CHARACTERIZATION OF THERMAL HYPERALGESIA, C-FOS EXPRESSION, AND ALTERATIONS IN NEUROPEPTIDES AFTER MECHANICAL IRRITATION OF THE DORSAL-ROOTGANGLION

Study Design. This study analyzed hypersensitization in sensory system s after mechanical irritation of the dorsal root ganglion. Objectives. To develop a reliable and reproducible animal model of hyperalgesia a rising from the dorsal root ganglion and to understand the unique cont ributions of the dorsal root ganglion to clinical manifestations of sc iatica. Summary of Background Data. The dorsal root ganglion likely pl ays an important role in disorders of sciatica. However, no previous s tudy has analyzed sciatica after irritation of the dorsal root ganglio n. Thermal hyperalgesia indicates a decrease in thermal nociceptive th reshold and hypersensitization in sensory systems. Methods. The left L 4 and L5 dorsal root ganglia in rats (n = 22) were exposed circumferen tially. Other rats (n = 22) also had the left L4 and L5 dorsal root ga nglia ligated loosely with two 4-0 chromic gut sutures. Changes in the rmal withdrawal latency were examined in the hindpaws across time. Sub stance P and vasoactive intestinal polypeptide contents were quantifie d 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 i n myelinated nerve content were examined in the dorsal root ganglion. Results. Thermal hyperalgesia occurred in rats with exposure of the do rsal root ganglion and in rats with loose ligation of the dorsal root ganglion, and was accompanied by an increase in c-fos expression and s pontaneous pain-related behaviors. Conclusions. This experimental mode l reliably produced a disorder resembling an acute phase sciatica and should help further advance the understanding of pathomechanisms of sp inal pain after irritation of the dorsal root ganglion in humans.