Motor cortex and pain control: exploring the descending relay analgesic pathways and spinal nociceptive neurons in healthy conscious rats

Anatomo-physiological basis and applied techniques of electrical neuromodulation in chronic pain

Chronic pain, a complex and debilitating condition, poses a significant challenge to both patients and healthcare providers worldwide. Conventional pharmacological interventions often prove inadequate in delivering satisfactory relief while carrying the risks of addiction and adverse reactions. In recent years, electric neuromodulation emerged as a promising alternative in chronic pain management. This method entails the precise administration of electrical stimulation to specific nerves or regions within the central nervous system to regulate pain signals. Through mechanisms that include the alteration of neural activity and the release of endogenous pain-relieving substances, electric neuromodulation can effectively alleviate pain and improve patients' quality of life. Several modalities of electric neuromodulation, with a different grade of invasiveness, provide tailored strategies to tackle various forms and origins of chronic pain. Through an exploration of the anatomical and physiological pathways of chronic pain, encompassing neurotransmitter involvement, this narrative review offers insights into electrical therapies' mechanisms of action, clinical utility, and future perspectives in chronic pain management.

Brain stimulation for chronic pain management: a narrative review of analgesic mechanisms and clinical evidence

Chronic pain constitutes one of the most common chronic complaints that people experience. According to the International Association for the Study of Pain, chronic pain is defined as pain that persists or recurs longer than 3 months. Chronic pain has a significant impact on individuals' well-being and psychosocial health and the economy of healthcare systems as well. Despite the availability of numerous therapeutic modalities, treatment of chronic pain can be challenging. Only about 30% of individuals with non-cancer chronic pain achieve improvement from standard pharmacological treatment. Therefore, numerous therapeutic approaches were proposed as a potential treatment for chronic pain including non-opioid pharmacological agents, nerve blocks, acupuncture, cannabidiol, stem cells, exosomes, and neurostimulation techniques. Although some neurostimulation methods such as spinal cord stimulation were successfully introduced into clinical practice as a therapy for chronic pain, the current evidence for brain stimulation efficacy in the treatment of chronic pain remains unclear. Hence, this narrative literature review aimed to give an up-to-date overview of brain stimulation methods, including deep brain stimulation, motor cortex stimulation, transcranial direct current stimulation, repetitive transcranial magnetic stimulation, cranial electrotherapy stimulation, and reduced impedance non-invasive cortical electrostimulation as a potential treatment for chronic pain.

Disruption of Periaqueductal Gray-default Mode Network Functional Connectivity in Patients with Crohn's Disease with Abdominal Pain

Abdominal pain in Crohn's disease (CD) has been known to be associated with changes in the central nervous system. The periaqueductal gray (PAG) plays a well-established role in pain processing. However, the role of PAG-related network and the effect of pain on the network in CD remain unclear.Resting-state functional magnetic imaging (fMRI) data were collected from 24 CD patients in remission with abdominal pain, 24 CD patients without abdominal pain and 28 healthy controls (HCs). Using the subregions of PAG (dorsomedial (dmPAG), dorsolateral (dlPAG), lateral (lPAG) and ventrolateral (vlPAG)) as seeds, the seed-based FC maps were calculated and one-way analysis of variance (ANOVA) was performed to investigate the differences among the three groups.Results showed that the group differences were mainly involved in the FC of the vlPAG with the precuneus, medial prefrontal cortex (mPFC) as well as orbitofrontal cortex (OFC), and the FC of the right lateral PAG (lPAG) with the precuneus, inferior parietal lobule (IPL), angular gyrus and premotor cortex. The FC values of all these regions decreased successively in the order of HCs, CD without abdominal pain and CD with abdominal pain. The pain score was negatively correlated with the FC of the l/vlPAG with the precuneus, angular gyrus and mPFC in CD patients with abdominal pain.This study implicated the disrupt communication between the PAG and the default mode network (DMN). These findings complemented neuroimaging evidence for the pathophysiology of visceral pain in CD patients.

