Chronic pain induces generalized enhancement of aversion

An ACC-VTA-ACC positive-feedback loop mediates the persistence of neuropathic pain and emotional consequences

The central mechanisms underlying pain chronicity remain elusive. Here, we identify a reciprocal neuronal circuit in mice between the anterior cingulate cortex (ACC) and the ventral tegmental area (VTA) that mediates mutual exacerbation between hyperalgesia and allodynia and their emotional consequences and, thereby, the chronicity of neuropathic pain. ACC glutamatergic neurons (ACCGlu) projecting to the VTA indirectly inhibit dopaminergic neurons (VTADA) by activating local GABAergic interneurons (VTAGABA), and this effect is reinforced after nerve injury. VTADA neurons in turn project to the ACC and synapse to the initial ACCGlu neurons to convey feedback information from emotional changes. Thus, an ACCGlu-VTAGABA-VTADA-ACCGlu positive-feedback loop mediates the progression to and maintenance of persistent pain and comorbid anxiodepressive-like behavior. Disruption of this feedback loop relieves hyperalgesia and anxiodepressive-like behavior in a mouse model of neuropathic pain, both acutely and in the long term.

Optimizing the use of ketamine to reduce chronic postsurgical pain in women undergoing mastectomy for oncologic indication: study protocol for the KALPAS multicenter randomized controlled trial

BACKGROUND

Mastectomies are commonly performed and strongly associated with chronic postsurgical pain (CPSP), more specifically termed postmastectomy pain syndrome (PMPS), with 25-60% of patients reporting pain 3 months after surgery. PMPS interferes with function, recovery, and compliance with adjuvant therapy. Importantly, it is associated with chronic opioid use, as a recent study showed that 1 in 10 patients continue to use opioids at least 3 months after curative surgery. The majority of PMPS patients are women, and, over the past 10 years, women have outpaced men in the rate of growth in opioid dependence. Standard perioperative multimodal analgesia is only modestly effective in prevention of CPSP. Thus, interventions to reduce CPSP and PMPS are urgently needed. Ketamine is well known to improve pain and reduce opioid use in the acute postoperative period. Additionally, ketamine has been shown to control mood in studies of anxiety and depression. By targeting acute pain and improving mood in the perioperative period, ketamine may be able to prevent the development of CPSP.

METHODS

Ketamine analgesia for long-lasting pain relief after surgery (KALPAS) is a phase 3, multicenter, randomized, placebo-controlled, double-blind trial to study the effectiveness of ketamine in reducing PMPS. The study compares continuous perioperative ketamine infusion vs single-dose ketamine in the postanesthesia care unit vs placebo for reducing PMPS. Participants are followed for 1 year after surgery. The primary outcome is pain at the surgical site at 3 months after the index surgery as assessed with the Brief Pain Inventory-short form pain severity subscale.

DISCUSSION

This project is part of the NIH Helping to End Addiction Long-term (HEAL) Initiative, a nationwide effort to address the opioid public health crisis. This study can substantially impact perioperative pain management and can contribute significantly to combatting the opioid epidemic.

TRIAL REGISTRATION

ClinicalTrials.gov NCT05037123. Registered on September 8, 2021.

Optogenetic Neuromodulation in Inflammatory Pain

Inflammatory pain is one of the most prevalent forms of pain and negatively influences the quality of life. Neuromodulation has been an expanding field of pain medicine and is accepted by patients who have failed to respond to several conservative treatments. Despite its effectiveness, neuromodulation still lacks clinically robust evidence on inflammatory pain management. Optogenetics, which controls particular neurons or brain circuits with high spatiotemporal accuracy, has recently been an emerging area for inflammatory pain management and studying its mechanism. This review considers the fundamentals of optogenetics, including using opsins, targeting gene expression, and wavelength-specific light delivery techniques. The recent evidence on application and development of optogenetic neuromodulation in inflammatory pain is also summarised. The current limitations and challenges restricting the progression and clinical transformation of optogenetics in pain are addressed. Optogenetic neuromodulation in inflammatory pain has many potential targets, and developing strategies enabling clinical application is a desirable therapeutic approach and outcome.

Exploring altered oscillatory activity in the anterior cingulate cortex after nerve injury: Insights into mechanisms of neuropathic allodynia

While neural oscillations play a critical role in sensory perception, it remains unclear how these rhythms function under conditions of neuropathic allodynia. Recent studies demonstrated that the anterior cingulate cortex (ACC) is associated with the affective-aversive component of pain, and plasticity changes in this region are closely linked to abnormal allodynic sensations. Here, to study the mechanisms of allodynia, we recorded local field potentials (LFPs) in the bilateral ACC of awake-behaving rats and compared the spectral power and center frequency of brain oscillations between healthy and CCI (chronic constriction injury) induced neuropathic pain conditions. Our results indicated that activation of the ACC occurs bilaterally in the presence of neuropathic pain, similar to the healthy condition. Furthermore, CCI affects both spontaneous and stimulus-induced activity of ACC neurons. Specifically, we observed an increase in spontaneous beta activity after nerve injury compared to the healthy condition. By stimulating operated or unoperated paws, we found more intense event-related desynchronization (ERD) responses in the theta, alpha, and beta frequency bands and faster alpha center frequency after CCI compared to before CCI. Although the behavioral manifestation of allodynia was more pronounced in the operated paw than the unoperated paw following CCI, there was no significant difference in the center frequency and ERD responses observed in the ACC between stimulation of the operated and unoperated limbs. Our findings offer evidence supporting the notion that aberrancies in ACC oscillations may contribute to the maintenance and development of neuropathic allodynia.

AMPAkines have site-specific analgesic effects in the cortex

Different brain areas have distinct roles in the processing and regulation of pain and thus may form specific pharmacological targets. Prior research has shown that AMPAkines, a class of drugs that increase glutamate signaling, can enhance descending inhibition from the prefrontal cortex (PFC) and nucleus accumbens. On the other hand, activation of neurons in the anterior cingulate cortex (ACC) is known to produce the aversive component of pain. The impact of AMPAkines on ACC, however, is not known. We found that direct delivery of CX516, a well-known AMPAkine, into the ACC had no effect on the aversive response to pain in rats. Furthermore, AMPAkines did not modulate the nociceptive response of ACC neurons. In contrast, AMPAkine delivery into the prelimbic region of the prefrontal cortex (PL) reduced pain aversion. These results indicate that the analgesic effects of AMPAkines in the cortex are likely mediated by the PFC but not the ACC.

Photic sensitization is mediated by cortico-accumbens pathway in rats with trigeminal neuropathic pain

Exposure to light stimuli may trigger or exacerbate perception of pain, also known as a common yet debilitating symptom of photophobia in patient with chronic orofacial pain. Mechanism underlying this phenomenon of photic sensitization in neuropathic condition remains elusive. Here, we found that rats developed hypersensitivity to normal light illumination after establishment of chronic constriction injury of infraorbital nerve (ION-CCI) model, which can be attenuated by blocking the exposure of photic stimulation. Additionally, this behavioral phenotype of light-sensitivity impairment was associated with overexpression of anterior cingulate cortex (ACC) c-fos positive neurons, enhancement of neural excitability in the ACC neurons and its excitatory synaptic transmission between nucleus accumbens (NAc). Optogenetic and chemogenic silencing of ACC-NAc pathway improved trigeminal sensitization in responses to light stimuli by decreasing spontaneous pain-like episodes in ION-CCI animals. In contrast, selective activation of ACC-to-NAc circuits enhanced photic hypersensitivity in dark environment. Thus, our data provided novel role of ACC and its projection to NAc in bidirectional modulation of photic sensation, which may contribute to the understanding of photic allodynia in trigeminal neuropathic pain status.

