Transection but not topical treatment of the sciatic nerve with capsaicin induces secondary hyperalgesia to mechanical stimulation in the saphenous nerve area of the rat

Topical treatment of the sciatic nerve with capsaicin as well as transection of the sciatic nerve has been shown to induce sprouting of A-fiber terminals into lamina II of the spinal dorsal horn that predominantly receives C-fiber input. It has been proposed that this type of spinal reorganisation might underly enhanced pain sensitivity adjacent to an injured nerve. In the present study we determined whether capsaicin treatment as well as transection of the sciatic nerve actually produce hyperalgesia and allodynia to mechanical stimulation in the saphenous nerve area. Following transection of the sciatic nerve, the threshold for the hindlimb withdrawal induced by mechanical stimulation of the ipsilateral saphenous nerve area was significantly decreased. However, following topical capsaicin treatment of the sciatic nerve no change was observed in the withdrawal threshold induced by mechanical stimulation of the saphenous nerve area. This difference in behavioral findings between rats with transection and capsaicin-treatment of the sciatic nerve suggests that the anatomical reorganisation of the A-fiber inputs to innervate nociceptive neurons of the spinal dorsal horn lamina II may not be sufficient for the development of secondary hyperalgesia to mechanical stimulation or that capsaicin is less effective in producing a functionally significant reorganization of the spinal dorsal horn than nerve transection.