EFFECT OF LUTEAL HORMONE ON THE BEHAVIOUR OF HYPOTHALAMIC NEURONES IN PSEUDOPREGNANT RATS

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Abstract
SUMMARY: A polygraphic recording method was used to study unit activity in the hypothalamus (mainly the lateral hypothalamic area) and thalamus (mainly N. anteroventralis and N. ventralis) in adult female rats under light urethane anaesthesia. The percentage of hypothalamic neurones excited by mechanical probing of the cervix in rats, 7–11 days pseudopregnant, was only one-fourth of that previously observed in cyclic rats, and a fourfold increase was procured by removing the pseudopregnant ovaries before commencing unit recording. In the ovariectomized rat initially responsive hypothalamic neurones ceased to be excited by cervix stimuli 30 min. after the i.v. injection of 400 μg. progesterone and regained their responsiveness in 1 hr. Only small differences were recorded in the percentage of hypothalamic neurones excited by pain or cold stimuli on the tail in cyclic, pseudopregnant and ovariectomized pseudopregnant rats. There was likewise little change in the small percentage of neurones inhibited by the test stimuli in the presence or absence of pseudopregnant ovaries. The percentage of thalamic neurones excited by cervical stimuli in pseudopregnant rats was not depressed below that of ovariectomized animals. The findings are thought to indicate that exogenous or endogenous progesterone selectively depresses the excitation of hypothalamic neurones by stimuli from the genital tract.