Inhibition of the Preovulatory Prolactin Surge in the Rat by Catechol Estrogens: Functional and Temporal Specificity*

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Abstract
Catechol estrogens, administered iv to cycling rats on the morning of proestrus, were able to block the preovulatory PRL surge on the afternoon of the same day. Only catechol estrogens with a low affinity for the estrogen receptor, such as 2-hydroxyesterone and 2-hydroxyestradiol-17.alpha. were effective in this respect, while the estrogenic catechol estrogen 2-hydroxyestradiol was unable to influence the PRL surge. The effectiveness of the PRL surge abolition was highly dependent on the state of the endogenous estradiol levels at the time of administraion. Only doses given just before the peak secretion of estradiol were effective in blocking the PRL surge. Despite similarities in the inhibition of the preovulatory LH and PRL surges by catechol estrogens, these are considered to occur by different mechanisms because the LH secretion is blocked by all catechol estrogens, while the PRL surge is affected only by catechol estrogens with no estrogen agonist properties. The catechol estrogen blockade of the PRL surge may have physiological parallels and provides a useful probe of the mechanisms of the estrogen-PRL axis.