A novel behavioral model of neuropathic pain disorders produced in rats by partial sciatic nerve injury

Partial nerve injury is the main cause of causalgiform pain disorders in humans. We present here a novel animal model of this condition. In rats we unilaterally ligated about half of the sciatic nerve high in the thigh. Within a few hours after the operation, and for several months thereafter, the rats developed guarding behavior of the ipsilateral hind paw and licked it often, suggesting the possibility of spontaneous pain. The plantar surface of the foot was evenly hyperesthetic to non-noxious and noxious stimuli. None of the rats autotomized. There was a sharp decrease in the withdrawal thresholds bilaterally in response to repetitive Von Frey hair stimulation at the plantar side. After a series of such stimuli in the operated side, light touch elicited aversive responses, suggesting allodynia to touch. The withdrawal thresholds to CO2 laser heat pulses were markedly lowered bilaterally. Suprathreshold noxious heat pulses elicited exaggerated responses unilaterally, suggesting thermal hyperalgesia. Pin-prick evoked such exaggerated responses bilaterally (mechanical hyperalgesia). In a companion report, we show that these abnormalities critically depend on the sympathetic outflow. Based on the immediate onset and long-lasting perpetuation of similar symptoms, such as touch-evoked allodynia and hyperalgesia, and the resemblance of the contralateral phenomena to 'mirror image' pains in some humans with causalgia, we suggest that this preparation may serve as a model for syndromes of the causalgiform variety that are triggered by partial nerve injury and maintained by sympathetic activity.