Postural and sympathetic influences on brain cooling during the ultradian wake-sleep cycle.

The influence of posture and tonic vasoconstrictor sympathetic outflow on systemic (ear pinna-environment) and selective (carotid rete-venous plexus) heat exchange underlying brain cooling was studied in cats chronically implanted with EEG and EMG electrodes, and transducers that measured hypothalamic, pontine and ear pinna temperatures across the ultradian wake-sleep cycle in a thermoneutral environment. Transmural pressure on heat exchanger vasculature was varied by keeping the animal's head above or at heart level. The vasoconstrictor sympathetic outflow to heat exchanger vasculature was varied both by keeping the animal's abdomen cool or warm and by means of bilateral common carotid ligature. The results show that a rise in transmural pressure enhances selective brain cooling and weakens systemic brain cooling. An increase in tonic vasoconstrictor sympathetic outflow decreases both systemic and selective brain cooling.