Modeling at-level allodynia after mid-thoracic contusion in the rat.

The rat mid-thoracic contusion model has been used to study at-level tactile allodynia, a common type of pain that develops after spinal cord injury (SCI). An important advantage of this model is that not all animals develop hypersensitivity. Therefore, it can be used to examine mechanisms that are strictly related to the development of pain-like behavior separately from mechanisms related to the injury itself. However, how to separate animals that develop hypersensitivity from those that do not is unclear. The aims of the current study were to identify where hypersensitivity and spasticity develop and use this information to identify metrics to separate animals which develop hypersensitivity from those that do not to study differences in their behavior. To accomplish these aims, a grid was used to localize hypersensitivity on the dorsal trunk relative to thoracic dermatomes, and supraspinal responses to tactile stimulation were tallied. These supraspinal responses were used to develop a hypersensitivity score to separate animals that develop hypersensitivity, or pain-like response to nonpainful stimuli. Similar to humans, the development of hypersensitivity could occur with the development of spasticity or hyperreflexia. Moreover, the time course and prevalence of hypersensitivity phenotypes (at-, above-, or below level) produced by this model were similar to that observed in humans with SCI. However, the amount of spared spinal matter in the cord did not explain the development of hypersensitivity, as previously reported. This approach can be used to study the mechanisms underlying the development of hypersensitivity separately from mechanisms related to injury alone.