Thermal and chemical stimulations of the hypothalamic heat detectors: the effects of the EEG.

In acute immobilized rats, the effect on the EEG of thermal and chemical (capsaicin microinjection) stimulation of the warm sensors in the preoptic region, mid-hypothalamic area and posterior hypothalamus were studied. Both localized heating and capsaicin resulted in a sleep-like EEG with spindles and slow waves. Stimulation of the posterior hypothalamus was the most effective and stimulation in the mid-hypothalamus was the least effective in inducing spindle activity. Since capsaicin is regarded as a specific stimulant for the hypothalamic warm sensors, the results suggest that the EEG effect, and probably the sleep-inducing effect, of heat are mediated via the central thermoreceptors, and cannot be due to a non-specific activation of the basal forebrain hypnogenic mechanisms.