Effect of Naloxone on Serum Luteinizing Hormone, Cortisol and Prolactin Concentrations in Anestrous Beef Cows1

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Abstract
Two experiments were conducted with the opioid antagonist naloxone to determine the effect of opioid receptor blockade on hormone secretion in postpartum beef cows. In Exp. 1, nine anestrous postpartum beef cows were used to measure the effect of naloxone on serum luteinizing hormone (LH), Cortisol and prolactin concentrations. Cows received either saline (n = 4) or 200 mg naloxone in saline (n = 5) iv. Blood samples were collected at 15-min intervals for 2 h before and after naloxone administration. Serum LH concentrations increased (P<.01) in naloxone-treated cows from 1.8 ± .04 ng/ml before treatment to 3.9 ± .7 ng/ml and 4.2 ± .5 ng/ml at 15 and 30 min, respectively, after naloxone administration. In contrast, LH remained unchanged in saline-treated cows (1.6 ± .3 ng/ml). Serum Cortisol and prolactin concentrations were not different between groups. In Exp. 2, 12 anestrous postpartum beef cows were used to examine the influence of days postpartum on the serum LH response to naloxone. Four cows each at 14 ± 1.2, 28 ± .3 and 42 ± 1.5 d postpartum received 200 mg of naloxone in saline iv. Blood samples were taken as in the previous experiment. A second dose of naloxone was administered 2 h after the first, and blood samples were collected for a further 2 h. Serum LH concentrations increased (P<.01) only in cows at 42 d postpartum. Serum LH concentrations in cows at 42 d postpartum increased from 2.1 ± .4 to 3.6 ± .6 ng/ml and from 2.2 ± .3 to 3.4 ± .5 ng/ml after the first and second administrations of naloxone, respectively. These results suggest that endogenous opioids influence LH secretion in the postpartum beef cow, and that the ability of naloxone to increase serum LH concentrations changes during the postpartum period. Copyright © 1986. American Society of Animal Science. Copyright 1986 by American Society of Animal Science

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