Effect of oestrogen on the responsiveness of neurones in the hypothalamus, septum and preoptic area of rats with light-induced persistent oestrus.

Abstract

SUMMARY The responses of septal, preoptic and hypothalamic neurones to pain, cold, changes in ocular illumination and probing of the cervix were recorded in adult female rats under light urethane anaesthesia. Responses in rats with light-induced persistent oestrus were compared with those obtained after ovariectomy, and after ovariectomy with oestrogen treatment. The majority of units in the lateral hypothalamic area were excited by pain, cold and cervical stimuli, whereas in the lateral septal area most were inhibited. The numbers of units in the anterior hypothalamic and preoptic areas that displayed excitation to these stimuli were approximately equal to those showing inhibition. The time-course of the responses to pain, cold and cervical stimuli of most hypothalamic and septal units closely corresponded to that of the associated EEG activation (frontal cortex), suggesting that they were non-specific arousal effects. Other responses, usually of brief duration, were not correlated with EEG changes. Endogenous and exogenous oestrogen increased the percentage of units in the lateral and anterior hypothalamic areas that were inhibited by the pain, cold and cervical stimuli, and decreased the number in the lateral septal area. Oestrogen enhanced the responsiveness of preoptic units to the cervical stimulus, but depressed their responsiveness to pain and cold. Hypothalamic units inhibited by the pain stimulus had mean 'spontaneous' firing rates of 4–5 spikes/sec. and those which were excited had rates of 1–2/sec. Light-sensitive units were found mainly in the lateral septal and anterior hypothalamic areas. The usual form of the response was a brief 'on-off' or 'off-on' discharge. The results are discussed in relation to the central nervous control of ovulation.