Characterization of thermal hyperalgesia, c-fos expression, and alterations in neuropeptides after mechanical irritation of the dorsal root ganglion

This study analyzed hypersensitization in sensory systems after mechanical irritation of the dorsal root ganglion. To develop a reliable and reproducible animal model of hyperalgesia arising from the dorsal root ganglion and to understand the unique contributions of the dorsal root ganglion to clinical manifestations of sciatica. The dorsal root ganglion likely plays an important role in disorders of sciatica. However, no previous study has analyzed sciatica after irritation of the dorsal root ganglion. Thermal hyperalgesia indicates a decrease in thermal nociceptive threshold and hypersensitization in sensory systems. The left L4 and L5 dorsal root ganglia in rats (n = 22) were exposed circumferentially. Other rats (n = 22) also had the left L4 and L5 dorsal root ganglia ligated loosely with two 4-0 chromic gut sutures. Changes in thermal withdrawal latency were examined in the hindpaws across time. Substance P and vasoactive intestinal polypeptide contents were quantified in the dorsal root ganglion and spinal cord. Substance P, calcitonin gene-related peptide, and c-fos expression also were examined in the spinal cord by immunohistochemistry. In addition, histologic changes in myelinated nerve content were examined in the dorsal root ganglion. Thermal hyperalgesia occurred in rats with exposure of the dorsal root ganglion and in rats with loose ligation of the dorsal root ganglion, and was accompanied by an increase in c-fos expression and spontaneous pain-related behaviors. This experimental model reliably produced a disorder resembling an acute phase sciatica and should help further advance the understanding of pathomechanisms of spinal pain after irritation of the dorsal root ganglion in humans.