Changes in alpha, theta, and gamma oscillations in distinct cortical areas are associated with altered acute pain responses in chronic low back pain patients

Introduction

Chronic pain negatively impacts a range of sensory and affective behaviors. Previous studies have shown that the presence of chronic pain not only causes hypersensitivity at the site of injury but may also be associated with pain-aversive experiences at anatomically unrelated sites. While animal studies have indicated that the cingulate and prefrontal cortices are involved in this generalized hyperalgesia, the mechanisms distinguishing increased sensitivity at the site of injury from a generalized site-nonspecific enhancement in the aversive response to nociceptive inputs are not well known.

Methods

We compared measured pain responses to peripheral mechanical stimuli applied to a site of chronic pain and at a pain-free site in participants suffering from chronic lower back pain (n = 15) versus pain-free control participants (n = 15) by analyzing behavioral and electroencephalographic (EEG) data.

Results

As expected, participants with chronic pain endorsed enhanced pain with mechanical stimuli in both back and hand. We further analyzed electroencephalographic (EEG) recordings during these evoked pain episodes. Brain oscillations in theta and alpha bands in the medial orbitofrontal cortex (mOFC) were associated with localized hypersensitivity, while increased gamma oscillations in the anterior cingulate cortex (ACC) and increased theta oscillations in the dorsolateral prefrontal cortex (dlPFC) were associated with generalized hyperalgesia.

Discussion

These findings indicate that chronic pain may disrupt multiple cortical circuits to impact nociceptive processing.

Anterior cingulate cortex regulates pain catastrophizing-like behaviors in rats

Negative pain expectation including pain catastrophizing is a well-known clinical phenomenon whereby patients amplify the aversive value of a painful or oftentimes even a similar, non-painful stimulus. Mechanisms of pain catastrophizing, however, remain elusive. Here, we modeled pain catastrophizing behavior in rats, and found that rats subjected to repeated noxious pin pricks on one paw demonstrated an aversive response to similar but non-noxious mechanical stimuli delivered to the contralateral paw. Optogenetic inhibition of pyramidal neuron activity in the anterior cingulate cortex (ACC) during the application of repetitive noxious pin pricks eliminated this catastrophizing behavior. Time-lapse calcium (Ca2+) imaging in the ACC further revealed an increase in spontaneous neural activity after the delivery of noxious stimuli. Together these results suggest that the experience of repeated noxious stimuli may drive hyperactivity in the ACC, causing increased avoidance of subthreshold stimuli, and that reducing this hyperactivity may play a role in treating pain catastrophizing.

Janus effect of the anterior cingulate cortex: Pain and emotion

Over the past 20 years, clinical and preclinical studies point to the anterior cingulate cortex (ACC) as a site of interest for several neurological and psychiatric conditions. The ACC plays a critical role in emotion, autonomic regulation, pain processing, attention, memory and decision making. An increasing number of studies have demonstrated the involvement of the ACC in the emotional component of pain and its comorbidity with emotional disorders such as anxiety and depression. Thanks to the development of animal models combined with state-of-the-art technologies, we now have a better mechanistic understanding of the functions of the ACC. Hence, the primary aim of this review is to compile the most recent preclinical studies on the role of ACC in the emotional component and consequences of chronic pain. Herein, we thus thoroughly describe the pain-induced electrophysiological, molecular and anatomical alterations in the ACC and in its related circuits. Finally, we discuss the next steps that are needed to strengthen our understanding of the involvement of the ACC in emotional and pain processing.

Perspectives on pain in Down syndrome

Down syndrome (DS) or trisomy 21 is a genetic condition often accompanied by chronic pain caused by congenital abnormalities and/or conditions, such as osteoarthritis, recurrent infections, and leukemia. Although DS patients are more susceptible to chronic pain as compared to the general population, the pain experience in these individuals may vary, attributed to the heterogenous structural and functional differences in the central nervous system, which might result in abnormal pain sensory information transduction, transmission, modulation, and perception. We tried to elaborate on some key questions and possible explanations in this review. Further clarification of the mechanisms underlying such abnormal conditions induced by the structural and functional differences is needed to help pain management in DS patients.

Cannabidiol modulates chronic neuropathic pain aversion behavior by attenuation of neuroinflammation markers and neuronal activity in the corticolimbic circuit in male Wistar rats

Chronic neuropathic pain (CNP) is a vast world health problem often associated with the somatosensory domain. This conceptualization is problematic because, unlike most other sensations that are usually affectively neutral and may present emotional, affective, and cognitive impairments. Neuronal circuits that modulate pain can increase or decrease painful sensitivity based on several factors, including context and expectation. The objective of this study was to evaluate whether subchronic treatment with Cannabidiol (CBD; 0.3, 3, and 10 mg/kg intraperitoneal route - i.p., once a day for 3 days) could promote pain-conditioned reversal, in the conditioned place preference (CPP) test, in male Wistar rats submitted to chronic constriction injury (CCI) of the sciatic nerve. Then, we evaluated the expression of astrocytes and microglia in animals treated with CBD through the immunofluorescence technique. Our results demonstrated that CBD promoted the reversal of CPP at 3 and 10 mg/kg. In CCI animals, CBD was able to attenuate the increase in neuronal hyperactivity, measured by FosB protein expression, in the regions of the corticolimbic circuit: anterior cingulate cortex (ACC), complex basolateral amygdala (BLA), granular layer of the dentate gyrus (GrDG), and dorsal hippocampus (DH) - adjacent to subiculum (CA1). CBD also prevented the increased expression of GFAP and IBA-1 in CCI animals. We concluded that CBD effects on CNP are linked to the modulation of the aversive component of pain. These effects decrease chronic neuronal activation and inflammatory markers in regions of the corticolimbic circuit.

Repetitive nociceptive stimulation elicits complex behavioral changes in Hirudo: evidence of arousal and motivational adaptations

Appropriate responses to real or potential damaging stimuli to the body (nociception) are critical to an animal's short- and long-term survival. The initial goal of this study was to examine habituation of withdrawal reflexes (whole-body and local shortening) to repeated mechanical nociceptive stimuli (needle pokes) in the medicinal leech, Hirudo verbana, and assess whether injury altered habituation to these nociceptive stimuli. While repeated needle pokes did reduce shortening in H. verbana, a second set of behavior changes was observed. Specifically, animals began to evade subsequent stimuli by either hiding their posterior sucker underneath adjacent body segments or engaging in locomotion (crawling). Animals differed in terms of how quickly they adopted evasion behaviors during repeated stimulation, exhibiting a multi-modal distribution for early, intermediate and late evaders. Prior injury had a profound effect on this transition, decreasing the time frame in which animals began to carry out evasion and increasing the magnitude of these evasion behaviors (more locomotory evasion). The data indicate the presence in Hirudo of a complex and adaptive defensive arousal process to avoid noxious stimuli that is influenced by differences in internal states, prior experience with injury of the stimulated areas, and possibly learning-based processes.

