Five of eight females with bilateral implants aimed at the VMN exhibited female sexual behavior in at least one of the two tests. Of these, four also showed proceptive behavior. Histological examination of brain sections indicated that behaviorally effective implants were located in, or adjacent to, the central portions of the VMN. Implants from nonreceptive animals were located at the extreme anterior or posterior aspects of the VMN. Our diffusion analysis showed that the mean spread was 0.42 mm laterally and 0.43 mm ventrally. Moreover, for 75% of the tips analyzed, the spread of [³H]E₂ was less than 0.50 mm. These data are consistent with the view that estrogen acts within a sharply defined region of the VMN to prime estrous behavior.