Effect of a long-lasting opioid receptor antagonist (naltrexone) on pulsatile LH release in early postpartum Holstein dairy cows

The present trial was the first one to investigate the effect of an intrave nous injection of naltrexone, an opioid receptor antagonist that has a long er duration of action than that of naloxone, on the LH pulse in early postp artum Holstein dairy cows. On Day 10 postpartum, blood samples were collect ed from cows at 10-min intervals for a period of 4 h before (pre-injection period) and a period of 5 h after (post-injection period) an intravenous in jection of 10 mt of saline (Control Group, n=5) or 300 mg of naltrexone in 10 mt of saline (Naltrexone Group, n=5). The plasma LH level was assayed by double antibody radioimmunoassay. The number of LH peaks per 1 h, the mean LH level, and the amplitude of LH peaks were analyzed utilizing the Pul sa r al sori thm, and data were compared by repeated measures ANOVA. No differ ences were observed in the parameters of LH pulse in the pre-injection peri od between the Control and the Naltrexone Groups (P>0.10). In the Naltrexon e Group, the number of LH peaks per 1 h and the mean LH level were signific antly higher in the postinjection period than in the pre-injection period ( 0.85 +/- 0.29 vs. 1.24 P<0.05, and 1.81 +/- 0.70 vs. 2.47 +/- 0.92 ng/ml, P <0.05, respectively), but there was no significant increase in the amplitud e of LH peaks (1.48 +/- 0.64 vs. 1.83 +/- 0.82 ng/ml, P>0.10). In constast, all of the parameters of LH pulse remained unchanged in the Control Group (P>0.10). These results suggested that an intravenous injection of naltrexo ne activates the LH pulse. (C) 2000 by Elsevier Science Inc.