Effects of Sex Hormones and Antifertility Steroids on Brain Thresholds in the Rabbit

Abstract

A study of the effects of antifertility norsteroids and progesterone derivatives on brain function in the rabbit reveals that in ovulation-blocking dosages both types of steroid elevate the threshold of an electrically stimulated cerebral response called the EEG [electroencephalogram] afterreaction. This response includes phases of slow wave sleep and paradoxical sleep. In anti-ovulatory dosages the norsteroids fail to influence the threshold of a second response known as EEG arousal The natural and synthetic progesterone derivatives generally synergize with estrogen to exert Diphasic influences on both responses, an initial depression followed by an elevation in threshold. The long-acting derivatives prolong both phases of the biphasic response. In the absence of estrogen, only the second or elevated phase is activated by the progesterone derivatives. In most instances the EEG arousal threshold changes are more closely related to estrous behavior than to blockade of pituitary-gonad function, whereas the EEG afterreaction threshold changes indicate that the antifertility steroids exert at least part of their ovulation-blocking effect on central nervous control of the adeno-hypophysis.