Changes in the characteristics of pulsatile LH secretion after estradiol implantation into the preoptic area and the basal hypothalamus in ovariectomized rats.

Intracranial actions of estradiol benzoate (EB) in modulating the characteristics of pulsatile LH secretion were examined in ovariectomized rats. A minute amount of crystalline EB was implanted into the medial basal hypothalamus (MBH) or the preoptic suprachiasmatic area (POSC) and blood samples were collected at 6-min intervals for radioimmunoassay of serum LH concentrations. In rats with EB implanted in the MBH, the LH pulse amplitude decreased while the frequency did not change for 3 h after implantation. In rats bearing EB implants in the POSC, in contrast, the LH pulse frequency decreased without a significant variation in the pulse amplitude. Control animals with empty cannulae inserted into the MBH or POSC exhibited unchanged amplitude and frequency of LH secretory pulses. These results indicate that estrogen can regulate the amplitude and frequency of pulsatile LH secretion via mechanisms independent of each other. It appears that MBH and/or the pituitary is a site of the action of estrogen in decreasing LH pulse amplitude, while the POSC is a site of the steroid action in decreasing the pulse frequency.