Differences between primary somatosensory cortex- and vertex-derived somatosensory-evoked potentials in the rat

High-frequency electrical stimulation increases cortical excitability and mechanical sensitivity in a chronic large animal model

ABSTRACT

Translational models of the sensitized pain system are needed to progress the understanding of involved mechanisms. In this study, long-term potentiation was used to develop a mechanism-based large-animal pain model. Event-related potentials to electrical stimulation of the ulnar nerve were recorded by intracranial recordings in pigs, 3 weeks before, immediately before and after, and 3 weeks after peripheral high-frequency stimulation (HFS) applied to the ulnar nerve in the right forelimb (7 pigs) or in control animals (5 pigs). Event-related potential recordings and peripheral HFS were done during anesthesia. Two weeks before and after the HFS, behavioral responses reflecting mechanical and thermal sensitivity were collected using brush, noxious limb-mounted pressure algometer, and noxious laser stimuli. The HFS intervention limb was progressively sensitized to noxious mechanical stimulation in week 1 and 2 compared with baseline ( P = 0.045) and the control group ( P < 0.034) but not significantly to laser or brush stimulation. The first negative (N1) peak of the event-related potential was increased 30 minutes after HFS compared with before ( P < 0.05). The N1 peak was also larger compared with control pigs 20 to 40 minutes after HFS ( P < 0.031) but not significantly increased 3 weeks after. The relative increase in N1 30 minutes after HFS and the degree of mechanical hyperalgesia 2 weeks post-HFS was correlated ( P < 0.033). These results show for the first time that the pig HFS model resembles the human HFS model closely where the profile of sensitization is comparable. Interestingly, the degree of sensitization was associated with the cortical signs of hyperexcitability at HFS induction.

Differences in intracortical responses following non-noxious and noxious stimulation in anaesthetized rats

Cortical responses have been proposed as a source for the extraction of unique and non-subjective sensory information. The present study aimed to investigate if it is possible to distinguish between non-noxious and noxious cortical responses with two different types of anesthesia. Sixteen rats were randomly allocated to receive either Hypnorm/Dormicum (HD) or isoflurane (ISO) anesthesia. Each animal had a custom-made microelectrode array implanted in the primary somatosensory cortex to record the local field potentials and a cuff electrode implanted around the sciatic nerve to deliver electrical stimulations. Three stimulation intensities were applied: 1x movement threshold (MT) (i.e., non-noxious activation), 5x MT (low intensity noxious activation), and 10x MT (high intensity noxious activation). The evoked potentials were assessed by extracting three features: 1) the negative peak (NP), 2) the positive peak (PP), and 3) the peak-to-peak (PtP) amplitudes. Our results showed that it was possible to distinguish between three levels of stimulation intensities based on the NP, PP, and PtP features for the HD group, whereas it was only possible to make the same differentiation with the use of PP and PtP when applying ISO. This work is believed to contribute to a basic understanding of how the cortical responses change in the hyperacute phase of pain and which cortical features may be suitable as objective measures of nociception.

Mapping of sheep sensory cortex with a novel microelectrocorticography grid

Microelectrocorticography (µECoG) provides insights into the cortical organization with high temporal and spatial resolution desirable for better understanding of neural information processing. Here we evaluated the use of µECoG for detailed cortical recording of somatosensory evoked potentials (SEPs) in an ovine model. The approach to the cortex was planned using an MRI-based 3D model of the sheep's brain. We describe a minimally extended surgical procedure allowing placement of two different µECoG grids on the somatosensory cortex. With this small craniotomy, the frontal sinus was kept intact, thus keeping the surgical site sterile and making this approach suitable for chronic implantations. We evaluated the procedure for chronic implantation of an encapsulated µECoG recording system. During acute and chronic recordings, significant SEP responses in the triangle between the ansate, diagonal, and coronal sulcus were identified in all animals. Stimulation of the nose, upper lip, lower lip, and chin caused a somatotopic lateral-to-medial, ipsilateral response pattern. With repetitive recordings of SEPs, this somatotopic pattern was reliably recorded for up to 16 weeks. The findings of this study confirm the previously postulated ipsilateral, somatotopic organization of the sheep's sensory cortex. High gamma band activity was spatially most specific in the comparison of different frequency components of the somatosensory evoked response. This study provides a basis for further acute and chronic investigations of the sheep's sensory cortex by characterizing its exact position, its functional properties, and the surgical approach with respect to macroanatomical landmarks.

