Characterization of primary sensory neurons mediating static and dynamic allodynia in rat chronic constriction injury model

Mechanical allodynia, such as static and dynamic allodynia, is a prominent feature of neuropathic pain syndromes. The aim of this study is to characterize primary sensory neurons mediating the mechanical allodynia in a rat chronic constriction injury (CCI) model with a combination of pharmacological and histological investigations. N-(4-Tertiarybutylphenyl)-4-(3-chloropyridin-2-yl) tetrahydropyrazine-1(2H)-carbox-amide (BCTC), a selective and competitive antagonist of the vanilloid receptor 1 (TRPV1), and resiniferatoxin, which causes desensitization of TRPV1-expressing fibres, suppressed static allodynia but not dynamic allodynia in CCI rats. Immunohistochemical studies of TRPV1 and NF200, an A-fibre marker 200 kDa neurofilament, in dorsal root ganglion neurons demonstrated that each 48% of the positive-stained neurons were immunoreactive only for TRPV1 or NF200. The other 4% of stained neurons were double-positive for TRPV1 and NF200. Of the TRPV1-positive neurons, more than 99% were small-(diameter <25 mu m) and medium- (25-45 mu m) sized. In contrast, 97% of NF200 single-labelled neurons were medium- and large- (>45 mu m) sized. These findings suggest that two types of mechanical allodynia are transmitted by different primary sensory neurons: static allodynia is mediated by TRPV1 positive small- and medium-sized neurons and dynamic allodynia might be signalled by TRPV1-negative medium- and large-sized neurons.