We studied the effect of unilateral
ligation of two spinal nerves on behavioral
pain responses evoked by various types of cutaneous stimuli in the adult rat. Furthermore, we determined the effect of spinal nerve
ligation on morphology of the peripheral nerves. The most consistent behavioral finding (83%) was a marked decrease in monofilament-induced hindlimb withdrawal thresholds (
mechanical allodynia) ipsilateral to the spinal nerve
ligation. This
mechanical allodynia was observed as early as during the 1st post-operative day and it persisted up to 2 months (the maximum length of the observation period). In contrast,
hyperalgesia to noxious mechanical stimulation (Randal-Sellitto test) was not observed in allodynic rats until the 3rd post-operative day. In a minority of rats (13%), spinal nerve
ligation-induced
mechanical hyperalgesia without a concomitant
mechanical allodynia. There was no corresponding heat
hyperalgesia in the injured hindlimb (hot water immersion-, radiant heat- or hot-plate-induced hindlimb withdrawal tests). In contrast, hypoalgesia to heat was observed on the 1st postoperative day, but not later. Neuropathological analysis of the peripheral nerves revealed a dramatic decrease in the number of myelinated nerve fibers distal to the spinal nerve
ligation site. The results support the previous evidence indicating that
ligation of spinal nerves induces a marked
allodynia to mechanical stimulation. However, this
mechanical allodynia may differentially dissociate from mechanical and
thermal hyperalgesia at various post-
operative time points. The marked
mechanical allodynia together with a dramatic decrease in the number of myelinated nerve fibers is paradoxical, since the activation of myelinated nerve fibers by monofilaments produced abnormally strong behavioral responses. This paradox may be explained by spinal nerve
ligation-induced amplification or disinhibition of tactile signals at central levels.