The analgesic effect of different interactive modes of virtual reality: A prospective functional near-infrared spectroscopy (fNIRS) study

Virtual reality has demonstrated its analgesic effectiveness. However, its optimal interactive mode for pain relief is yet unclear, with rare objective measurements that were performed to explore its neural mechanism.

OBJECTIVE

This study primarily aimed at investigating the analgesic effect of different VR interactive modes via functional near-infrared spectroscopy (fNIRS) and exploring its correlations with the subjectively reported VR experience through a self-rating questionnaire.

METHODS

Fifteen healthy volunteers (Age: 21.93 ± 0.59 years, 11 female, 4 male) were enrolled in this prospective study. Three rounds of interactive mode, including active mode, motor imagery (MI) mode, and passive mode, were successively facilitated under consistent noxious electrical stimuli (electrical intensity: 23.67 ± 5.69 mA). Repeated-measures of analysis of variance (ANOVA) was performed to examine its pain relief status and cortical activation, with post hoc analysis after Bonferroni correction performed. Spearman's correlation test was conducted to explore the relationship between VR questionnaire (VRQ) items and cortical activation.

RESULTS

A larger analgesic effect on the active (-1.4(95%CI, -2.23 to -0.57), p = 0.001) and MI modes (-0.667(95%CI, -1.165 to -0.168), p = 0.012) was observed compared to the passive mode in the self-rating pain score, with no significant difference reported between the two modes (-0.733(95%CI, -1.631 to.165), p = 0.131), associated with diverse activated cortical region of interest (ROI) in charge of motor and cognitive functions, including the left primary motor cortex (LM1), left dorsal-lateral prefrontal cortex (LDLPFC), left primary somatosensory cortex (LS1), left visual cortex at occipital lobe (LOL), and left premotor cortex (LPMC). On the other hand, significant correlations were found between VRQ items and different cortical ROIs (r = -0.629 to 0.722, p < 0.05) as well as its corresponding channels (r = -0.599 to 0.788, p < 0.05).

CONCLUSION

Our findings suggest that VR can be considered as an effective non-invasive approach for pain relief by modulating cortical pain processing. A better analgesic effect can be obtained by exciting and integrating cortical ROIs in charge of motor and cognitive functions. The interactive mode can be easily tailored to be in line with the client's characteristics, in spite of the diverse cortical activation status when an equivalent analgesic effect can be obtained.

Upregulation of the hypothalamo-neurohypophysial system and activation of vasopressin neurones attenuates hyperalgesia in a neuropathic pain model rat

Arginine vasopressin (AVP) is a hypothalamic neurosecretory hormone well known as an antidiuretic, and recently reported to be involved in pain modulation. The expression kinetics of AVP and its potential involvement in the descending pain modulation system (DPMS) in neuropathic pain (NP) remains unclear. We investigated AVP expression and its effects on mechanical and thermal nociceptive thresholds using a unilateral spinal nerve ligation (SNL) model. All rats with SNL developed NP. Intensities of enhanced green fluorescent protein (eGFP) in the supraoptic and paraventricular nuclei, median eminence, and posterior pituitary were significantly increased at 7 and 14 days post-SNL in AVP-eGFP rats. In situ hybridisation histochemistry revealed significantly increased AVP mRNA expression at 14 days post-SNL compared with the sham control group. The chemogenetic activation of AVP neurones significantly attenuated mechanical and thermal hyperalgesia with elevated plasma AVP concentration. These analgesic effects were suppressed by pre-administration with V1a receptor antagonist. AVP neurones increased the neuronal activity of serotonergic dorsal raphe, noradrenergic locus coeruleus, and inhibitory interneurones in the spinal dorsal horn. These results suggest that the hypothalamo-neurohypophysial system of AVP is upregulated in NP and activated endogenous AVP exerts analgesic effects via the V1a receptors. AVP neurones may activate the DPMS.

