Abstract
BackgroundMany 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.MethodsNociception 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.ResultsThere was clear daily variation in response latency. Reaction time was shortest a few hours before lights-on and longest at the light-dark transition.ConclusionNociception 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.
Highlights
Many behavioral and physiological variables exhibit daily rhythmicity
Studies on rats [2] and golden hamsters [3] have indicated the occurrence of greater pain sensitivity during the dark phase of the light-dark cycle, another study on rats indicated the occurrence of greater sensitivity during the light phase [4], and a study on mice indicated the occurrence of two daily peaks in sensitivity, one during the light phase and one during the dark phase [5]
Reaction time was longest at the transition from light to darkness and shortest a few hours before the transition from darkness to light. This suggests greater pain sensitivity late in the dark phase, which is in agreement with previous studies in rats and golden hamsters [2,3]
Summary
Male albino Wistar rats were purchased from the Chellamuthu Trust, Madurai. They were housed in microlon cages maintained at 25 ± 1°C under an L12:D12 lightdark cycle. The latency between the noxious stimulus and the animal's response was recorded as reaction time. Rats previously adapted to an L12:D12 light-dark cycle were divided into 7 groups of 6 animals and tested at one of 7 times of day 4 hours apart. DFiagiluyrrehy1thmicity of nociception in rats Daily rhythmicity of nociception in rats The figure shows the daily variation in reaction time to nociceptive stimulation. The horizontal bar at the top indicates the timing of the light-dark cycle
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