Comparative effect of thermal, mechanical, and electrical noxious stimuli on the electroencephalogram of the rat

Thermal, mechanical, and electrical stimuli are often used in acute pain studies and cause qualitatively different pain sensations. Yet, the comparative electroencephalogram (EEG) changes caused by these stimuli have not been studied. We hypothesized that because these stimuli cause different pain sensations, EEG responses would also differ.

Anaesthesia was maintained with halothane in 46 male Sprague-Dawley rats. The EEG was recorded from the primary somatosensory cortices and vertex. Supramaximal noxious stimuli were applied to the tail and comprised mechanical (forceps clamp 20 N), thermal (52 degrees C water bath), and electrical (50 V, 50 Hz for 2 s) stimuli. The EEG descriptors median frequency (F50), spectral edge frequency (F95), and total power (P(tot)) recorded before (baseline) and after noxious stimulation were compared. Data were analysed using two-way factorial ANOVA (stimulus, EEG channel) followed by Bonferroni adjusted post-tests (P < 0.05).

F50 increased during electrical stimulation compared with all baseline periods in all EEG channels, increases from baseline ranging from 115.3 (SD 34.8) to 122.1 (39.6)% for the various channels. A significant increase in F50 during thermal stimulation was identified in some EEG channels, whereas no changes in F50 during mechanical stimulation occurred. Changes in F95 during any stimulus compared with baseline were not significant.

Different noxious stimuli caused differing EEG changes. As the somatosensory cortex contains relatively few exclusively nociceptive neurons, the EEG recorded from this region during the application of predominantly noxious stimuli (mechanical and thermal) may demonstrate minimal cortical activation compared with non-specific electrical noxious stimuli.