We investigated pain evoked activity in the human secondary sensory cortex (SII) following clonidine administration in six healthy volunteers using multi-channel magnetoencephalography (MEG). Pain was elicited by electrical shocks applied intracutaneously to the fingertip. Subjects rated pain intensity and perceptions of tiredness and passiveness by numerical ranking scales. Each subject underwent two investigations, one week apart from each other, with clonidine doses of 1.5 or 3.0microg/kg, administered intravenously in a random order and double-blinded. We applied a total number of seven blocks, each consisting of 60 painful stimuli, with one adaptation block, one pre-medication block, four post-medication blocks and one recovery block at the end of the session. MEG data were analysed by dipole reconstruction using CURRY(R) (Neuroscan, Hamburg) software package. Cortical activity in the contralateral SII cortex appeared with peak latencies of 118.5+/-10ms. This activity was significantly reduced by clonidine, in parallel with a reduction of pain intensity and enhancement of subjective tiredness and passiveness. There was, however, no significant correlation between MEG and subjective effects. Although both clonidine doses had similar effects, the higher dose induced longer changes. Results indicate that intravenous clonidine is able to relieve pain, but the exact mechanism of clonidine at the level of the SII cortex remains unclear. It is possible that clonidine interacts with the brainstem ascending system regulating vigilance and arousal which would explain the observed decrement of pain induced activity in SII. An additional more specific analgesic action at spinal level cannot be excluded.
