The Neural Origin of Nociceptive-Induced Gamma-Band Oscillations

Deciphering Authentic Nociceptive Thalamic Responses in Rats

The thalamus and its cortical connections play a pivotal role in pain information processing, yet the exploration of its electrophysiological responses to nociceptive stimuli has been limited. Here, in 2 experiments we recorded neural responses to nociceptive laser stimuli in the thalamic (ventral posterior lateral nucleus and medial dorsal nucleus) and cortical regions (primary somatosensory cortex [S1] and anterior cingulate cortex) within the lateral and medial pain pathways. We found remarkable similarities in laser-evoked brain responses that encoded pain intensity within thalamic and cortical regions. Contrary to the expected temporal sequence of ascending information flow, the recorded thalamic response (N1) was temporally later than its cortical counterparts, suggesting that it may not be a genuine thalamus-generated response. Importantly, we also identified a distinctive component in the thalamus, i.e., the early negativity (EN) occurring around 100 ms after the onset of nociceptive stimuli. This EN component represents an authentic nociceptive thalamic response and closely synchronizes with the directional information flow from the thalamus to the cortex. These findings underscore the importance of isolating genuine thalamic neural responses, thereby contributing to a more comprehensive understanding of the thalamic function in pain processing. Additionally, these findings hold potential clinical implications, particularly in the advancement of closed-loop neuromodulation treatments for neurological diseases targeting this vital brain region.

The Bayliss-Starling Prize Lecture: The developmental physiology of spinal cord and cortical nociceptive circuits

When do we first experience pain? To address this question, we need to know how the developing nervous system processes potential or real tissue-damaging stimuli in early life. In the newborn, nociception preserves life through reflex avoidance of tissue damage and engagement of parental help. Importantly, nociception also forms the starting point for experiencing and learning about pain and for setting the level of adult pain sensitivity. This review, which arose from the Bayliss-Starling Prize Lecture, focuses on the basic developmental neurophysiology of early nociceptive circuits in the spinal cord, brainstem and cortex that form the building blocks of our first pain experience.

Ventral Hippocampal CA1 Pyramidal Neurons Encode Nociceptive Information

As a main structure of the limbic system, the hippocampus plays a critical role in pain perception and chronicity. The ventral hippocampal CA1 (vCA1) is closely associated with negative emotions such as anxiety, stress, and fear, yet how vCA1 neurons encode nociceptive information remains unclear. Using in vivo electrophysiological recording, we characterized vCA1 pyramidal neuron subpopulations that exhibited inhibitory or excitatory responses to plantar stimuli and were implicated in encoding stimuli modalities in naïve rats. Functional heterogeneity of the vCA1 pyramidal neurons was further identified in neuropathic pain conditions: the proportion and magnitude of the inhibitory response neurons paralleled mechanical allodynia and contributed to the confounded encoding of innocuous and noxious stimuli, whereas the excitatory response neurons were still instrumental in the discrimination of stimulus properties. Increased theta power and theta-spike coupling in vCA1 correlated with nociceptive behaviors. Optogenetic inhibition of vCA1 pyramidal neurons induced mechanical allodynia in naïve rats, whereas chemogenetic reversal of the overall suppressed vCA1 activity had analgesic effects in rats with neuropathic pain. These results provide direct evidence for the representations of nociceptive information in vCA1.

Sleep disturbance correlated with severity of neuropathic pain in sciatic nerve crush injury model

The association between sleep and pain has been investigated widely. However, inconsistent results from animal studies compared with human data show the need for a validated animal model in the sleep-pain association field. Our study aims to validate common neuropathic pain models as a tool for evaluating the sleep-pain association. Electrodes electroencephalogram (EEG) and electromyogram (EMG) were surgically implanted to measure sleep. The von Frey test was used to measure pain sensitivity. Following the baseline data acquisition, two pain-modelling procedures were performed: sciatic nerve crush injury (SCI) and common peroneal nerve ligation (CPL). Post-injury measurements were performed on days 1, 5, 10, and 15 post-surgery. The results presented decreased paw withdrawal thresholds and reduced NREM sleep duration in both models on the first post-surgery day. In the SCI model, NREM sleep duration was negatively correlated with paw withdrawal thresholds (p = 0.0466), but not in the CPL model. Wake alpha and theta EEG powers were also correlated with the pain threshold. The results confirm that the SCI model shows disturbed sleep patterns associated with increased pain sensitivity, suggesting it is a reliable tool for investigating sleep disturbances associated with neuropathic pain.

