The GnRH pulse generator

Abstract

The notion of an oscillator or signal generator in the central nervous system that controls the rhythmic release of GnRH and, thereby, the pulsatile secretion of the gonadotropic hormones, originated in the finding of strikingly abrupt and rhythmic fluctuations in the concentration of LH in the plasma of ovariectomized monkeys. These oscillations had a period of about one hour when blood samples were obtained at 10 to 20 minute intervals. I These surprising observations were presaged by reports of seemingly random, major fluctuations in plasma gonadotropin concentrations in gonadectomized and rats1 as well as in women.' In these earlier studies the sampling intervals employed were never less than one hour and unable, therefore, to reveal the orderly, rhythmic events that occur with frequencies of one event per hour, or more. In the original study describing pulsatile LH secretion in ovariectomized monkeys the possibility was considered that the circhoral discharges of LH may be initiated by autoregulatory mechanisms involving long or short feedback loops, but the more likely view that these discharges were due to intermittent signals from the central nervous system unrelated to circulating LH levels that resulted in putative increments in GnRH release was favored. Nevertheless, the possible role of autoregulatory mechanisms in the control of pulsatile LH secretion was repeatedly considered,6 but finally laid to rest in the relatively recent past.7 Conversely, the view that each pulse of LH released from the pituitary gland is the consequence of a bolus of GnRH secreted into the pituitary portal system has been unequivocally upheld by the demonstration of synchronous increments ofGnRH assessed in the pituitary portal circulation and of LH measured in samples of peripheral blood obtained simultaneously.s, 9 In any case, the phenomenon of pulsatile gonadotropic hormone secretion was rapidly extended to most vertebrate species studied in this regard, including our own, and