Adjacent neuropathic hyperalgesia in rats: A model for sympathetic independent pain

Following sciatic nerve section in rats a saphenous nerve mediated adjacent neuropathic hyperalgesia (ANH) develops in response to pressure. This study demonstrates that a similar ANH to heat stimuli develops over the medial dorsum of the hindpaw (MDH) following sciatic section. A similar neuropathic hyperalgesia, with an associated pain syndrome, is seen in man following peripheral nerve section. When chronic bilateral ANH rats were unilaterally injected intradermally over the MDH with noradrenaline (NA), and with vehicle over the contralateral MDH, no side to side pressure or heat withdrawal threshold differences were observed. Likewise, no changes in pressure or heat withdrawal thresholds occurred in chronic ANH rats following chemical sympathectomy with 6-hydroxydopamine (6-OHDA). A similar pattern was seen in control rats treated with NA and 6-OHDA. The pressure and heat ANH seen in the rat was not dependent on sympathetic outflow, and thus provides an animal model for sympathetic independent pain (SIP). Sympathetic modulation was also ineffective on control rat nociceptive thresholds, which is in agreement with prior behavioral findings (Nature, 323 (1986) 158–160).