Cyclic Gonadotropin Release in the Presence and Absence of Estrogenic Feedback in Ovariectomized Golden Hamsters

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Abstract
Two or 3 weeks after ovariectomy, adult hamsters housed on 14 h of light/day (lights 0600-2000 h) a) received a 5 mm Silastic implant containing crystalline-estradiol-17β, b) received an empty implant or c) received no implant. Following decapitation 1, 3 or 5 days later, serum luteinizing hormone (LH) and follicle stimulating hormone (FSH) were measured by radioimmunoassay. All animals demonstrated a peak release of LH between 1300 and 1800 h. Similar results were observed in females sampled by cardiac puncture 5 weeks after ovariectomy. FSH release was more varied; a cyclic pattern was not discernable in every group. Exposure to estradiol for 1 day induced levels of LH similar to those seen during spontaneous LH release on the afternoon of proestrus. FSH in these animals was elevated 3 times over proestrous levels. Prolonged exposure to estradiol was correlated with reduction of peak levels of LH and FSH and reduction of basal FSH levels. In castrates with empty implants or with no implants, peak serum LH levels were 35-40% of those observed in normal animals during the proestrous surge. Peak serum FSH levels in these animals were 3-4 times those observed in intact proestrous hamsters. In all ovariectomized groups, pituitary levels of LH and FSH were significantly elevated over those observed in intact females sampled over a similar period on the day of proestrus. In castrates with empty implants or with no implants, pituitary LH was elevated 3-5 times and pituitary FSH 13-18 times over levels observed in intact proestrous hamsters. Exposure to estradiol for 1 day resulted in a significant increase in pituitary LH, but not FSH. Pituitary gonadotropin content diminished with prolonged exposure to estradiol, though even after 5 days, glandular content remained significantly greater than that in intact proestrous hamsters. These results suggest that an endogenous pacemaker times gonadotropin release in reproductively mature female hamsters and that estradiol acts to alter synthesis and release of LH and FSH.