Pain-preferential thalamocortical neural dynamics across species

Searching for pain-preferential neural activity is essential for understanding and managing pain. Here, we investigated the preferential role of thalamocortical neural dynamics in encoding pain using human neuroimaging and rat electrophysiology across three studies. In study 1, we found that painful stimuli preferentially activated the medial-dorsal (MD) thalamic nucleus and its functional connectivity with the dorsal anterior cingulate cortex (dACC) and insula in two human functional magnetic resonance imaging (fMRI) datasets (n = 399 and n = 25). In study 2, human fMRI and electroencephalography fusion analyses (n = 220) revealed that pain-preferential MD responses were identified 89-295 ms after painful stimuli. In study 3, rat electrophysiology further showed that painful stimuli preferentially activated MD neurons and MD-ACC connectivity. These converging cross-species findings provided evidence for pain-preferential thalamocortical neural dynamics, which could guide future pain evaluation and management strategies.

Pinprick-induced gamma-band oscillations are not a useful electrophysiological marker of pinprick hypersensitivity in humans

OBJECTIVE

This study aimed to investigate scalp gamma-band oscillations (GBOs) induced by mechanical stimuli activating skin nociceptors before and after the induction of mechanical hypersensitivity using high-frequency electrical stimulation (HFS) of the skin.

METHODS

In twenty healthy volunteers, we recorded the electroencephalogram during robot-controlled mechanical pinprick stimulation (512 mN) applied at the right ventral forearm before and after HFS.

RESULTS

HFS induced a significant increase in mechanical pinprick sensitivity, but this increased pinprick sensitivity was, at the group level, not accompanied by a significant increase in GBOs. Visual inspection of the individual data revealed that possible GBOs were present in eight out of twenty participants (40%) and the frequency of these GBOs varied substantially across participants.

CONCLUSIONS

Based on the low number of participants showing GBOs we question the (clinical) utility of mechanically-induced GBOs as an electrophysiological marker of pinprick hypersensitivity in humans.

SIGNIFICANCE

Mechanical pinprick-induced scalp GBOs are not useful for evaluating mechanical pinprick hypersensitivity in humans.

Pain sensitivity related to gamma oscillation of parvalbumin interneuron in primary somatosensory cortex in Dync1i1-/- mice

Cytoplasmic dynein is an important intracellular motor protein that plays an important role in neuronal growth, axonal polarity formation, dendritic differentiation, and dendritic spine development among others. The intermediate chain of dynein, encoded by Dync1i1, plays a vital role in the dynein complex. Therefore, we assessed the behavioral and related neuronal activities in mice with dync1i1 gene knockout. Neuronal activities in primary somatosensory cortex were recorded by in vivo electrophysiology and manipulated by optogenetic and chemogenetics. Nociception of mechanical, thermal, and cold pain in Dync1i1^-/- mice were impaired. The activities of parvalbumin (PV) interneurons and gamma oscillation in primary somatosensory were also impaired when exposed to mechanical nociceptive stimulation. This neuronal dysfunction was rescued by optogenetic activation of PV neurons in Dync1i1^-/- mice, and mimicked by suppressing PV neurons using chemogenetics in WT mice. Impaired pain sensations in Dync1i1^-/- mice were correlated with impaired gamma oscillations due to a loss of interneurons, especially the PV type. This genotype-driven approach revealed an association between impaired pain sensation and cytoplasmic dynein complex.

