Further Studies on the Regulation of FSH Secretion

In the present study we wished to determine: a) the physiological importance of the dorsal hypothalamic area (DAHA) in the regulation of the ovulatory discharge of LH/FSH, and b) whether the DAHA is involved in the negative feedback action of estradiol on pituitary FSH secretion. Large bilateral lesions were produced in the DAHA and the animals were maintained (18–20 days) until vaginal cyclicity had resumed. In such rats at proestrus, temporal patterns of LH were identical to normal controls but peak plasma concentrations were reduced. All DAHA-lesioned rats ovulated fully (8–12 eggs). In control Nembutal®-anesthetized proestrous rats which were sequentially bled for 6 h beginning at 1300 h, plasma LH/FSH remained at basal concentrations. Two other groups of Nembutal-blocked rats were either ovariectomized (OVX) or a bolus of purified rat LH was injected iv at 1300 h. In both studies, FSH rose significantly from baseline within 3 h and thereafter increased linearly for the next 3 h. Basal plasma LH levels in OVX rats were unaltered throughout the same sampling times. In other rats, Silastic capsules containing estradiol benzoate (EB) were implanted sc at the time of gonadectomy or LH injection (1300 h of proestrus) or 100 ng of 17β-estradiol was injected at 1400 and at 1500 h in a group of OVX rats. In all estrogen-treated groups, only a partial reduction in the elevated plasma FSH levels which follows OVX occurred. In a final group of rats, the DAHA was electrolytically destroyed and when such animals exhibited vaginal cyclicity, OVX was performed at 1300 h of proestrus. Increases in FSH plasma levels occurred at times similar to controls. However, if Silastic EB capsules were implanted at the time of OVX of die DAHA-lesioned animals (1300 h proestrus), no elevations in plasma FSH/LH were observed. Thus, the DAHA does not appear to be essential for spontaneous LH/FSH surges or for the negative feedback actions of estrogen to occur. In the absence of the DAHA, estrogen suppresses the plasma FSH rises which follow OVX. Seemingly, two levels of control for regulation of FSH secretion are present within the CNS, one estrogen-sensitive (preoptico-tuberal system) and the other estrogeninsensitive (DAHA).