Difference in Luteinizing Hormone Response to an Opioid Antagonist in Beef Heifers and Cows1

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Abstract
In three experiments, we examined endogenous opioid inhibition of luteinizing hormone (LH) secretion during the bovine estrous cycle. An increase in serum LH in response to the opioid antagonist naloxone (Na; 1 mg/kg i.v.) was the criterion for opioid inhibition. Estrous cycles were synchronized via prostaglandin administration. In Experiment 1, mean serum LH was not different during the luteal phase in yearling heifers (n = 6/group) at Hour 1 after Nal (2.1 ng/ml) compared to controls (1.8 ng/ml). However, LH peak amplitude was increased (p < 0.05) in the Na1 compared to the control group. Serum LH was increased (p < 0.01) during the follicular phase in heifers at Hour 1 post-Na1 compared to controls (4.7 and 3.5 ng/ml, respectively). Again, NaI administration was followed by increased (p < 0.05) LH pulse amplitude compared to control. In Experiment 2, no effect of Nal upon serum LH was detected in cows (n = 9) during proestrus, metestrus, midluteal and late luteal portions of the estrous cycle. In Experiment 3, the LH response to Nal was examined simultaneously in yearling heifers and cows (n = 5/group) during the luteal and follicular phases. Serum LH increased (p < 0.001) during Hour 1 post-Nal in heifers compared to cows during the follicular (3.4 vs. 1.7 ng/ml) but not during the luteal phase. LH pulse amplitude also increased (p < 0.05) during the Hour 1 post-Nal in heifers compared to cows during the luteal (2.5 vs. 1.1 ng/l) and follicular (2.5 vs. 1.3 ng/ml) phases. Therefore opioid inhibition of LH secretion was demonstrated during the estrous cycle in yearling heifers but not in cows.