Nociception and conditioned fear in rats: strains matter

When using rats in pain research, strain-related differences in outcomes of tests for pain and nociception are acknowledged. However, very little is known about the specific characteristics of these strain differences. In this study four phylogenetically distant inbred rat strains, i.e. Wistar Kyoto (WKY), Fawn Hooded (FH), Brown Norway (BN) and Lewis (LE), were investigated in different tests related to pain and nociception. During Pavlovian fear conditioning, the LE and WKY showed a significantly longer duration of freezing behaviour than the FH and BN. Additionally, differences in c-Fos expression in subregions of the prefrontal cortex and amygdala between rat strains during retrieval and expression of conditioned fear were found. For example, the BN did not show recruitment of the basolateral amygdala, whereas the WKY, FH and LE did. During the hot plate test, the WKY and LE showed a lower thermal threshold compared to the BN and FH. In a follow-up experiment, the two most contrasting strains regarding behaviour during the hot plate test and Pavlovian fear conditioning (i.e. FH and WKY) were selected and the hot plate test, Von Frey test and somatosensory-evoked potential (SEP) were investigated. During the Von Frey test, the WKY showed a lower mechanical threshold compared to the FH. When measuring the SEP, the FH appeared to be less reactive to increasing stimulus intensities when considering both peak amplitudes and latencies. Altogether, the combined results indicate various differences between rat strains in Pavlovian fear conditioning, nociception related behaviours and nociceptive processing. These findings demonstrate the necessity of using multiple rat strains when using tests including noxious stimuli and suggest that the choice of rat strains should be considered. When selecting a strain for a particular study it should be considered how this strain behaves during the tests used in that study.

Predictability of painful stimulation modulates the somatosensory-evoked potential in the rat

Somatosensory-evoked potentials (SEPs) are used in humans and animals to increase knowledge about nociception and pain. Since the SEP in humans increases when noxious stimuli are administered unpredictably, predictability potentially influences the SEP in animals as well. To assess the effect of predictability on the SEP in animals, classical fear conditioning was applied to compare SEPs between rats receiving SEP-evoking electrical stimuli either predictably or unpredictably. As in humans, the rat’s SEP increased when SEP-evoking stimuli were administered unpredictably. These data support the hypothesis that the predictability of noxious stimuli plays a distinctive role in the processing of these stimuli in animals. The influence of predictability should be considered when studying nociception and pain in animals. Additionally, this finding suggests that animals confronted with (un)predictable noxious stimuli can be used to investigate the mechanisms underlying the influence of predictability on central processing of noxious stimuli.

Coupling between somatosensory evoked potentials and hemodynamic response in the rat

We studied the relationship between somatosensory evoked potentials (SEP) recorded with scalp electroencephalography (EEG) and hemoglobin responses recorded non-invasively with diffuse optical imaging (DOI) during parametrically varied electrical forepaw stimulation in rats. Using these macroscopic techniques we verified that the hemodynamic response is not linearly coupled to the somatosensory evoked potentials, and that a power or threshold law best describes the coupling between SEP and the hemoglobin response, in agreement with the results of most invasive studies. We decompose the SEP response in three components (P1, N1, and P2) to determine which best predicts the hemoglobin response. We found that N1 and P2 predict the hemoglobin response significantly better than P1 and the input stimuli (S). Previous electrophysiology studies reported in the literature show that P1 originates in layer IV directly from thalamocortical afferents, while N1 and P2 originate in layers I and II and reflect the majority of local cortico-cortical interactions. Our results suggest that the evoked hemoglobin response is driven by the cortical synaptic activity and not by direct thalamic input. The N1 and P2 components, and not P1, need to be considered to correctly interpret neurovascular coupling.

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

BACKGROUND

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.

METHODS

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).

RESULTS

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.

CONCLUSIONS

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.

Is it possible to generate cerebral evoked potentials with a mechanical stimulus from the duodenum in rats?

The study aim was to develop a model to generate cerebral evoked potentials (CEPs) by mechanical distention of the duodenum in rats. Twenty Sprague-Dawley rats were anaesthetized and the EEG recorded from the left and right somatosensory cortices (S1L, S1R). A balloon catheter was implanted into the duodenum. A pneumatic device, triggered by data acquisition software, inflated the balloon for 200 ms every 3s to deliver a repeatable noxious stimulus. EEG was recorded for 100 ms before and 500 ms after onset of inflation and the response to 512 stimuli averaged to generate a CEP. Two CEPs were generated in each animal and data summed to calculate a single CEP for each channel. Data were excluded when the signal to noise ratio was < 2, therefore data are presented from 11 animals. A repeatable CEP was identified in waveforms recorded from S1L. The mean (S.D.) CEP comprised a triphasic waveform (P1, N1, P2) with latencies of 246.0 (24.7), 289.3 (12.8) and 321.5 (13.2)ms, respectively. We are the first group to have generated and characterized a CEP following mechanical stimulation of the duodenum. This model can be applied to further elucidate the mechanisms leading to visceral pain perception.

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

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.

Neurophysiological techniques to assess pain in animals

Neurophysiological techniques are widely applied to animals, both in the search as a monitor for adequacy of anaesthesia, and studies to assess the efficacy of analgesic agents. Laboratory animals have been extensively used in models to investigate pain in man. However a substantial number of studies have also used neurophysiological techniques to increase knowledge of pain in specific animal species, with the aim of improving animal welfare. This review provides an overview of neurophysiological techniques involving the brain that have been used in the assessment of pain in animals. An explanation of the methodology of EEG recording, with particular emphasis on veterinary studies, is given. Neurophysiological models developed to assess pain in different species are described, and their relevance to advancements in animal welfare or best clinical practice indicated.

