Brain temperature as proxy for brain state and oscillatory activity in the mouse.

Brain temperature and brain activity are in a complex, bidirectional relationship. Changes in brain temperature impact brain functioning and, conversely, brain activity generates heat. The latter can be illustrated by the characteristic changes in brain temperature that accompany the transitions between the brain states wakefulness, NREM sleep, and REM sleep. Here we show in the mouse that these typical temperature changes are sufficiently consistent to predict brain state. To gain further insight into this relationship, we quantified the effects of specific EEG activity patterns characteristic of sleep-wake states on temperature. We found that occurrences of spindles (11-15 Hz) during NREM sleep and of theta (7-9 Hz) and gamma (55-85 Hz) activity during wakefulness and REM sleep, were followed by increases in cortical temperature with a 10-14 s delay. In contrast, temperature decreased during the theta-rich cataplexy-associated state (CAS) observed in mice lacking the hypocretin gene, shedding new light on this non-physiological state. Our results show that brain temperature can be used as a reliable and accessible proxy of brain state and the accompanying oscillatory activity.