Effects of heterotopic conditioning stimuli on first and second pain: a psychophysical evaluation in humans.

Psychophysical experiments were carried out on 7 human participants to determine the extent to which experimentally produced first or second pain is reduced by concomitant nociceptive stimulation of body regions remote from those at which test stimuli are presented. This form of pain reduction has been termed diffuse noxious inhibitory controls (DNIC). Test stimuli used to evoke first and second pain consisted of intense electrical pulses delivered to the ankle area by subepithelial electrodes. Conditioning stimuli were 10 sec thermal stimuli (43, 47, 51 degrees C) applied to 1 cm2 areas of skin of either the dorsal surface of the contralateral foot or the abdominal region. The perceived magnitudes of first and second pains evoked by test stimuli were rated on visual analogue scales (VAS). Frankly noxious conditioning thermal stimuli (47 and 51 degrees C), but not innocuous thermal stimuli (43 degrees C), applied to the contralateral foot or abdomen reliably inhibited both first and second pain. However, the degree of inhibition was significantly greater for second pain that for first pain. In addition, the inhibitory effects did not outlast the duration of the conditioning stimulus. All of these results closely parallel electrophysiological observations about DNIC in primates. Since the extent of reduction of first pain is relatively weak and the durations of all inhibitory effects are very brief, it is unlikely that DNIC subserves the functions of relieving pain or providing a mechanism of coding pain. The spatial and temporal pattern of DNIC indicates that it may be a phenomenon associated more with the organization and production of withdrawal reflexes than with the relief of pain or pain coding.