AMPLITUDE AND FREQUENCY-MODULATION OF PULSATILE LUTEINIZING-HORMONE-RELEASING HORMONE-RELEASE

1. A variety of neuroendocrine approaches has been used to characteriz e cellular mechanisms governing luteinizing hormone-releasing hormone (LHRH) pulse generation. We review recent in vivo microdialysis, in vi tro superfusion, and in situ hybridization experiments in which we tes ted the hypothesis that the amplitude and frequency of LHRH pulses are subject to independent regulation via distinct and identifiable cellu lar pathways. 2. Augmentation of LHRH pulse amplitude is proposed as a central feature of preovulatory LHRH surges, Three mechanisms are des cribed which may contribute to this increase in LHRH pulse amplitude: (a) increased LHRH gene expression, (b) augmentation of facilitatory n eurotransmission, and (c) increased responsiveness of LHRH neurons to afferent synaptic signals, Neuropeptide Y (NPY) is examined as a proto typical afferent transmitter regulating the generation of LHRH surges through the latter two mechanisms. 3. Retardation of LHRH pulse genera tor frequency is postulated to mediate negative feedback actions of go nadal hormones, Evidence supporting this hypothesis is reviewed, inclu ding results of in vivo monitoring experiments in which LHRH pulse fre quency, but not amplitude, is shown to be increased following castrati on. A role for noradrenergic neurons as intervening targets of gonadal hormone negative feedback actions is discussed. 4. Future directions for study of the LHRH pulse generator are suggested.