Drug sensitivities of the primary afferents and their changes after inflammation

This study explored the possibility that altered sensitivities of the dorsal root and dorsal root ganglion to neuroactive substances released in inflamed tissue may be involved in radicular pain. The chemical sensitivities of the dorsal root and ganglion of rats were examined by monitoring nerve membrane potential. Endogenous pain inducing substances such as bradykinin, serotonin, acetylcholine, and histamine caused depolarizations of the dorsal root and the ganglion. Application of bradykinin or capsaicin to the dorsal root and ganglion on the isolated spinal cord preparation evoked spinal reflex activities in the lumbar ventral root. These results suggest that, when pain inducing substances are released at the dorsal root or its ganglion, they may initiate action potentials and cause pain. As an inflammation model, chromic gut was tied loosely around the lower lumbar nerve root. The dorsal root of the surgically treated rats showed an increased sensitivity to bradykinin when compared with sham operated rats. In contrast, the sensitivity of the dorsal root to gamma-aminobutyric acid, a major inhibitory transmitter in the spinal cord, was decreased. This result suggests that these reciprocal changes in the sensitivities of the dorsal root may play an important role in the pathogenesis of chemical radiculitis.