Electrical low-frequency stimulation (LFS) of nociceptive afferents evokes long-term depression (LTD) of nociception and pain. Whereas in-vitro studies in spinal cord slices indicate homosynaptic LTD in rodents, spatial organization of LTD in man still awaits further investigation. This study in man addresses the hypothesis that electrically induced LTD is restricted to the peripheral cutaneous receptive field where LFS is applied.

Twenty-one psychophysical experiments were performed in seven healthy volunteers. Painful electrical test stimulation (0.05 Hz, 10 pulses, 3-fold pain threshold (I_P)) and LFS (1 Hz, 1200 pulses, 3xI_P) were applied by a multiarray electrode to the left forearm. This custom-made electrode consisted of 6 rows with 4 pin electrodes each. Cathode pin electrodes were connected to one anode (7.5 x 10 cm²) that was fixed on the opposite side of the forearm. Interspaces between pin electrodes were 1.5 cm each, resulting in an electrode size of 4.5 x 7.5 cm². In order to operate pin electrodes separately each row was energized by a separate stimulator that powers 4 pin electrodes. Cutaneous receptive field (RF) size on the forearm was detected by a two-point-discrimination test applying dividers with sharp tips. The multiarray electrode was placed in such a way that it covered the center of one RF (cRF), its peripheral edge (pRF), and an area outside the corresponding RF (oRF). In order to investigate the influence of LFS application on pain perception test stimulation was applied before and after LFS. Test stimulation was always performed within cRF whereas LFS was applied to electrodes placed in cRF, pRF, or oRF within three different experimental sessions. Test and conditioning stimulation were applied to 4 pin electrodes arranged in a squared design throughout all sessions. Volunteers rated pain perception according to a verbal rating scale (0 to 100).

Pain ratings of test stimulation series significantly decreased after LFS application to cRF (-27.3%, p<0.01). A significant reduction of pain perception was also demonstrated after LFS to pRF (-23.3%, p<0.05). In contrast, LFS application to oRF did not induce any significant changes in test stimulation rating.