Age-dependent responses to nerve injury-induced mechanical allodynia.

BACKGROUND Developmental differences in responses to acute and chronic nerve injury have received minimal attention. This study examines developmental differences in behavioral responses to a proximal (closer to the spinal cord) (L5 and L6 spinal nerve root ligation) or to a more distal (closer to peripheral innervation) (partial sciatic nerve ligation) nerve injury in rats paralleling the infant to young adult human. METHODS Withdrawal thresholds to von Frey filament testing in the hind paw were determined before and various times after either spinal nerve root ligation or partial sciatic nerve ligation in rats aged 2, 4, and 16 weeks. Control rats of these ages were observed serially without surgery. Times for withdrawal thresholds to mechanical stimuli to return to 80% of that of the hind paw in the control animals were compared among the different ages in the two models. RESULTS Baseline withdrawal thresholds in younger rats were lower (P < 0.05). In the 2-week-old animals, distal injury partial sciatic nerve ligation did not cause a reduction in withdrawal threshold from baseline. This was different from the spinal nerve root ligation group and the older animals in the partial sciatic nerve ligation group. However, when compared with age-matched control animals, both nerve injuries resulted in reduced withdrawal thresholds (P < 0.05). The resolution of hypersensitivity to mechanical stimulation, as measured by return of threshold to 80% of controls, occurred more quickly in 2-week-old than in 4- and 16-week-old animals in both injury models (P < 0.05). CONCLUSION These data suggest that resolution of sensitization to A-fiber input occurs more rapidly in young animals. In addition, distal injury has less of a sensitizing effect on A-fiber input than proximal injury in the younger animals. The authors speculate that neuroimmune responses, especially at the site of injury, are developmentally regulated and less likely to produce chronic pain when injury occurs at a young age.