Transient gamma events delineate somatosensory modality in S1

Gamma band activity localized to the primary somatosensory cortex (S1) in humans and animals is implicated in the higher order neural processing of painful and tactile stimuli. However, it is unclear if gamma band activity differs between these distinct somatosensory modalities. Here, we coupled a novel behavioral approach with chronic extracellular electrophysiology to investigate differences in S1 gamma band activity elicited by noxious and innocuous hind paw stimulation in transgenic mice. Like prior studies, we found that trial-averaged gamma power in S1 increased following both noxious and innocuous stimuli. However, on individual trials, we noticed that evoked gamma band activity was not a continuous oscillatory signal but a series of transient spectral events. Upon further analysis we found that there was a significantly higher incidence of these gamma band events following noxious stimulation than innocuous stimulation. These findings suggest that somatosensory stimuli may be represented by specific features of gamma band activity at the single trial level, which may provide insight to mechanisms underlying acute pain.

Dynamic peripheral nerve stimulation can produce cortical activation similar to punctate mechanical stimuli

During contact, phasic and tonic responses provide feedback that is used for task performance and perceptual processes. These disparate temporal dynamics are carried in peripheral nerves, and produce overlapping signals in cortex. Using longitudinal intrafascicular electrodes inserted into the median nerve of a nonhuman primate, we delivered composite stimulation consisting of onset and release bursts to capture rapidly adapting responses and sustained stochastic stimulation to capture the ongoing response of slowly adapting receptors. To measure the stimulation's effectiveness in producing natural responses, we monitored the local field potential in somatosensory cortex. We compared the cortical responses to peripheral nerve stimulation and vibrotactile/punctate stimulation of the fingertip, with particular focus on gamma band (30-65 Hz) responses. We found that vibrotactile stimulation produces consistently phase locked gamma throughout the duration of the stimulation. By contrast, punctate stimulation responses were phase locked at the onset and release of stimulation, but activity maintained through the stimulation was not phase locked. Using these responses as guideposts for assessing the response to the peripheral nerve stimulation, we found that constant frequency stimulation produced continual phase locking, whereas composite stimulation produced gamma enhancement throughout the stimulus, phase locked only at the onset and release of the stimulus. We describe this response as an "Appropriate Response in the gamma band" (ARγ), a trend seen in other sensory systems. Our demonstration is the first shown for intracortical somatosensory local field potentials. We argue that this stimulation paradigm produces a more biomimetic response in somatosensory cortex and is more likely to produce naturalistic sensations for readily usable neuroprosthetic feedback.

Gamma-band oscillations of pain and nociception: A systematic review and meta-analysis of human and rodent studies

Pain-induced gamma-band oscillations (GBOs) are one of the most promising biomarkers of the pain experience. Although GBOs reliably encode pain perception across different individuals and species, considerable heterogeneity could be observed in the characteristics and functions of GBOs. However, such heterogeneity of GBOs and its underlying sources have rarely been detailed previously. Here, we conducted a systematic review and meta-analysis to characterize the temporal, frequential, and spatial characteristics of GBOs and summarize the functional significance of distinct GBOs. We found that GBO heterogeneity was mainly related to pain types, with a higher frequency (∼66 Hz) GBOs at the sensorimotor cortex elicited by phasic pain and a lower frequency (∼55 Hz) GBOs at the prefrontal cortex associated with tonic and chronic pains. Positive correlations between GBO magnitudes and pain intensity were observed in healthy participants. Notably, the characteristics and functions of GBOs seemed to be phylogenetically conserved across humans and rodents. Altogether, we provided a comprehensive description of heterogeneous GBOs in pain and nociception, laying the foundation for clinical applications of GBOs.

