Effects of lowering ambient temperature on pain-related behaviors in a rat model of neuropathic pain.

To clarify the mechanism by which changes in chronic pain are induced by cold environments, rats rendered neuropathic by a chronic constriction injury (CCI) to the sciatic nerve were exposed to low ambient temperature (LT; 7 degrees C decrease from 22 degrees C) in a climate-controlled room. LT exposure aggravated pain-related behaviors in CCI rats, i.e., decreased the threshold to von Frey hair and paw pressure stimulation, prolonged the duration of foot withdrawal to pinprick stimulation, and increased the cumulative duration of guarding posture. Lumbar sympathectomy (SYX) did not inhibit LT-induced augmentations of pain-related behaviors in CCI rats. LT exposure decreased the skin temperatures of both hind paws to the same degree in the sham-operated control and SYX rats, while in the CCI and SYX+CCI rats it caused a larger temperature decrease in the injured paw than in the uninjured one. These results indicate that LT exposure augments abnormalities in pain-related behaviors of neuropathic rats, and also suggest that sympathetic nervous activity is not a predominant factor in the augmenting mechanism.