Flinching in phase 2 of the formalin test using rat neuropathic pain models

Abstract

We used electroencephalography (EEG) to determine whether the frequency of flinching in phase 2 of the formalin test using rat neuropathic pain models is appropriate as a parameter of pain. The partial sciatic nerve ligation model rats (PSL group) showed a significant increase in the frequency of flinching in phase 2, a change to rapid EEG waves, and a significant increase in the fast Fourier transform (FFT) power for β waves compared with the controls. The flinching and EEG results suggest hyperalgesia to inflammatory stimulation in the PSL group. The chronic constriction injury model rats (CCI group) showed a significant decrease in the frequency of flinching compared with the control group. These flinching results suggest hypoalgesia to inflammatory stimulation in the CCI group. However, the EEG results suggested pain in the CCI group, as in the controls. Thus, in the CCI group, there was dissociation between the interpretation of the flinching results and that of the EEG results. We concluded that the use of flinching in phase 2 of the formalin test based on the rat neuropathic pain model may be inappropriate as a parameter of pain.