The question of a decrease in sensitivity to the inhibitory effects of estradiol (E2) on LH release in relation to puberty was re-examined. The rats were fitted with indwelling venous catheters making it possible to monitor plasma LH levels by radioimmunoassay longitudinally in individual animals and to administer E2 as a virtually continuous infusion. The rats were ovx at 19 (prepuberal) or 45-55 days of age (postpuberal), and one week later a polyethylene catheter was introduced into the vena cava via the right external jugular vein and an infusion of E2 or saline started (rate = 1.14 ml/24 hr.). The infusion was interrupted daily between 1600-1800 hr (time of preovulatory LH surge of intact adults) for 10 min for blood collection. In control pre and postpuberal rats infused with saline plasma LH levels exceeded 160 ng/ml (LER 1213A) one week after ovx. In rats ovx after puberty LH was not suppressed by infusion of 0.32 μg E2/100 g/day, while in the spayed prepuberal rats, plasma LH was lowered by a quarter of this amount. In a number of the prepuberal rats as the infusion with this suppressive dose of E2 (0.08 μg/100 g/day) was continued, there occurred an abrupt, secondary rise in LH which in some rats coincided with vaginal opening. The secondary rise in LH was thought to be due to a decrease in the sensitivity to the inhibitory effect of E2. With higher doses of E2 plasma LH levels were suppressed for shorter periods of time and the secondary rise in LH and vaginal opening occurred earlier suggesting a doserelated effect of E2 on the maturational change in sensitivity to the inhibitory effects of E2 on LH secretion.
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