Changes in exploratory behavior as a measure of chronic central pain following spinal cord injury

Spinal cord injury (SCI) produces abnormal pain syndromes in patients that lead to changes in evoked and spontaneous behaviors. To test if a spontaneous component of pain-like behavior could be measured in a rodent model of chronic central pain (CCP), exploratory behavior (rearing events, rearing time, active time, rest time, distance traveled, and total activity) of adult male rats, subjected to sham surgery or spinal cord contusion injury treated with either vehicle (saline) or gabapentin (30 mg/kg, i.p.), was recorded. SCI was produced at spinal segment TIO using the NYU impactor device (10-g rod, 2.0-mm diameter, 12.5-mm drop height). Activity measures were collected on postsurgical days (PSD) 14, 28, and 60, and compared to presurgical activity. Sham control activity was not significantly different compared to presurgical activity in any measured parameter. SCI vehicle-treated rats demonstrated a significant decrease in total rearing time on PSD 14 and by PSD 28 significant differences in total activities where seen in all parameters measured. SCI gabapentin-treated rats did not display differences in total rearing time until PSD 28 and a significant difference in total activity of all measured parameters was not seen until PSD 60. No difference in hindlimb locomotor ability between SCI groups or sedation effects of gabapentin was found using open field BBB scores. We interpret the differences in exploratory behavior to reflect spontaneous behavioral changes due to CCP since (1) when locomotor ability was greatest, activity was lowest and (2) gabapentin attenuates the temporal decrease in activity. This study demonstrates that spontaneous as well as evoked behaviors may be used to evaluate CCP following SCI.