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The Journal of Neurophysiology Vol. 87 No. 2 February 2002, pp. 721-731
Copyright ©2002 by the American Physiological Society
- and C-Fibers Innervating the Plantar
Rat Hindpaw One Day After an Incision
1Department of Anesthesia and 2Department of Pharmacology, College of Medicine, The University of Iowa, Iowa City, Iowa 52242
Pogatzki, Esther M.,
G.
F. Gebhart, and
Timothy J. Brennan.
Characterization of A- and C-Fibers Innervating the Plantar
Rat Hindpaw One Day After an Incision. J. Neurophysiol. 87: 721-731, 2002. Primary hyperalgesia after
tissue injury is suggested to result from sensitization of primary
afferent fibers, but sensitization to mechanical stimuli has been
difficult to demonstrate. In the companion study, sensitization of
mechano-responsive A
- and C-fibers did not explain pain behaviors 45 min after an incision in the rat hindpaw. In the present study, we
examined mechanical response properties of A- and C-fibers
innervating the glabrous skin of the plantar hindpaw in rats 1 day
after an incision or sham procedure. In behavioral experiments, median
withdrawal thresholds to von Frey filaments were reduced from 522 mN
before to 61 mN 2 and 20 h after incision; median withdrawal
thresholds after sham procedure were stable (522 mN). Responses to a
nonpunctate mechanical stimulus were increased after incision. In
neurophysiological experiments in these same rats, 67 single afferent
fibers were characterized from the left tibial nerve 1 day after sham
procedure (n = 39) or incision (n = 28); electrical stimulation was used as the search stimulus to identify
a representative population of A- and C-fibers. In the incision
group, 11 fibers (39%) had spontaneous activity with frequencies
ranging from 0.03 to 39.3 imp/s; none were present in the sham group.
The median response threshold of A-fibers was less in the incision
(56 mN, n = 13) compared with sham (251 mN,
n = 26) group, mainly because the proportion of
mechanically insensitive afferents (MIAs) was less (8 vs. 54% after
sham procedure). Median C-fiber response thresholds were similar in
incised (28 mN, n = 15) and sham rats (56 mN,
n = 13). Responsiveness to monofilaments was
significantly enhanced in A-fibers 1 day after incision; stimulus
response functions of C-fibers after incision and after sham procedure
did not differ significantly. Only A-fibers but not C-fibers
sensitized to the nonpunctate mechanical stimulus. The size of
receptive fields was increased in A- and C-fibers 1 day after
incision. The results indicate that sensitization of A- and C-fibers
is apparent 1 day after incision. Because sensitization of afferent
fibers to mechanical stimuli correlated with behavioral results,
sensitization may contribute to the reduced withdrawal threshold after
incision. Spontaneous activity in A- and C-fibers may account for
nonevoked pain behavior and may also contribute to mechanical
hyperalgesia by amplifying responses centrally.
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