Transient analgesia evoked by noxious stimulus offset.

Pain has long been thought to wax and wane in relative proportion to fluctuations in the intensity of noxious stimuli. Dynamic aspects of nociceptive processing, however, remain poorly characterized. Here we show that small decreases (+/-1-3 degrees C) in noxious stimulus temperatures (47-50 degrees C) evoked changes in perceived pain intensity that were as much as 271% greater than those of equal magnitude increases. These decreases in perceived pain intensity were sufficiently large to be indistinguishable from those evoked by 15 degrees C decreases to clearly innocuous levels. Furthermore, decreases in pain ratings following noxious stimulus offset were significantly greater than those occurring during adaptation to constant temperature stimuli. Together, these findings indicate that an analgesic mechanism is activated during noxious stimulus offset. This analgesic phenomenon may serve as a temporal contrast enhancement mechanism to amplify awareness of stimulus offset and to reinforce escape behaviors. Disruption of this mechanism may contribute importantly to chronic pain.