Electroencephalographic responses to the formalin test in rats

The formalin test is an animal model of persistent pain. Although biphasic behavioral responses to formalin injection have been well described, the si gnificance of the biphasic time course of the pain behaviors has not been e stablished. To explore the significance of the behavioral responses to the formalin injection, we measured and analyzed cortical electrorncephalogram (EEG) during the formalin tests in rats. Formalin was injected subcutaneous ly in the hindpaw of freely moving rats, and behavioral responses were visu ally counted and recorded. Results were compared with a control group which received saline injection. Neoeortical EEG was recorded from implanted dur al surface electrodes and analyzed using a Fast Fourier Transformation. For malin produced biphasic pain behaviors with a transient pause between two p hases. Cortical EEG recordings showed a biphasic change; a vigilant pattern (a low amplitude high frequency activity) followed by a non-vigilant patte rn (a high amplitude low frequency activity), showing a good correlation wi th apparent arousal states of rats. Observed discrepancies between pain beh aviors and EEG-measured vigilance stages included (1) a vigilant EEG patter n persisted during the transient pause of pain behavior, and (2) pain behav iors persisted even after non-vigilant EEG pattern became dominant. The res ults of the current study showed that there are temporal discrepancies betw een the pain behaviors and EEG-measured vigilance during the formalin test in rats. The temporal relationship between the 'pain' behaviors and nocicep tion per se may not be as solid as believed. (C) 1999 International Associa tion for the Study of Pain. Published by Elsevier Science B.V.