Peripheral nerves can regenerate, and when injured may cause neuropathic pain. We propose that the active regeneration process plays a pivotal role in the maintenance of neuropathic pain. In one commonly used rodent neuropathic pain model, pronounced pain behaviors follow ligation and cutting of the L5 spinal nerve. We found that the injured nerve regenerates into the sciatic nerve, and functionally reinnervates target tissues: the regenerated nerve conducts electrical signals, mechanical responses, and tracers between the leg/hindpaw and axotomized sensory ganglion. The regenerating nerve is the primary source of abnormal spontaneous activity detected in vivo. Disrupting the regeneration inhibited pain: First, semaphorin 3A, an inhibitory axonal guidance molecule, reduced functional regeneration, spontaneous activity, and pain behaviors when applied to the injury site in vivo. Second, knockdown of the upregulated growth-associated protein 43 (GAP43) with siRNA injected into the axotomized sensory ganglion reduced pain behaviors. We next examined the spared nerve injury model, in which pain behaviors are essentially permanent. The regeneration resulted in tangled GAP43-positive neuromas at the nerve injury site without target reinnervation. Perfusing the nerve stump with semaphorin 3A, but not removal of the tangled fibers, prevented or reversed pain behaviors. This effect far outlasted the semaphorin 3A perfusion. Hence in this model the long-lasting chronic pain may reflect the anatomical inability of regenerating nerves to successfully reinnervate target tissues, resulting in an ongoing futile regeneration process. We propose that specifically targeting the regeneration process may provide effective long-lasting pain relief in patients when functional reinnervation becomes impossible.
Significance Statement After peripheral nerve injury, an active regeneration process may play a pivotal role in maintaining neuropathic pain. In two different rat neuropathic pain models, genetic or pharmacological blockade of regeneration reduced pain behaviors. In the spinal nerve injury model, functional regeneration of the ligated spinal nerve was observed, contrary to long-held assumptions. In the long-lasting spared nerve injury model, a tangled neuroma rather than effective target reinnervation was observed. In vivo perfusion of the neuroma with semaphorin 3A, an inhibitor of regeneration, also reversed established pain. The persistent neuropathic pain in this model may reflect the anatomical inability of the regenerating nerves to successfully reinnervate target tissues. Specifically targeting the regeneration process may provide long-lasting pain relief.
The authors declare no competing financial interests.
HHS | NIH | National Institute of Neurological Disorders and Stroke (NINDS) [NS045594]; HHS | NIH | National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS) [AR068989]