Pain-related gamma band activity is dependent on the features of nociceptive stimuli: a comparison of laser and contact heat

Painful contact heat and laser stimulation offer an avenue to characterize nociceptive pathways involved in acute pain processing, by way of evoked potentials. Direct comparisons of radiant laser and contact heat are limited, particularly in context of examining time-frequency responses to stimulation. This is important in light of recent evidence to suggest that gamma band oscillations (GBOs) represent a functionally heterogeneous measure of pain. The purpose of the current study was to investigate differences in GBOs generated in response to laser and contact heat stimulation of the nondominant forearm. Following intensity matching to pain ratings, evoked electroencephalography (EEG) responses to laser and contact heat stimulation were examined in the time-frequency domain in the same participants (19 healthy adults) across two sessions. At ∼200 ms, both contact heat and laser stimulation resulted in significant, group-level event-related synchronization (ERS) in the low gamma band (i.e., 30-60 Hz) in central electrode locations (Cc, Cz, Ci). Laser stimulation also generated ERS in the 60-100 Hz range (i.e., high gamma), at ∼200 ms, while contact heat led to a significant period of desynchronization in the high gamma range between 400 and 600 ms. Both contact heat and laser GBOs were stronger on the central electrodes contralateral to the stimulated forearm, indicative of primary somatosensory cortex involvement. Based on our findings, and taken in conjunction with previous studies, laser and contact heat stimulation generate characteristically different responses in the brain, with only the former leading to high-frequency GBOs characteristic of painful stimuli.NEW & NOTEWORTHY Despite matching pain perception between noxious laser and contact heat stimuli, we report notable differences in gamma band oscillations (GBO), measured via electroencephalography. GBOs produced following contact heat more closely resembled that of nonnoxious stimuli, while GBOs following laser stimuli were in line with previous reports. Taken together, laser and contact heat stimulation generate characteristically different responses in the brain, with only the former leading to high-frequency GBOs characteristic of painful stimuli.

Somatosensory cortical signature of facial nociception and vibrotactile touch-induced analgesia

Pain relief by vibrotactile touch is a common human experience. Previous neurophysiological investigations of its underlying mechanism in animals focused on spinal circuits, while human studies suggested the involvement of supraspinal pathways. Here, we examine the role of primary somatosensory cortex (S1) in touch-induced mechanical and heat analgesia. We found that, in mice, vibrotactile reafferent signals from self-generated whisking significantly reduce facial nociception, which is abolished by specifically blocking touch transmission from thalamus to the barrel cortex (S1B). Using a signal separation algorithm that can decompose calcium signals into sensory-evoked, whisking, or face-wiping responses, we found that the presence of whisking altered nociceptive signal processing in S1B neurons. Analysis of S1B population dynamics revealed that whisking pushes the transition of the neural state induced by noxious stimuli toward the outcome of non-nocifensive actions. Thus, S1B integrates facial tactile and noxious signals to enable touch-mediated analgesia.

Early Life Pain Experience Changes Adult Functional Pain Connectivity in the Rat Somatosensory and the Medial Prefrontal Cortex

Early life pain (ELP) experience alters adult pain behavior and increases injury-induced pain hypersensitivity, but the effect of ELP on adult functional brain connectivity is not known. We have performed continuous local field potential (LFP) recording in the awake adult male rats to test the effect of ELP on functional cortical connectivity related to pain behavior. Primary somatosensory cortex (S1) and medial prefrontal cortex (mPFC) LFPs evoked by mechanical hindpaw stimulation were recorded simultaneously with pain reflex behavior for 10 d after adult incision injury. We show that, after adult injury, sensory evoked S1 LFP δ and γ energy and S1 LFP δ/γ frequency coupling are significantly increased in ELP rats compared with controls. Adult injury also induces increases in S1-mPFC functional connectivity, but this is significantly prolonged in ELP rats, lasting 4 d compared with 1 d in controls. Importantly, the increases in LFP energy and connectivity in ELP rats were directly correlated with increased behavioral pain hypersensitivity. Thus, ELP alters adult brain functional connectivity, both within and between cortical areas involved in sensory and affective dimensions of pain. The results reveal altered brain connectivity as a mechanism underlying the effects of ELP on adult pain perception.SIGNIFICANCE STATEMENT Pain and stress in early life has a lasting impact on pain behavior and may increase vulnerability to chronic pain in adults. Here, we record pain-related cortical activity and simultaneous pain behavior in awake adult male rats previously exposed to pain in early life. We show that functional connectivity within and between the somatosensory cortex and the medial prefrontal cortex (mPFC) is increased in these rats and that these increases are correlated with their behavioral pain hypersensitivity. The results reveal that early life pain (ELP) alters adult brain connectivity, which may explain the impact of childhood pain on adult chronic pain vulnerability.

