Lack of involvement of capsaicin-sensitive primary afferents in nerve-ligation injury induced tactile allodynia in rats.

Tactile allodynia and thermal hyperalgesia, two robust signs of neuropathic pain associated with experimental nerve injury, have been hypothesized to be mechanistically distinguished based on (a) fiber types which may be involved in the afferent input, (b) participation of spinal and supraspinal circuitry in these responses, and (c) sensitivity of these endpoints to pharmacological agents. Here, the possibility that nerve-injury induced tactile allodynia and thermal hyperalgesia may be mediated via different afferent fiber input was tested by evaluating these responses in sham-operated or nerve-injured (L5/L6) rats before or after a single systemic injection of resiniferatoxin (RTX), an ultrapotent analogue of the C-fiber specific neurotoxin, capsaicin. Tactile allodynia, and three measures of thermal nociception, tail-flick, paw-flick and hot-plate responses, were determined before and at various intervals for at least 40 days after RTX injection. Nerve-injured, but not sham-operated, rats showed a long-lasting tactile allodynia and thermal hyperalgesia (paw-flick) within 2-3 days after surgery; responses to other noxious thermal stimuli (i.e., tail-flick and hot-plate tests) did not distinguish the two groups at the stimulus intensities employed. RTX treatment resulted in a significant and long-lasting (i.e. essentially irreversible) decrease in sensitivity to thermal noxious stimuli in both sham-operated and nerve-injured rats; thermal hyperalgesia was abolished and antinociception produced by RTX. In contrast, RTX treatment did not affect the tactile allodynia seen in the same nerve-injured rats. These data support the concept that thermal hyperalgesia seen after nerve ligation, as well as noxious thermal stimuli, are likely to be mediated by capsaicin-sensitive C-fiber afferents. In contrast, nerve-injury related tactile allodynia is insensitive to RTX treatment which clearly desensitizes C-fibers and, therefore such responses are not likely to be mediated through C-fiber afferents. The hypothesis that tactile allodynia may be due to inputs from large (i.e. A beta) afferents offers a mechanistic basis for the observed insensitivity of this endpoint to intrathecal morphine in this nerve-injury model. Further, these data suggest that clinical treatment of neuropathic pains with C-fiber specific agents such as capsaicin are unlikely to offer significant therapeutic benefit against mechanical allodynia.