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Influence of the Ovary and Suckling on Luteinizing Hormone Response to Naloxone in Postpartum Beef Cows¹

University of Georgia, Athens 30602

Abstract

The influence of the suckling stimulus and ovarian secretions on LH response to naloxone was studied in 16 postpartum anestrous beef cows that were assigned randomly to one of four groups (n = 4/group): intact suckled (IS), intact nonsuckled (IN), ovariectomized suckled (OS) or ovariectomized nonsuckled (ON). Ovariectomy (OS + ON) and calf removal (IN + ON) were performed on d 2, 3 or 4 after parturition. Jugular venous blood was collected at 15-min intervals for 4 h before and 4 h after administration of naloxone (1 mg/kg BW, i.v.) on d 14 and d 28 after parturition. Gonadotropin-releasing hormone (5 µg, i.v.) was given 3 h after naloxone. Both IN and OS increased (P < .05) mean pretreatment LH above IS values (mean ± SE, ng/ml; IS 1.6 ± .1 vs IN 2.5 ± .3 and OS 2.7 ± .4; P < .01), whereas ON increased (P < .01) LH (3.7 ± .3 ng/ml) even further. Mean LH increased (P < .05) after naloxone administration in all treatment groups. However, magnitude of this response was variable and dependent on ovarian status. Amplitude of the naloxone-induced LH response was greater (P < .05) for ovariectomized (5.9 ± 1.1 ng/ml) than for intact groups (2.7 ± .5 ng/ml). Gonadotropin-releasing hormone increased mean LH concentrations in all groups. We suggest that ovarian secretions and the suckling stimulus contribute to endogenous opioid inhibition of LH during the postpartum interval. However, the ability of naloxone to increase LH in ovariectomized, nonsuckled cows indicates that opioid inhibition of LH secretion was not entirely dependent on the ovary and suckling.

¹ This research was conducted with State, Hatch and USDA funds and USDA Grant #85-CRCR-M854. LH for iodination was donated by D. J. Bolt, USDA-ARS, Beltsville, MD. The authors thank R. R. Tackett, J. W. Crim, Univ. of Georgia and R. R. Kraeling, USDA-ARS, for their advice in conducting this research.

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

³ Dept. of Physiol. and Pharmacol., College of Vet Med.