Chronic spinal cord injury induced changes in the responses of thalamic neurons.

Sensory disturbances following spinal cord injury (SCI) include chronic pain, which is often localized at spinal levels just rostral to the lesion (referred to as at-level neuropathic pain) and not effectively relieved by traditional treatments. In the present study, a clinically relevant spinal contusion injury was made at the spinal T8 level in 11 deeply anesthetized male rats. Behavioral testing just prior to terminal electrophysiological experiments (done at 30 days post-injury) demonstrated at-level sensitivity to touching the trunk (i.e., allodynia) in 64% of the animals. Electrophysiological data (urethane anesthesia) were obtained for 218 single somatovisceral convergent neurons that were located throughout 12 subregions of the thalamus. In total, 90% (197 of 218) responded to noxious at-level pinch, compared to 52% for pinching the dorsal trunk at the same level in uninjured controls (our previously published data--recorded from 133 total neurons). In addition, 33% of the total neurons tested also responded to gentle touch (dorsal trunk) versus 9% in controls. A comparison of electrophysiological and behavioral data for each individual animal reveals novel tactile neuronal responses within ventral and posterior thalamic subnuclei for those rats showing signs of at-level allodynia. These data suggest that neurons in specific regions of thalamus undergo significant changes in responsiveness following severe chronic SCI. The observed plasticity and ensuing hypersensitivity are likely part of the central reorganization producing the multitude of sensory disturbances that surface following SCI.