Sexually Dimorphic Pattern of Pain Mitigation Following Prophylactic Regenerative Peripheral Nerve Interface (RPNI) in a Rat Neuroma Model

Abstract

BACKGROUND: Treating neuroma pain is a clinical challenge. Identification of sex-specific nociceptive pathways allows a more individualized pain management. The Regenerative Peripheral Nerve Interface (RPNI) consists of a neurotized autologous free muscle using a severed peripheral nerve to provide physiological targets for the regenerating axons. OBJECTIVE: To evaluate prophylactic RPNI to prevent neuroma pain in male and female rats. METHODS: F344 rats of each sex were assigned to neuroma, prophylactic RPNI, or sham groups. Neuromas and RPNIs were created in both male and female rats. Weekly pain assessments including neuroma site pain and mechanical, cold, and thermal allodynia were performed for 8 weeks. Immunohistochemistry was used to evaluate macrophage infiltration and microglial expansion in the corresponding dorsal root ganglia and spinal cord segments. RESULTS: Prophylactic RPNI prevented neuroma pain in both sexes; however, female rats displayed delayed pain attenuation when compared with males. Cold allodynia and thermal allodynia were attenuated exclusively in males. Macrophage infiltration was mitigated in males, whereas females showed a reduced number of spinal cord microglia. CONCLUSION: Prophylactic RPNI can prevent neuroma site pain in both sexes. However, attenuation of both cold allodynia and thermal allodynia occurred in males exclusively, potentially because of their sexually dimorphic effect on pathological changes of the central nervous system.

Regenerative Peripheral Nerve Interface Surgery for the Management of Chronic Posttraumatic Neuropathic Pain

“…In RPNI studies performed in a rat model, pain was assessed with either "autotomy" (a self-mutilation rat behavior due to neuroma irritation after nerve transection) or pain withdrawal response from neuroma site tapping. [28][29][30] "Autonomy" behavior and withdrawal responses were seen in 70 to 80% of times in control rats which was significantly higher than the rates seen in rats who had underwent RPNI surgery. 28,29 Clinical Evidence Clinical studies in human patients have also demonstrated improved pain outcomes following RPNI surgery (►Fig.…”

“…[28][29][30] "Autonomy" behavior and withdrawal responses were seen in 70 to 80% of times in control rats which was significantly higher than the rates seen in rats who had underwent RPNI surgery. 28,29 Clinical Evidence Clinical studies in human patients have also demonstrated improved pain outcomes following RPNI surgery (►Fig. 2).…”

“…In rat models, histology and microscopy images of RPNI demonstrated robust neovascularization, muscle fiber regeneration, nerve axonal sprouting and myelinations, and novel neuromuscular junction formation. 14,[27][28][29] Through successful reinnervation of the muscle graft by the transected nerve, neuroma formation is effectively mitigated, and this was confirmed through histologic images and in vivo rat studies with ultrasound. 14,[27][28][29] Detailed analysis shows not only decreases in nerve stump diameter, but also decreases in axonal density and neural fibrosis at the nerve stump when comparing RPNI groups to control groups.…”

“…14,[27][28][29] Through successful reinnervation of the muscle graft by the transected nerve, neuroma formation is effectively mitigated, and this was confirmed through histologic images and in vivo rat studies with ultrasound. 14,[27][28][29] Detailed analysis shows not only decreases in nerve stump diameter, but also decreases in axonal density and neural fibrosis at the nerve stump when comparing RPNI groups to control groups. 28 Histologic staining of proximal nerve ends within the RPNI also showed more organized and regular sized regenerated nerves with less surrounding fibrous connective tissue invasion compared to nerve stumps of transected nerves without RPNI.…”

“…28 Although the full molecular mechanism of RPNI surgery is not yet fully understood, exploration into pain pathways and immune response reveals that RPNI surgery may lead to reduced expression of neuronal growth factors and receptors, decreased neuroinflammation, and attenuation of dorsal root ganglia macrophage proliferation that typically play a role in neuropathic pain with neuroma formation. 29 There is also evidence that RPNI surgery is associated with upregulation of glial cell line-derived neurotrophic factor (GDNF) in the dorsal root ganglia which is not seen in control groups. 28 Because GDNF has been found to reduce or eliminate neuropathic pain after peripheral nerve injury in previous studies and is produced mostly in skeletal muscle, it is theorized that the RPNI reinnervated muscle graft may secrete GDNF which directly decreases neuropathic pain after the protein factor is transported to the dorsal root ganglia.…”