Chronic Central Pain after Spinal Cord Injury

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Abstract
Spinal cord injury (SCI) frequently results in dysesthesias that have remained refractory to clinical treatments despite a variety of interventions. The failure of therapeutic strategies to treat dysesthesias after SCI is due to the lack of attention given to mechanisms that elicit chronic pain following SCI. An overview of the literature with respect to the development of chronic pain in the SCI patient population will be given. In addition, a mammalian model of chronic central pain following spinal cord trauma will be presented. The model is characterized by the development of mechanical and thermal allodynia, as demonstrated by measuring the thresholds of accepted nociceptive tests, the paw withdrawal responses accompanied by changes in behavior consistent with the experience of noxious stimulis. In addition, vocalization responses that are accompanied by postural and behavioral changes consistent with the receipt of a noxious stimulus and involving supraspinal pathways are measured. Locomotor function was also tested and scored using the Basso, Beattie, and Bresnahan (BBB) open field test scale. Our data indicate that somatosensory thresholds for both mechanical and thermal stimuli that elicit paw withdrawal (flexor reflex) or vocalizations, accompanied by complex changes in behavior, are significantly different following SCI. These changes represent the development of mechanical and thermal allodynia. To determine the underlying mechanism for the altered sensory responses, we used electrophysiological techniques to determine if nociceptive dorsal horn neurons demonstrated increased excitability to peripheral stimulation as evidenced by increased responses to natural somatosensory stimuli. The data presented support the development of central sensitization of dorsal horn neurons after spinal cord hemisection. This provides a mechanism for the development of mechanical and thermal allodynia after SCI. Hypotheses that account for the development of the central pain state after SCI, as well as therapeutic interventions to ameliorate the pain state, are discussed. Key words: chronic pain; spinal cord injury; rats, nociception; wide dynamic range neurons; pain