Abstract
Disturbances of circadian rhythm and dysregulation of clock gene expression are involved in the induction of various neurological disorder states, including chronic pain. However, the relationship between the CNS circadian-clock gene system and nociception remains poorly defined. Significant circadian oscillations of Period (Per1, Per2), Bmal1 and Cryptochrome 1 (Cry1) mRNA expression have been observed in the lumbar spinal dorsal horn of naïve mice. The current study examined the expression of clock genes in the lumbar spinal dorsal horn of mice with neuropathic pain due to a partial sciatic nerve ligation (PSNL). Seven days after PSNL, the mice displayed a robust unilateral hind paw mechanical hypersensitivity. The normal circadian oscillations of Per1, Per2 and Cry1, but not Bmal1, mRNA expression were significantly suppressed in the ipsilateral lumbar spinal dorsal horn of PSNL mice 7days following surgery. The circadian expression of PER1 protein, in particular, was also significantly suppressed in the ipsilateral spinal dorsal horn of PSNL mice. Double-labeling immunohistochemistry revealed downregulation of PER1 in neurons and astrocytes, but not microglia. Knockdown of Per1 expression by intrathecal treatment with Per1 siRNA also induced mechanical hypersensitivity, phosphorylation of c-jun N-terminal kinase (JNK) and the upregulation of chemokine (C-C motif) ligand 2 (CCL2) production in the lumbar spinal dorsal horn. Per1 siRNA-induced mechanical hypersensitivity was attenuated with intrathecal treatment of either the JNK inhibitor SP600125 or the selective CCL2 receptor (CCR2) antagonist RS504393, indicating that these intracellular messengers are crucial in mediating the mechanical hypersensitivity following the downregulation of PER1 expression. These results suggest that the downregulation of the spinal dorsal horn clock genes such as Per1 expressed could be crucial in the induction of neuropathic pain following peripheral nerve injury. Modulating clock gene Per1 expression could be a novel therapeutic strategy in alleviating neuropathic pain.