EFFECT OF NALOXONE ON SERUM LUTEINIZING-HORMONE CONCENTRATION IN ANOVULATORY HOLSTEIN COWS DURING THE EARLY POSTPARTUM PERIOD

This experiment was conducted to investigate the effect of naloxone (N AL), an opioid receptor antagonist, on pituitary LH secretion in anovu latory Holstein cows during the early postpartum period when cows were expected to be in negative net energy balance. Twenty-three cows (11 primiparous) received either saline (n = 12) or 1 mg/kg BW NAL i.v. (n = 11) on Day 14 or 15 postpartum Jugular blood samples were collected at 15-min intervals for 2 hr before and 2.5 hr after NAL or saline. A ll cows received 3 ug gonadotropin releasing hormone (GnRH) at 2.5 hr post-NAL or -saline and blood collection was continued for I hr. Mean serum progesterone concentration was 0.33 +/- 0.2 ng/ml. Mean net ener gy balance for all cows was -5.5 +/- 0.6 Mcal/day. Naloxone caused a t ransient increase (P < 0.05) in serum LH concentrations in both primi- and multiparous cows within 45 min after administration. In contrast, serum LII concentrations remained unchanged in saline-treated cows. G nRH increased (P < 0.05) LH and there was no effect of treatment. Thes e results suggest that modulation of LH secretion, at least in part, m ay be mediated via endogenous opioids in dairy cows before first postp artum ovulation. (C) Elsevier Science Inc. 1998.