Effects of hormones and vaginal stimulation on the EEG and hypothalamic units in rats.

RAMIREZ, V. DOMINGO, BARRY R. KOMISARUK, DAVID I. WHITMOYER, AND CHARLES H. SAWYER. Effects of hormones and vaginal stimulation on the EEG and hypothalamic units in rat. Am. J. Physiol. 2 I 2(6) : I 376-1384. rg67.-In female rats under light urethane anesthesia, a study has been made of the effects of vaginal stimulation and intravenous injections of progesterone and luteinizing hormone (LH) on cerebral activity as represented by both the cortical EEG and hypothalamic unit firing rates. The results reveal that: z) Stimulation of the vaginal cervix during late diestrus-early proestrus induces an “afterreaction” characterized by the induction of a “sleepy” EEG and a temporally correlated effect on unit firing rates, usually involving a depression in frequency. 2) Intravenous injections of progesterone in dosages as low as IOO pg exert brief generalized anesthetic effects on EEG and unit firing rates. 3) Intravenous LH exerts a relatively specific effect on basal hypothalamic neuron firing rates independent of EEG changes, an effect consistent with the hypothesis that the LH is acting as an internal negative feedback agent on the hypothalamic neurons controlling pituitary LH secretion. The results emphasize that in studies of specific actions of hormones on individual neurons it is essential to monitor the EEG as a control for generalized influences. stimulation have been recorded in the cat by Porter et al. (I 7), the rabbit by Sawyer and Kawakami (23), and the rat by Barraclough (I). Estrogens have lowered and androgens elevated such EEG thresholds as the afterreaction and arousal thresholds to electrical stimulation of the brain in the rabbit (14) and the seizure threshold in the rat (25). Barraclough and Cross (2) have introduced unit recording methods to this type of study. With microelectrodes in the lateral hypothalamus of the rat they reported that exogenous progesterone rather specifically inhibited the unit response to vaginal stimulation. Later, Cross and Silver (7) f ound that endogenous progesterone in natural pseudopregnancy also reduced the percent of lateral hypothalamic units responding to stimulation of the vaginal cervix and that ovariectomy reversed this condition. Since the steroid did not affect thalamic units in the same manner, they suggested that progesterone might be acting specifically on the hypothalamic neurons from which the units were being recorded.