Thermogenesis elicited by skin cooling in anaesthetized rats: lack of contribution of the cerebral cortex.

Non-noxious cooling stimuli were delivered to the shaved back of urethane-chloralose-anaesthetized, artificially ventilated rats using a plastic bag containing water at 24-40 degrees C. Cooling of the skin by 2-6 degrees C increased the rate of whole body oxygen consumption (.V(O(2)) and triggered electromyographic (EMG) activity recorded from the neck or femoral muscles. The cooling-induced (.V(O(2)) responses did not depend on core (colonic) temperature and followed skin temperature in a graded manner. Pretreatment with the beta-blocker propranolol (10 mg kg(-1), i.v.) greatly attenuated the (.V(O(2)) response but did not affect the EMG response. On the other hand, pretreatment with the muscle relaxant pancuronium bromide (2 mg kg(-1), i.v.) affected the (.V(O(2)) response very slightly but completely abolished the EMG activity. Accordingly, the cooling stimulus activated mainly non-shivering thermogenesis. Next, the contribution of the cerebral cortex to the cooling-induced thermogenesis was examined. Power spectral analysis of the electroencephalogram (EEG) showed that the cooling stimulus largely inhibited delta (0.5-3 Hz) waves, enhanced theta (3-8 Hz) waves, and slightly increased frequencies higher than 8 Hz. Pinching the hindpaw elicited changes in EEG similar to those elicited by skin cooling but did not increase the (.V(O(2)). Therefore, there was no relationship between changes in the EEG and the magnitude of thermogenesis. Finally, skin cooling increased the (.V(O(2)) of decorticated rats but did not increase that of decerebrated rats. The results suggest that the subcortical forebrain structure, but not cortical activation, is indispensable for non-shivering thermogenesis elicited by cooling stimulation of the skin.