Homotopic and heterotopic effects of endogenous analgesia in healthy volunteers

Although research on DNIC has revealed the inhibitory effect occurring between two remote pain stimuli, the interrelation between two adjacent painful stimuli has not yet been characterized. In the present study, we used a sample of 40 healthy volunteers to examine the effect of 30-s immersion of the fingers in water of 1°C, as a conditioning stimulus, on pain intensities produced by conditioned mechanical punctuate stimuli, applied both adjacent and contralateral to the cooled area. There was a significant decrease in mechanical pain intensities from 17.23 ± 2.39 at baseline to 12.45 ± 2.39 when stimulating immediately after the cold immersion at an adjacent site, and from 20.00 ± 2.39 to 15.08 ± 2.39 at remote sites (F = 20.02, p < 0.0001). A significant positive correlation between the extent of pain reduction in the cooled and in the uncooled hand was found (r_s = 0.59, p = 0.0001). The extent of pain reduction following cooling in the cooled and in the uncooled hand was also found to be similar for males and for females (p = 0.63). It is concluded that under the conditions of this experiment, EA affects heterotopic and homotopic regions similarly and without gender differences.