A somatosensory cortex input to the caudal dorsolateral striatum controls comorbid anxiety in persistent pain

A systematic review of the pain-related emotional and cognitive impairments in chronic inflammatory pain induced by CFA injection and its mechanism

Emotional and cognitive impairments are comorbidities commonly associated with chronic inflammatory pain. To summarize the rules and mechanisms of comorbidities in a complete Freund's adjuvant (CFA)-induced pain model, we conducted a systematic review of 66 experimental studies identified in a search of three databases (PubMed, Web of Science, and ScienceDirect). Anxiety-like behaviors developed at 1- or 3-days post-CFA induction but also appeared between 2- and 4 weeks post-induction. Pain aversion, pain depression, and cognitive impairments were primarily observed within 2 weeks, 4 weeks, and 2-4 weeks post-CFA injection, respectively. The potential mechanisms underlying the comorbidities between pain and anxiety predominantly involved heightened neuronal excitability, enhanced excitatory synaptic transmission, and neuroinflammation of anterior cingulate cortex (ACC) and amygdala. The primary somatosensory cortex (S1)Glu→caudal dorsolateral striatum (cDLS)GABA, medial septum (MS)CHAT→rACC, rACCGlu→thalamus, parabrachial nucleus (PBN)→central nucleus amygdala (CeA), mediodorsal thalamus (MD)→basolateral amygdala (BLA), insular cortex (IC)→BLA and anteromedial thalamus nucleus (AM)CaMKⅡ→midcingulate cortex (MCC)CaMKⅡ pathways are enhanced in the pain-anxiety comorbidity. The ventral hippocampal CA1 (vCA1)→BLA and BLA→CeA pathways were decreased in the pain-anxiety comorbidity. The BLA→ACC pathway was enhanced in the pain-depression comorbidity. The infralimbic cortex (IL)→locus coeruleus (LC) pathway was enhanced whereas the vCA1→IL pathway was decreased, in the pain-cognition comorbidity. Inflammation/neuroinflammation, oxidative stress, apoptosis, ferroptosis, gut-brain axis dysfunction, and gut microbiota dysbiosis also contribute to these comorbidities.

Distinct functional cerebral hypersensitivity networks during incisional and inflammatory pain in rats

Although the pathophysiology of pain has been investigated tremendously, there are still many open questions with regard to specific pain entities and their pain-related symptoms. To increase the translational impact of (preclinical) animal neuroimaging pain studies, the use of disease-specific pain models, as well as relevant stimulus modalities, are critical. We developed a comprehensive framework for brain network analysis combining functional magnetic resonance imaging (MRI) with graph-theory (GT) and data classification by linear discriminant analysis. This enabled us to expand our knowledge of stimulus modalities processing under incisional (INC) and pathogen-induced inflammatory (CFA) pain entities compared to acute pain conditions. GT-analysis has uncovered specific features in pain modality processing that align well with those previously identified in humans. These include areas such as S1, M1, CPu, HC, piriform, and cingulate cortex. Additionally, we have identified unique Network Signatures of Pain Hypersensitivity (NSPH) for INC and CFA. This leads to a diminished ability to differentiate between stimulus modalities in both pain models compared to control conditions, while also enhancing aversion processing and descending pain modulation. Our findings further show that different pain entities modulate sensory input through distinct NSPHs. These neuroimaging signatures are an important step toward identifying novel cerebral pain biomarkers for certain diseases and relevant outcomes to evaluate target engagement of novel therapeutic and diagnostic options, which ultimately can be translated to the clinic.

The primary somatosensory sensory cortex-basolateral amygdala pathway contributes to comorbid depression in spared nerve injury-induced neuropathic pain

Comorbid depression in chronic pain is a prevalent health problem, yet the underlying neural mechanisms remain largely unexplored. This study identified a dedicated neural circuit connecting the hind limb region of the primary somatosensory cortex (S1HL) to the basolateral amygdala (BLA) that mediated neuropathic pain-induced depression. We demonstrated that depressive-like behaviors in the chronic phase of a mouse neuropathic pain model were associated with heightened activity in the S1HL and BLA. Using viral tracing and RNAscope in situ hybridization, we characterized the circuit architecture of S1HL glutamatergic projections to BLA cholecystokinin (CCK) neurons (S1HLGlu → BLACCK). In vivo fiber photometry calcium imaging revealed that both the S1HL BLA-projecting afferents and the BLA S1HL-innervating neurons exhibited hyperactivity in neuropathic pain-induced depressive states. Chemogenetic inhibition of the S1HL → BLA circuit could block neuropathic pain-induced depressive-like behaviors. In addition, specific knockdown of CCK expression in BLA S1HL-innervating neurons alleviated these depressive-like behaviors. Our findings demonstrated that the cortical-amygdala circuit S1HLGlu → BLACCK drove the transition from chronic pain to depression, thus suggesting a potential neural circuit basis for treating chronic pain-related depressive disorders.

