Objective: To investigate the expression of protein kinase C (PKC) in the spinal dorsal horn of rats with formalin-induced pain and the effect of intrathecal ketamine on PKC expression.
Methods: Thirty-two SD rats were randomly divided into 4 equal groups, namely the control group, intrathecal saline group (NS), 50 µg ketamine group (K1) and 100 µg ketamine group (K2). The rats were anesthetized with 10% chloral hydrate, and a microspinal catheter was inserted intrathecally into the lumbar region. Five days later, the rats in groups, K1 and K2 were subjected to intrathecal administration of 50 and 100 µg ketamine (10 µl), respectively, followed by 10 µl saline, and those in NS group received 20 µl saline only. Thirty minutes later, 5% formalin (50 µl) was subcutaneously injected into the left hindpaw. The pain intensity score (PIS) was utilized to assess antinociceptive behavior within 1 h after formalin injection. Twenty-four hours later, the left hindpaw thickness was measured and the expression of PKC in the spinal dorsal horn in the L5 segment was assayed using immunohistochemistry.
Results: Compared to group NS, groups K1 and K2 showed significantly decreased PIS (P<0.01) in the second phase of formalin-induced pain; 24 h later, the left hindpaw thickness of group NS increased obviously in comparison with that in the control group (P<0.01), whereas the thickness was significantly reduced in group K1 and K2 as compared to that in group NS (P<0.05). The number of immunoreactive cells and the immunohistochemical score of PKC in the spinal dorsal horn were significantly higher in group NS than in group C (P<0.01), but significantly lower in groups K1 and K2 than in group NS (P<0.05).
Conclusion: Intrathecal ketamine produces obvious antinociception against formalin-induced pain in rats and inhibits the enhanced PKC expression in the spinal dorsal horn in response to formalin-induced pain, suggesting the important role of PKC in nociceptive signal transmission and modulation in the spinal cord.