BIOLOGY OF REPRODUCFION 41, 431-437 (1989) 431 Difference in Luteinizing Hormone Response to an Opioid Antagonist in Beet Heiters and Cows1 A. I. MAHMOUD,3’4 F. N. THOMPSON,2’4 D. D. PECK,4 K. M. MIZINGA,4 L. S. LESHIN,5 L. A. RUND,5 J. A. STUEDEMANN,6 and T. E. KISER5 Department of Physiology and Pharmacology4 College of Veterinary Medicine University of Georgia Athens, Georgia 30602 Department of Animal and Dairy Science5 College of Agriculture University of Georgia, Athens, Georgia 30602 and Agricultural Research Service6 USDA Wa:kinsville, Georgia 30677 ABSTRACT In three experiments, we examined endogenous opioid inhibition of luteinizing hormone (LII) secretion during the bovine estrous cycle. An increase in serum LII 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 I, mean serum LH was not different during the lzaeal phase in yearling he(fers (n = 6/group) at Hour) after Nal (2.1 ag/mi) compared to controls (1.8 ng/m!). However, UI peak amplitude was increased (p<O.O5) in the Na! compared to the control group. Serum UI was increased (p<O.Ol) during the follicular phase in he(fers at Hour I post-Na! compared to controls (4.7 and 3.5 ng/ml, respectively). Again, Na! administration was followed by increased (p<0.OS) LH pulse amplitude compared to control. In Experiment 2, no effect of Na! 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 UI response to Na! was exwnined simultaneously in yearling he4fers and cows (n = 5/group) during the luteal and follicular phases. Serum LII increased (p’<O.OOl) during Hour I post-Na! in heifers com- pared to cows during the follicular (3.4 vs. 1.7 ng/ml) but not during the luteal phase. LH pulse amplitude also increased (p<O.OS) during Hour 1 post-Na! in heifers compared to cows during the luteal (2.5 vs. 1.1 ag/I) and follicular (2.5 vs. 13 ng/ml) phases. Therefore opioid inhibition of UI secretion was demonstrated during the estrous cycle in yearling heifers but not in cows. INTRODUCTION An interaction exists in several species between en- dogenous opioid peptides and gonadal steroids in the Accepted May 2, 1989. Received February 10, 1989. ‘Supported by funds from The University of Georgia Veterinary Medical and Agricultural Experiment Stations and USDA Grant #85-CRCR-1-1854. 2Reprmnt requests: Frederick N. Thompson, DVM, PhD., Depart. of Physiology and Pharmacology, College of Veterinary Medicine, The Univer- sity of Georgia, Athens, GA 30602. 3Present address: Department of Medical Sciences, School of Veterinary medicine. University of Wisconsin, Madison, WI 53706. control of luteinizing hormone (LH) secretion. In wom- en, naloxone (Na), an opioid antagonist, increases LH secretion during the late follicular and midluteal phases of the menstrual cycle (Quigley and Yen, 1980), whereas in monkeys Nal increases LH secretion during the luteal phase of the menstrual cycle (Van Vught et al., 1983; Orstead et a!., 1987). Similarly, guts respond to Na! with increased serum LH concentrations only during the luteal phase (Barb et a!., 1986). Nal given to rats results in increased LH secretion throughout the cycle (Piva et a!., 1985; Pelraglia et a!., 1986). Al- though an LH response to Na! is absent in ovariecto- Downloaded from https://academic.oup.com/biolreprod/article-abstract/41/3/431/2930716 by guest on 23 May 2020