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 intravenous injection of naltrexone, an opioid receptor antagonist that has a longer duration of action than that of naloxone, on the LH pulse in early postpartum Holstein dairy cows. On Day 10 postpartum, blood samples were collected 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 injection of 10 mL of saline (Control Group, n=5) or 300 mg of naltrexone in 10 mL 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 Pulsar algorithm, and data were compared by repeated measures ANOVA. No differences were observed in the parameters of LH pulse in the pre-injection period between the Control and the Naltrexone Groups (P>0.10). In the Naltrexone Group, the number of LH peaks per 1 h and the mean LH level were significantly higher in the post-injection period than in the pre-injection period (0.85 +/- 0.29 vs. 1.24 +/- 0.17, 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 amplitude of LH peaks (1.48 +/- 0.64 vs. 1.83 +/- 0.82 ng/ml, P>0.10). In contrast, all of the parameters of LH pulse remained unchanged in the Control Group (P>0.10). These results suggested that an intravenous injection of naltrexone activates the LH pulse.