Cholinergic basal forebrain nucleus of Meynert regulates chronic pain-like behavior via modulation of the prelimbic cortex

The basal nucleus of Meynert (NBM) subserves critically important functions in attention, arousal and cognition via its profound modulation of neocortical activity and is emerging as a key target in Alzheimer's and Parkinson's dementias. Despite the crucial role of neocortical domains in pain perception, however, the NBM has not been studied in models of chronic pain. Here, using in vivo tetrode recordings in behaving mice, we report that beta and gamma oscillatory activity is evoked in the NBM by noxious stimuli and is facilitated at peak inflammatory pain-like behavior. Optogenetic and chemogenetic cell-specific, reversible manipulations of NBM cholinergic-GABAergic neurons reveal their role in endogenous control of nociceptive hypersensitivity, which are manifest via projections to the prelimbic cortex, resulting in layer 5-mediated antinociception. Our data unravel the importance of the NBM in top-down control of neocortical processing of pain-like behavior.

The role of gamma oscillations in central nervous system diseases: Mechanism and treatment

Gamma oscillation is the synchronization with a frequency of 30–90 Hz of neural oscillations, which are rhythmic electric processes of neuron groups in the brain. The inhibitory interneuron network is necessary for the production of gamma oscillations, but certain disruptions such as brain inflammation, oxidative stress, and metabolic imbalances can cause this network to malfunction. Gamma oscillations specifically control the connectivity between different brain regions, which is crucial for perception, movement, memory, and emotion. Studies have linked abnormal gamma oscillations to conditions of the central nervous system, including Alzheimer’s disease, Parkinson’s disease, and schizophrenia. Evidence suggests that gamma entrainment using sensory stimuli (GENUS) provides significant neuroprotection. This review discusses the function of gamma oscillations in advanced brain activities from both a physiological and pathological standpoint, and it emphasizes gamma entrainment as a potential therapeutic approach for a range of neuropsychiatric diseases.

Neurophysiological characteristics in the periventricular/periaqueductal gray correlate with pain perception, sensation, and affect in neuropathic pain patients

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The PAG/PVG carries out its biology function by oscillatory network.

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Three distinct local networks of oscillations involved in pain perception, sensory and affective.

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The delta oscillation is a key hub for coding pain perception.

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The high-gamma oscillation is a key hub for coding sensory pain.

The periventricular/periaqueductal gray (PAG/PVG) is critical for pain perception and is associated with the emotional feelings caused by pain. However, the electrophysiological characteristics of the PAG/PVG have been little investigated in humans with chronic pain. The present study analyzed the oscillatory characteristics of local field potentials (LFPs) in the PAG/PVG of eighteen neuropathic pain patients. Power spectrum analysis and neural state analysis were applied to the PAG/PVG LFPs. Neural state analysis is based on a dynamic neural state identification approach and discriminates the LFPs into different neural states, including a single neural state based on one oscillation and a combinational neural state based on two paired oscillations. The durations and occurrence rates were used to quantify the dynamic features of the neural state. The results show that the combined neural state forms three local networks based on neural oscillations that are responsible for the perceptive, sensory, and affective components of pain. The first network is formed by the interaction of the delta oscillation with other oscillations and is responsible for the coding of pain perception. The second network is responsible for the coding of sensory pain information, uses high gamma as the main node, and is widely connected with other neural oscillations. The third network is responsible for the coding of affective pain information, and beta oscillations play an important role in it. This study suggested that the combination of two neural oscillations in the PAG/PVG is essential for encoding perceptive, sensory, and affective measures of pain.

