MECHANICAL AND THERMAL ALLODYNIA IN CHRONIC CENTRAL PAIN FOLLOWING SPINAL-CORD INJURY

Spinal cord injury (SCI) results in variable motor recoveries and chro nic central pain syndromes develop in the majority of SCI patients. To provide a basis for further studies, we report a new rodent model of chronic central pain following spinal cord trauma. Male Sprague-Dawley rats (N = 10) were hemisectioned at T13 and were tested both preopera tively and postoperatively and compared to sham-operated controls (N = 10) for locomotor function, and mechanical and thermal thresholds of both paw withdrawal and supraspinal responses. Results support the dev elopment and persistence of allodynia which persists for 160 days. Loc omotor function was tested using the Basso, Beattie and Bresnahan (BBB ) open field test and only the limb ipsilateral to the hemisection was affected, demonstrating acute flaccid paralysis with motor recovery w hich approached normal values by postoperative day (POD) 15. Prior to the hemisection, the rats showed little to no paw withdrawal response to von Prey stimulation of 4.41 mN or 9.41 mN in both forelimbs and hi ndlimbs. Postoperatively, responses in both ipsilateral and contralate ral forelimbs and hindlimbs increased over time and the increase was s tatistically significant compared to intra-animal presurgical and sham control values (P < 0.05). There were no significant side-to-side dif ferences in limb responses preoperatively or beyond POD 15. The foreli mbs and hindlimbs responded to von Frey hair strengths of 122 mN preop eratively and postoperatively with similar withdrawal frequencies that were not statistically significant. Preoperatively, the paw withdrawa l latency to heat stimuli was 22.9 +/- 3.0 (mean +/- SE) and 20.1 +/- 3.1 sec for the hindlimbs and forelimbs, respectively. Postoperatively , the mean hindlimb and forelimb latency of paw withdrawals decreased to 11.9 +/- 1.8 and 9.2 +/- 2.5 sec, respectively. This decrease in th ermal thresholds is statistically significant when compared to intra-a nimal preoperative and sham control values (P < 0.05). These data indi cate that somatosensory thresholds for non-noxious mechanical and radi ant heat which elicit paw withdrawal (flexor reflex) are significantly lowered following SCI. To further support the development and persist ence of chronic pain following hemisection, supraspinal responses such as paw lick, head turns, attacking the stimulus, and vocalizations we re elicited in response to mechanical and thermal stimuli and were sta tistically significant compared to presurgical intra-animal or sham co ntrol values (P < 0.05). Hemisected animals vocalized to von Frey hair bending forces of 49.8 with a mean of 6.0 +/- 1.2 times out of 10 sti muli compared to intra-animal presurgical and sham control values of z ero. Supraspinal responses of hemisected animals to thermal stimuli oc curred at lower temperatures that were statistically significant compa red to sham control or preoperative values (P < 0.05). These chronic c hanges in thresholds to both mechanical and thermal stimuli represent the development and persistence of mechanical and thermal allodynia af ter SCI.