Effects of selective spinal nerve ligation on acetic acid-induced nociceptive responses and ASIC3 immunoreactivity in the rat dorsal root ganglion

We investigated changes in pain behavior after injection of acetic acid in the hindpaws of rats with L5 spinal nerve ligation (SNL)-induced neuropathy. We also examined immunoreactivity for acid-sensing ion channel 3 (ASIC3) in the dorsal root ganglion (DRG) of rats with L5 SNL. Two weeks after SNL, the withdrawal threshold to a mechanical stimulus was significantly lower in the SNL group than in the sham-operated group (n = 9 per group, P < 0.01). After acetic acid injection, spontaneous pain responses in the SNL group were significantly increased compared to those in the sham-operated group (n = 5, P < 0.05). L5 SNL significantly increased the proportion of total ASIC3-immunoreactive (ir) neurons in the ipsilateral L4 DRG compared to that in sham-operated rats (n = 4, P < 0.01). Analysis of cell size showed that the proportion of large (> 1200 mm²) ASIC3-ir neurons in the ipsilateral L4 DRG significantly increased after L5 SNL (P < 0.05). In the ipsilateral L5 DRG, the proportion of ASIC3-ir neurons was not significantly affected by treatment. However, L5 SNL significantly increased (P < 0.01) the proportion of small (< 1200 mm²) ASIC3-ir neurons and significantly decreased (P < 0.01) the proportion of large ASIC3-ir neurons compared to proportions in sham-operated animals. These findings suggest that ASIC3 is associated with hyperalgesia in response to a chemical stimulus in the L5 SNL rat model.