Formalin-induced central sensitization in the rat: somatosensory evoked potential data

Cortical somatosensory evoked potentials were used to measure secondary hyperaesthesia resulting from subcutaneous 1 and 5% formalin in unanesthetized rats with permanently implanted electrodes. Near field responses were evoked by contralateral non-noxious electrical stimulation of the middle third of the tail. 0.05 ml 5% formalin injected subcutaneously at the base of the tail increased the amplitude of P1-N1 a maximum of 158.5 +/- 10.91% and N2 a maximum of 150.4 +/- 21.40% compared to controls (P < 0.05 and P < 0.01). Amplitudes were increased from 5 min after injection to the end of the 70 min test period. The effect of 1% formalin was equivalent to 5% formalin. This increase was prevented by pretreatment with 5 mg/kg ketamine or 5 mg/kg morphine, in agreement with behavioral and electrophysiological data. Cortical somatosensory evoked potentials are objective measures of central sensitization which may usefully complement current behavioral models for the evaluation of analgesic drugs. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.