Ovariectomy Increases in vivo Luteinizing Hormone-Releasing Hormone Release in Pubertal, but not Prepubertal, Female Rhesus Monkeys

Abstract

In pubertal, but not prepubertal, monkeys ovariectomy (OVX) results in an elevation of circulating luteinizing hormone (LH) levels. To determine if the castration-induced LH increase in pubertal monkeys is due to an increase in pulsatile LH-releasing hormone (LHRH) release, effects of OVX on in vivo LHRH release in the stalk-median eminence were examined in fully conscious monkeys using a push-pull perfusion method. The average ages (+/- SEM) of female rhesus monkeys in each group at OVX were 14.5 +/- 0.6 months (n = 6; prepubertal), 25.0 +/- 1.3 months (n = 5; early pubertal) and 37.8 +/- 2.1 months (n = 6; midpubertal). Perfusate samples from the stalk-median eminence were obtained in 10-min fractions for 6 h in the morning (0600 to 1200 h) and 6 h in the evening (1800 to 2400 h), from the same subjects before OVX, and at 29 days and approximately 100 days after OVX. LHRH levels in perfusates were measured by radioimmunoassay. LH levels throughout the experiment were monitored by periodic blood sampling. OVX resulted in a significant LH increase in early and midpubertal monkeys (P < 0.001 for both), but not in prepubertal monkeys. Similarly, OVX in early and midpubertal monkeys increased mean LHRH release when examined 29 days after surgery (P < 0.05 and P < 0.01, respectively). The OVX-induced LHRH increases in early and midpubertal monkeys remained elevated at approximately 100 days postcastration. Furthermore, it was found that effects of OVX on the increased LHRH release were primarily due to the elevation of basal release and pulse amplitude, but not pulse frequency. In contrast, OVX did not cause any significant effects on pulsatile LHRH release in prepubertal monkeys. The results indicate that an increase in LHRH release and a concomitant increase in circulating LH occurs after OVX in pubertal monkeys, but not in prepubertal monkeys. These data are consistent with the hypothesis that the low level of LH in circulation before the onset of puberty is due to a low amount of LHRH release which is independent of ovarian steroid feedback and that the maturity of the neuronal control system for the pulsatile LHRH release is responsible for the onset of puberty. After the onset of puberty, the negative feedback of ovarian steroid hormones becomes important to the regulation of gonadotropin release.