PRE-INJURY MECHANORECEPTOR ABLATION REDUCES NOCICEPTOR-DRIVEN SPINAL CORD INJURY-INDUCED NEUROPATHIC PAIN
Evidence from previous studies supports the concept that spinal cord injury (SCI) induced neuropathic pain (NP) has its neural roots in the peripheral nervous system. There is uncertainty about how and to which degree nociceptors and mechanoreceptors contribute. Sensorimotor activation-based interventions (e.g. treadmill training) have been shown to reduce NP following experimental SCI, suggesting transmission of pain-alleviating signals through mechanoreceptors. At the same time, nociceptors have been shown to become hyperexcitable early after SCI and peptidergic axons sprout into deeper laminae of the below injury level dorsal horn. The aim of the present study is to comprehensively understand the relative contribution of each pathway in respect to NP presentation in a moderate mouse contusion SCI model. After genetic ablation of tropomyosin receptor kinase B (TrkB) expressing mechanoreceptors before SCI mechanical allodynia was reduced. The identical genetic ablation after SCI did not yield any change in pain behavior. CGRP sprouting into lamina III/IV below injury level as a consequence of SCI was not altered by either mechanoreceptor ablation. Moreover, detection of hyperexcitability in nociceptors, not in mechanoreceptors, in skin-nerve preparations of contusion SCI mice 7 days after injury makes a substantial direct contribution of mechanoreceptors to NP maintenance unlikely. SNS reporter mice allowing specific visualization of the entire nociceptor population confirmed significant sprouting of respective neurons into laminae III/IV as early as 5 days post-injury. Genetic ablation of SNS-Cre mice severely affected their overall health condition, which precluded them to undergo experimental SCI and subsequent further analysis. Complementing animal data, quantitative sensory testing in human SCI subjects indicated reduced mechanical pain thresholds, whereas the mechanical detection threshold was not altered. Taken together, early mechanoreceptor ablation modulates pain behavior, most likely through indirect mechanisms. Hyperexcitable nociceptors with consecutive peptidergic fiber sprouting in the dorsal horn are confirmed as the likely main driver of SCI-induced NP. Future studies need to focus on injury-derived factors triggering early onset nociceptor hyperexcitability, which could serve as targets for more effective therapeutic interventions.