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 transectio n of the sciatic nerve has been shown to induce sprouting of A-fiber termin als into lamina II of the spinal dorsal horn that predominantly receives C- fiber input. It has been proposed that this type of spinal reorganisation m ight underly enhanced pain sensitivity adjacent to an injured nerve. In the present study we determined whether capsaicin treatment as well as transec tion of the sciatic nerve actually produce hyperalgesia and allodynia to me chanical stimulation in the saphenous nerve area. Following transection of the sciatic nerve, the threshold for the hindlimb withdrawal induced by mec hanical stimulation of the ipsilateral saphenous nerve area was significant ly decreased. However, following topical capsaicin treatment of the sciatic nerve no change was observed in the withdrawal threshold induced by mechan ical stimulation of the saphenous nerve area. This difference in behavioral findings between rats with transection and capsaicin-treatment of the scia tic nerve suggests that the anatomical reorganisation of the A-fiber inputs to innervate nociceptive neurons of the spinal dorsal horn lamina II may n ot be sufficient for the development of secondary hyperalgesia to mechanica l stimulation or that capsaicin is less effective in producing a functional ly significant reorganization of the spinal dorsal horn than nerve transect ion.