Effects of 2-Hydroxyestradiol-17β, 2-Hydroxyestradiol-17α, and 4-Hydroxyestrone on the Preovulatory Luteinizing Hormone Surge in the Rat: Agonist and Antagonist Actions*

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Abstract
Four-day cycling rats equipped with intracardiac catheters were injected with 2-hydroxyestradiol-17.beta. (20HE2-17.beta.) at 0800, 0900, 1000 and 1200 h on the morning of proestrus. The administration at 0800 and 0900 h resulted in abolition of the afternoon preovulatory luteinizing hormone [LH] surge in virtually all animals. Injections given at 1000 and 1200 h were ineffective in this respect. The isomeric catechol estrogen 4-hydroxyestrone effectively inhibited the LH surge when given at 0900 h, but not if injected at 1000 or 1200 h. The nonestrogenic 2-hydroxyestradiol-17.alpha. was effective in blocking the LH surge when given at 0900 or 1000 h. Rats treated with 20HE2-17.beta. at 1000 h responded normally to exogenous LHRH administration in the afternoon, indicating that the action of 20HE2-17.beta. is at the hypothalamic level. 20HE2-17.beta. and 4-hydroxyestrone, which are potent estrogens, may act in the instance first as estrogen agonists, advancing the time window when their catechol antagonist properties in blocking the LH surge can be functional. The nonuterotropic 2-hydroxyestradiol-17.alpha. and 2-hydroxyestrone act solely as catechol estrogens and inhibit the preovulatory LH surge if administered at a time when they may be present during this physiological window, which is thought to involve events at the estrogen-catecholaminergic interphase.