Inhibition of prolactin secretion delays extinction of circadian LH surges in ovariectomized rats bearing estradiol implants.

Abstract

Ovariectomized rats bearing Silastic capsules containing estradiol exhibit a daily afternoon surge of luteinizing hormone (LH) which decreases with time until it is undetectable by Day 10 after implantation of estradiol. Increases in basal prolactin levels as well as afternoon surges are also observed. To determine if increased prolactin secretion contributed to the extinction of the circadian LH surges, we examined the patterns of LH and prolactin secretion in rats in which prolactin was suppressed by bromocriptine treatment. In vehicle-treated control rats, the magnitude of the LH surges decreased with time. Large LH surges were observed on Days 2 and 4. A significant decrease in the surge occurred on Day 6, and it disappeared by Day 10. Animals treated with bromocriptine also exhibited large LH surges on Days 2 and 4, and in addition, secreted a greater amount of LH than the control group on Days 6, 8, and 10. In ovariectomized rats bearing estradiol implants, large afternoon surges in prolactin were observed and by Day 6, basal prolactin levels were also elevated. Bromocriptine treatment completely suppressed prolactin secretion through Day 6, but a small afternoon rise was observed on Days 8 and 10. These findings suggest that elevated prolactin secretion may be one factor contributing to the extinction of circadian LH surges in the estrogen-treated rat.