Study objective: The liver, like brown adipose tissue, is an important source of nonshivering thermogenesis. We tested the hypothesis that the thermal state of the liver is preserved during slow wave sleep but deteriorates during paradoxical sleep.
Methods: Twelve adult male Wistar rats were equipped with electrodes and thermistor probes--which measured cortical (Tco), interscapular brown adipose tissue (Tibat), and liver (Tl) temperatures--for thermal and sleep studies. Six rats were exposed to thermoneutral (24 degrees C) and 6 to low (9 degrees C) ambient temperature. Isolated paradoxical sleep episodes preceded and followed by more than 3 minutes of wakefulness or slow wave sleep were analyzed. The Tco, Tibat, and Tl levels were also assessed during the slow wave sleep that preceded each isolated paradoxical sleep episode.
Results: During slow wave sleep, the Tibat and Tl were clearly higher in rats exposed to the cold than in those exposed to the thermoneutral condition (< 1 degrees C). During paradoxical sleep, however, these 2 temperatures decreased in rats exposed to the cold condition, and the changes observed for Tco were completely inversed.
Conclusions: These results support the hypothesis that the thermal state in the liver of rats deteriorates during paradoxical sleep and suggest that nonshivering thermogenesis in the liver contributes to the defense of global thermal homeostasis in the sleeping endothermic organism, especially during slow wave sleep.