Upregulation of calcium channel alpha-2-delta-1 subunit in dorsal horn contributes to spinal cord injury-induced tactile allodynia.

Spinal cord injury (SCI) commonly results not only in motor paralysis but also in the emergence of neuropathic pain (NeuP), both of which can impair the quality of life for patients with SCI. In the clinical field, it is well known that pregabalin, which binds to the voltage-gated calcium channel alpha-2-delta-1 (α2δ-1) subunit has therapeutic effects on NeuP after SCI. A previous study has demonstrated that SCI increased α2δ-1 in the L4-L6 dorsal spinal cord of SCI rats by Western blot analysis and that the increase of α2δ-1 was correlated with tactile allodynia of the hind paw. However, the detailed feature of an increase in α2δ-1 protein in the spinal dorsal horn and the mechanism of pregabalin effect on SCI-induced NeuP have not been fully examined. This study aimed to examine the detailed distribution of α2δ-1 expression in the lumbar spinal cord after thoracic SCI in rats and the correlation of the therapeutic effect of pregabalin in SCI rats. Male Sprague-Dawley rats underwent thoracic (T10) spinal cord contusion injury using the IH impactor device. Spinal cord injury rats received pregabalin (30 mg/kg) once a day for 2 weeks over a 4-week period after SCI. The mechanical threshold in the rat hind paw was measured over 4 weeks. Alpha-2-delta-1 expression in the lumbar spinal cord and in the dorsal root ganglion (DRG) was analyzed using immunohistochemistry and in situ hybridization histochemistry. A significant reduction of the withdrawal threshold of mechanical stimuli to the hind paw was observed for 2 weeks and continued at least 4 weeks after SCI. In the control rats, expression of α2δ-1 immunoreactivity was detected mainly in laminae I and II in the lumbar dorsal horn. Thoracic SCI significantly increased α2δ-1 immunoreactivity in laminae I and II in the lumbar dorsal horn 4 weeks after SCI; however, thoracic SCI did not affect the expression of α2δ-1 mRNA in the L4 and L5 DRGs. Meanwhile, the signal intensity of α2δ-1 mRNAs in the lumbar spinal cord increased from Day 7 and continued for at least 28 days after SCI. Cellular analysis showed that SCI increased the number of α2δ-1-expressing cells in laminae I and II. The tactile allodynia of the hind paw in the SCI rats was reversed after pregabalin treatment and was maintained for 21 days. This administration of pregabalin decreased the α2δ-1 immunoreactivity significantly in the lumbar dorsal horn of thoracic SCI rats at 28 days after SCI. The present study results suggest that an increase of α2δ-1 in the L4 and L5 dorsal horns after thoracic SCI is derived from the increase in the expression in lumbar spinal neurons. This increase may be involved in the development of NeuP in the hind paws and the therapeutic effect of pregabalin on central NeuP after SCI.