Neural activity in the VMH associated with suppression of the circulatory system in rats

Spontaneous neural activity within the ventromedial nucleus of the hypothalamus (VMH) was monitored in rats to search for neurons regulating the autonomic nervous system. By means of multiple unit activity (MUA) recording method, unique explosive rises in neural activity (MUA volleys), 1 to 4 min in duration, were recorded in conscious freely moving animals. Heart rate was monitored as an autonomic parameter and found to decrease when MUA volleys appeared. These MUA volleys also occurred under urethane anesthesia, and blood pressure and heart rate decreased simultaneously with the volleys, but body temperature remained constant. This fall in blood pressure (but not heart rate) was replicated by electrical stimulation through the electrodes that recorded MUA volleys, suggesting that the neurons responsible for MUA volleys can suppress the circulatory system. The frequency of MUA volleys exhibited a clear diurnal variation: they appeared every 15 or 30 min in the light phase but only seldom in the dark. This diurnal variation seems to be an endogenous circadian rhythm because it was indicated to freerun after blinding the animals. These results suggest that there is a discrete population of neurons in the VMH that fires predominantly during the light phase in an episodic manner and suppresses the circulatory system.