Hemiovariectomized rats 30–31 days of age were implanted with FSH in the medial basal hypothalamic region (MBH) or the adenohypophysis, and the hemicastrated controls received LH, LtH or cocoa butter in the MBH. They then were injected sc daily for 10 days with estradiol benzoate (Eb) 0.1 μg, or with oil. At the 11th day post implantation in rats bearing MBH-LtH or LH implants or no implant, Eb treatment prevented the compensatory ovarian hypertrophy (COH) normally observed in oil-treated controls (p <0.01). Eb-treated rats with an MBH-FSH implant showed COH similar to oil-treated controls. Rats with intrapituitary FSH and Eb treatment had significant COH reduction as compared to MBH-FSH-Eb rats (p <0.01). Some rats receiving ip 25 μCi of 17Î-6, 73H-estradiol the 11th day post implantation were killed 1 hr later, and the hypothalamic uptake of 3H-estradiol was determined. Animals bearing MBH-FSH implants had significantly lower 3H-estradiol uptake than hemiovariectomized rats without implants or with MBH-cocoa butter implants (p <0.01). It seems that FSH in immature rats lacks a negative feedback effect for its own regulation. The results support our concept that FSH has a positive feedback action in the female rat and suggest that FSH may compete with estrogen for the same hypothalamic receptor.
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