In search of a composite biomarker for chronic pain by way of EEG and machine learning: where do we currently stand?

Machine learning is becoming an increasingly common component of routine data analyses in clinical research. The past decade in pain research has witnessed great advances in human neuroimaging and machine learning. With each finding, the pain research community takes one step closer to uncovering fundamental mechanisms underlying chronic pain and at the same time proposing neurophysiological biomarkers. However, it remains challenging to fully understand chronic pain due to its multidimensional representations within the brain. By utilizing cost-effective and non-invasive imaging techniques such as electroencephalography (EEG) and analyzing the resulting data with advanced analytic methods, we have the opportunity to better understand and identify specific neural mechanisms associated with the processing and perception of chronic pain. This narrative literature review summarizes studies from the last decade describing the utility of EEG as a potential biomarker for chronic pain by synergizing clinical and computational perspectives.

Oxytocin promotes prefrontal population activity via the PVN-PFC pathway to regulate pain

Neurons in the prefrontal cortex (PFC) can provide top-down regulation of sensory-affective experiences such as pain. Bottom-up modulation of sensory coding in the PFC, however, remains poorly understood. Here, we examined how oxytocin (OT) signaling from the hypothalamus regulates nociceptive coding in the PFC. In vivo time-lapse endoscopic calcium imaging in freely behaving rats showed that OT selectively enhanced population activity in the prelimbic PFC in response to nociceptive inputs. This population response resulted from the reduction of evoked GABAergic inhibition and manifested as elevated functional connectivity involving pain-responsive neurons. Direct inputs from OT-releasing neurons in the paraventricular nucleus (PVN) of the hypothalamus are crucial to maintaining this prefrontal nociceptive response. Activation of the prelimbic PFC by OT or direct optogenetic stimulation of oxytocinergic PVN projections reduced acute and chronic pain. These results suggest that oxytocinergic signaling in the PVN-PFC circuit constitutes a key mechanism to regulate cortical sensory processing.

Plasticity of cortico-striatal neurons of the caudal anterior cingulate cortex during experimental neuropathic pain

Maladaptive neuronal plasticity is a main mechanism for the development and maintenance of pathological pain. Affective, motivational and cognitive deficits that are comorbid with pain involve cellular and synaptic modifications in the anterior cingulate cortex (ACC), a major brain mediator of pain perception. Here we use a model of neuropathic pain (NP) in male mice and ex-vivo electrophysiology to investigate whether layer 5 caudal ACC (cACC) neurons projecting to the dorsomedial striatum (DMS), a critical region for motivational regulation of behavior, are involved in aberrant neuronal plasticity. We found that while the intrinsic excitability of cortico-striatal cACC neurons (cACC-CS) was preserved in NP animals, excitatory postsynaptic potentials (EPSP) induced after stimulation of distal inputs were enlarged. The highest synaptic responses were evident both after single stimuli and in each of the EPSP that compose responses to trains of stimuli, and were accompanied by increased synaptically-driven action potentials. EPSP temporal summation was intact in ACC-CS neurons from NP mice, suggesting that the plastic changes were not due to alterations in dendritic integration but rather through synaptic mechanisms. These results demonstrate for the first time that NP affects cACC neurons that project to the DMS and reinforce the notion that maladaptive plasticity of the cortico-striatal pathway may be a key factor in sustaining pathological pain.

Interhemispheric neural characteristics of noxious mechano-nociceptive stimulation in the anterior cingulate cortex

Background

Pain is an unpleasant sensory and emotional experience. One of the most critical regions of the brain for pain processing is the anterior cingulate cortex (ACC). Several studies have examined the role of this region in thermal nociceptive pain. However, studies on mechanical nociceptive pain have been very limited to date. Although several studies have investigated pain, the interactions between the two hemispheres are still not clear. This study aimed to investigate nociceptive mechanical pain in the ACC bilaterally.

Methods

Local field potential (LFP) signals were recorded from seven male Wistar rats' ACC regions of both hemispheres. Mechanical stimulations with two intensities, high-intensity noxious (HN) and non-noxious (NN) were applied to the left hind paw. At the same time, the LFP signals were recorded bilaterally from awake and freely moving rats. The recorded signals were analyzed from different perspectives, including spectral analysis, intensity classification, evoked potential (EP) analysis, and synchrony and similarity of two hemispheres.

Results

By using spectro-temporal features and support vector machine (SVM) classifier, HN vs. no-stimulation (NS), NN vs. NS, and HN vs. NN were classified with accuracies of 89.6, 71.1, and 84.7%, respectively. Analyses of the signals from the two hemispheres showed that the EPs in the two hemispheres were very similar and occurred simultaneously; however, the correlation and phase locking value (PLV) between the two hemispheres changed significantly after HN stimulation. These variations persisted for up to 4 s after the stimulation. In contrast, variations in the PLV and correlation for NN stimulation were not significant.

Conclusions

This study showed that the ACC area was able to distinguish the intensity of mechanical stimulation based on the power activities of neural responses. In addition, our results suggest that the ACC region is activated bilaterally due to nociceptive mechanical pain. Additionally, stimulations above the pain threshold (HN) significantly affect the synchronicity and correlation between the two hemispheres compared to non-noxious stimuli.

Learning pain in context: Response-conditioned placebo analgesia and nocebo hyperalgesia in male rats with chronic neuropathic pain

BACKGROUND

Animal models of placebo analgesia and nocebo hyperalgesia have great potential to assist in the development of novel treatments for chronic pain that exploit or inhibit these phenomena. This study sought to elicit both conditioned placebo analgesia and conditioned nocebo hyperalgesia in rats with chronic neuropathic pain using non-pharmacological, contextual conditioning approaches, similar to those most often used in humans.

METHODS

Sciatic nerve-injured male Sprague-Dawley rats (n = 80), and sham controls (n = 16), underwent a conditioning procedure in which three different thermal stimulus intensities (4 °C, 20 °C or 30 °C) were paired with contextual cues. Injured hind paw withdrawal behaviours were used to determine pain sensitivity, and either conditioned analgesia or conditioned hyperalgesia was evoked by re-exposing the rats to the same context with either an increased or decreased thermal stimulus, respectively.

RESULTS

Stronger conditioned analgesia and conditioned hyperalgesia were seen when rats were conditioned in a more complex environment, highlighting the importance of context in these processes. Rats that did not undergo conditioning procedures showed fewer hind paw withdrawals, indicating a learned component to these pain behaviours.

CONCLUSIONS

Our data call attention to context and learning as two critical factors in the development of placebo and nocebo effects in male rodents with a neuropathic injury. Additionally, the response-conditioning model we present in this study affords better comparisons between human and animal studies, in particular for those seeking to identify commonalities in the neurobiological mechanisms of placebo and nocebo responses.

