SINGLE-UNIT COMPONENTS OF THE HYPOTHALAMIC MULTIUNIT ELECTRICAL-ACTIVITY ASSOCIATED WITH THE CENTRAL SIGNAL GENERATOR THAT DIRECTS THE PULSATILE SECRETION OF GONADOTROPIC-HORMONES

Vertebrate reproduction is dependent on the operation of a central sig nal generator that directs the episodic release of gonadotropin-releas ing hormone, a neuropeptide that stimulates secretion of the pituitary gonadotropic hormones and, thereby, controls gonadal function. The el ectrophysiological correlates of this pulse generator are characterize d by abrupt increases in hypothalamic multiunit electrical activity (M UA volleys) invariably associated with the initiation of secretory epi sodes of luteinizing hormone. Using cluster analysis, we extracted sin gle units from the multiunit signals recorded from the mediobasal hypo thalamus of four intact and four ovariectomized rhesus monkeys. Of the 40 individual units identified in this manner, 24 increased their fre quency with the MUA volleys. The onset and termination of these single -unit bursts occurred coincidently with those of the MUA volleys in bo th intact and ovariectomized animals, indicating that the longer durat ion of the MUA volleys characteristic of the gonadectomized animals wa s due not to the sequential activation of different units but to the l onger bursts of the individual cells. Four other units showed decrease s in firing rate during the MUA volleys, while the frequency of the re mainder did not change. All the examined units were active during the intervals between the volleys of electrical activity. The results indi cate that the MUA volleys associated with the activity of the gonadotr opin-releasing hormone pulse generator represent the simultaneous incr ease in firing rate of some individual hypothalamic neurons and the de crease in the frequency of others.