HORMONAL MODULATION OF HYPOTHALAMIC UNIT ACTIVITY AND EEG RESPONSE TO VAGINAL STIMULATION IN THE DEERMOUSE

SUMMARY The EEG after-reaction to vaginal stimulation occurred during oestrus and was inhibited during dioestrus or abolished by pregnancy or ovariectomy. Hypothalami of oestrous animals were more responsive to vaginal stimulation than the hypothalami of dioestrous or pregnant animals. The increase in the number of responsive neurones in oestrous animals was attributed to an increase in the number of neurones that exhibited a delayed decline in their discharge rate in response to vaginal probing. These neurones were located along the midline in the anterior and basal hypothalamus. Administration of oestrogen to ovariectomized animals was moderately successful in reinstating the delayed inhibitory response of the hypothalamic neurones to vaginal stimulation while the EEG response was almost completely restored. The importance of this hypothalamic activity is discussed in relation to the luteinizing hormone short loop feedback mechanism and the EEG after-reaction.