This study aimed to determine whether neuropathic pain is associated with a plasticity change in the number of synapses in the spinal dorsal horn. 12 normal adult SD rats were randomly divided into two groups: 7 animals were subjected to unilateral loose ligation (to induce chronic constriction injury) of the sciatic nerve (CCI group) and 5 animals subjected to unilateral sham-operation (sham-operated group). 28 days after operation, the L4-L6 segment of the spinal cord was removed, and paraffin-embedded sections were prepared and stained with Nissl's method and synaptophysin immunohistochemistry. The numbers of neurons and synapses in the spinal dorsal horn were estimated using a contemporary stereological technique-the optical disector. An 86% increase in the number (per unit length of the spinal cord) of synapses or 98% increase in the ratio between the numbers of synapses and neurons in the spinal dorsal horn was found in the middle tissue block but not in both the rostral and caudal tissue blokes cut from L4-L6 segment of the spinal cord. The results suggest that neuropathic pain, as established by the CCI model, is associated with a plasticity change in the spinal dorsal horn: increase in the number of synapses.
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