UNIT ACTIVITY IN THE HYPOTHALAMUS OF THE CYCLIC FEMALE RAT: EFFECT OF GENITAL STIMULI AND PROGESTERONE

Abstract

SUMMARY Unit activity in the hypothalamus and other diencephalic regions was recorded with stereotaxically oriented steel microelectrodes in adult female rats under light urethane anaesthesia. Spontaneous firing rates of neurones varied from < 1/10 sec. to > 50/sec., but the majority fired at 1–10/sec. Some variations in the pattern of firing are described. Acceleration of firing rate was most readily induced by pain stimuli (64% of neurones) and then by cold (60%), probing the cervix (47%), smell (20%), light (5%) and noise (3%) in that order. A minority of neurones were inhibited by the stimuli. Many neurones responded to several different stimuli, most commonly by accelerating to cervical probing, cold and pain. Inhibitory convergence was also observed, e.g. blockade of the response to cervical probing by an olfactory stimulus, and inhibition by cervical probing of the response to cold or pain. The proportion of neurones excited by smell in prooestrous rats was more than double that in oestrous or dioestrous rats. Oestrous rats had relatively more neurones which were unresponsive or inhibited by the test stimuli. Slow intravenous injection of 400 μg. progesterone induced a selective depression of the response of lateral hypothalamic neurones to cervical probing. The effect was maximal at about 30 min. and full recovery occurred in 1 hr. The possible significance of these observations is discussed with particular reference to the neural control of luteotrophin secretion.