Sexual Differentiation of Positive Feedback: Effect of Hour of Castration at Birth on Estradiol-Induced Luteinizing Hormone Secretion in Immature Male Rats*

Abstract

In the male rat, a dramatic increase in hypothalamic testosterone and estradiol concentrations occurs during the 1st few hours of postnatal life. Whether such increases participate in the defeminization of positive estrogen feedback effects on LH [luteinizing hormone] secretion was determined. Newborn male rats were castrated either in utero (0 h males), or 10 or 24 h after birth. Some males were castrated at 0 h in utero and injected at the time of surgery with 1, 2, 5, or 5 .mu.g testosterone proprionate. A group of females was ovariectomized at 0 h in utero (0 h females). The control group consisted of male and female rats sham gonadectomized at 0 h in utero which were either gonadectomized at 21 days of age or left intact. The experimental groups were challenged before puberty to determine if estrogen induced a release of LH using 2 different types of estrogen treatment. The 1st treatment consisted of an injection of 0.2 .mu.g estradiol benzoate (EB) on day 28 followed by a 2nd 10 .mu.g injection of EB on day 29. This treatment resulted on the afternoon of day 30 in a surge of LH in intact females. Normal males, 0 h males, or females castrated at 21 days did not have a significant LH surge. The 2nd test consisted of the daily injection of 0.05 .mu.g EB on days 23-27; on day 28 the rats were injected with 2.5 .mu.g EB. Zero hour male and female rats showed a large LH surge on the afternoon of day 29; sham castrated males never responded to this treatment. No sex difference was observed in the mean size of the LH surge providing the males were castrated at 0 h in utero. The effect of the hour of castration on the day of birth was also studied. Males castrated at 10 or 24 h after birth showed either no LH surge or the magnitude of the surge was greatly reduced compared to that obtained in the 0 h males (P < 0.001). The fact that 0 h males injected with 1 .mu.g testosterone propionate never showed an LH surge after prepuberal treatment with estrogen suggests that 0 h is a time during which the newborn is sensitive to the defeminizing effect of androgens. Apparently the testicular hyperactivity which occurs at the time of birth could influence the defeminization of the LH surge mechanisms.