Cortical stimulation for chronic pain: from anecdote to evidence

Epidural stimulation of the motor cortex (eMCS) was devised in the 1990's, and has now largely supplanted thalamic stimulation for neuropathic pain relief. Its mechanisms of action involve activation of multiple cortico-subcortical areas initiated in the thalamus, with involvement of endogenous opioids and descending inhibition toward the spinal cord. Evidence for clinical efficacy is now supported by at least seven RCTs; benefits may persist up to 10 years, and can be reasonably predicted by preoperative use of non-invasive repetitive magnetic stimulation (rTMS). rTMS first developed as a means of predicting the efficacy of epidural procedures, then as an analgesic method on its own right. Reasonable evidence from at least six well-conducted RCTs favors a significant analgesic effect of high-frequency rTMS of the motor cortex in neuropathic pain (NP), and less consistently in widespread/fibromyalgic pain. Stimulation of the dorsolateral frontal cortex (DLPFC) has not proven efficacious for pain, so far. The posterior operculo-insular cortex is a new and attractive target but evidence remains inconsistent. Transcranial direct current stimulation (tDCS) is applied upon similar targets as rTMS and eMCS; it does not elicit action potentials but modulates the neuronal resting membrane state. tDCS presents practical advantages including low cost, few safety issues, and possibility of home-based protocols; however, the limited quality of most published reports entails a low level of evidence. Patients responsive to tDCS may differ from those improved by rTMS, and in both cases repeated sessions over a long time may be required to achieve clinically significant relief. Both invasive and non-invasive procedures exert their effects through multiple distributed brain networks influencing the sensory, affective and cognitive aspects of chronic pain. Their effects are mainly exerted upon abnormally sensitized pathways, rather than on acute physiological pain. Extending the duration of long-term benefits remains a challenge, for which different strategies are discussed in this review.

Effects of Exercise-induced Hypoalgesia and Its Neural Mechanisms

PURPOSE

Exercise-induced hypoalgesia is frequently documented in the literature. However, the underlying neural mechanism of this phenomenon remains unclear. Here, we explored the effects of different intensities of isometric exercise on pain perception with a randomized controlled design and investigated its neural mechanisms through tracing the dynamic changes of heat-evoked brain responses.

METHODS

Forty-eight participants were randomly assigned to one of the three groups with different exercise intensities (i.e., high, low, and control). Their subjective pain reports and brain responses elicited by heat stimuli before and after exercise were assessed.

RESULTS

We observed 1) the increased pressure pain thresholds and heat pain thresholds on the dorsal surface of the hand and the biceps brachii muscle of the exercised limb (closed to the contracting muscle), and the decreased pressure pain ratings at the indexed finger of the unexercised limb; 2) more reduction of pain sensitivity on both the biceps brachii muscle and the dorsal surface of the hand induced by the high-intensity isometric exercise than the low-intensity isometric exercise; and 3) both the high-intensity and the low-intensity isometric exercise induced the reduction of N2 amplitudes and N2-P2 peak-to-peak amplitudes, as well as the reduction of event-related potential magnitudes elicited by the heat stimuli on the exercised limb.

CONCLUSIONS

The hypoalgesic effects induced by the isometric exercise were not only localized to the moving part of the body but also can be extended to the distal part of the body. The exercise intensities play a vital role in modulating these effects. Exercise-induced hypoalgesia could be related to the modulation of nociceptive information transmission via a spinal gating mechanism and also rely on a top-down descending pain inhibitory mechanism.

