Journal of neurophysiology
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Peripheral nociceptor sensitization is accepted as an important mechanism of cutaneous primary hyperalgesia, but secondary hyperalgesia has been attributed to central mechanisms since evidence for sensitization of primary afferents has been lacking. In this study, microneurography was used to test for changes in sensitivity of C nociceptors in the area of secondary hyperalgesia caused by intradermal injection of capsaicin in humans. Multiple C units were recruited by electrical stimulation of the skin at 0.25 Hz and were identified as discrete series of dots in raster plots of spike latencies. ⋯ Capsaicin, injected 10-50 mm away from the site of electrical stimulation, had no effect on any of 29 CM units, but induced bursts of activity in 11 of 15 CM(i) units, after delays ranging from 0.5 to 18 min. The capsaicin injections also sensitized a majority of the CM(i) units, so that 11 of 17 developed immediate or delayed responsiveness to mechanical stimuli. This sensitization may contribute a peripheral C fiber component to secondary hyperalgesia.