Non-noxious A fiber afferent input enhances capsaicin-induced mechanical hyperalgesia in the rat

Intradermal injection of capsaicin induces primary hyperalgesia at the inje ction site and secondary hyperalgesia in the surrounding undamaged skin. Th e secondary hyperalgesia is thought to be due to central sensitization of t he dorsal horn neurons while primary hyperalgesia is caused by sensitizatio n of nociceptors in the damaged skin. In this study, we asked if additional non-noxious afferent input from the undamaged skin influences the already developed secondary hyperalgesia, which follows an intradermal injection of capsaicin. Capsaicin dissolved in olive oil was injected into the middle o f the hind paw of male Sprague-Dawley rats (250-300 a) under gaseous anesth esia. This produced a decrease in the mechanical threshold at the base of t he toes for hind limb withdrawals lasting for 1-2 h, thus showing a short-l asting (hours) secondary hyperalgesia. When the capsaicin injection was imm ediately followed by repeated nonnoxious mechanical stimuli or weak electri cal stimuli (A fiber strength) applied to the area of secondary hyperalgesi a (toes) for 30 min, the reduction of the mechanical threshold lasted longe r than 24 h. These results suggest that non-noxious A fiber afferent input can powerfully modulate central sensitization in the spinal dorsal hom, cau sing the duration of the secondary hyperalgesia to be greatly extended. (C) 2001 International Association for the Study of Pain. Published by Elsevie r Science B.V. All rights reserved.