Evaluations of inhibitory effect on the motor cortex by cutaneous pain via application of capsaicin

To evaluation the effect of experimental tonic cutaneous pain on motor cortex excitability by means of transcranial magnetic stimulation (TMS). Thirty healthy adults (male: 12, female: 18) were tested with TMS. Cutaneous pain was induced by topical application of capsaicin cream (0.025%) on the skin overlying the flexor carpi radialis of dominant limb. And the cream had been under an occlusive dressing for 40 minutes until we removed it. Magnetic stimulation was performed to the contralateral cortex with a circular coil at 80% intensity of maximum stimulator output. MEPs were recorded at flexor carpi radialis (FCR), extensor carpi radialis (ECR) and opponens pollicis (OP) with constant isometric contraction of 10%, 30% maximal voluntary contraction (MVC). Main outcome measures the intensity of pain by visual analog scale (VAS). The latency, amplitude and silent period of motor evoked potentials (MEPs). Values at baseline and 20, 40, 60, 80 minutes from applying capsaicin were compared. VAS score reached the peak value at 40 minutes. Amplitudes of MEPs at FCR progressively decreased up to 40 minutes than returned to nearly baseline value at 80 minutes. The decrease in MEP amplitude at FCR was accompanied by the increase in VAS score significantly and sustained for 20 min after washing out capsaicin cream. At ECR & OP, we could not demonstrate any changes in amplitude with time. Silent period at FCR only was significantly prolonged with the increase in VAS score. The present findings support that noxious cutaneous stimulation inhibit motor cortex excitability by so-called cortico-cortical circuits.