Temporomandibular disorders and mental health: shared etiologies and treatment approaches

The biopsychosocial model suggests that temporomandibular disorders (TMDs) often coexist with mental health disorders, particularly depression and anxiety, affecting a significant portion of the global population. The interplay between TMDs and mental health disorders contributes to a complex comorbidity, perpetuating a cycle of mutual influence and reinforcement. This review investigates the neurobiological mechanisms and epidemiological evidence supporting the shared etiology of TMDs and mental health disorders, exploring potential shared vulnerabilities and bidirectional causal relationships. Shared vulnerabilities between TMDs and mental health disorders may stem from genetic and epigenetic predispositions, psychosocial factors, and behavioral aspects. Inflammatory cytokines, neurotransmitters, neurotrophins, and neuropeptides play pivotal roles in both peripheral and central sensitization as well as neuroinflammation. Brain imaging studies suggest that TMDs and mental health disorders exhibit overlapping brain regions indicative of reward processing deficits and anomalies within the triple network model. Future research efforts are crucial for developing a comprehensive understanding of the underlying mechanisms and confirming the reciprocal causal effects between TMDs and mental health disorders. This review provides valuable insights for oral healthcare professionals, stressing the importance of optimizing treatment strategies for individuals dealing with concurrent TMDs and mental health issues through a personalized, holistic, and multidisciplinary approach.

Pde4b-regulated cAMP signaling pathway in the AUDGABA-S1TrSst circuit underlies acute-stress-induced anxiety-like behavior

Acute-stress-induced anxiety helps animals avoid danger, but the neural and molecular mechanisms controlling this behavior remain largely elusive. Here, we find that acute physical stress activates many neurons in the primary somatosensory cortex, trunk region (S1Tr). Single-cell sequencing reveals that the S1Tr c-fos-positive neurons activated by acute stress are largely GABAergic somatostatin (Sst) neurons. These S1TrSst neurons desensitize during subsequent anxiety-like behavior tests. Inhibiting or inducing apoptosis of S1TrSst neurons mimics acute-stress effects and induces anxiety, while activating these neurons reduces acute-stress-induced anxiety. S1TrSst cells receive inputs from secondary auditory cortex, dorsal area (AUD) GABAergic neurons to modulate this anxiety. Spatial transcriptome sequencing and targeted Pde4b protein knockdown show that acute stress reduces Pde4b-regulated cAMP signaling in AUDGABA-S1TrSst projections, leading to decreased S1TrSst neuron activity in subsequent behavioral tests. Our study reports a neural and molecular mechanism for acute-stress-induced anxiety, providing a basis for treating anxiety disorders.

Effect of Painful Stimuli on PVNCRH Neurons: Implications for States of Consciousness Under Isoflurane Anesthesia

BACKGROUND

Many patients undergoing surgery experience accompanying pain symptoms preoperatively. The impact of painful stimuli on general anesthesia remains largely unknown. Corticotrophin-releasing hormone neurons in the paraventricular nucleus of the hypothalamus (PVNCRH neurons) are crucial central stress hubs that respond to painful stimuli. These neurons also participate in regulating processes such as sleep and anesthesia. Natural reward can inhibit PVNCRH neurons to relieve stress-induced behavioral changes, but the effect of natural reward on the anesthesia process in patients with pain is not clear. In this study, we assessed the impact of painful stimuli on isoflurane anesthesia and its potential neural mechanism. We also investigated the potential of natural reward therapy for alleviating the impact of painful stimuli on isoflurane anesthesia.

METHODS

The righting reflex test and cortical electroencephalography (EEG) were used as measures of consciousness in complete Freund's adjuvant (CFA)-injected mice during isoflurane anesthesia. EEG and burst-suppression ratios (BSR) were used to assess the depth of anesthesia. The expression of c-Fos, fiber photometry recording, and brain slice electrophysiology were used to determine neuronal activity changes in PVNCRH neurons after CFA injection or 10% sucrose treatment. Finally, chemogenetic technology was used to specifically manipulate PVNCRH neurons.

RESULTS

Compared to the saline-injected mice, the CFA-injected mice exhibited an increased the mean[SD] induction time of isoflurane anesthesia (354[48] s vs 258[30] s, P = .0001) and a reduced BSR of isoflurane anesthesia (60.1[10.3] % vs 81.5[9.76] %, P = .002). CFA injection increased PVN c-Fos expression (3667[706] vs 1735[407], P = .0002) and enhanced the population activity of PVNCRH neurons (33.4[13.6] % vs 1.23[3.57] %, P = .0009). Chemogenetic suppression of PVNCRH neurons reversed the anesthesia abnormalities in CFA-injected mice. Natural reward accelerated the induction time of isoflurane anesthesia (252[24] s vs 324[36] s, P = .003) and increased the BSR of isoflurane anesthesia (84.8[5.36] % vs 57.7[14.3] %, P = .0005). Chemogenetic activation of PVNCRH neurons reversed the effect of natural reward on isoflurane anesthesia in CFA-injected mice.

CONCLUSIONS

Painful stimuli affect the process of isoflurane anesthesia by activating PVNCRH neurons, which implies that these neurons modulate isoflurane anesthesia. Additionally, natural reward alleviates the impact of painful stimuli on isoflurane anesthesia by inhibiting PVNCRH neurons.

Modulating Anxiety-Like Behaviors in Neuropathic Pain: Role of Anterior Cingulate Cortex Astrocytes Activation

AIMS

The comorbidity of anxiety-like symptoms in neuropathic pain (NP) is a significant yet often overlooked health concern. Anxiety sufferers may have a lower tolerance for pain, but which is difficult to treat. Accumulating evidence suggests a strong link between astrocytes and the manifestation of NP with concurrent anxiety-like behaviors. And the anterior cingulate cortex (ACC) has emerged as a key player in pain modulation and related emotional processing. However, the complex mechanisms that astrocytes in ACC influence anxiety behavior in mouse models of NP remain largely unexplored.