Green light induces antinociception via visual-somatosensory circuits

Light has been shown to relieve pain, but the underlying neural mechanisms remain unknown. Here, we show that low-intensity (200 lux) green light treatment exerts antinociceptive effects through a neural circuit from the visual cortex projecting to the anterior cingulate cortex (ACC) in mice. Specifically, viral tracing, in vivo two-photon calcium imaging, and fiber photometry recordings show that green light activated glutamatergic projections from the medial part of the secondary visual cortex (V2MGlu) to GABAergic neurons in the ACC, which drives inhibition of local glutamatergic neurons (V2MGlu→ACCGABA→Glu). Optogenetic or chemogenetic activation of the V2MGlu→ACCGABA→Glu circuit mimics green-light-induced antinociception in both neuropathic and inflammatory pain model mice. Artificial inhibition of ACC-projecting V2MGlu neurons abolishes the antinociception induced by green light. Taken together, our study shows the V2M-ACC circuit as a potential candidate mediating green-light-induced antinociceptive effects.

A prototype closed-loop brain-machine interface for the study and treatment of pain

Chronic pain is characterized by discrete pain episodes of unpredictable frequency and duration. This hinders the study of pain mechanisms and contributes to the use of pharmacological treatments associated with side effects, addiction and drug tolerance. Here, we show that a closed-loop brain-machine interface (BMI) can modulate sensory-affective experiences in real time in freely behaving rats by coupling neural codes for nociception directly with therapeutic cortical stimulation. The BMI decodes the onset of nociception via a state-space model on the basis of the analysis of online-sorted spikes recorded from the anterior cingulate cortex (which is critical for pain processing) and couples real-time pain detection with optogenetic activation of the prelimbic prefrontal cortex (which exerts top-down nociceptive regulation). In rats, the BMI effectively inhibited sensory and affective behaviours caused by acute mechanical or thermal pain, and by chronic inflammatory or neuropathic pain. The approach provides a blueprint for demand-based neuromodulation to treat sensory-affective disorders, and could be further leveraged for nociceptive control and to study pain mechanisms.

Adrenergic signalling to astrocytes in anterior cingulate cortex contributes to pain-related aversive memory in rats

Pain contains both sensory and affective dimensions. We identify the role of norepinephrine in colorectal distention (sub-threshold for acute pain) induced conditioned place avoidance and plasticity gene expression in the anterior cingulate cortex (ACC). Activating locus coeruleus (LC)-projecting ACC neurons facilitates pain-evoked aversive consolidation and memory, while inhibiting LC-projecting ACC neurons reversibly blocks it. Optogenetic activation of ACC astrocytes facilitates aversive behaviour. ACC astrocytic Gi manipulation suppressed aversive behaviour and early plasticity gene expression induced by opto-activation of LC neurons projecting to ACC. Evidences for the critical role of β2AR in ACC astrocytes were provided using AAV encoding β2AR miRNAi to knockdown β2AR in astrocytes. In contrast, opto-activation of ACC astrocytic β2ARs promotes aversion memory. Our findings suggest that projection-specific adrenergic astrocytic signalling in ACC is integral to system-wide neuromodulation in response to visceral stimuli, and plays a key role in mediating pain-related aversion consolidation and memory formation.

Temporal pain processing in the primary somatosensory cortex and anterior cingulate cortex

Pain is known to have sensory and affective components. The sensory pain component is encoded by neurons in the primary somatosensory cortex (S1), whereas the emotional or affective pain experience is in large part processed by neural activities in the anterior cingulate cortex (ACC). The timing of how a mechanical or thermal noxious stimulus triggers activation of peripheral pain fibers is well-known. However, the temporal processing of nociceptive inputs in the cortex remains little studied. Here, we took two approaches to examine how nociceptive inputs are processed by the S1 and ACC. We simultaneously recorded local field potentials in both regions, during the application of a brain-computer interface (BCI). First, we compared event related potentials in the S1 and ACC. Next, we used an algorithmic pain decoder enabled by machine-learning to detect the onset of pain which was used during the implementation of the BCI to automatically treat pain. We found that whereas mechanical pain triggered neural activity changes first in the S1, the S1 and ACC processed thermal pain with a reasonably similar time course. These results indicate that the temporal processing of nociceptive information in different regions of the cortex is likely important for the overall pain experience.

The rs216009 single-nucleotide polymorphism of the CACNA1C gene is associated with phantom tooth pain

Phantom tooth pain (PTP) is a rare and specific neuropathic pain that occurs after pulpectomy and tooth extraction, but its cause is not understood. We hypothesized that there is a genetic contribution to PTP. The present study focused on the CACNA1C gene, which encodes the α1C subunit of the Cav1.2 L-type Ca2+ channel (LTCC) that has been reported to be associated with neuropathic pain in previous studies. We investigated genetic polymorphisms that contribute to PTP. We statistically examined the association between genetic polymorphisms and PTP vulnerability in 33 patients with PTP and 118 patients without PTP but with pain or dysesthesia in the orofacial region. From within and around the CACNA1C gene, 155 polymorphisms were selected and analyzed for associations with clinical data. We found that the rs216009 single-nucleotide polymorphism (SNP) of the CACNA1C gene in the recessive model was significantly associated with the vulnerability to PTP. Homozygote carriers of the minor C allele of rs216009 had a higher rate of PTP. Nociceptive transmission in neuropathic pain has been reported to involve Ca2+ influx from LTCCs, and the rs216009 polymorphism may be involved in CACNA1C expression, which regulates intracellular Ca2+ levels, leading to the vulnerability to PTP. Furthermore, psychological factors may lead to the development of PTP by modulating the descending pain inhibitory system. Altogether, homozygous C-allele carriers of the rs216009 SNP were more likely to be vulnerable to PTP, possibly through the regulation of intracellular Ca2+ levels and affective pain systems, such as those that mediate fear memory recall.

Pain, from perception to action: A computational perspective

Pain is driven by sensation and emotion, and in turn, it motivates decisions and actions. To fully appreciate the multidimensional nature of pain, we formulate the study of pain within a closed-loop framework of sensory-motor prediction. In this closed-loop cycle, prediction plays an important role, as the interaction between prediction and actual sensory experience shapes pain perception and subsequently, action. In this Perspective, we describe the roles of two prominent computational theories-Bayesian inference and reinforcement learning-in modeling adaptive pain behaviors. We show that prediction serves as a common theme between these two theories, and that each of these theories can explain unique aspects of the pain perception-action cycle. We discuss how these computational theories and models can improve our mechanistic understandings of pain-centered processes such as anticipation, attention, placebo hypoalgesia, and pain chronification.

A novel implantable device for sensory and affective assessment of orofacial pain in rats

Background and objective

Orofacial pain, in particular, chronic orofacial pain remains a great challenge in clinical practice. To better understand the underlying mechanism of disease, it is essential to apply a feasible and stable preclinical measurement of facial pain. Here, we introduced a novel electrical noxious stimulator in freely behavioral rodents and examined its validation in both naïve and chronic orofacial pain animals.

