Arcuate area of the female rat maintained in vitro exhibits increased afternoon electrical activity

The arcuate area of the rat hypothalamus has previously been shown to have increased neuronal activity in vivo corresponding to the estradiol-induced luteinizing hormone rise in the afternoon. The present experiments investigate the arcuate area to determine whether afternoon increases in activity occur in vitro and are dependent on estradiol pretreatment. Light-dark entrained pubertal rats were ovariectomized and implanted with estradiol- or cholesterol-filled silastic capsules. Two days later they were sacrificed at 10.00 h and coronal sections containing the arcuate area were maintained in oxygenated artificial cerebrospinal fluid at 34 degrees C. With estrogen pretreatment, units recorded from 11.00 to 15.00 h had a slow discharge with hourly mean rates ranging from 0.7 to 0.86 spikes/s. However, in the next hour the mean firing rate doubled (p less than 0.01) and an increase in units encountered per electrode penetration was observed. In tissue from animals sacrificed 3 h later, an increase in firing rate was also noted after 15.00 h, suggesting that a diurnal mechanism rather than a technical artifact was responsible for the timing of the activity increase. In control experiments with cholesterol capsules, electrical activity did not have a diurnal pattern. In summary, the arcuate area of the rat isolated in vitro was observed to display an increase in neuronal activity during the afternoon that was dependent on estrogen pretreatment and correlated to the time of day that luteinizing hormone rises are seen in intact animals. Although the arcuate area is involved in numerous functions, this electrical activity may represent interneurons controlling the release of luteinizing hormone-releasing hormone from axonal endings in the median eminence.