Four experiments were conducted to investigate the effect of naloxone, an opioid
receptor antagonist, on pituitary LH secretion in Holstein cows during two periods after
parturition and two phases of the estrous cycle. In experiment 1, 24 cows (12
primiparous; 12 multiparous) received either saline (n = 12) or 1 mg/kg naloxone (n = 12)
i. v. at 14 1 days postpartum. Blood samples were collected at 15-minute intervals for
2 hours before and 2.5 hours after naloxone or saline. Serum LH concentrations
increased (P < .05) in response to naloxone injection in both primi- and multiparous
cows. Saline injection did not affect LH concentrations. In experiment 2, 27 cows (13
primiparous; 14 multiparous) received either saline (n=14) or 1 mg/kg naloxone (n=13)
i. v. at 28 ± 1 days postpartum. Blood samples were collected as in the previous
experiment. Naloxone did not affect serum LH concentrations in either primi- or
mUltiparous cows at 28 days postpartum. In experiment 3, estrous cycles were
synchronized via prostaglandin administration (25 mg) in 22 cows (10 primiparous; 12
multiparous). Cows received either saline (n=11) or 1 mg/kg naloxone (n=11) Lv.
during the luteal phase of the estrous cycle. Blood samples were collected as in the previous experiments. Luteinizing hormone concentrations were not affected by naloxone
in either primi- or mUltiparous cows during the luteal phase of the estrous cycle. In
experiment 4, the same cows used in experiment 3 received a second dose of
prostaglandin (25 mg). Thirty-six hours later, during the follicular phase of the estrous
cycle, the cows received either saline (n =9) or 1 mg/kg naloxone (n = 11) i. v. Naloxone
increased (P < .05) serum LH concentrations in both primi- and mUltiparous cows in
the follicular phase. These results suggest that LH release in the early postpartum dairy
cow is regulated, at least in part, by endogenous opioid pep tides , and the ability of
naloxone to affect LH secretion may change as days postpartum increases, perhaps due
to changes in degree of inhibition by endogenous opioid peptides, and (or) changes in
serum progesterone concentration due to onset of ovarian activity during postpartum
period. It appears that the modulation of LH secretion may be mediated via opioids
during the follicular phase of the estrous cycle. However, an opioid-mediated mechanism
for modulation of LH secretion was absent or overridden by progesterone feedback
during the luteal phase of the estrous cycle.
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