Summary
Female rats were ovariectomized at 31 days of age, on the day of proestrus, or on
the day of the first vaginal estrus if corpora lutea were seen in the ovaries. Immediately
after castration, estradiol-17β (E2) or oil was administered via s.c. silastics capsules,
or a 1:240 mixture of estradiol benzoate (EB) and cholesterol or cholesterol alone
was unilaterally implanted into the hypothalamic ventromedial-arcuate region. Forty-eight
hours after surgery the rats were decapitated and the serum concentrations of LH and
FSH estimated. In rats implanted s.c. with E2 in anestrus, proestrus or estrus, estrogen
treatment reduced the circulating LH level to 9.4; 60.4 and 33.6% and that of FSH
to 49.5; 120.3 and 63%, respectively, of the concentration recorded in the corresponding
controls implanted with oil. Following the intrahypothalamic implantation of EB, the
serum concentration of LH was lowered to 20.8; 67.4 and 68.1% and that of FSH to 48.8;
87.0 and 62.0% as compared to the cholesterol-implanted controls. The findings clearly
suggest that a major part of the change in sensitivity to the negative feedback of
estrogen occurs prior to the first preovulatory surge of gonadotrophins.
Key words
Desensitization - negative feedback - estrogen - prepubertal - rat
