PROGESTERONE, 17-BETA-ESTRADIOL, AND OPIOID NEUROPEPTIDES MODULATE PATTERN OF LUTEINIZING-HORMONE IN CIRCULATION OF THE COW

The working hypothesis for this study was that 17beta-estradiol (E2), progesterone (P4), and opioid neuropeptides modulate frequency of pulsatile release of LH during the luteal phase of the bovine estrous cycle. On Day 8 of the estrous cycle (Day 0 = estrus), 20 cows were ovariectomized (OVX) and immediately received one of three steroid replacement treatments: P4 alone (n = 6), E2 alone (n = 7), or P4 and E2 (n = 7). To characterize the pattern of LH in peripheral circulation, serial blood samples were collected (12-min intervals for 22 h) on the fifth day following OVX. LHRH was administered after the 22-h sample was taken. Naloxone (opioid neuropeptide antagonist; 0.5 mg/kg) was administered on the sixth day after OVX, and change of concentration of LH in peripheral circulation was monitored. Pulse frequency of LH and mean concentration of LH were lower (p < 0.01) in cows treated with P4 and E2 compared to cows treated with P4 alone. Moreover, cows treated with E2 alone had a greater (p < 0.01) frequency of LH pulses and mean concentration of LH compared to cows treated with P4 alone. The greatest amplitude (p < 0.01) of LH release after LHRH was in cows treated with E2 alone. The greatest percentage increase (p < 0.01) in LH after administration of naloxone occurred in cows treated with P4 alone. In conclusion, in cows treated with doses of P4 and E2 that resulted in circulating concentrations of these hormones typical of the midluteal phase of the estrous cycle, pulses of LH were fewer than in cows treated with P4 alone; in cows treated with P4 alone, pulses of LH were fewer than in cows treated with E2 alone. The responsiveness of the pituitary to LHRH is greater in cows treated with E2 alone than in cows treated with P4 alone or with the combination of P4 and E2. Endogenous opioid inhibition of LH secretion was greater in cows treated with P4 alone compared to cows treated with P4 and E2 or with E2 alone.