Daily rhythm of nociception in rats

Abstract

Abstract Background Many behavioral and physiological variables exhibit daily rhythmicity. Few investigations of the daily rhythmicity in nociception have been conducted, and conflicting results have been obtained. The present study evaluated the daily rhythmicity in nociception in Wistar rats. Methods Nociception was investigated by Eddy's hot plate method, tail immersion method, and tail clip method. The latency between the noxious stimulus and the animal's response was recorded as reaction time. Separate groups of rats were tested in 4-hour intervals for 24 hours. Results There was clear daily variation in response latency. Reaction time was shortest a few hours before lights-on and longest at the light-dark transition. Conclusion Nociception exhibits robust daily rhythmicity in rats. Sensitivity to pain is highest late in the dark phase of the light-dark cycle and lowest at the light-dark transition.