Methods

One subcutaneous device of electrical stimulator was implanted in the facial region for delivery of the nociceptive input. The sensory component of orofacial pain was assessed by response scoring tool, and conditioned place aversion (CPA) paradigm for pain affect respectively. To confirm its usage in chronic pain state, the chronic constriction injury of the infraorbital nerve (ION-CCI) model was then applied.

Results

We found that responsive scores increased with stimulation intensity, and acted in a dosage-dependent manner, which can be attenuated by the administration of morphine intraperitoneally. Naïve rats displayed significant aversive reaction to the noxious electrical stimulation (25V) in the CPA testing. In addition, an obvious sensory hypersensitivity to electrical stimulation was confirmed by the increased response scores in ION-CCI rats. Furthermore, ION-CCI animal showed significant avoidance to electrical stimulation at relatively low intensity (10V), which was innoxious to naïve rats.

Conclusion

Our findings may provide an alternative pre-clinical measurement of orofacial pain, to quantitively assess both sensory and affective component of orofacial pain.

Activation of VIP interneurons in the prefrontal cortex ameliorates neuropathic pain aversiveness

While dysfunction of the medial prefrontal cortex (mPFC) has been implicated in chronic pain, the underlying neural circuits and the contribution of specific cellular populations remain unclear. Using in vivo Ca²⁺ imaging, we report that in both male and female mice, peripheral nerve injury-induced neuropathic pain causes a marked reduction of vasoactive intestinal polypeptide (VIP)-expressing interneuron activity in the prelimbic area of the mPFC, which contributes to decreased prefrontal cortical outputs. Moreover, prelimbic glutamatergic projections to GABAergic interneurons in the anterior cingulate cortex (ACC) are diminished, leading to loss of cortical-cortical inhibition and increased pyramidal neuron activity in the ACC. Chemogenetic activation of prelimbic VIP interneurons restores neuronal responses in the mPFC-ACC pathway and attenuates pain-like behaviors in mice. Furthermore, restoration of prelimbic outputs to the ACC reverses nerve injury-induced ACC hyperactivation. These findings reveal mPFC circuit changes associated with neuropathic pain and highlight VIP interneurons as potential therapeutic targets for pain treatment.

Closed-loop stimulation using a multiregion brain-machine interface has analgesic effects in rodents

Effective treatments for chronic pain remain limited. Conceptually, a closed-loop neural interface combining sensory signal detection with therapeutic delivery could produce timely and effective pain relief. Such systems are challenging to develop because of difficulties in accurate pain detection and ultrafast analgesic delivery. Pain has sensory and affective components, encoded in large part by neural activities in the primary somatosensory cortex (S1) and anterior cingulate cortex (ACC), respectively. Meanwhile, studies show that stimulation of the prefrontal cortex (PFC) produces descending pain control. Here, we designed and tested a brain-machine interface (BMI) combining an automated pain detection arm, based on simultaneously recorded local field potential (LFP) signals from the S1 and ACC, with a treatment arm, based on optogenetic activation or electrical deep brain stimulation (DBS) of the PFC in freely behaving rats. Our multiregion neural interface accurately detected and treated acute evoked pain and chronic pain. This neural interface is activated rapidly, and its efficacy remained stable over time. Given the clinical feasibility of LFP recordings and DBS, our findings suggest that BMI is a promising approach for pain treatment.

Sex-Specific Transcriptomic Signatures in Brain Regions Critical for Neuropathic Pain-Induced Depression

Neuropathic pain is a chronic debilitating condition with a high comorbidity with depression. Clinical reports and animal studies have suggested that both the medial prefrontal cortex (mPFC) and the anterior cingulate cortex (ACC) are critically implicated in regulating the affective symptoms of neuropathic pain. Neuropathic pain induces differential long-term structural, functional, and biochemical changes in both regions, which are thought to be regulated by multiple waves of gene transcription. However, the differences in the transcriptomic profiles changed by neuropathic pain between these regions are largely unknown. Furthermore, women are more susceptible to pain and depression than men. The molecular mechanisms underlying this sexual dimorphism remain to be explored. Here, we performed RNA sequencing and analyzed the transcriptomic profiles of the mPFC and ACC of female and male mice at 2 weeks after spared nerve injury (SNI), an early time point when the mice began to show mild depressive symptoms. Our results showed that the SNI-induced transcriptomic changes in female and male mice were largely distinct. Interestingly, the female mice exhibited more robust transcriptomic changes in the ACC than male, whereas the opposite pattern occurred in the mPFC. Cell type enrichment analyses revealed that the differentially expressed genes involved genes enriched in neurons, various types of glia and endothelial cells. We further performed gene set enrichment analysis (GSEA), which revealed significant de-enrichment of myelin sheath development in both female and male mPFC after SNI. In the female ACC, gene sets for synaptic organization were enriched, and gene sets for extracellular matrix were de-enriched after SNI, while such signatures were absent in male ACC. Collectively, these findings revealed region-specific and sexual dimorphism at the transcriptional levels induced by neuropathic pain, and provided novel therapeutic targets for chronic pain and its associated affective disorders.

Increased burst coding in deep layers of the ventral anterior cingulate cortex during neuropathic pain

Neuropathic pain induces changes in neuronal excitability and synaptic connectivity in deep layers of the anterior cingulate cortex (ACC) that play a central role in the sensory, emotional and affective consequences of the disease. However, how this impacts ACC in vivo activity is not completely understood. Using a mouse model, we found that neuropathic pain caused an increase in ACC in vivo activity, as measured by the indirect activity marker c-Fos and juxtacellular electrophysiological recordings. The enhanced firing rate of ACC neurons in lesioned animals was based on a change in the firing pattern towards bursting activity. Despite the proportion of ACC neurons recruited by noxious stimuli was unchanged during neuropathic pain, responses to noxious stimuli were characterized by increased bursting. Thus, this change in coding pattern may have important implications for the processing of nociceptive information in the ACC and could be of great interest to guide the search for new treatment strategies for chronic pain.

Disrupted population coding in the prefrontal cortex underlies pain aversion

The prefrontal cortex (PFC) regulates a wide range of sensory experiences. Chronic pain is known to impair normal neural response, leading to enhanced aversion. However, it remains unknown how nociceptive responses in the cortex are processed at the population level and whether such processes are disrupted by chronic pain. Using in vivo endoscopic calcium imaging, we identify increased population activity in response to noxious stimuli and stable patterns of functional connectivity among neurons in the prelimbic (PL) PFC from freely behaving rats. Inflammatory pain disrupts functional connectivity of PFC neurons and reduces the overall nociceptive response. Interestingly, ketamine, a well-known neuromodulator, restores the functional connectivity among PL-PFC neurons in the inflammatory pain model to produce anti-aversive effects. These results suggest a dynamic resource allocation mechanism in the prefrontal representations of pain and indicate that population activity in the PFC critically regulates pain and serves as an important therapeutic target.

Li et al. reveal that inflammatory pain disrupts the functional connectivity of neurons in the prelimbic prefrontal cortex (PL-PFC) and the overall nociceptive response. Ketamine, meanwhile, restores the functional connectivity of neurons in the PL-PFC in the inflammatory pain state to produce anti-aversive effects.

