Naloxone antagonizes the morphine suppression of the oestradiol-induced surge of luteinising hormone in dairy cows

Abstract

The study was designed to evaluate the effects of exogenous opiates on the release of luteinising hormone (LH) surge in response to oestradiol. Four intact and three ovariectomized cows released a normal LH surge after oestradiol benzoate alone. Four intact cows given morphine plus naloxone intravenously, 17 h after oestradiol benzoate, i.e. just before the expected time of onset of the LH surge, produced a normal LH surge. However, two of the four cows given morphine alone, 17 h after oestradiol, had no LH surge; one cow had a reduced response (peak value 21.6 ng/ml), the fourth had a normal LH surge. In the two latter cows, the surge of LH had either already started (LH values 7.7 and 10.3 ng/ml) before the morphine treatment, or was scheduled to occur after the effect of morphine was over, and remained unaffected or partially affected. In all other cows the LH values were basal (2.7 to 5.7 ng/ml) at this time. Similarly, an LH surge which started before or just after the morphine treatment 17 h after oestradiol in the three ovariectomized cows was not blocked but had a reduced amplitude. It is concluded that morphine can block, or suppress, the LH surge depending upon the time of treatment in relation to onset of the surge. This, and the antagonism of the effect by naloxone, supports the suggestion that in cattle the LH surge can be effected by the opioidergic interference, which might also be involved in aberrations of LH release in stressful situations that reduce fertility.