METHODS

Utilizing the traditional spared nerve injury (SNI) surgical model, we employed chemogenetic approaches, immunofluorescence, and western blot to investigate the functional significance and interactive dynamics between ACC astrocytes and excitatory neurons.

RESULTS

Our results revealed that SNI surgery induces NP and delayed anxiety-like behaviors, accompanied by increased astrocyte activity in the ACC. Chemogenetic manipulation demonstrated that inhibiting astrocytes alleviates anxiety symptoms, while activating them exacerbates anxiety-like behaviors, affecting local excitatory neurons and synapse density. Direct manipulation of ACC excitatory neurons also significantly impacted anxiety-like behaviors.

CONCLUSION

Our results highlight the pivotal role of ACC astrocytes in modulating anxiety-like behavior, suggesting a novel therapeutic strategy for anxiety associated with NP by targeting astrocyte function.

Berberine alleviates chronic pain-induced anxiety-like behaviors by inhibiting the activation of VLT-projecting cACC (Cg2) neurons

Chronic pain is often accompanied by anxiety, and gradually increasing anxiety makes the pain itself more protracted. Berberine has been found to be able to cross the blood-brain barrier to treat psychiatric disorders, but its neurocirculatory mechanisms remain unclear. Here, we found that neurons in cingulate area 2 (Cg2) of the caudal anterior cingulate cortex (cACC), but not in Cg1 of the cACC, projected to the ventral lateral thalamus (VLT). Next, we induced chronic inflammatory pain by plantar injection of complete Freund's adjuvant (CFA) and observed stable anxiety-like behaviors until two weeks postinjection. We specifically activated VLT-projecting cACC (Cg2) neurons in one-week-old CFA-induced mice without anxiety-like behaviors and in normal control mice to induce anxiety-like behaviors. We inhibited the activation of VLT-projecting cACC (Cg2) neurons in two-week-old CFA-treated mice with anxiety-like behaviors and observed that their anxiety-like behaviors were alleviated. On this basis, we further screened the effective dose of berberine for anxiolysis in two-week-old CFA-treated mice. We observed that the effective dose of berberine obtained above decreased the activity of VLT-projecting cACC (Cg2) neurons. The activation of VLT-projecting cACC (Cg2) neurons abrogated the anxiolytic effect of berberine in two-week-old CFA-treated mice.

A thalamic nucleus reuniens-lateral septum-lateral hypothalamus circuit for comorbid anxiety-like behaviors in chronic itch

Chronic itch often clinically coexists with anxiety symptoms, creating a vicious cycle of itch-anxiety comorbidities that are difficult to treat. However, the neuronal circuit mechanisms underlying the comorbidity of anxiety in chronic itch remain elusive. Here, we report anxiety-like behaviors in mouse models of chronic itch and identify γ-aminobutyric acid-releasing (GABAergic) neurons in the lateral septum (LS) as the key player in chronic itch-induced anxiety. In addition, chronic itch is accompanied with enhanced activity and synaptic plasticity of excitatory projections from the thalamic nucleus reuniens (Re) onto LS GABAergic neurons. Selective chemogenetic inhibition of the Re → LS circuit notably alleviated chronic itch-induced anxiety, with no impact on anxiety induced by restraint stress. Last, GABAergic neurons in lateral hypothalamus (LH) receive monosynaptic inhibition from LS GABAergic neurons to mediate chronic itch-induced anxiety. These findings underscore the potential significance of the Re → LS → LH pathway in regulating anxiety-like comorbid symptoms associated with chronic itch.

Propofol Alleviates Anxiety-Like Behaviors Associated with Pain by Inhibiting the Hyperactivity of PVNCRH Neurons via GABAA Receptor β3 Subunits

Pain, a comorbidity of anxiety disorders, causes substantial clinical, social, and economic burdens. Emerging evidence suggests that propofol, the most commonly used general anesthetic, may regulate psychological disorders; however, its role in pain-associated anxiety is not yet described. This study investigates the therapeutic potential of a single dose of propofol (100 mg kg-1) in alleviating pain-associated anxiety and examines the underlying neural mechanisms. In acute and chronic pain models, propofol decreased anxiety-like behaviors in the elevated plus maze (EPM) and open field (OF) tests. Propofol also reduced the serum levels of stress-related hormones including corticosterone, corticotropin-releasing hormone (CRH), and norepinephrine. Fiber photometry recordings indicated that the calcium signaling activity of CRH neurons in the paraventricular nucleus (PVNCRH) is reduced after propofol treatment. Interestingly, artificially activating PVNCRH neurons through chemogenetics interfered with the anxiety-reducing effects of propofol. Electrophysiological recordings indicated that propofol decreases the activity of PVNCRH neurons by increasing spontaneous inhibitory postsynaptic currents (sIPSCs). Further, reducing the levels of γ-aminobutyric acid type A receptor β3 (GABAAβ3) subunits in PVNCRH neurons diminished the anxiety-relieving effects of propofol. In conclusion, this study provides a mechanistic and preclinical rationale to treat pain-associated anxiety-like behaviors using a single dose of propofol.

