Myelinated afferents sprout into lamina II of L3-5 dorsal horn following chronic constriction nerve injury in rats

In order to investigate the consequences of chronic constriction injury (CCI) to nerve, we explored the relationship between the development of mechanical allodynia and the reorganization of primary afferent terminals in the sensory lamina of the rat spinal cord dorsal hem. Following sciatic CCI neuropathy, mechanical allodynia developed in the corresponding footpad within two weeks and persisted throughout the experimental period which extended for an additional two weeks. The neuropathy of the sciatic injury includes extensive Wallerian-like degeneration of myelinated fibers but relative sparing of unmyelinated fibers. We observed that there was no significant change in the dorsal horn termination of unmyelinated C fibers in lamina II of the dorsal hem, using nerve injections of wheat germ agglutin-horseradish peroxidase for transganglionic axonal tracing of these fibers from the nerve injury site, and no evidence of sprouting into adjacent lamina. In contrast, myelinated afferent fibers were observed to be sprouting into lamina II of the dorsal hem, as indicated by cholera toxin P-subunit-horseradish peroxidase retrograde axonal tracings. This region of the dorsal horn is associated with nociceptive-specific neurons that are not generally associated with myelinated fiber input from mechanical and proprioceptive receptors. As previously suggested in nerve transection and crush injuries, and now demonstrated in CCI neuropathy, these morphological changes may have significance in the pathogenesis of chronic mechanical allodynia. (C) 1999 Elsevier Science B.V. All rights reserved.