Capsaicin-sensitive afferents mediate chronic cold, but not mechanical, allodynia-like behavior in spinally injured rats.

Pain-like responses to cold or innocuous mechanical stimuli were observed chronically in rats after spinal cord ischemia. These resembled the symptoms of mechanical allodynia and cold hyperalgesia that are frequently observed in spinally injured patients. We evaluated the involvement of capsaicin-sensitive afferents in mediating these responses. A single subcutaneous injection of resiniferatoxin (RTX), an ultrapotent capsaicin analogue, produced hypoalgesia to noxious heat stimulus and normalized the enhanced response to cold stimulus. In contrast, the mechanical allodynia-like response was not influenced by RTX. Thus, the enhanced response to cold, but not light touch, is mediated by capsaicin-sensitive afferents. Capsaicin and related compounds may have therapeutic potential in treating neuropathic pain elicited by some, but not all, modalities of stimulation.