HCN channels in the lateral habenula regulate pain and comorbid depressive-like behaviors in mice

AIMS

Comorbid anxiodepressive-like symptoms (CADS) in chronic pain are closely related to the overactivation of the lateral habenula (LHb). Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels have been implicated to play a key role in regulating neuronal excitability. However, the role of HCN channels in the LHb during CADS has not yet been characterized. This study aimed to investigate the effect of HCN channels in the LHb on CADS during chronic pain.

METHODS

After chronic neuropathic pain induction by spared nerve injury (SNI), mice underwent a sucrose preference test, forced swimming test, tail suspension test, open-field test, and elevated plus maze test to evaluate their anxiodepressive-like behaviors. Electrophysiological recordings, immunohistochemistry, Western blotting, pharmacological experiments, and virus knockdown strategies were used to investigate the underlying mechanisms.

RESULTS

Evident anxiodepressive-like behaviors were observed 6w after the SNI surgery, accompanied by increased neuronal excitability, enhanced HCN channel function, and increased expression of HCN2 isoforms in the LHb. Either pharmacological inhibition or virus knockdown of HCN2 channels significantly reduced LHb neuronal excitability and ameliorated both pain and depressive-like behaviors.

CONCLUSION

Our results indicated that the LHb neurons were hyperactive under CADS in chronic pain, and this hyperactivation possibly resulted from the enhanced function of HCN channels and up-regulation of HCN2 isoforms.

Impaired neuronal macroautophagy in the prelimbic cortex contributes to comorbid anxiety-like behaviors in rats with chronic neuropathic pain

A large proportion of patients with chronic pain experience co-morbid anxiety. The medial prefrontal cortex (mPFC) is proposed to underlie this comorbidity, but the molecular and neuronal mechanisms are not fully understood. Here, we reported that impaired neuronal macroautophagy in the prelimbic cortical (PrL) subregion of the mPFC paralleled the occurrence of anxiety-like behaviors in rats with chronic spared nerve injury (SNI). Intriguingly, such macroautophagy impairment was mainly observed in a FOS/c-Fos+ neuronal subpopulation in the PrL. Chemogenetic inactivation of this comorbid anxiety-related neuronal ensemble relieved pain-induced anxiety-like behaviors. Rescuing macroautophagy impairment in this neuronal ensemble relieved chronic pain-associated anxiety and mechanical allodynia and restored synaptic homeostasis at the molecular level. By contrast, artificial disruption of macroautophagy induced early-onset co-morbid anxiety in neuropathic rats, but not general anxiety in normal rats. Taken together, our work identifies causal linkage between PrL neuronal macroautophagy dysfunction and comorbid anxiety in neuropathic pain and provides novel insights into the role of PrL by differentiating its contribution in pain-induced comorbid anxiety from its modulation over general anxiety-like behaviors.Abbreviation: AAV: adeno-associated viruses; ACC: anterior cingulate cortex; ATG5: autophagy related 5; ATG7: autophagy related 7; ATG12: autophagy related 12; CAMK2/CaMKII: calcium/calmodulin-dependent protein kinase II; CNO: clozapine-N-oxide; CQ: chloroquine; DIA: data independent acquisition; DIO: double floxed inverse orf; DLG4/PSD-95: discs large MAGUK scaffold protein 4; Dox: doxycycline; GABA: γ-aminobutyric acid; GFP: green fluorescent protein; GO: gene ontology; Gi: inhibitory guanine nucleotide-binding proteins; HsCHRM4/M4D: human cholinergic receptor muscarinic 4; HsSYN: human synapsin; KEGG: Kyoto encyclopedia of genes and genomes; LAMP1: lysosomal-associated membrane protein 1; LC3-II: PE conjugated microtubule-associated protein 1 light chain3; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; mPFC: medial prefrontal cortex; P2A: 2A self-cleaving peptide; PPI: protein-protein interaction networks; PrL: prelimbic cortex; RBFOX3/NeuN: RNA binding protein, fox-1 homolog (C. elegans) 3; rtTA: reverse tetracycline-transactivator; SDS-PAGE: sodium dodecylsulfate-polyacrylamide gel electrophoresis; SHANK3: SH3 and multiple ankyrin repeat domains 3; SLC1A1/EAAC1: solute carrier family 1 (neuronal/epithelial high affinity glutamate transporter, systemXag), member 1; SNAP23: synaptosomal-associated protein 23; SNI:spared nerve injury; SQSTM1/p62: sequestosome 1; SYT3: synaptotagmin 3; TRE: tetracycline-responsive element; TRE3G: third-generation tetracycline-responsive element.

Vibration-reduced anxiety-like behavior relies on ameliorating abnormalities of the somatosensory cortex and medial prefrontal cortex

Tibetan singing bowls emit low-frequency sounds and produce perceptible harmonic tones and vibrations through manual tapping. The sounds the singing bowls produce have been shown to enhance relaxation and reduce anxiety. However, the underlying mechanism remains unclear. In this study, we used chronic restraint stress or sleep deprivation to establish mouse models of anxiety that exhibit anxiety-like behaviors. We then supplied treatment with singing bowls in a bottomless cage placed on the top of a cushion. We found that unlike in humans, the combination of harmonic tones and vibrations did not improve anxiety-like behaviors in mice, while individual vibration components did. Additionally, the vibration of singing bowls increased the level of N-methyl-D-aspartate receptor 1 in the somatosensory cortex and prefrontal cortex of the mice, decreased the level of γ-aminobutyric acid A (GABA) receptor α 1 subtype, reduced the level of CaMKII in the prefrontal cortex, and increased the number of GABAergic interneurons. At the same time, electrophysiological tests showed that the vibration of singing bowls significantly reduced the abnormal low-frequency gamma oscillation peak frequency in the medial prefrontal cortex caused by stress restraint pressure and sleep deprivation. Results from this study indicate that the vibration of singing bowls can alleviate anxiety-like behaviors by reducing abnormal molecular and electrophysiological events in somatosensory and medial prefrontal cortex.

