Effects of diffuse noxious inhibitory controls (DNICs) on the sensory-discriminative dimension of pain perception.

We have recently demonstrated that humans report heat stimuli as less painful when presented concurrently with a second noxious stimulus applied to another part of the body. Previous neurophysiological studies have shown that similar heterotopically applied noxious stimuli selectively and completely inhibit the activity of wide-dynamic-range (WDR) neurons in the dorsal horn - a phenomenon termed diffuse noxious inhibitory controls (DNICs). Taken together, these 2 lines of evidence suggest that activation of WDR cells may be necessary for normal perception of pain. Recent studies in the behaving monkey have additionally shown that WDR neurons respond to small changes in noxious heat stimuli better than do high threshold neurons, thus indicating a more specific role for WDR neurons in sensory-discriminative aspects of pain perception. If DNICs do indeed selectively and completely inhibit the activity of WDR neurons, then a heterotopically applied noxious stimulus should selectively interfere with a subject's ability to discriminate noxious stimuli. This hypothesis was tested using a noxious heat discrimination task and a cold water (5 degrees C) diffuse noxious stimulus. We found that the ability to detect small changes (0.4-0.8 degrees C) in painful heat stimuli applied to the face decreases when the person's hand is submerged in painfully cold water (P = 0.005) and that this effect persists, to a lesser extent, after the hand is removed from water. Control tasks, using visual stimuli, demonstrated that the modulation of nociceptive discrimination was not a generalized effect on sensory perception; other control measures indicated that the results could not be attributed to distraction, fatigue or changes in response bias.(ABSTRACT TRUNCATED AT 250 WORDS)

Diffuse noxious inhibitory controls (DNICs): psychophysical evidence in man for intersegmental suppression of noxious heat perception by cold pressor pain.

Counterirritation, the phenomenon of one painful stimulus reducing pain caused by a second noxious stimulus, has been recognized clinically for decades. Recently a physiological mechanism to explain counterirritation was described and termed diffuse noxious inhibitory controls (DNICs). Nevertheless, few psychophysical studies have examined systematically the effects of a noxious conditioning stimulus on pain perception. The present study examined the perception of painful heat stimuli on the face before, during and after the subject submerged a hand in painfully cold water (5 degrees C) for 5 min (cold pressor pain). We found that the subjects' ratings of the heat stimuli were significantly, although not completely, reduced during the cold pressor pain; this attenuation of pain perception continued after the noxious conditioning stimulus was withdrawn. Similarly, the pain threshold was significantly increased from 45.7 degrees C to 47.3 degrees C while the hand was in cold water and this threshold remained elevated after the cold water was terminated. Since DNICs have been found to completely and selectively inhibit the activity of only one type of pain transmission neuron (wide dynamic range), our data suggest that these neurons are involved in the perception of pain intensity. However, the persistence of residual pain perception in the presence of noxious conditioning stimuli indicates the importance of other nociceptive pathways.