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
Yi. Kim et al., 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, The Journal of neuroscience, 15(6), 1995, pp. 4133-4139
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.