Homeostatic mechanisms in the hypothalamus

Abstract

1. The homeostatic readjustments associated with neuronal activity were studied through an analysis of local blood flow, temperature and electrical activity (EEG) in the hypothalamus of unrestrained cats with permanently implanted probes. 2. Three characteristic types of correlations among the three parameters were distinguished during the arousal response. Type 1 consisted of an increase in blood flow and temperature coinciding with EEG arousal. Type 2 occurred with prolonged, very mild reticular stimulation. During maintained EEG activation, the temperature and blood flow increased but only after a long latency. Type 3 was characterized by an immediate, transient increase in blood flow. 3. Small volumes of hypothalamic tissue were gently heated without stimulation by passing a train of square waves. This temperature rise represents physical heat production. When the hypothalamus was heated with stimulation, the hypothalamic temperature rose in two distinct steps. The first step was similar to the temperature rise due to physical heat production, the second step, representing metabolic heat production. The second temperature step was markedly depressed by small doses of pentobarbital; however, no significant changes in the first temperature step were observed. 4. It is concluded that metabolic heat production rather than thermal clearance probably determines the physiological temperature rise and the sudden-decrease type of blood flow response represents a sympathetic cerebrovascular reaction, and the occurrence of the immediate (first type) or of the delayed (second type) of response pattern depends on the rapidity of accumulation of vasodilator substances and on the rate of metabolic heat production.