The role of ionotropic glutamate receptors in chronic central pain after spinal cord injury

Spinal cord injury (SCI) results in both loss of function and chronic central pain syndromes. In the clinical population, pain is characterized based on anatomical location: 1) Below-level pain – located at dermatomes corresponding to spinal segments caudal to the injury site, 2) At-level pain – located at dermatomes corresponding to spinal segments immediately adjacent to the injury site, 3) Above-level pain – located at segments rostral to the injury site (in area of sensory preservation). \r\n A contusion model of SCI was first characterized behaviorally and electrophysiologically. A contusion at spinal segment T10 at 150 kdynes of force and a 1 second dwell time resulted in the pain like behavior in the hindlimbs, thoracic region, and forelimbs 35 days post-injury. These contused animals exhibited spinal hyperexcitability during extracellular single-unit electrophysiological spinal recordings from the dorsal horn of the lumbar enlargement (below-level), thoracic cord (at-level; immediately rostral to injury site), and brachial enlargement (above level). \r\n In models of peripheral injury, increased ionotropic glutamate receptor mediated activity results in spinal central sensitization. Extracellular single-unit recordings from all three regions of the spinal cord (lumbar enlargement, thoracic cord, and brachial enlargement) were made on both contused and non-contused animals during ionotropic glutamate antagonist treatment (D-AP5 or NBQX). The thoracic cord, which is nearest to the site of injury, showed the greatest increase in ionotropic glutamate receptor mediated activity. \r\n Calcium-calmodulin protein kinase II (CaMKII) has been shown to be responsible for enhancing ionotropic glutamate receptor mediated activity. CaMKII also has been shown to be a molecular intermediate in both long-term potentiation (LTP) and peripherally induced central sensitization. After contusive SCI, the segments immediately rostral to the injury site show an increase in activated CaMKII. Application of CaMKII inhibitor, KN-93, during recording 35 days post injury, reduces spinal hyperexcitability induced by SCI. \r\n