UNIT ACTIVITY IN THE HYPOTHALAMUS, SEPTUM AND PREOPTIC AREA OF THE RAT: CHARACTERISTICS OF SPONTANEOUS ACTIVITY AND THE EFFECT OF OESTROGEN

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Abstract
SUMMARY: The frequency characteristics of the neuronal activity in the hypothalamus, septum and preoptic area of the forebrain were recorded with stereotaxically orientated steel microelectrodes in adult female rats under light urethane anaesthesia. An exponential type of relationship was observed between the mean discharge rate of units and the frequency with which they were encountered in all areas. Thus, 40–60 % of units had firing rates of less than 1 spike/sec. and only 10–15% had rates exceeding 6/sec. However, units with a mean discharge rate of 2–4/sec. contributed most to the total spike activity. The form of the interspike interval distributions was found to vary with mean firing rate, and the variability of the interspike interval decreased as the firing rate of the unit increased. The levels of 'spontaneous' activity in the anterior hypothalamic, preoptic and septal areas of rats with light-induced persistent oestrus were lower than those observed after ovariectomy. The administration of 10 μg. oestradiol benzoate/day to the ovariectomized animals depressed the levels of activity towards those observed in the persistently oestrous animals. Reciprocal effects were observed in the lateral hypothalamic area, i.e. decreased activity in the ovariectomized rats. The results are discussed in relation to unit recording procedures and the feedback effects of oestrogen.