Descending pain modulatory efficiency in healthy subjects is related to structure and resting connectivity of brain regions

The descending pain modulatory system in humans is commonly investigated using conditioned pain modulation (CPM). Whilst variability in CPM efficiency, i.e., inhibition and facilitation, is normal in healthy subjects, exploring the inter-relationship between brain structure, resting-state functional connectivity (rsFC) and CPM readouts will provide greater insight into the underlying CPM efficiency seen in healthy individuals. Thus, this study combined CPM testing, voxel-based morphometry (VBM) and rsFC to identify the neural correlates of CPM in a cohort of healthy subjects (n =40), displaying pain inhibition (n = 29), facilitation (n = 10) and no CPM effect (n = 1). Clusters identified in the VBM analysis were implemented in the rsFC analysis alongside key constituents of the endogenous pain modulatory system. Greater pain inhibition was related to higher volume of left frontal cortices and stronger rsFC between the motor cortex and periaqueductal grey. Conversely, weaker pain inhibition was related to higher volume of the right frontal cortex - coupled with stronger rsFC to the primary somatosensory cortex, and rsFC between the amygdala and posterior insula. Overall, healthy subjects showed higher volume and stronger rsFC of brain regions involved with descending modulation, while the lateral and medial pain systems were related to greater pain inhibition and facilitation during CPM, respectively. These findings reveal structural alignments and functional interactions between supraspinal areas involved in CPM efficiency. Ultimately understanding these underlying variations and how they may become affected in chronic pain conditions, will advance a more targeted subgrouping in pain patients for future cross-sectional studies investigating endogenous pain modulation.

Acupuncture With deqi Modulates the Hemodynamic Response and Functional Connectivity of the Prefrontal-Motor Cortical Network

As a world intangible cultural heritage, acupuncture is considered an essential modality of complementary and alternative therapy to Western medicine. Despite acupuncture’s long history and public acceptance, how the cortical network is modulated by acupuncture remains largely unclear. Moreover, as the basic acupuncture unit for regulating the central nervous system, how the cortical network is modulated during acupuncture at the Hegu acupoint is mostly unclear. Here, multi-channel functional near-infrared spectroscopy (fNIRS) data were recorded from twenty healthy subjects for acupuncture manipulation, pre- and post-manipulation tactile controls, and pre- and post-acupuncture rest controls. Results showed that: (1) acupuncture manipulation caused significantly increased acupuncture behavioral deqi performance compared with tactile controls. (2) The bilateral prefrontal cortex (PFC) and motor cortex were significantly inhibited during acupuncture manipulation than controls, which was evidenced by the decreased power of oxygenated hemoglobin (HbO) concentration. (3) The bilateral PFC’s hemodynamic responses showed a positive correlation trend with acupuncture behavioral performance. (4) The network connections with bilateral PFC as nodes showed significantly increased functional connectivity during acupuncture manipulation compared with controls. (5) Meanwhile, the network’s efficiency was improved by acupuncture manipulation, evidenced by the increased global efficiency and decreased shortest path length. Taken together, these results reveal that a cooperative PFC-Motor functional network could be modulated by acupuncture manipulation at the Hegu acupoint. This study provides neuroimaging evidence that explains acupuncture’s neuromodulation effects on the cortical network.

The primary motor cortex electrical and chemical stimulation attenuates the chronic neuropathic pain by activation of the periaqueductal grey matter: The role of NMDA receptors

BACKGROUND

Motor cortex stimulation (MCS) is proper as a non-pharmacological therapy for patients with chronic and neuropathic pain (NP).

AIMS

This work aims to investigate if the MCS in the primary motor cortex (M₁) produces analgesia and how the MCS could interfere in the MCS-induced analgesia. Also, to elucidate if the persistent activation of N-methyl-d-aspartic acid receptor (NMDAr) in the periaqueductal grey matter (PAG) can contribute to central sensitisation of the NP.

METHODS

Male Wistar rats were submitted to the von Frey test to evaluate the mechanical allodynia after 21 days of chronic constriction injury (CCI) of the sciatic nerve. The MCS was performed with low-frequency (20 μA, 100 Hz) currents during 15 s by a deep brain stimulation (DBS) device. Moreover, the effect of M₁-treatment with an NMDAr agonist (at 2, 4, and 8 nmol) was investigated in CCI rats. The PAG dorsomedial column (dmPAG) was pretreated with the NMDAr antagonist LY 235959 (at 8 nmol), followed by MCS.

