Naloxone infusion increases pulsatile luteinizing hormone release in postpartum beef cows

Abstract

The limiting factor in the return to cyclicity in the postpartum cow appears to be the lack of pulsatile luteinizing hormone (LH) secretion. To test the role of endogenous opioids in regulating pulsatile LH release, naloxone, an opioid antagonist, was infused into postpartum cows. Eight cows (39.3 ± 2.1 d postpartum) received either a constant infusion of saline or 50 mg/hr of naloxone dissolved in saline for eight hr. Blood samples were taken at 15 min intervals for determination of serum LH concentrations and to determine frequency and amplitude of the LH pulses. Frequency of LH pulses was greater (P<.01) in naloxone infused cows ( 3.0 ± 0.2 8 hr ) compared to saline infused cows ( 0.5 ± .1 8 hr ). Amplitude of LH pulses averaged 4.0 ± .9 ng/ml and 5.8 ± 1.3 ng/ml in the saline and naloxone groups respectively (P>.05). Mean serum LH concentrations were greater (P<.01) in the naloxone infused group (2.7 ± .3 ng/ml) compared to the saline infused group (1.9 ± .4 ng/ml). These data indicate that endogenous opioids inhibit pulsatile LH secretion in the anestrous postpartum beef cow and that naloxone infusion increases mean serum LH concentrations by increasing the frequency of LH pulses.