Studies on the feedback regulation of pulsatile luteinizing hormone secretion: effect of ovarian steroid implantation into the forebrain and basal hypothalamus on pulsatile LH secretion in ovariectomized rats

Abstract

Recent studies have demonstrated that ovarian steroids are involved in the regulation of pulsatile LH secretion. In order to identify the site of action of ovarian steroids in modulating pulsatile LH secretion, the effect of local administration of estradiol benzoate (EB) or progesterone (P) into various brain regions on the characteristic of LH pulses was investigated in ovariectomized rats. Female rats of the Wistar strain were ovariectomized about 3-4 weeks before the experiment. Blood samples were obtained at 6-min intervals for 4h without anesthesia through the indwelling atrial catheter. The steroid was implanted into the brain via the chronically-implanted cannula 1h after the initiation of the bleeding. Serum LH concentrations were determined by radioimmunoassay. The following results were obtained. Pulsatile LH secretion occurred at intervals of approximately 20-30 min and mean LH pulse amplitude was 4.85-5.27 ng/ml h in intact ovariectomized rats. Implantation of cholesterol, as a control, in various brain areas did not induce any changes in the pattern of pulsatile LH secretion. Implantation of EB into the preoptic suprachiasmatic area (POSC) rapidly decreased the mean serum concentration of LH within 1h as compared to the pre-implantation value. The LH pulse frequency, but not the amplitude, was also decreased rapidly and significantly within 1h after EB was implanted into the POSC. In rats with EB implanted into the diagonal band of Broca (DBB), LH pulse frequency began to decrease with 2h, followed by a decline in the mean LH concentration within 3h after the implantation. Implantation of EB into the medial part of the amygdala (m-AMYG) decreased the pulse frequency within 2h, and lowered the average LH level within 3h. The mean amplitude of LH pulses did not change after the implantation. Mean LH concentrations and LH pulse amplitudes began to decrease 1-2h after EB was implanted into the medial basal hypothalamus (MBH), whereas there was no change in the pulse frequency. Rats with the EB implant in the bed nucleus of the stria terminaris, the medial preoptic area, the medial septal nucleus or the anterior hypothalamic area did not show any changes in either the amplitude or the frequency of pulsatile LH secretion. Implantation of P into the DBB, POSC or MBH of ovariectomized rats did not induce any significant change in the pattern of pulsatile LH secretion. These results suggest that the sites of estradiol action in modulating characteristics of pulsatile Lh secretion are not widespread but rather concentrated within the specific brain regions.(ABSTRACT TRUNCATED AT 400 WORDS)