Effects of diffuse noxious inhibitory controls on temporal summation of the RIII reflex in humans

The aim of this study was to investigate the effects of diffuse noxious inhibitory controls (DNICs) on the temporal summation of the nociceptive flexion reflex (RIII reflex) in humans. Recordings were obtained from 36 healthy adults (16 M, 20 F), and the area and temporal summation threshold (TST) of the RIII reflex were measured. The subjective intensity of the painful sensation was rated on an 11-point visual analogue scale (VAS). Neurophysiological and VAS measurements were recorded after activation of DNICs by means of the cold pressor test (CPT), which involved immersing the hand in cold water (2-4 degrees C). A slight significant lower TST was found in the females versus the males. In all the subjects, the CPT induced a significant TST increase and RIII area reduction compared with the control session. The VAS results paralleled those of the RIII reflex area and TST. During the CPT, a significant difference in the percentage TST increase emerged between females and males, being lower in the former. Similarly, we found a significantly lower percentage reduction of the RIII area in women than in men during the CPT. To summarize, activation of DNICs through the CPT significantly increased the TST of the RIII reflex in healthy subjects. This inhibitory effect was gender-specific. Whereas other findings are based on psychophysical evaluations, the results of this experimental study provide an objective neurophysiological demonstration that DNICs attenuate temporal summation in humans and confirm the presence of significant differences in pain modulation mechanisms between men and women.