Nociceptive responsiveness during slow-wave sleep and waking in the rat

Brainstem neurons that are thought to modulate pain are reported to have state-dependent discharge rates. Yet, the effect of behavioral state upon nociceptive transmission has not been well studied. Therefore, we examined responses to noxious thermal stimulation of the rat hindpaw presented during different behavioral states. Noxious thermal stimuli were applied to rats as they spontaneously cycled through waking and sleeping states. Two different methods of heating the paw - a focused light bulb ("radiant heat") and a CO2 laser ("laser heat")-were employed. Regardless of the heating method used, rats withdrew from noxious thermal stimulation when it was applied in each behavioral state tested. When rats were tested with radiant heat, the withdrawal latency from noxious heat was shorter during slow-wave sleep than during waking. In contrast, when tested with laser heat, there was no difference in either the response latency or magnitude evoked by noxious heat across sleep/wake states. Despite the fact that rats withdrew from noxious heat (using either method of application) applied during sleep, the rats quickly returned to sleep afterwards. The latency to sleep after noxious stimulation was significantly greater during waking than during sleeping. The behavioral response to noxious thermal stimulation includes both an initial motor withdrawal which is enhanced during sleep and arousal or alerting which is suppressed during sleep. Therefore, pain evokes at least two distinct reactions that are differentially modulated across sleep/wake cycles.