INHIBITION OF THE RELEASE OF PITUITARY OVULATORY HORMONE IN THE RAT BY MORPHINE1

Abstract

When mature cyclic female Sprague-Dawley rats were injected with morphine prior to 2 P.M. on the day of proestrus, ovulation was blocked in 13 of 16 cases, but if treatment was delayed until 4 P.M. ovulation occurred in 10 of 10 rats. Rats which failed to ovulate the first night did ovulate the next night if no further treatment was given. Daily administration of the drug prior to 2 P.M. blocked the mechanism for periods up to 25 days after which new corpora lutea indicated a "break through" of the ovulatory mechanism even though normal vaginal cycles had not been restored. That the weight loss in chronically-treated rats is not the important factor in preventing ovulation is indicated by the observation that normal animals, pair-fed to morphine-treated rats, retained cycle regularity and ovulated normally.