Role of spinal voltage-dependent calcium channel α₂δ-1 subunit in the expression of a neuropathic pain-like state in mice

The present study was undertaken to investigate the role of spinal voltage-dependent calcium channel α₂δ-1 subunit in the expression of a neuropathic pain-like state induced by partial sciatic nerve ligation in mice. In cultured spinal neurons, gabapentin (GBP), which displays the inhibitory effect of α₂δ-1 subunit, suppressed the extracellular Ca²⁺ influx induced by KCl, whereas it failed to inhibit the intracellular Ca²⁺ release induced by inositol-1,4,5-triphosphate. Seven days after sciatic nerve ligation, the protein level of α₂δ-1 subunit in the ipsilateral spinal cord was clearly increased compared to that observed in sham-operated mice. In addition, the mRNA level of α₂δ-1 subunit was significantly increased in the dorsal root ganglion, but not in the spinal cord, of nerve-ligated mice. Under these conditions, a marked decrease in the latency of paw-withdrawal against a thermal stimulation and tactile stimulation, induced by sciatic nerve ligation was abolished by repeated intrathecal (i.t.) treatment with GBP. Additionally, the persistent reduction in the nociceptive threshold by i.t. treatment with GBP at the early stage of the neuropathic pain-like state was maintained for 7 days even after GBP withdrawal. It is of interest to note that a single i.t. post-injection of GBP showed a marked and transient inhibitory effect on the developed neuropathic pain-like state, whereas repeated i.t. post-treatment with GBP produced a persistent inhibitory effect during the treatment. In conclusion, we propose here that the neuropathic pain-like state with sciatic nerve ligation is associated with the increased level of the α₂δ-1 subunit of Ca²⁺ channels at the sensory nerve terminal in the spinal dorsal horn of mice. Furthermore, the present data provide evidence that the neuropathic pain may be effectively controlled by repeated treatment with GBP at the early stage.