Analysis of the behavioral, cellular and molecular characteristics of pain in severe rodent spinal cord injury

Human SCI is frequently associated with chronic pain that is severe and refractory to medical therapy. Most rodent models used to assess pain outcomes in SCI apply moderate injuries to lower thoracic spinal levels, whereas the majority of human lesions are severe in degree and occur at cervical or upper thoracic levels. To better model and understand mechanisms associated with chronic pain after SCI, we subjected adult rats to T3 severe compression or complete transection lesions, and examined pain-related behaviors for three months. Within one week after injury, rats developed consistent forepaw pain-related behaviors including increased spontaneous lifts, tactile allodynia and cold sensitivity that persisted for three months. Place escape avoidance testing confirmed that withdrawal of the forepaws from a von Frey stimulus represented active pain-related aversion. Spontaneous and evoked pain-related measures were attenuated by gabapentin, further indicating that these behaviors reflect development of pain. Spinal level of injury was relevant: rats with T11 severe SCI did not exhibit forepaw pain-related behaviors. Immunoblotting and immunofluorescence of C6-C8 spinal dorsal horn, reflecting sensory innervation of the forepaw, revealed: 1) expansion of CGRP immunoreactivity in lamina I/II; 2) increased GAP-43 expression; and 3) increased IBA1, GFAP and connexin-43 expression. These findings indicate that aberrant pain fiber sprouting and gliopathy occur after severe SCI. Notably, satellite glial cells (SGCs) in C6-C8 DRGs exhibited increases in GFAP and connexin-43, suggesting ongoing peripheral sensitization. Carbenoxolone, a gap junction inhibitor, and specific peptide inhibitors of connexin-43, ameliorated established tactile allodynia after severe SCI. Collectively, severe T3 SCI successfully models persistent pain states and could constitute a useful model system for examining candidate translational pain therapies after SCI.