Effects of estradiol-17beta on the induction of gonadotropin release by electrical stimulation of the hypothalamus in rhesus monkeys

Abstract

Serum LH and FSH were measured at 60, 30, and 0 min before, at 5, 15, and 30 min during, and at 10, 45, and 90 min after bilateral electrical stimulation (ES) of various hypothalamic regions in 12 unanesthetized ovariectomized rhesus monkeys. ES of the arcuate-ventromedial nuclei (medial basal hypothalamus; MBH) induced a prompt increase in serum LH that persisted throughout stimulation and returned to basal levels within 90 min thereafter. FSH was also released, but the release was slower and less dramatic than that of LH. Sham stimulation (0muA) caused no change in serum gonadotropins. The amount of LH released after MBH-ES depended upon current strength (1.0 mA greater than 0,5 or 0.7 mA). Three sequential 30-min MBH-ES trials at 90-min intervals induced comparable LH responses and 3 h of continuous MBH-ES maintained elevated serum LH levels throughout the stimulation period, suggesting that these stimulation period, suggesting that these stimulation parameters did not completely deplete pituitary stores of releasable LH. The character of the LH response was similar in individual monkeys through 3 to 24 trials during 4 to 18 months. Comparisons were made of the effects of estradiol-17beta (E2) treatment at different doses and for different intervals of time before MBH-ES. ES-induced LH release was not affected by low levels (25 and 55 pg/ml) ofE2 for 48 h, but was reduced by higher E2 concentrations (100 or 230 pg/ml). E2 concentrations of 100 pg/ml had no effect at 24 h, but reduced MBH-ES-activated LH release at 48 to 96 h; the degree of depression was time-related (48 h less than 72 h less than 96 h). ES of the preoptic-suprachiasmatic region (rostral hypothalamus; RH) in non-E2-treated monkeys also released LH, but this increase was less than after MBH-ES. FSH release was not measurable after RH-ES. In contrast to the depressed LH response to MBH-ES after 48 h of E2 (100 pg/ml), the response to RH-ES was not inhibited by this E2 regimen. These data suggest that ES of an area extending caudally from the rostral hypothalamus to the arcurate-median eminence region will evoke LH release in rhesus monkeys. This electrically induced gonadotropin release was affected by administration of physiological levels of E2 but the nature of effect depended on the specific region stimulated: distinct inhibition of the gonadotropic response to MBH-ES and slight facilitation of the response to RH-ES.