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Opioid Inhibition of Luteinizing Hormone Secretion during the Postpartum Period in Suckled Beef Cows¹

University of Georgia, Athens 30602

Abstract

Recent studies have shown that naloxone (N), an opioid antagonist, increases concentrations of luteinizing hormone (LH) in the postpartum anestrous beef cow. However, the LH response to N was influenced by the postpartum interval. For example, a significant LH response to 200 mg of N occurred on d 42 but not on d 14 or 28 postpartum. The present study was conducted to determine the effect of different doses of N on LH secretion during the postpartum period of beef cows. Twelve cows were given 200, 400 or 800 mg of N on d 14, 28 and 42 postpartum in a Latin square design with repeat measures within cells. On d 14, serum concentrations of LH increased (P<.01) from .5 ± .1 ng/ml (mean ± SE) before N to a peak of 2.0 ± .5 and 1.4 ± .5 ng/ml for cows given 400 and 800 mg of N, respectively. In contrast, 200 mg of N had no effect on serum concentrations of LH. On d 28 and 42 all three doses of N elevated (P<.01) serum concentrations of LH. Therefore, a larger dose of N was required to increase serum concentrations of LH on d 14 postpartum compared with d 28 and 42. Based on these data we suggest that endogenous opioids participate in the regulation of LH secretion in the early postpartum period. The differential response to naloxone may be due to changes in endogenous opioid inhibition of LH secretion during the postpartum period.

¹ This research was conducted with State, Hatch and USDA funds. LH for iodination was donated by Dr. D. J. Bolt USDA-ARS, Beltsville, MD. The authors thank Dr. G. B. Rampacek and Dr. J. W. Crim, Univ. of Georgia and Dr. R. R. Kraeling, USDA-ARS for their advice in conducting this research.

² Dept. of Anim. and Dairy Sci., College of Agr.

³ Dept. of Physiol and Pharmacol., College of Vet. Med., Univ. of Georgia, Athens, GA 30602.

⁴ USDA-ARS, Russell Res. Center, Athens, GA 30613.