Regulation of Plasma LH by Estradiol and Progesterone in Ovariectomized Mares1

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Abstract
Twelve long term ovariectomized (OVX) mares were randomized into 4 groups which received the following treatments s.c. daily (for 10 or 15 days in 2 experiments) during the breeding season (summer): estradiol-17.beta. (E2), 1 mg; progesterone (P4), 100 mg; E2, 1 mg and P4, 100 mg; vehicle, 3 ml. Blood samples were taken daily for the duration of the experiments (15 or 21 days). E2 increased (P < 0.05) plasma LH [lutropin] concentrations within 6 days. P4 decreased (P < 0.05) plasma LH and E2 plus P4 resulted in further depression of plasma LH. These effects were seen for as long as treatments were given. Plasma LH concentrations reverted to the original LH concentrations within a few days after cessation of treatment. The same 12 OVX mares were randomized into 4 groups and given the same steroid treatments during the nonbreeding season (winter). Treatments were given on days 1-16 and blood samples were collected on days 1-22. Mean plasma LH concentration for E2-treated mares increased (P < 0.05) over baseline on day 7. The LH concentration remained high thereafter, but appeared to fluctuate, until day 20 when an apparent decline began. Mean plasma LH concentration by day 22 was not different from pretreatment. Mean plasma LH concentrations for the other 3 groups remained at a low level typical of OVX mares during the nonbreeding season and no significant changes due to treatment were detected. In the OVX mares, concentrations of LH follow a seasonal pattern (low basal concentrations in winter: high basal concentrations in summer). E2 elevated the low and high basal LH concentrations, P4 depressed the high basal LH concentration and P4 plus E2 caused a greater depression. The cyclic changes in LH concentrations during the breeding season probably result from the modulation of high basal LH concentrations by ovarian steroids. In the mare, the regulation of LH secretion involves 1st, a CNS pituitary component responsible for a basal circannual rhythm of LH release, entrained to an environmental zeitgeber (most probably photoperiod) and independent of ovarian influences and 2nd, an ovarian (steroidal) component which modifies the primary LH rhythm during the breeding season.