NEUROENDOCRINE CORRELATES OF CHANGES IN BRAIN ACTIVITY THRESHOLDS BY SEX STEROIDS AND PITUITARY HORMONES1

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Abstract
A neurophysiological study has been made in rabbits of the effects of sex steroids and pituitary and placental hormones on sex behavior and pituitary function as related to changes in thresholds of two opposing cerebral systems: the EEG arousal threshold involving the brain stem reticular formation (I), and the EEG after-reaction threshold involving the rhinencephalon and hypothalamus (II). In the estrous or estrogen-primed female rabbit, progesterone at first lowers both thresholds for a few hours and subsequently elevates them to supranormal levels until withdrawal or rebound brings them down again. The early phase is related to estrus and a lowered threshold of pituitary activation, whereas the elevated thresholds correlate with anestrus and pituitary inhibition. Testosterone, certain pituitary and placental hormones, seasonal influences and even prolonged treatment with female sex steroids affect the two thresholds differentially. Relating the effects on thresholds to behavior and to known thresholds of pituitary activation, one may conclude that changes in I are more closely related to sexual behavior, whereas alterations in II are correlated with pituitary thresholds for the release of ovulating hormone. It is con-cluded that both the facilitatory and inhibitory effects of sex steroids on pituitary activation, as well as the influences on behavior, are mediated by means of altered thresholds of cerebral activity.