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

Abstract

We analyzed electroencephalograms to investigate whether the frequency of flinching in phase 2 of the formalin test in rat neuropathic pain models is an appropriate index of pain. Electroencephalograms show high-amplitude slow waves at rest, and low-amplitude fast waves in the presence of various stimuli including pain. When an electroencephalogram consists of a fast wave, it is not appropriate to evaluate the reduction of pain-related behaviors as hyposensitivity or pain reduction. We found that the chronic constriction injury (CCI) model showed an electroencephalogram consisting of a fast wave despite the reduction of flinching in phase 2 of the formalin test. After oral administration of indomethacin, an electroencephalogram consisting of a slow wave was detected. We concluded that flinching in phase 2 of the formalin test in the CCI model may not be a useful index of pain.