Decoding pain from brain activity

Pain is a dynamic, complex and multidimensional experience. The identification of pain from brain activity as neural readout may effectively provide a neural code for pain, and further provide useful information for pain diagnosis and treatment. Advances in neuroimaging and large-scale electrophysiology have enabled us to examine neural activity with improved spatial and temporal resolution, providing opportunities to decode pain in humans and freely behaving animals. This topical review provides a systematical overview of state-of-the-art methods for decoding pain from brain signals, with special emphasis on electrophysiological and neuroimaging modalities. We show how pain decoding analyses can help pain diagnosis and discovery of neurobiomarkers for chronic pain. Finally, we discuss the challenges in the research field and point to several important future research directions.

Optogenetic Stimulation of the Anterior Cingulate Cortex Modulates the Pain Processing in Neuropathic Pain: A Review

Neuropathic pain is characterized by hypersensitivity, hyperalgesia, and allodynia, which is caused by damage to the somatosensory nervous system. It substantially impairs the quality of life. The management of neuropathic pain is challenging and should comprise alternative therapies. Researchers working on neural modulation methods in the field of optogenetics have recently referred to novel techniques that involve the activation or inhibition of signaling proteins by specific wavelengths of light. The use of optogenetics in neuropathic pain facilitates the investigation of pain pathways involved in chronic pain and has the potential for therapeutic use. Neuropathic pain is often accompanied by negative stimuli involving a broad network of brain regions. In particular, the anterior cingulate cortex (ACC) is a part of the limbic system that has highly interconnected structures involved in processing components of pain. The ACC is a key region for acute pain perception as well as the development of neuropathic pain, characterized by long-term potentiation induced in pain pathways. The exact mechanism for neuropathic pain in the ACC is unclear. Current evidence supports the potential of optogenetics methods to modulate the neuronal activity in the ACC for neuropathic pain. We anticipate the neuronal modulation in the ACC will be used widely to manage neuropathic pain.

Frequency Dependent Electrical Stimulation of PFC and ACC for Acute Pain Treatment in Rats

As pain consists of both sensory and affective components, its management by pharmaceutical agents remains difficult. Alternative forms of neuromodulation, such as electrical stimulation, have been studied in recent years as potential pain treatment options. Although electrical stimulation of the brain has shown promise, more research into stimulation frequency and targets is required to support its clinical applications. Here, we studied the effect that stimulation frequency has on pain modulation in the prefrontal cortex (PFC) and the anterior cingulate cortex (ACC) in acute pain models in rats. We found that low-frequency stimulation in the prelimbic region of the PFC (PL-PFC) provides reduction of sensory and affective pain components. Meanwhile, high-frequency stimulation of the ACC, a region involved in processing pain affect, reduces pain aversive behaviors. Our results demonstrate that frequency-dependent neuromodulation of the PFC or ACC has the potential for pain modulation.

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.

Pelvic Pain Alters Functional Connectivity Between Anterior Cingulate Cortex and Hippocampus in Both Humans and a Rat Model

The anterior cingulate cortex (ACC) and hippocampus (HIPP) are two key brain regions associated with pain and pain-related affective processing. However, whether and how pelvic pain alters the neural activity and connectivity of the ACC and HIPP under baseline and during social pain, and the underlying cellular and molecular mechanisms, remain unclear. Using functional magnetic resonance imaging (fMRI) combined with electrophysiology and biochemistry, we show that pelvic pain, particularly, primary dysmenorrhea (PDM), causes an increase in the functional connectivity between ACC and HIPP in resting-state fMRI, and a smaller reduction in connectivity during social exclusion in PDM females with periovulatory phase. Similarly, model rats demonstrate significantly increased ACC-HIPP synchronization in the gamma band, associating with reduced modulation by ACC-theta on HIPP-gamma and increased levels of receptor proteins and excitation. This study brings together human fMRI and animal research and enables improved therapeutic strategies for ameliorating pain and pain-related affective processing.

An orbitofrontal cortex to midbrain projection modulates hypersensitivity after peripheral nerve injury

Neuropathic pain is a debilitating condition that is often refractory to treatment. The network of neural substrates for pain transmission and control within the brain is complex and remains poorly understood. Through a combination of neuronal tracing, optogenetics, chemogenetics, electrophysiological recordings, and behavioral assessment, we demonstrate that activation of layer 5 pyramidal neurons in the ventrolateral orbitofrontal cortex (vlOFC) attenuates mechanical and thermal hypersensitivity and cold allodynia in mice with neuropathic pain induced by spared nerve injury (SNI). These vlOFC output neurons project to the posterior ventrolateral periaqueductal gray (vlPAG) region and receive inputs from the ventromedial thalamus (VM). Specific optogenetic and chemogenetic activation of the vlOFC-vlPAG and the VM-vlOFC circuits inhibits hypersensitivity associated with neuropathy. Thus, we reveal a modulatory role of the vlOFC and its projections to the vlPAG circuit in the processing of hypersensitive nociception.

AMPAkines potentiate the corticostriatal pathway to reduce acute and chronic pain

The corticostriatal circuit plays an important role in the regulation of reward- and aversion-types of behaviors. Specifically, the projection from the prelimbic cortex (PL) to the nucleus accumbens (NAc) has been shown to regulate sensory and affective aspects of pain in a number of rodent models. Previous studies have shown that enhancement of glutamate signaling through the NAc by AMPAkines, a class of agents that specifically potentiate the function of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, reduces acute and persistent pain. However, it is not known whether postsynaptic potentiation of the NAc with these agents can achieve the full anti-nociceptive effects of PL activation. Here we compared the impact of AMPAkine treatment in the NAc with optogenetic activation of the PL on pain behaviors in rats. We found that not only does AMPAkine treatment partially reconstitute the PL inhibition of sensory withdrawals, it fully occludes the effect of the PL on reducing the aversive component of pain. These results indicate that the NAc is likely one of the key targets for the PL, especially in the regulation of pain aversion. Furthermore, our results lend support for neuromodulation or pharmacological activation of the corticostriatal circuit as an important analgesic approach.

Pharmacological restoration of anti-nociceptive functions in the prefrontal cortex relieves chronic pain

Chronic pain affects one in four adults, and effective non-sedating and non-addictive treatments are urgently needed. Chronic pain causes maladaptive changes in the cerebral cortex, which can lead to impaired endogenous nociceptive processing. However, it is not yet clear if drugs that restore endogenous cortical regulation could provide an effective therapeutic strategy for chronic pain. Here, we studied the nociceptive response of neurons in the prelimbic region of the prefrontal cortex (PL-PFC) in freely behaving rats using a spared nerve injury (SNI) model of chronic pain, and the impact of AMPAkines, a class of drugs that increase central glutamate signaling, on such response. We found that neurons in the PL-PFC increase their firing rates in response to noxious stimulations; chronic neuropathic pain, however, suppressed this important cortical pain response. Meanwhile, CX546, a well-known AMPAkine, restored the anti-nociceptive response of PL-PFC neurons in the chronic pain condition. In addition, both systemic administration and direct delivery of CX546 into the PL-PFC inhibited symptoms of chronic pain, whereas optogenetic inactivation of the PFC neurons or administration of AMPA receptor antagonists in the PL-PFC blocked the anti-nociceptive effects of CX546. These results indicate that restoration of the endogenous anti-nociceptive functions in the PL-PFC by pharmacological agents such as AMPAkines constitutes a successful strategy to treat chronic neuropathic pain.

