Time Course of the Negative Feedback Effect of Testosterone on Serum Gonadotropiiis in Prepubertal Male Rats

Abstract

The difference in the time course of the negative feedback effect of a single dose of 20 mg/kg testosterone propionate (TP) on gonadotropin secretion was studied in two groups of prepubertal (22-and 28-day-old) male rats. In both age groups serum testosterone concentration was highest 10 hr after the injection of TP and the level decreased steadily and at a similar extent to the control level by day 5. The injection of TP produced a significant rise in the serum FSH level 24 hr after the injection in 28-day-old rats but no significant effect of TP on serum FSH was observed during the first 24 hr in 22-day-old rats. At 48 hr, serum FSH levels decreased significantly in both age groups and the level returned to the control level on day 5 in 22-day-old rats but remained low in 28-day-old rats. In contrast, serum LH levels decreased already 10 hr after the injection of TP in both age groups. However, the LH level in 22-day-old rats tended to increase over the control level 5 days after administration of TP, while the level remained low in 28-day-old rats. The weight of the seminal vesicle and ventral prostate showed a significant increase from 24 or 48 hr after the injection of TP in both age groups. The magnitude of the increase in the weight of these organs in 28-day-old rats was much greater than that seen in 22-day-old rats. An apparent age difference in the time course of the negative feedback of exogenous testosterone on the gonadotropin secretion, particularly on the FSH release, may correlate with the appearance of physiological changes in the sensitivity and function of the hypothalamic-pituitary-gonadal system which may occur during prepubertal period for the onset of puberty.