RESULTS

The MCS decreased the mechanical allodynia in rats with chronic NP. The M₁-treatment with an NMDA agonist at 2 and 8 nmol reduced the mechanical allodynia in CCI rats. In addition, dmPAG-pretreatment with LY 235959 at 8 nmol attenuated the mechanical allodynia evoked by MCS.

CONCLUSION

The M₁ cortex glutamatergic system is involved in the modulation of chronic NP. The analgesic effect of MCS may depend on glutamate signaling recruitting NMDAr located on PAG neurons in rodents with chronic NP.

Motor cortex stimulation for chronic neuropathic pain: results of a double-blind randomized study

Motor cortex stimulation (MCS) via surgically implanted electrodes has been used as an off-label treatment for chronic neuropathic pain (cNeP) but its efficacy has not been fully established. We aimed to objectively study the efficacy of MCS and characterize potential predictors of response. In this randomised, double-blind, sham-controlled, single centre trial, we recruited 18 cNeP patients who did not adequately respond to conventional treatment and had a numerical rating pain scale (NRS) score ≥ 6. Patients were initially assigned to receive three months of active ("on") or sham ("off") stimulation in a double-blind cross-over phase. This was followed by a 3-month single-blind phase, and 6 months of open-label follow-up. A meaningful response in our trial was defined as a ≥ 30% or 2-point reduction in NRS scores during active stimulation. Using Bayesian statistics, we found a 41.4% probability of response towards "on" vs. "off" MCS. The probability of improvement during active stimulation (double-blind, single-blind and open label phases) compared to baseline was of 47.2-68.5%. 39% of patients were long-term responders, 71.4% of whom had facial pain, phantom limb pain, or complex regional pain syndrome. In contrast, 72.7% of non-responders had either post-stroke pain or pain associated with brachial plexus avulsion. 39% of patients had a substantial post-operative analgesic effect after electrode insertion in the absence of stimulation. Individuals with diagnoses associated with a good postoperative outcome or those who developed an insertional effect had a near 100% probability of response to MCS. In summary, we found that approximately 40% of patients responded to MCS, particularly those who developed an insertional effect or had specific clinical conditions that seemed to predict an appropriate postoperative response.

Increasing Serotonin to Reduce Parkinsonian Tremor

While current dopamine-based drugs seem to be effective for most Parkinson's disease (PD) motor dysfunctions, they produce variable responsiveness for resting tremor. This lack of consistency could be explained by considering recent evidence suggesting that PD resting tremor can be divided into different partially overlapping phenotypes based on the dopamine response. These phenotypes may be associated with different pathophysiological mechanisms produced by a cortical-subcortical network involving even non-dopaminergic areas traditionally not directly related to PD. In this study, we propose a bio-constrained computational model to study the neural mechanisms underlying a possible type of PD tremor: the one mainly involving the serotoninergic system. The simulations run with the model demonstrate that a physiological serotonin increase can partially recover dopamine levels at the early stages of the disease before the manifestation of overt tremor. This result suggests that monitoring serotonin concentration changes could be critical for early diagnosis. The simulations also show the effectiveness of a new pharmacological treatment for tremor that acts on serotonin to recover dopamine levels. This latter result has been validated by reproducing existing data collected with human patients.

Motor Cortex Stimulation Reversed Hypernociception, Increased Serotonin in Raphe Neurons, and Caused Inhibition of Spinal Astrocytes in a Parkinson's Disease Rat Model

