Long term depression of human nociceptive skin senses induced by thin fibre stimulation.

We have recently shown that stimulation, through a multi-electrode array, of thin nerve fibres close to the dermo-epidermal junction in the skin, produces powerful inhibition of itch and, to a lesser degree, cutaneous pain in humans. Here, we have studied the induction time and frequency dependency (range 1-10Hz) of the inhibitory effects of such stimulation on itch, mechanical, and thermal pain, in 20 healthy subjects. Sixteen electrodes applied on the skin were consecutively stimulated using a method termed cutaneous field stimulation (CFS). The results show that different treatment periods with CFS were required for the induction of significant inhibitory effects on different nociceptive qualities: 1st heat pain (1 min), itch (3 min), 2nd heat pain (6 min), pinch evoked pain (8 min). Six to ten minutes stimulation sufficed to induce peak inhibitory effects on all these sensory qualities while longer stimulation (up to 40 min) did not cause significantly stronger inhibition. The effects on itch, 1st and 2nd heat pain lasted over 55 min after termination of CFS. There was no effect on prickle. No significant difference in inhibitory effects of different stimulation frequencies (1, 4 and 10Hz/electrode) was found. The induction time and effective stimulation frequencies may suggest that the underlying mechanisms are similar to those of long term depression (LTD) previously described in the spinal cord in animal experiments.