Neocortical circuits in pain and pain relief

The sensory, associative and limbic neocortical structures play a critical role in shaping incoming noxious inputs to generate variable pain perceptions. Technological advances in tracing circuitry and interrogation of pathways and complex behaviours are now yielding critical knowledge of neocortical circuits, cellular contributions and causal relationships between pain perception and its abnormalities in chronic pain. Emerging insights into neocortical pain processing suggest the existence of neocortical causality and specificity for pain at the level of subdomains, circuits and cellular entities and the activity patterns they encode. These mechanisms provide opportunities for therapeutic intervention for improved pain management.

Cortical interaction of bilateral inputs is similar for noxious and innocuous stimuli but leads to different perceptual effects

The cerebral integration of somatosensory inputs from multiple sources is essential to produce adapted behaviors. Previous studies suggest that bilateral somatosensory inputs interact differently depending on stimulus characteristics, including their noxious nature. The aim of this study was to clarify how bilateral inputs evoked by noxious laser stimuli, noxious shocks, and innocuous shocks interact in terms of perception and brain responses. The experiment comprised two conditions (right-hand stimulation and concurrent stimulation of both hands) in which painful laser stimuli, painful shocks and non-painful shocks were delivered. Perception, somatosensory-evoked potentials (P45, N100, P260), laser-evoked potentials (N1, N2 and P2) and event-related spectral perturbations (delta to gamma oscillation power) were compared between conditions and stimulus modalities. The amplitude of negative vertex potentials (N2 or N100) and the power of delta/theta oscillations were increased in the bilateral compared with unilateral condition, regardless of the stimulus type (P < 0.01). However, gamma oscillation power increased for painful and non-painful shocks (P < 0.01), but not for painful laser stimuli (P = 0.08). Despite the similarities in terms of brain activity, bilateral inputs interacted differently for painful stimuli, for which perception remained unchanged, and non-painful stimuli, for which perception increased. This may reflect a ceiling effect for the attentional capture by noxious stimuli and warrants further investigations to examine the regulation of such interactions by bottom-up and top-down processes.

Neurobiology of brain oscillations in acute and chronic pain

Pain is a complex perceptual phenomenon. Coordinated activity among local and distant brain networks is a central element of the neural underpinnings of pain. Brain oscillatory rhythms across diverse frequency ranges provide a functional substrate for coordinating activity across local neuronal ensembles and anatomically distant brain areas in pain networks. This review addresses parallels between insights from human and rodent analyses of oscillatory rhythms in acute and chronic pain and discusses recent rodent-based studies that have shed light on mechanistic underpinnings of brain oscillatory dynamics in pain-related behaviors. We highlight the potential for therapeutic modulation of oscillatory rhythms, and identify outstanding questions and challenges to be addressed in future research.

The Prediction of Acute Postoperative Pain Based on Neural Oscillations Measured before the Surgery

Even with an improved understanding of pain mechanisms and advances in perioperative pain management, inadequately controlled postoperative pain remains. Predicting acute postoperative pain based on presurgery physiological measures could provide valuable insights into individualized, effective analgesic strategies, thus helping improve the analgesic efficacy. Considering the strong correlation between pain perception and neural oscillations, we hypothesize that acute postoperative pain could be predicted by neural oscillations measured shortly before the surgery. Here, we explored the relationship between neural oscillations 2 hours before the thoracoscopic surgery and the subjective intensity of acute postoperative pain. The spectral power density of resting-state beta and gamma band oscillations at the frontocentral region was significantly different between patients with different levels of acute postoperative pain (i.e., low pain vs. moderate/high pain). A positive correlation was also observed between the spectral power density of resting-state beta and gamma band oscillations and subjective reports of postoperative pain. Then, we predicted the level of acute postoperative pain based on features of neural oscillations using machine learning techniques, which achieved a prediction accuracy of 92.54% and a correlation coefficient between the real pain intensities and the predicted pain intensities of 0.84. Altogether, the prediction of acute postoperative pain based on neural oscillations measured before the surgery is feasible and could meet the clinical needs in the future for better control of postoperative pain and other unwanted negative effects. The study was registered on the Clinical Trial Registry (https://clinicaltrials.gov/ct2/show/NCT03761576?term=NCT03761576&draw=2&rank=1) with the registration number NCT03761576.