Vertex-recorded, rather than primary somatosensory cortex-recorded, somatosensory-evoked potentials signal unpleasantness of noxious stimuli in the rat

In the present study, we investigated in the rat whether vertex- or primary somatosensory cortex-recorded somatosensory-evoked potentials (Vx-SEP/SI-SEP, respectively) signal unpleasantness of noxious stimuli. Therefore, initially we characterised fentanyl effects (0, 20, 40 or 50 microg/kg/h) on somatosensory and auditory processing by recording Vx-/SI-SEPs and vertex- and primary auditory cortex-recorded auditory-evoked potentials (Vx-/AI-AEPs, respectively). Subsequently, in a separate experiment, the animals were subjected to a Pavlovian fear-conditioning paradigm. The noxious stimuli applied to evoke Vx-/SI-SEPs (unconditioned stimulus (US)) were paired to a tone (conditioned stimulus (CS)) under 'steady state' conditions of 0, 20, 40 or 50 microg/kg/h fentanyl. Vx-/SI-SEPs were recorded simultaneously during these trials. After CS-US presentation, CS-induced fear-conditioned behaviour was analysed in relation to the SEPs recorded during CS-US presentation and the AEPs recorded in the first experiment. While the SI-SEP and AI-AEP were minimally but significantly affected, fentanyl dose-dependently decreased the Vx-SEP and Vx-AEP. The decrease of the Vx-SEP and Vx-AEP was parallelled by the dose-dependent decrease of the amount of CS-induced fear-conditioned behaviour. These results suggest that the dose-dependent decrease of the Vx-SEP amplitude, rather than of the SI-SEP, indicates that the US was experienced as less unpleasant. Next to an altered US processing, altered CS processing contributed to the decrease of the amount of CS-induced fear-conditioned behaviour as indicated by the dose-dependent decrease of the Vx-AEP.

Differences between somatosensory-evoked potentials recorded from the ventral posterolateral thalamic nucleus, primary somatosensory cortex and vertex in the rat

Somatosensory-evoked potential (SEP) components recorded over the primary somatosensory cortex (SI) and vertex in the rat within the 10-30 ms latency range were characterised with respect to the anatomy and function of the primary somatosensory pathway. To this aim, these components were compared to SEP components in the similar latency range recorded from the ventral posterolateral thalamic (VPL) nucleus, a nucleus known to be part of the subcortical structure of the primary somatosensory pathway and were described with respect to their stimulus-response characteristics and their response to the mu-opioid agonist fentanyl. The VPL positive (P)11-negative (N)18-P22 and SI P13-N18-P22 differed with respect to peak occurrence (P11 versus P13, respectively) and waveform morphology, but did not differ with respect to stimulus-response characteristics and their response to fentanyl. When compared to the vertex P15-N19-P26, the VPL P11-N18-P22 and SI P13-N18-P22 complex follow a relatively fast acquisition in stimulus intensity-response and were affected significantly less to increasing stimulus frequencies and to fentanyl. These results demonstrated that when compared to the VPL-SEP and SI-SEP, the Vx-SEP was modulated differently by the experimental conditions. It is suggested that this may be related to involvement of neural structures within different functional somatosensory pathways.

Development of a rat model to assess the efficacy of the somatosensory-evoked potential as indicator of analgesia

Drug-induced changes in somatosensory-evoked potentials (SEPs) are considered to reflect an altered nociceptive state. Therefore, the SEP is proposed to be a parameter of analgesic efficacy. However, at present, SEPs have not been studied in relation to animal pain. The present study aims to develop a rat model in which this relationship can be studied based on Pavlovian fear conditioning. Therefore, rats, implanted with epidural electro-encephalogram recording electrodes, were randomly assigned to either a paired or random-control group and subjected to an aversive-to-appetitive transfer paradigm. During the aversive phase, the SEP-stimulation paradigm (5 mA square wave pulses, n = 72, of 2 ms duration each, with a stimulus frequency of 0.5 Hz; total duration 144 s) was used as the unconditioned stimulus (US), while a tone (40 s, 1500 Hz, 85 dB sound pressure level) was used as the conditioned stimulus (CS). During the appetitive phase, the CS was presented paired to the presentation of a sugar pellet. When compared to the random-control group, the paired group showed significantly more freezing behavior and significantly less reward-directed behavior in response to the CS in the appetitive phase. In addition, SEPs were not significantly affected by fear conditioning. Based on these results, we conclude that the SEP-stimulation paradigm can be successfully employed as a US in fear conditioning. In future studies, fear conditioning can be carried out under different levels of an analgesic regimen to allow the changes in SEP parameters to be compared to changes in fear-induced behavior making this model potentially useful to validate SEP parameters as indicators of analgesia.