Electroacupuncture Exerts Analgesic Effects by Restoring Hyperactivity via Cannabinoid Type 1 Receptors in the Anterior Cingulate Cortex in Chronic Inflammatory Pain

As one of the commonly used therapies for pain-related diseases in clinical practice, electroacupuncture (EA) has been proven to be effective. In chronic pain, neurons in the anterior cingulate cortex (ACC) have been reported to be hyperactive, while the mechanism by which cannabinoid type 1 receptors (CB1Rs) in the ACC are involved in EA-mediated analgesic mechanisms remains to be elucidated. In this study, we investigated the potential central mechanism of EA analgesia. A combination of techniques was used to detect the expression and function of CB1R, including quantitative real-time PCR (q-PCR), western blot (WB), immunofluorescence (IF), enzyme-linked immunosorbent assay (ELISA), and in vivo multichannel optical fibre recording, and neuronal activity was examined by in vivo two-photon imaging and in vivo electrophysiological recording. We found that the hyperactivity of pyramidal neurons in the ACC during chronic inflammatory pain is associated with impairment of the endocannabinoid system. EA at the Zusanli acupoint (ST36) can reduce the hyperactivity of pyramidal neurons and exert analgesic effects by increasing the endocannabinoid ligands anandamide (AEA), 2-arachidonoylglycerol (2-AG) and CB1R. More importantly, CB1R in the ACC is one of the necessary conditions for the EA-mediated analgesia effect, which may be related to the negative regulation of the N-methyl-D-aspartate receptor (NMDAR) by the activation of CB1R downregulating NR1 subunits of NMDAR (NR1) via histidine triad nucleotide-binding protein 1 (HINT1). Our study suggested that the endocannabinoid system in the ACC plays an important role in acupuncture analgesia and provides evidence for a central mechanism of EA-mediated analgesia.

A neural circuit associated with anxiety-like behaviors induced by chronic inflammatory pain and the anxiolytic effects of electroacupuncture

AIMS

Negative emotions induced by chronic pain are a serious clinical problem. Electroacupuncture (EA) is a clinically proven safe and effective method to manage pain-related negative emotions. However, the circuit mechanisms underlying the effect of EA treatment on negative emotions remain unclear.

METHODS

Plantar injection of complete Freund's adjuvant (CFA) was performed to establish a rat model of chronic inflammatory pain-induced anxiety-like behaviors. Adeno-associated virus (AAV) tracing was used to identify excitatory synaptic transmission from the rostral anterior cingulate cortex (rACC) to the dorsal raphe nucleus (DRN). Employing chemogenetic approaches, we examined the role of the rACC-DRN circuit in chronic pain-induced anxiety-like behaviors and investigated whether EA could reverse chronic pain-induced dysfunctions of the rACC-DRN circuit and anxiety-like behaviors.

RESULTS

We found that chemogenetic activation of the rACC-DRN circuit alleviated CFA-induced anxiety-like behaviors, while chemogenetic inhibition of the rACC-DRN circuit resulted in short-term CFA-induced anxiety-like behaviors. Further research revealed that the development of CFA-induced anxiety-like behaviors was attributed to the dysfunction of rACC CaMKII neurons projecting to DRN serotonergic neurons (rACCCaMKII-DRN5-HT neurons) but not rACC CaMKII neurons projecting to DRN GABAergic neurons (rACCCaMKII-DRNGABA neurons). This is supported by the findings that chemogenetic activation of the rACCCaMKII-DRN5-HT circuit alleviates anxiety-like behaviors in rats with chronic pain, whereas neither chemogenetic inhibition nor chemogenetic activation of the rACCCaMKII-DRNGABA circuit altered CFA chronic pain-evoked anxiety-like behaviors in rats. More importantly, we found that EA could reverse chronic pain-induced changes in the activity of rACC CaMKII neurons and DRN 5-HTergic neurons and that chemogenetic inhibition of the rACCCaMKII-DRN5-HT circuit blocked the therapeutic effects of EA on chronic pain-induced anxiety-like behaviors.

CONCLUSIONS

Our data suggest that the reversal of rACCCaMKII-DRN5-HT circuit dysfunction may be a mechanism underlying the therapeutic effect of EA on chronic pain-induced anxiety-like behaviors.

Inferior social hierarchy is vulnerable to anxiety-like behavior in chronic pain mice: Potential role of gut microbiota and metabolites

Social dominance is a universal phenomenon among grouped animals that profoundly affects survival, health, and reproductive success by determining access to resources, and exerting a powerful influence on subsequent behavior. However, the understanding of pain and anxiety comorbidities in dominant or subordinate animals suffering from chronic pain is not well-defined. Here, we provide evidence that subordinate mice are more susceptible to pain-induced anxiety compared to dominant mice. We propose that the gut microbiota may play a mediating role in this mechanism. Our findings demonstrate that transplantation of fecal microbiota from subordinate mice with chronic inflammatory pain, but not dominant mice, into antibiotics-treated pseudo-germ-free mice significantly amplifies anxiety-like phenotypes, highlighting the critical involvement of gut microbiota in this behavioral response. Using chronic inflammatory pain model, we carried out 16S rRNA sequencing and untargeted metabolomic analyses to explore the relationship between microbiota and metabolites in a stable social hierarchy of mice. Interestingly, anxiety-like behaviors were directly associated with some microbial genera and metabolites, especially bile acid metabolism. Overall, we have demonstrated a close relationship between social status and anxiety susceptibility, highlighting the contributions of gut microbiota and the associated metabolites in the high-anxiety state of subordinate mice with chronic inflammatory pain.

