The Effect of Naloxone on Endogenous Opioid Regulation of Pituitary Gonadotropins and Prolactin during the Menstrual Cycle*

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Abstract
To determine the influence of ovarian sex steroid hormones on endogenous opioid regulation of pituitary FSH, LH, and PRL secretion, six women were studied during the follicular phase (days 8–9) and luteal phase (days 21–23) of their menstrual cycles. An iv bolus dose of 10 mg of the opiate antagonist naloxone was given, and plasma FSH, LH, and PRL were measured at −30, −15, 0, 15, 30, 45, 60, 90, 120, and 180 min. During the follicular phase, baseline plasma FSH and LH levels were 10.7 ± 0.9 and 16.7 ± 2.0 mlU/ml (mean ± SEM), respectively; the plasma PRL level was 11.7 ± 1.2 ng/ml. Nal-oxone did not significantly alter plasma FSH, LH, or PRL during the follicular phase. Basal levels of LH were significantly lower during the luteal phase than during the follicular phase (P < 0.01). During the luteal phase, plasma LH increased significantly from a basal level of 10.0 ± 1.0 to 20.8 ± 3.0 mlU at 30 min (P < 0.001) and remained significantly elevated at 90 min. Similarly, plasma PRL increased significantly from a basal level of 11.0 ± 0.7 to 16.2 ± 2.7 ng/ml at 30 min (P < 0.025), but decreased by 90 min to 12.5 ± 1.5 ng/ml. Plasma FSH did not change after naloxone treatment. Our results suggest that en-dogenous opiates have a prominent inhibitory effect on pituitary gonadotropin and PRL secretion only during the luteal phase of the menstrual cycle.