The effects of capsaicin on the sensory neuropeptides substance P and
calcitonin gene-related peptide were analyzed in the ankle joints and
dorsal root ganglia (L2-L6) of adult female Lewis rats. The study incl
uded 23 normal rats and 23 arthritic rats, all injected subcutaneously
with capsaicin (total dose 200 mg/kg bw). Another two groups of anima
ls from a previous study, i.e.,, 23 normal rats and 23 arthritic rats
not given capsaicin served as controls. Adjuvant arthritis was induced
by inoculation with heat-killed mycobacteria. The morphological distr
ibution of sensory neuropeptides was assessed by immunohistochemistry
and the tissue concentrations were determined by radioimmunoassay. In
normal rats, capsaicin significantly reduced the concentrations of sub
stance P and calcitonin gene-related peptide in ankle joints (54 respe
ctively) as well as dorsal root ganglia (40 and 54%, respectively). In
arthritic rats those pretreated with capsaicin had significantly lowe
r concentrations of substance P and calcitonin gene-related peptide in
dorsal root ganglia (19 and 42%, respectively) compared to the arthri
tic controls. In the ankle joints, however, only the SP concentration
was reduced (42%). Notably, this was accompanied by a 40% reduction in
inflammatory response as assessed by comparing the ankle joint weight
s of the experimental groups. In general, there was a good correlation
between the neuropeptide concentrations in ipsilateral ankle joints a
nd the corresponding dorsal root ganglia as assessed in individual rat
s. The present study of adjuvant induced arthritis shows that capsaici
n administration reduces the otherwise up-regulated levels of sensory
neuropeptides in dorsal root ganglia and ankle joints. However, capsai
cin at the dose given can only mitigate, not completely prevent the de
velopment of joint inflammation. Nonetheless, the findings suggest tha
t antineuronal therapy targeted against specific neurotransmitters may
prove useful in inflammatory joint disease.