Corticotropin-releasing hormone neurons control trigeminal neuralgia-induced anxiodepression via a hippocampus-to-prefrontal circuit

Anxiety and depression are frequently observed in patients suffering from trigeminal neuralgia (TN), but neural circuits and mechanisms underlying this association are poorly understood. Here, we identified a dedicated neural circuit from the ventral hippocampus (vHPC) to the medial prefrontal cortex (mPFC) that mediates TN-related anxiodepression. We found that TN caused an increase in excitatory synaptic transmission from vHPCCaMK2A neurons to mPFC inhibitory neurons marked by the expression of corticotropin-releasing hormone (CRH). Activation of CRH+ neurons subsequently led to feed-forward inhibition of layer V pyramidal neurons in the mPFC via activation of the CRH receptor 1 (CRHR1). Inhibition of the vHPCCaMK2A-mPFCCRH circuit ameliorated TN-induced anxiodepression, whereas activating this pathway sufficiently produced anxiodepressive-like behaviors. Thus, our studies identified a neural pathway driving pain-related anxiodepression and a molecular target for treating pain-related psychiatric disorders.

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.

Interleukin-17: A Putative Novel Pharmacological Target for Pathological Pain

Pathological pain imposes a huge burden on the economy and the lives of patients. At present, drugs used for the treatment of pathological pain have only modest efficacy and are also plagued by adverse effects and risk for misuse and abuse. Therefore, understanding the mechanisms of pathological pain is essential for the development of novel analgesics. Several lines of evidence indicate that interleukin-17 (IL-17) is upregulated in rodent models of pathological pain in the periphery and central nervous system. Besides, the administration of IL-17 antibody alleviated pathological pain. Moreover, IL-17 administration led to mechanical allodynia which was alleviated by the IL-17 antibody. In this review, we summarized and discussed the therapeutic potential of targeting IL-17 for pathological pain. The upregulation of IL-17 promoted the development of pathological pain by promoting neuroinflammation, enhancing the excitability of dorsal root ganglion neurons, and promoting the communication of glial cells and neurons in the spinal cord. In general, the existing research shows that IL-17 is an attractive therapeutic target for pathologic pain, but the underlying mechanisms still need to be investigated.

Electroacupuncture alleviates mechanical allodynia and anxiety-like behaviors induced by chronic neuropathic pain via regulating rostral anterior cingulate cortex-dorsal raphe nucleus neural circuit

AIMS

Epidemiological studies in patients with neuropathic pain have demonstrated a strong association between neuropathic pain and psychiatric conditions such as anxiety. Preclinical and clinical work has demonstrated that electroacupuncture (EA) effectively alleviates anxiety-like behaviors induced by chronic neuropathic pain. In this study, a potential neural circuitry underlying the therapeutic action of EA was investigated.

METHODS

The effects of EA stimulation on mechanical allodynia and anxiety-like behaviors in animal models of spared nerve injury (SNI) were examined. EA plus chemogenetic manipulation of glutamatergic (Glu) neurons projecting from the rostral anterior cingulate cortex (rACCGlu ) to the dorsal raphe nucleus (DRN) was used to explore the changes of mechanical allodynia and anxiety-like behaviors in SNI mice.

RESULTS

Electroacupuncture significantly alleviated both mechanical allodynia and anxiety-like behaviors with increased activities of glutamatergic neurons in the rACC and serotoninergic neurons in the DRN. Chemogenetic activation of the rACCGlu -DRN projections attenuated both mechanical allodynia and anxiety-like behaviors in mice at day 14 after SNI. Chemogenetic inhibition of the rACCGlu -DRN pathway did not induce mechanical allodynia and anxiety-like behaviors under physiological conditions, but inhibiting this pathway produced anxiety-like behaviors in mice at day 7 after SNI; this effect was reversed by EA. EA plus activation of the rACCGlu -DRN circuit did not produce a synergistic effect on mechanical allodynia and anxiety-like behaviors. The analgesic and anxiolytic effects of EA could be blocked by inhibiting the rACCGlu -DRN pathway.

CONCLUSIONS

The role of rACCGlu -DRN circuit may be different during the progression of chronic neuropathic pain and these changes may be related to the serotoninergic neurons in the DRN. These findings describe a novel rACCGlu -DRN pathway through which EA exerts analgesic and anxiolytic effects in SNI mice exhibiting anxiety-like behaviors.

