Changes in spinal cord met-enkephalin levels and mechanical threshold values of pain after pulsed radio frequency in a spared nerve injury rat model.

OBJECTIVES: The present study investigated changes in the met-enkephalin (M-ENK) levels in the spinal cord. We also determined the mechanical threshold value of pain in spared nerve injury (SNI) rats after applying pulsed radiofrequency (PRF) on L5 dorsal root ganglion (DRG). METHODS: Sixty-four rats were divided into four groups: the normal group (n = 16), the control group (n = 16), the sham intervention group (n = 16), and the PRF group (n = 16). With exception for the normal group, the other three groups were treated with an established SNI model. After 7 days, PRF or sham intervention was applied on the right L5 DRG. The M-ENK levels in the spinal cord were examined by radioimmunoassay 24 hours after applying PRF or sham operation. Mechanical threshold values of pain were also tested 1 day before SNI procedure, 1 and 2 days after SNI procedure, and 2 and 24 hours after applying PRF or sham operation. RESULTS: Twenty-four hours after treatment with PRF, M-ENK levels in spinal cord increased significantly, while no changes were detected in the sham intervention group. Hyperalgesia was found in rats 1-2 days after SNI procedure and was improved by PRF. This was demonstrated by an increased mechanical threshold of pain 2 and 24 hours after the PRF. The sham intervention group showed no change in the mechanical threshold of pain. CONCLUSION: This study demonstrates that applying PRF on the DRG can improve hyperalgesia and increase M-ENK levels in the spinal cord of SNI rats within 24 hours. These findings indicate that the endogenous M-ENK in the spinal cord is involved in the mechanism of PRF on the therapy of neuropathic pain.