DOPAMINERGIC STRUCTURES IN THE OVINE HYPOTHALAMUS MEDIATING ESTRADIOL NEGATIVE FEEDBACK IN ANESTROUS EWES

This study examined the role of two dopaminergic (DA) cell groups, the A-14 and A-15 DA groups, in the seasonal shift in the response of LH to estradiol negative feedback in ewes. Radiofrequency lesions were placed bilaterally, in the area of the A-15 or the ventromedial A-14 cell groups of ovariectomized ewes, while control animals underwent sham neurosurgery. The effect of estrogen was tested in anestrus by analyzing LH pulse patterns before and 3 and 10 days after the insertion of estradiol implants. To evaluate the effects of these lesions on DA inhibition of LH secretion, LH pulse patterns were compared before and after an iv injection of the DA antagonist pimozide on day 3 of estradiol treatment. LH pulses were also examined in these ewes during the breeding season before and 3 days after the insertion of estradiol implants. Also, the effect of the DA receptor agonist apomorphine was tested to determine any effect of lesions on DA receptors inhibitory to LH. Lesions in either the A-14 or A-15 area decreased, but did not completely abolish, estradiol inhibition of LH pulse frequency in anestrus. Both types of lesions also blocked the stimulatory effects of pimozide on LH pulse frequency in estradiol-treated ovariectomized anestrous ewes. During the breeding season, estrogen decreased LH pulse amplitude, but not frequency, in all groups. The DA receptor agonist apomorphine decreased LH pulse frequency in all groups. Furthermore, immunohistochemistry for tyrosine hydroxylase revealed catecholaminergic fibers apparently connecting the caudal A-14 and the rostral A-15 areas. These results suggest that both the A-14 and A-15 DA cell groups are involved in the inhibition of LH by estradiol in anestrous, but not breeding season, ewes. Seasonal shifts in the activity of these DA neurons may, thus, play a role in the annual reproductive cycle of the ewe.