Somatosensory-evoked potentials indicate increased unpleasantness of noxious stimuli in response to increasing stimulus intensities in the rat

Abstract

Recently, it has been shown in rats that specific characteristics of somatosensory-evoked potentials (SEPs) recorded from different sites on the scalp correlate differently to the amount of unpleasantness experienced by the animal following noxious stimulation. It was shown that the SEP recorded from vertex (Vx-SEP) did correlate with the unpleasantness, whereas the SEP recorded from the primary somatosensory cortex (SI-SEP) did not. In the present study, we further investigated the relationship between the Vx-SEP, SI-SEP and the unpleasantness of noxious stimuli. Therefore, different groups of rats were subjected to a SEP fear-conditioning paradigm in which the unconditioned stimulus (US), represented by noxious stimuli applied to evoke SEPs, was paired to a conditioned stimulus (CS) represented by a tone. Different stimulus intensities of the US were applied in the different groups. After CS-US presentation, CS-induced fear-conditioned behaviour was analysed in relation to the characteristics of the Vx-and SI-SEP during CS-US presentation. Results showed that increasing stimulus intensities led to increased SEP amplitudes, which were paralleled by an increased amount of CS-induced fear-conditioned behaviour. No differences between Vx-SEP and SI-SEP were found. The increase in the SEPs in parallel with the increased amount of fear-induced behaviour further supports the SEP to be a potentially valuable tool for studying acute pain and analgesia in animals.

Nociception and Conditioned Fear in Rats: Strains Matter

“…Furthermore, this paradigm has also been used to correlate cortical processing of nociceptive stimuli and pain unpleasantness in rats [7]. In this paradigm, animals are trained to associate a neutral stimulus (conditioned stimulus (CS)) with an aversive (typically painful) stimulus (unconditioned stimulus (US)).…”

“…Furthermore, the expression of CR has been shown to correlate with the cortical processing of the US (i.e. nociceptive stimuli) [7]. Although the exact neural substrates and their roles involved in expression of CR are not known, several brain areas are suggested to play a role, including the amygdala, hippocampus and prefrontal cortex [9]–[13].…”

“…The SEP waveform is described by the latency (the time of occurrence) and amplitude (height) of peaks of interest, both of which can be analyzed in a highly standardized and objective way. The SEP represents the processing of noxious stimuli [16], correlates well to adversity to pain in animals [7], [17] and is altered by anaesthetic and analgesic treatment [18], [19]. Earlier studies showed that lower stimulus intensities and higher dosages of anaesthetics lead to longer latency times and lower amplitudes [18], [20], [21].…”

Research Tools for the Measurement of Pain and Nociception

“…Electrophysiological techniques commonly used in studies of nociception and pain include many different techniques such as recording from peripheral nerves [13], analysis of the electroencephalogram (EEG) [40], recording of evoked potentials, usually the somatosensory-evoked potential (SEP) [41]. Electrophysiological techniques can be used in many ways, and a full consideration of them is beyond the scope of this review.…”

“…A study utilizing evoked potentials derived from the EEG [41], investigated the degree to which the magnitude of a noxious stimulus and its perceived unpleasantness are linked. The methodology of evoked potentials and their use in pain assessment in animals has been reviewed elsewhere [39].…”