Persistent pain is a prevalent symptom of Parkinson’s disease (PD), which is related to the loss of monoamines and neuroinflammation. Motor cortex stimulation (MCS) inhibits persistent pain by activating the descending analgesic pathways; however, its effectiveness in the control of PD-induced pain remains unclear. Here, we evaluated the analgesic efficacy of MCS together with serotonergic and spinal glial modulation in an experimental PD (ePD) rat model. Wistar rats with unilateral striatal 6-OHDA and MCS were assessed for behavioral immobility and nociceptive responses. The immunoreactivity of dopamine in the substantia nigra and serotonin in the nucleus raphe magnus (NRM) and the neuronal, astrocytic, and microglial activation in the dorsal horn of the spinal cord were evaluated. MCS, without interfering with dopamine loss, reversed ePD-induced immobility and hypernociception. This response was accompanied by an exacerbated increase in serotonin in the NRM and a decrease in neuronal and astrocytic hyperactivation in the spinal cord, without inhibiting ePD-induced microglial hypertrophy and hyperplasia. Taken together, MCS induces analgesia in the ePD model, while restores the descending serotonergic pathway with consequent inhibition of spinal neurons and astrocytes, showing the role of MCS in PD-induced pain control.

Suppression of neuropathic pain and comorbidities by recurrent cycles of repetitive transcranial direct current motor cortex stimulation in mice

Transcranial, minimally-invasive stimulation of the primary motor cortex (M1) has recently emerged to show promise in treating clinically refractory neuropathic pain. However, there is a major need for improving efficacy, reducing variability and understanding mechanisms. Rodent models hold promise in helping to overcome these obstacles. However, there still remains a major divide between clinical and preclinical studies with respect to stimulation programs, analysis of pain as a multidimensional sensory-affective-motivational state and lack of focus on chronic phases of established pain. Here, we employed direct transcranial M1 stimulation (M1 tDCS) either as a single 5-day block or recurring blocks of repetitive stimulation over early or chronic phases of peripherally-induced neuropathic pain in mice. We report that repeated blocks of stimulation reverse established neuropathic mechanical allodynia more strongly than a single 5-day regime and also suppress cold allodynia, aversive behavior and anxiety without adversely affecting motor function over a long period. Activity mapping revealed highly selective alterations in the posterior insula, periaqueductal gray subdivisions and superficial spinal laminae in reversal of mechanical allodynia. Our preclinical data reveal multimodal analgesia and improvement in quality of life by multiple blocks of M1 tDCS and uncover underlying brain networks, thus helping promote clinical translation.

Pain at the Slaughterhouse in Ruminants with a Focus on the Neurobiology of Sensitisation

Simple Summary

We pose based on a fundamental science examination that events that occur around the time of slaughter have the potential to intensify the pain response, through a process called sensitisation, or an exaggerated response to painful stimuli. Health conditions which result in inflammation, injuries arising from transport and handling and exaggerated fear responses may all be present at the slaughterhouse. Whilst there is limited evidence of a direct effect of these on the processes of sensitisation in animals at slaughter, by analogy with the human neurobiology literature the connection seems plausible. In this review we outline the biology of such a response, and the rationale for suggestion of a possible linkage between events at slaughter and a heightened animal pain response.

Abstract

We pose, based on a neurobiological examination, that events that occur around the time of slaughter have the potential to intensify the pain response, through the processes of sensitisation and enhanced transmission. Sensitisation, or an enhanced response to painful stimuli, is a well-discussed phenomenon in the human medical literature, which can arise from previous injury to an area, inflammatory reactions, or previous overstimulation of the stress axes. A number of events that occur prior to arrival at, or in the slaughterhouse, may lead to presence of these factors. This includes previous on-farm pathology, injuries arising from transport and handling and lack of habituation to humans. Whilst there is limited evidence of a direct effect of these on the processes of sensitisation in animals at slaughter, by analogy with the human neurobiology literature the connection seems plausible. In this review a neurobiological approach is taken to discuss this hypothesis in the light of basic science, and extrapolations from existing literature on the slaughter of ruminants. To confirm the postulated link between events at slaughter, and processes of hypersensitisation, further dedicated study is required.

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.

