Diurnal rhythm in sensitivity of a nociceptive spinal reflex

To detect possible diurnal variation in the nociceptive threshold of male Wistar rats the latency between onset of noxious radiant heat and the tail-flick response was measured in day (a.m.) and night experiments. The response latency was significant (P less than 0.001) longer, i.e. the response was less sensitive, at night in both normal and spinalized rats. This is the first indication of a diurnal rhythm in the sensitivity of a spinal reflex to noxious stimulation.