Frequency and Amplitude of Gonadotropin-Releasing Hormone Stimulation and Gonadotropin Secretion in the Rhesus Monkey*

TLDR

In adult ovariectomized rhesus monkeys with hypothalamic lesions that suppress LH and FSH, sustained gonadotropin levels can be restored by intermittent GnRH infusions of 1 µg/min for 6 min hourly, and the physiological implications of this are discussed. The study examined how varying the frequency or amplitude of GnRH pulses affects gonadotropin secretion in these animals. Higher GnRH pulse frequencies and lower infusion rates both caused marked declines in plasma LH and FSH, with the most extreme reductions at the highest frequencies and lowest rates, while lower frequencies increased FSH relative to LH, demonstrating that frequency and amplitude of GnRH stimulation profoundly alter gonadotropin levels and FSH/LH ratios.

Abstract

In adult ovariectomized rhesus monkeys bearing hypothalamic lesions which reduced circulating LH and FSH to undetectable levels, sustained elevated gonadotropin concentrations were reestablished by the intermittent administration of gonadotropin-releasing hormone (GnRH) at the rate of 1 microgram/min for 6 min once every hour. The effects of varying either the frequency or the amplitude of these GnRH pulses on gonadotropin secretion were examined in such animals. Increasing the frequency of GnRH administration from the physiological one pulse per h to two, three, or five pulses h while maintaining a constant infusion rate and pulse duration resulted in gradual declines in plasma gonadotropin concentrations. These declines were most profound at the highest frequencies and the consequence of reduced pituitary responses to individual GnRH pulses. Decreasing the frequency of GnRH pulses from one per h to one every 3 h led to variable declines in plasma LH levels, but circulating FSH invariably rose. Reducing the GnRH infusion rate from 1 to 0.1 mg/min while maintaining constant frequency and pulse duration resulted in abrupt declines in plasma LH and FSH to immeasurable levels, although pulsatile increments in circulating GnRH concentrations without a concomitant reduction in plasma LH concentrations, which remained unchanged. An infusion rate of 0.5 microgram/min resulted in unstable plasma LH and FSH levels. These results demonstrate that changes in the frequency or amplitude of hypophysiotropic stimulation have profound effects on plasma gonadotropin levels as well as on FSH to LH ratios in the circulation. The physiological implications of these observations are discussed.