Effect of Electroacupuncture on Pain Perception and Pain-Related Affection: Dissociation or Interaction Based on the Anterior Cingulate Cortex and S1

Electroacupuncture (EA) can effectively modulate pain perception and pain-related negative affect; however, we do not know whether the effect of EA on sensation and affect is parallel, or dissociated, interactional. In this study, we observed the effects of the anterior cingulate cortex (ACC) lesion and the primary somatosensory cortex (S1) activation on pain perception, pain-related affection, and neural oscillation in S1. ACC lesions did not affect pain perception but relieved pain-paired aversion. S1 activation increased pain perception and anxious behavior. EA can mitigate pain perception regardless of whether there is an ACC lesion. Chronic pain may increase the delta and theta band oscillatory activity in the S1 brain region and decrease the oscillatory activity in the alpha, beta, and gamma bands. EA intervention may inhibit the oscillatory activity of the alpha and beta bands. These results suggest that EA may mitigate chronic pain by relieving pain perception and reducing pain-related affection through different mechanisms. This evidence builds upon findings from previous studies of chronic pain and EA treatment.

Ketamine normalizes high-gamma power in the anterior cingulate cortex in a rat chronic pain model

Chronic pain alters cortical and subcortical plasticity, causing enhanced sensory and affective responses to peripheral nociceptive inputs. Previous studies have shown that ketamine had the potential to inhibit abnormally amplified affective responses of single neurons by suppressing hyperactivity in the anterior cingulate cortex (ACC). However, the mechanism of this enduring effect has yet to be understood at the network level. In this study, we recorded local field potentials from the ACC of freely moving rats. Animals were injected with complete Freund’s adjuvant (CFA) to induce persistent inflammatory pain. Mechanical stimulations were administered to the hind paw before and after CFA administration. We found a significant increase in the high-gamma band (60–100 Hz) power in response to evoked pain after CFA treatment. Ketamine, however, reduced the high-gamma band power in response to evoked pain in CFA-treated rats. In addition, ketamine had a sustained effect on the high-gamma band power lasting up to five days after a single dose administration. These results demonstrate that ketamine has the potential to alter maladaptive neural responses in the ACC induced by chronic pain.

A Predictive Coding Model for Evoked and Spontaneous Pain Perception

Pain is a complex multidimensional experience, and pain perception is still incompletely understood. Here we combine animal behavior, electrophysiology, and computer modeling to dissect mechanisms of evoked and spontaneous pain. We record the local field potentials (LFPs) from the primary somatosensory cortex (S1) and anterior cingulate cortex (ACC) of freely behaving rats during pain episodes, and develop a predictive coding model to investigate the temporal coordination of oscillatory activity between the S1 and ACC. Our preliminary results from computational simulations support the experimental findings and provide new predictions.

Mapping Cortical Integration of Sensory and Affective Pain Pathways

Pain is an integrated sensory and affective experience. Cortical mechanisms of sensory and affective integration, however, remain poorly defined. Here, we investigate the projection from the primary somatosensory cortex (S1), which encodes the sensory pain information, to the anterior cingulate cortex (ACC), a key area for processing pain affect, in freely behaving rats. By using a combination of optogenetics, in vivo electrophysiology, and machine learning analysis, we find that a subset of neurons in the ACC receives S1 inputs, and activation of the S1 axon terminals increases the response to noxious stimuli in ACC neurons. Chronic pain enhances this cortico-cortical connection, as manifested by an increased number of ACC neurons that respond to S1 inputs and the magnified contribution of these neurons to the nociceptive response in the ACC. Furthermore, modulation of this S1→ACC projection regulates aversive responses to pain. Our results thus define a cortical circuit that plays a potentially important role in integrating sensory and affective pain signals.

Granger causality analysis of rat cortical functional connectivity in pain

OBJECTIVE

The primary somatosensory cortex (S1) and the anterior cingulate cortex (ACC) are two of the most important cortical brain regions encoding the sensory-discriminative and affective-emotional aspects of pain, respectively. However, the functional connectivity of these two areas during pain processing remains unclear. Developing methods to dissect the functional connectivity and directed information flow between cortical pain circuits can reveal insight into neural mechanisms of pain perception.

APPROACH

We recorded multichannel local field potentials (LFPs) from the S1 and ACC in freely behaving rats under various conditions of pain stimulus (thermal versus mechanical) and pain state (naive versus chronic pain). We applied Granger causality (GC) analysis to the LFP recordings and inferred frequency-dependent GC statistics between the S1 and ACC.

MAIN RESULTS

We found an increased information flow during noxious pain stimulus presentation in both S1[Formula: see text]ACC and ACC[Formula: see text]S1 directions, especially at theta and gamma frequency bands. Similar results were found for thermal and mechanical pain stimuli. The chronic pain state shares common observations, except for further elevated GC measures especially in the gamma band. Furthermore, time-varying GC analysis revealed a negative correlation between the direction-specific and frequency-dependent GC and animal's paw withdrawal latency. In addition, we used computer simulations to investigate the impact of model mismatch, noise, missing variables, and common input on the conditional GC estimate. We also compared the GC results with the transfer entropy (TE) estimates.

SIGNIFICANCE

Our results reveal functional connectivity and directed information flow between the S1 and ACC during various pain conditions. The dynamic GC analysis support the hypothesis of cortico-cortical information loop in pain perception, consistent with the computational predictive coding paradigm.

An Update on the Basic and Clinical Science of Ketamine Analgesia

OBJECTIVE

In the context of the current opioid epidemic, there has been a renewed interest in the use of ketamine as an analgesic agent.

METHODS

We reviewed ketamine analgesia.

RESULTS

Ketamine is well-known as an antagonist for N-methyl-D-aspartate receptors. In addition, it can regulate the function of opioid receptors and sodium channels. Ketamine also increases signaling through α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors. These myriad of molecular and cellular mechanisms are responsible for a number of pharmacological functions including pain relief and mood regulation. Clinically, a number of studies have investigated the role of ketamine in the setting of acute and chronic pain, and there is evidence that ketamine can provide analgesia in a variety of pain syndromes.

DISCUSSION

In this review, we examined basic mechanisms of ketamine and its current clinical use and potential novel use in pain management.

Scaling Up Cortical Control Inhibits Pain

Acute pain evokes protective neural and behavioral responses. Chronic pain, however, disrupts normal nociceptive processing. The prefrontal cortex (PFC) is known to exert top-down regulation of sensory inputs; unfortunately, how individual PFC neurons respond to an acute pain signal is not well characterized. We found that neurons in the prelimbic region of the PFC increased firing rates of the neurons after noxious stimulations in free-moving rats. Chronic pain, however, suppressed both basal spontaneous and pain-evoked firing rates. Furthermore, we identified a linear correlation between basal and evoked firing rates of PFC neurons, whereby a decrease in basal firing leads to a nearly 2-fold reduction in pain-evoked response in chronic pain states. In contrast, enhancing basal PFC activity with low-frequency optogenetic stimulation scaled up prefrontal outputs to inhibit pain. These results demonstrate a cortical gain control system for nociceptive regulation and establish scaling up prefrontal outputs as an effective neuromodulation strategy to inhibit pain.

