Influence of Estradiol Benzoate on Pituitary Responsiveness to LH-RH at Different Stages of the Estrous Cycle in Rats

Abstract

The effect of estradiol ben-zoate (EB) on the pituitary response to LH-releasing hormone (LH-RH) was studied in the different stages of the estrous cycle of rats. An iv injection of 0.4 μg LH-RH always raised serum LH in the oil-treated control group. The magnitude of the increase varied according to the stage of the estrous cycle, being greatest in Proestrus and Estrus, and lowest in Diestrous II (DII). EB injected 24 hr before increased the pituitary response to LH-RH regardless of the stages of the cycle, but did not modify the fluctuation of pituitary responsiveness. The basal levels of LH at 9 am throughout the cycle in the oil-treated control rats were highest in Diestrus I and lowest in Diestrus II. Pretreatment with EB lowered the basal LH levels only on the day of DI. On the day of Estrus, the pituitary response was greatest at 4 hr, and the lowest level at 14 hr in the control rats. Injection of EB suppressed the pituitary response to LH-RH at 4 and 9 hr, and did not alter this response at 14 hr as compared with the response in the oil-injected control rats. The results provide further evidence for the complex interactions of estrogen with other endogenous hormones in the pituitary responsiveness to LH-RH.