Repetitive non-invasive prefrontal stimulation reverses neuropathic pain via neural remodelling in mice

Chronic neuropathic pain presents a major challenge to pharmacological therapy and neurostimulation-based alternatives are gaining interest. Although invasive and non-invasive motor cortex stimulation has been the focus of several studies, very little is known about the potential of targeting the prefrontal cortex. This study was designed to elucidate the analgesic potential of prefrontal stimulation in a translational context and to uncover the neural underpinnings thereof. Here, we report that non-invasive, repetitive direct anodal current transcranial stimulation (tDCS) of the prefrontal cortex exerted analgesia in mice with neuropathic pain for longer than a week. When applied at chronic stages of neuropathic pain, prefrontal tDCS reversed established allodynia and suppressed aversion and anxiety-related behaviours. Activity mapping as well as in vivo electrophysiological analyses revealed that although the cortex responds to acute tDCS with major excitation, repetitive prefrontal tDCS brings about large-scale silencing of cortical activity. Different classes of different classes of GABAergic interneurons and classes of excitatory neurons differs dramatically between single, acute vs and repetitive tDCS. Repetitive prefrontal tDCS alters basal activity as well as responsivity of a discrete set of distant cortical and sub-cortical areas to tactile stimuli, namely the rostral anterior cingulate cortex, the insular cortex, the ventrolateral periaqueductal grey and the spinal dorsal horn. This study thus makes a strong case for harnessing prefrontal cortical modulation for non-invasive transcranial stimulation paradigms to achieve long-lasting pain relief in established neuropathic pain states and provides valuable insights gained on neural mechanistic underpinnings of prefrontal tDCS in neuropathic pain.

Distinct Age-Dependent C Fiber-Driven Oscillatory Activity in the Rat Somatosensory Cortex

When skin afferents are activated, the sensory signals are transmitted to the spinal cord and eventually reach the primary somatosensory cortex (S1), initiating the encoding of the sensory percept in the brain. While subsets of primary afferents mediate specific somatosensory information from an early age, the subcortical pathways that transmit this information undergo striking changes over the first weeks of life, reflected in the gradual emergence of specific sensory behaviors. We therefore hypothesized that this period is associated with differential changes in the encoding of incoming afferent volleys in S1. To test this, we compared S1 responses to A fiber skin afferent stimulation and A + C skin afferent fiber stimulation in lightly anaesthetized male rats at postnatal day (P)7, P14, P21, and P30. Differences in S1 activity following A and A + C fiber stimulation changed dramatically over this period. At P30, A + C fiber stimulation evoked significantly larger γ, β, and α energy increases compared with A fiber stimulation alone. At younger ages, the changes in S1 oscillatory activity evoked by the two afferent volleys were not significantly different. Silencing TRPV1+ C fibers with QX-314 significantly reduced the γ and β S1 oscillatory energy increases evoked by A + C fibers, at P30 and P21, but not at younger ages. Thus, C fibers differentially modulate S1 oscillatory activity only from the third postnatal week, well after the functional maturation of the somatosensory cortex. This age-related change in afferent evoked S1 oscillatory activity may underpin the maturation of sensory discrimination in the developing brain.

Neural Oscillations: Understanding a Neural Code of Pain

Neural oscillations play an important role in the integration and segregation of brain regions that are important for brain functions, including pain. Disturbances in oscillatory activity are associated with several disease states, including chronic pain. Studies of neural oscillations related to pain have identified several functional bands, especially alpha, beta, and gamma bands, implicated in nociceptive processing. In this review, we introduce several properties of neural oscillations that are important to understand the role of brain oscillations in nociceptive processing. We also discuss the role of neural oscillations in the maintenance of efficient communication in the brain. Finally, we discuss the role of neural oscillations in healthy and chronic pain nociceptive processing. These data and concepts illustrate the key role of regional and interregional neural oscillations in nociceptive processing underlying acute and chronic pains.