Electrophysiological correlates of pulsatile gonadotropin release in rats

In order to reveal the electrophysiological correlates of pulsatile LH release in ovariectomized rats, changes in multiple unit activity (MUA) in the arcuate nucleus (ARC), the medial preoptic area, the ventromedial nucleus, the anterior hypothalamic area and other areas of the brain were recorded in lightly anesthetized female rats. Rats were ovariectomized at least 7 weeks prior to the experiment. 0.1 -ml blood samples were withdrawn from a cardiac catheter at 10-min intervals during the recording in order to measure the changes in the plasma LH concentration by radioimmunoassay. The LH concentration showed pulsatile changes under continuous intravenous infusion of thiopental sodium while cortical EEG was stable throughout the experiment. The firing rate in the middle part of the ARC (8 recordings) changed in parallel with the increase in the plasma LH concentration and was abruptly raised just prior to an increase in LH secretion. The number of MUA spike discharges per minute increased to 1.5-5 times that of the basal level. MUA started to increase 2-9 min prior to the LH rise and the high level of MUA lasted for 1-5 min. The firing rate in the ARC (14 recordings) and in other areas (54 recordings) recorded did not change periodically in parallel with the increase in LH. As these changes in MUA were localized in the ARC and were not reflected in cortical EEG, they might be specific local changes distinguished from generalized systemic effects involving the whole brain. These extreme increases in MUA seem to be due to the high activity either of neurons that secrete LH-RH or those that control the secretion of LH-RH.