NIRS measures in pain and analgesia: Fundamentals, features, and function

Current pain assessment techniques based only on clinical evaluation and self-reports are not objective and may lead to inadequate treatment. Having a functional biomarker will add to the clinical fidelity, diagnosis, and perhaps improve treatment efficacy in patients. While many approaches have been deployed in pain biomarker discovery, functional near-infrared spectroscopy (fNIRS) is a technology that allows for non-invasive measurement of cortical hemodynamics. The utility of fNIRS is especially attractive given its ability to detect specific changes in the somatosensory and high-order cortices as well as its ability to measure (1) brain function similar to functional magnetic resonance imaging, (2) graded responses to noxious and innocuous stimuli, (3) analgesia, and (4) nociception under anesthesia. In this review, we evaluate the utility of fNIRS in nociception/pain with particular focus on its sensitivity and specificity, methodological advantages and limitations, and the current and potential applications in various pain conditions. Everything considered, fNIRS technology could enhance our ability to evaluate evoked and persistent pain across different age groups and clinical populations.

Acupuncture Induces Reduction in Limbic-Cortical Feedback of a Neuralgia Rat Model: A Dynamic Causal Modeling Study

Background

Neuropathic pain after brachial plexus avulsion remained prevalent and intractable currently. However, the neuroimaging study about neural mechanisms or etiology was limited and blurred.

Objective

This study is aimed at investigating the effect of electroacupuncture on effective connectivity and neural response in corticolimbic circuitries during implicit processing of nociceptive stimulus in rats with brachial plexus pain.

Methods

An fMRI scan was performed in a total of 16 rats with brachial plexus pain, which was equally distributed into the model group and the electroacupuncture group. The analysis of task-dependent data determined pain-related activation in each group. Based on those results, several regions including AMY, S1, and h were recruited as ROI in dynamic causal modeling (DCM) analysis comparing evidence for different neuronal hypotheses describing the propagation of noxious stimuli in regions of interest and horizontal comparison of effective connections between the model and electroacupuncture groups.

Results

In both groups, DCM revealed that noxious stimuli were most likely driven by the somatosensory cortex, with bidirectional propagation with the hypothalamus and amygdala and the interactions in them. Also, the 3-month intervention of acupuncture reduced effective connections of h-S1 and AMY-S1.

Conclusions

We showed an evidence that a full connection model within the brain network of brachial plexus pain and electroacupuncture intervention reduces effective connectivity from h and AMY to S1. Our study for the first time explored the relationship of involved brain regions with dynamic causal modeling. It provided novel evidence for the feature of the organization of the cortical-limbic network and the alteration caused by acupuncture.

Connectivity Patterns of Subthalamic Stimulation Influence Pain Outcomes in Parkinson's Disease

Background: Pain is highly prevalent in Parkinson's disease and is associated with significant reduction in health-related quality of life. Subthalamic deep brain stimulation can produce significant pain relief in a subset of patients after surgery. However, the mechanism by which deep brain stimulation modulates sensory function in Parkinson's disease remains uncertain.

Objective: To describe the motor and pain outcomes of deep brain stimulation applied to a series of patients with Parkinson's disease and to determine whether the structural connectivity between the volume of tissue activated and different regions of the brain was associated with the changes of these outcomes after surgery.

Methods: Data from a long-term prospective cohort of 32 Parkinson's disease patients with subthalamic stimulation were combined with available human connectome to identify connections consistently associated with clinical improvement (Unified Parkinson Disease Rating Scale), pain intensity, and experimental cold pain threshold after surgery.

Results: The connectivity between the volume of tissue activated and a distributed network of sensory brain regions (prefrontal, insular and cingulate cortex, and postcentral gyrus) was inversely correlated with pain intensity improvement and reduced sensitivity to cold pain after surgery (p < 0.01). The connectivity strength with the supplementary motor area positively correlated with motor and pain threshold improvement (p < 0.05).

Conclusions: These data suggest that the pattern of the connectivity between the region stimulated and specific brain cortical area might be responsible, in part, for the successful control of motor and pain symptoms by subthalamic deep brain stimulation in Parkinson's disease.