Upregulation of Calhm2 in the anterior cingulate cortex contributes to the maintenance of bilateral mechanical allodynia and comorbid anxiety symptoms in inflammatory pain conditions

Peripheral inflammation-induced chronic pain tends to evoke concomitant anxiety disorders. It's common knowledge that the anterior cingulate cortex (ACC) plays a vital role in maintaining pain modulation and negative emotions. However, the potential mechanisms of chronic inflammation pain and pain-related anxiety remain elusive. Here, it was reported that injecting complete Freund's adjuvant (CFA) unilaterally resulted in bilateral mechanical allodynia and anxiety-like symptoms in mice via behavioral tests. In addition, CFA induced the bilateral upregulation and activation of calcium homeostasis modulator 2 (Calhm2) in ACC pyramidal neurons by quantitative analysis and double immunofluorescence staining. The knockdown of Calhm2 in the bilateral ACC by a lentiviral vector harboring ribonucleic acid (RNA) interference sequence reversed CFA-induced pain behaviors and neuronal sensitization. Furthermore, the modulating of ACC pyramidal neuronal activities via a designer receptor exclusively activated by designer drugs (DREADD)-hM4D(Gi) greatly changed Calhm2 expression, mechanical paw withdrawal thresholds (PWTs) and comorbid anxiety symptoms. Moreover, it was found that Calhm2 regulates inflammation pain promoting the upregulation of N-methyl-D-aspartic acid (NMDA) receptor 2B (NR2B) subunits. Calhm2 knockdown in ACC exhibited a significant decrease in NR2B expression. These results demonstrated that Calhm2 in ACC pyramidal neurons modulates chronic inflammation pain and pain-related anxiety symptoms, which provides a novel underlying mechanism for the development of inflammation pain.

Pain in Huntington's disease and its potential mechanisms

Pain is common and frequent in many neurodegenerative diseases, although it has not received much attention. In Huntington's disease (HD), pain is often ignored and under-researched because attention is more focused on motor and cognitive decline than psychiatric symptoms. In HD progression, pain symptoms are complex and involved in multiple etiologies, particularly mental issues such as apathy, anxiety and irritability. Because of psychiatric issues, HD patients rarely complain of pain, although their bodies show severe pain symptoms, ultimately resulting in insufficient awareness and lack of research. In HD, few studies have focused on pain and pain-related features. A detailed and systemic pain history is crucial to assess and explore pain pathophysiology in HD. This review provides an overview concentrating on pain-related factors in HD, including neuropathology, frequency, features, affecting factors and mechanisms. More attention and studies are still needed in this interesting field in the future.

Dim artificial light at night alters immediate early gene expression throughout the avian brain

Artificial light at night (ALAN) is a pervasive pollutant that alters physiology and behavior. However, the underlying mechanisms triggering these alterations are unknown, as previous work shows that dim levels of ALAN may have a masking effect, bypassing the central clock. Light stimulates neuronal activity in numerous brain regions which could in turn activate downstream effectors regulating physiological response. In the present study, taking advantage of immediate early gene (IEG) expression as a proxy for neuronal activity, we determined the brain regions activated in response to ALAN. We exposed zebra finches to dim ALAN (1.5 lux) and analyzed 24 regions throughout the brain. We found that the overall expression of two different IEGs, cFos and ZENK, in birds exposed to ALAN were significantly different from birds inactive at night. Additionally, we found that ALAN-exposed birds had significantly different IEG expression from birds inactive at night and active during the day in several brain areas associated with vision, movement, learning and memory, pain processing, and hormone regulation. These results give insight into the mechanistic pathways responding to ALAN that underlie downstream, well-documented behavioral and physiological changes.

A model for irritable bowel syndrome and anxiety comorbidities in relation to alcohol use disorders

About 95% of human body serotonin synthesis occurs in the gastrointestinal tract (GI). Lack of sufficient serotonin levels is thought to play a key role in mood disorders, including anxiety disorders. In this study, we looked at a disorder affecting the GI tract, irritable bowel syndrome (IBS), and aimed to determine whether IBS is differentially associated with anxiety disorders in 252 chronic pain patients in the presence of a history of alcohol use disorders (AUD) given that alcohol is an extremely aggressive substance for the GI mucosa. We found that while the prevalence of IBS was not affected by the presence of AUD in chronic pain patients, IBS had significantly higher comorbidity with anxiety disorders in chronic pain patients with comorbid alcohol use disorders. We argue that these findings highlight mechanistic differences in the comorbidity of anxiety disorders with chronic pain and AUD, implicating a central role for GI problems stemming from chronic alcohol use. The findings may have important implications for the treatment of IBS patients with AUD who commonly present with anxiety disorders which could motivate the continuation of problematic drinking and impede recovery success. We propose that addressing GI problems in patients with AUD may help manage AUD and recovery more effectively.

Microglia sustain anterior cingulate cortex neuronal hyperactivity in nicotine-induced pain

BACKGROUND

Long-term smoking is a risk factor for chronic pain, and chronic nicotine exposure induces pain-like effects in rodents. The anterior cingulate cortex (ACC) has been demonstrated to be associated with pain and substance abuse. This study aims to investigate whether ACC microglia are altered in response to chronic nicotine exposure and their interaction with ACC neurons and subsequent nicotine-induced allodynia in mice.

METHODS

We utilized a mouse model that was fed nicotine water for 28 days. Brain slices of the ACC were collected for morphological analysis to evaluate the impacts of chronic nicotine on microglia. In vivo calcium imaging and whole-cell patch clamp were used to record the excitability of ACC glutamatergic neurons.

