Evidence for an increased opioid inhibition of LH secretion in hyperprolactinaemic ovariectomized rats

The effects of acute and sub-chronic hyperprolactinaemia on the positive feedback action of progesterone in oestrogen-primed ovariectomized rats have been compared. A single injection of ovine prolactin administered with progesterone had no effect on the LH surge measured 5 h later although hyperprolactinaemia induced by 5-day treatment with the dopamine antagonist, domperidone, markedly attenuated the surge. Repeated injections of naloxone (5 mg/kg) during the development of the progesterone-stimulated LH surge completely reversed this inhibitory effect of hyperprolactinaemia, but had no apparent effect on the positive feedback action in control animals. In oestrogen-primed animals similar treatment with naloxone (0.4 and 5 mg/kg) stimulated LH secretion but the increase was significantly smaller than that observed after injecting progesterone. It is suggested that hyperprolactinaemia increases the inhibitory opioid modulation of LH release and that this effect is responsible for the impairment of the positive feedback action of progesterone.