The role of hedonics in the Human Affectome

Experiencing pleasure and displeasure is a fundamental part of life. Hedonics guide behavior, affect decision-making, induce learning, and much more. As the positive and negative valence of feelings, hedonics are core processes that accompany emotion, motivation, and bodily states. Here, the affective neuroscience of pleasure and displeasure that has largely focused on the investigation of reward and pain processing, is reviewed. We describe the neurobiological systems of hedonics and factors that modulate hedonic experiences (e.g., cognition, learning, sensory input). Further, we review maladaptive and adaptive pleasure and displeasure functions in mental disorders and well-being, as well as the experience of aesthetics. As a centerpiece of the Human Affectome Project, language used to express pleasure and displeasure was also analyzed, and showed that most of these analyzed words overlap with expressions of emotions, actions, and bodily states. Our review shows that hedonics are typically investigated as processes that accompany other functions, but the mechanisms of hedonics (as core processes) have not been fully elucidated.

Increased CXCL13 and CXCR5 in Anterior Cingulate Cortex Contributes to Neuropathic Pain-Related Conditioned Place Aversion

Pain consists of sensory-discriminative and emotional-affective components. The anterior cingulate cortex (ACC) is a critical brain area in mediating the affective pain. However, the molecular mechanisms involved remain largely unknown. Our recent study indicated that C-X-C motif chemokine 13 (CXCL13) and its sole receptor CXCR5 are involved in sensory sensitization in the spinal cord after spinal nerve ligation (SNL). Whether CXCL13/CXCR5 signaling in the ACC contributes to the pathogenesis of pain-related aversion remains unknown. Here, we showed that SNL increased the CXCL13 level and CXCR5 expression in the ACC after SNL. Knockdown of CXCR5 by microinjection of Cxcr5 shRNA into the ACC did not affect SNL-induced mechanical allodynia but effectively alleviated neuropathic pain-related place avoidance behavior. Furthermore, electrophysiological recording from layer II-III neurons in the ACC showed that SNL increased the frequency and amplitude of spontaneous excitatory postsynaptic currents (sEPSCs), decreased the EPSC paired-pulse ratio, and increased the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor/N-methyl-D-aspartate receptor ratio, indicating enhanced glutamatergic synaptic transmission. Finally, superfusion of CXCL13 onto ACC slices increased the frequency and amplitude of spontaneous EPSCs. Pre-injection of Cxcr5 shRNA into the ACC reduced the increase in glutamatergic synaptic transmission induced by SNL. Collectively, these results suggest that CXCL13/CXCR5 signaling in the ACC is involved in neuropathic pain-related aversion via synaptic potentiation.

Cortical Pain Processing in the Rat Anterior Cingulate Cortex and Primary Somatosensory Cortex

Pain is a complex multidimensional experience encompassing sensory-discriminative, affective-motivational and cognitive-emotional components mediated by different neural mechanisms. Investigations of neurophysiological signals from simultaneous recordings of two or more cortical circuits may reveal important circuit mechanisms on cortical pain processing. The anterior cingulate cortex (ACC) and primary somatosensory cortex (S1) represent two most important cortical circuits related to sensory and affective processing of pain. Here, we recorded in vivo extracellular activity of the ACC and S1 simultaneously from male adult Sprague-Dale rats (n = 5), while repetitive noxious laser stimulations were delivered to animalÕs hindpaw during pain experiments. We identified spontaneous pain-like events based on stereotyped pain behaviors in rats. We further conducted systematic analyses of spike and local field potential (LFP) recordings from both ACC and S1 during evoked and spontaneous pain episodes. From LFP recordings, we found stronger phase-amplitude coupling (theta phase vs. gamma amplitude) in the S1 than the ACC (n = 10 sessions), in both evoked (p = 0.058) and spontaneous pain-like behaviors (p = 0.017, paired signed rank test). In addition, pain-modulated ACC and S1 neuronal firing correlated with the amplitude of stimulus-induced event-related potentials (ERPs) during evoked pain episodes. We further designed statistical and machine learning methods to detect pain signals by integrating ACC and S1 ensemble spikes and LFPs. Together, these results reveal differential coding roles between the ACC and S1 in cortical pain processing, as well as point to distinct neural mechanisms between evoked and putative spontaneous pain at both LFP and cellular levels.

Upregulation of tumor necrosis factor-alpha in the anterior cingulate cortex contributes to neuropathic pain and pain-associated aversion

Injury associated pain involves subjective perception and emotional experience. The anterior cingulate cortex (ACC) is a key area involved in the affective component of pain processing. However, the neuroimmune mechanisms underlying enhanced ACC excitability following peripheral nerve injury are still not fully understood. Our previous work has shown that tumor necrosis factor-alpha (TNF-α) overexpression leads to peripheral afferent hyperexcitability and synaptic transmission potentiation in spinal cord. Here, we aimed to reveal the potential role of ACC TNF-α in ACC hyperexcitability and neuropathic pain. c-Fos, a widely used neuronal activity marker, was induced especially in contralateral ACC early [postoperative (PO) 1 h] and later (PO day 7 and 10) during the development of neuropathic pain. Spared nerve injury (SNI) elevated TNF-α level in contralateral ACC from PO day 5 to 14, delayed relative to decreased ipsilateral paw withdrawal threshold apparent from PO day 1 to 14. Microinjection of anti-TNF-α antibody into the ACC completely eliminated c-Fos overexpression and greatly attenuated pain aversion and mechanical allodynia induced by SNI, suggesting an important role of ACC TNF-α in the pain aversiveness and pain maintenance. Furthermore, modulating ACC pyramidal neurons via a Gi-coupled human M4 muscarinic receptor (hM4Di) or a Gq-coupled human M3 muscarinic receptor (hM3Dq), a type of designer receptors exclusively activated by designer drugs (DREADD), greatly changed the ACC TNF-α level and the mechanical paw withdrawal threshold. The positive interactions between TNF-α and ACC neurons might modulate the cytokine microenvironment thus contribute to the neuropathic pain.

Pros and Cons of Clinically Relevant Methods to Assess Pain in Rodents

The primary objective of preclinical pain research is to improve the treatment of pain. Decades of research using pain-evoked tests has revealed much about mechanisms but failed to deliver new treatments. Evoked pain-tests are often limited because they ignore spontaneous pain and motor or disruptive side effects confound interpretation of results. New tests have been developed to focus more closely on clinical goals such as reducing pathological pain and restoring function. The objective of this review is to describe and discuss several of these tests. We focus on: Grimace Scale, Operant Behavior, Wheel Running, Burrowing, Nesting, Home Cage Monitoring, Gait Analysis and Conditioned Place Preference/ Aversion. A brief description of each method is presented along with an analysis of the advantages and limitations. The pros and cons of each test will help researchers identify the assessment tool most appropriate to meet their particular objective to assess pain in rodents. These tests provide another tool to unravel the mechanisms underlying chronic pain and help overcome the translational gap in drug development.