CRITICAL ROLE OF THE CAPSAICIN-SENSITIVE NERVE-FIBERS IN THE DEVELOPMENT OF THE CAUSALGIC SYMPTOMS PRODUCED BY TRANSECTING SOME BUT NOT ALLOF THE NERVES INNERVATING THE RAT TAIL

We investigated the role of capsaicin-sensitive small diameter fibers in the development of the thermal and mechanical allodynia in a new ra t model for neuropathic pain, produced by transecting some but not all of the nerves innervating the tail. Capsaicin (50 mg/kg, s.c,) inject ed neonatally prior to the nerve injury produced thermal hypoalgesia i n the tail the degree of which was variable across individual rats, pr esumably as a result of variable degeneration of the small diameter fi bers, When subjected to the nerve injury, the animals with moderate th ermal hypoalgesia exhibited signs of pain (e.g., tail flick) to normal ly innocuous mechanical stimuli applied to the tail with von Frey hair s (4.9 mN or 19.6 mN bending force), but not to thermal stimuli given by immersion of the tail into cold (4 degrees C) or warm (40 degrees C ) water, The animals with marked thermal hypoalgesia, on the other han d, exhibited no signs of pain either to the mechanical or to the therm al stimuli, These results suggest that the capsaicin-sensitive fibers are critical in the development of both the mechanical and thermal all odynia, It is hypothesized that the destruction of A delta- and C-noci ceptive fibers by capsaicin prevented activities induced in these fibe rs by the nerve injury from producing a central sensitization and thus allodynia.