NEURAL ACTIVATION DURING ACUTE CAPSAICIN-EVOKED PAIN AND ALLODYNIA ASSESSED WITH PET

The PET (H2O)-O-15-bolus method was used to image regional brain activ ity in normal human subjects during intense pain induced by intraderma l injection of capsaicin and during post-capsaicin mechanical allodyni a (the perception of pain from a normally non-painful stimulus), Image s of regional cerebral blood flow were acquired during six conditions: (i) rest; (ii) light brushing of the forearm; (iii) forearm intraderm al injection of capsaicin, (iv) and (v) the waning phases of capsaicin pain; and (vi) allodynia, Allodynia was produced by light brushing ad jacent to the capsaicin injection site after ongoing pain from the cap saicin injection had completely subsided. Capsaicin treatment produced activation in many discrete brain regions which we classified as subs erving four main functions: sensation-perception (primary somatosensor y cortex, thalamus and insula); attention (anterior cingulate cortex); descending pain control (periaqueductal grey); and an extensive netwo rk related to sensory-motor integration (supplementary motor cortex, b ilateral putamen and insula, anterior lobe and vermis of the cerebellu m and superior colliculus), Comparison of the noxious and non-noxious stimuli yielded several new insights into neural organization of pain and tactile sensations. Capsaicin pain, which had no concomitant tacti le component, produced little or no activation in secondary somatosens ory cortex (SII), whereas light brushing produced a prominent activati on of SII, suggesting a differential sensitivity of Sn: to tactile ver sus painful stimuli. The cerebellar vermis was strongly activated by c apsaicin, whereas light brush and experimental allodynia produced litt le or no activation, suggesting a selective association with C-fibre s timulation and nociceptive second-order spinal neurons. The experiment al allodynia activated a network that partially overlapped those activ ated by both pain and light brush alone. Unlike capsaicin-induced pain , allodynia was characterized by bilateral activation of inferior pref rontal cortex, suggesting that prefrontal responses to pain are contex t dependent.