RESULTS

Compared to the vehicle control, the branch endpoints and the length of ACC microglial processes decreased in nicotine-treated mice, coinciding with the hyperactivity of glutamatergic neurons in the ACC. Inhibition of ACC glutamatergic neurons alleviated nicotine-induced allodynia and reduced microglial activation. On the other hand, reactive microglia sustain ACC neuronal excitability in response to chronic nicotine, and pharmacological inhibition of microglia by minocycline or liposome-clodronate reduces nicotine-induced allodynia. The neuron-microglia interaction in chronic nicotine-induced allodynia is mediated by increased expression of neuronal CX3CL1, which activates microglia by acting on CX3CR1 receptors on microglial cells.

CONCLUSION

Together, these findings underlie a critical role of ACC microglia in the maintenance of ACC neuronal hyperactivity and resulting nociceptive hypersensitivity in chronic nicotine-treated mice.

Acid-sensing ion channel 1a in the central nucleus of the amygdala regulates anxiety-like behaviors in a mouse model of acute pain

Pain is commonly comorbid with anxiety; however, the neural and molecular mechanisms underlying the comorbid anxiety symptoms in pain (CASP) have not been fully elucidated. In this study, we explored the role of acid-sensing ion channel 1a (ASIC1a), located in GABAergic neurons from the central nucleus of the amygdala (GABA^CeA), in the regulation of CASP in an acute pain mouse model. We found that the mice displayed significant mechanical pain sensitization and anxiety-like behaviors one day post injection of complete Freud's adjuvant (CFA1D). Electrophysiological recordings from acute brain slices showed that the activity of GABA^CeA neurons increased in the CFA1D mice compared with that in the saline mice. In addition, chemogenetic inhibition of GABA^CeA neurons relieved mechanical pain sensitization and anxiety-like behaviors in the CFA1D mice. Interestingly, through pharmacological inhibition and genetic knockdown of ASIC1a in the central nucleus amygdala, we found that downregulation of ASIC1a relieved the hypersensitization of mechanical stimuli and alleviated anxiety-related behaviors, accompanied with reversing the hyperactivity of GABA^CeA neurons in the CFA 1D mice. In conclusion, our results provide novel insights that ASIC1a in GABA^CeA neurons regulates anxiety-like behaviors in a mouse model of acute pain.

Activation of the VpdmVGLUT1-VPM pathway contributes to anxiety-like behaviors induced by malocclusion

Occlusal disharmony has a negative impact on emotion. The mesencephalic trigeminal nucleus (Vme) neurons are the primary afferent nuclei that convey proprioceptive information from proprioceptors and low-threshold mechanoreceptors in the periodontal ligament and jaw muscles in the cranio-oro-facial regions. The dorsomedial part of the principal sensory trigeminal nucleus (Vpdm) and the ventral posteromedial nucleus (VPM) of thalamus have been proven to be crucial relay stations in ascending pathway of proprioception. The VPM sends numerous projections to primary somatosensory areas (SI), which modulate emotion processing. The present study aimed to demonstrate the ascending trigeminal-thalamic-cortex pathway which would mediate malocclusion-induced negative emotion. Unilateral anterior crossbite (UAC) model created by disturbing the dental occlusion was applied. Tract-tracing techniques were used to identify the existence of Vme-Vpdm-VPM pathway and Vpdm-VPM-SI pathway. Chemogenetic and optogenetic methods were taken to modulate the activation of Vpdm^VGLUT1 neurons and the Vpdm-VPM pathway. Morphological evidence indicated the involvement of the Vme-Vpdm-VPM pathway, Vpdm-VPM-SI pathway and Vpdm^VGLUT1-VPM pathway in orofacial proprioception in wild-type mice and vesicular glutamate transporter 1 (^VGLUT1): tdTomato mice, respectively. Furthermore, chemogenetic inhibition of Vpdm^VGLUT1 neurons and the Vpdm-VPM pathway alleviated anxiety-like behaviors in a unilateral anterior crossbite (UAC) model, whereas chemogenetic activation induced anxiety-like behaviors in controls and did not aggravate these behaviors in UAC mice. Finally, optogenetic inhibition of the Vpdm^VGLUT1-VPM pathway in VGLUT1-IRES-Cre mice reversed UAC-induced anxiety comorbidity. In conclusion, these results suggest that the Vpdm^VGLUT1-VPM neural pathway participates in the modulation of malocclusion-induced anxiety comorbidity. These findings provide new insights into the links between occlusion and emotion and deepen our understanding of the impact of occlusal disharmony on brain dysfunction.

The anterior cingulate cortex contributes to the analgesic rather than the anxiolytic effects of duloxetine in chronic pain-induced anxiety

Mood disorders, such as anxiety and depression, are commonly found in people suffering from chronic pain. Serotonin-norepinephrine reuptake inhibitors (SNRIs) are potential in alleviating chronic pain and are the first-line option for anxiety disorder. The anterior cingulate cortex (ACC) plays a vital role in chronic pain-induced anxiety, but its role in the therapeutic effects of SNRIs remains largely unclear. We used complete Freund's adjuvant (CFA) in this current study to induce chronic inflammatory pain. Von Frey test was used to measure the mechanical withdrawal threshold. The elevated plus maze test (EPM) and the novelty-suppressed feeding test (NSF) were used to measure anxiety-like behaviors. Twenty-one days after the modeling, anxiety-like behaviors were successfully induced in CFA mice, and a 3-day intraperitoneal injection of duloxetine attenuated such behaviors. While, mechanical hyperalgesia was also improved. Then, we locally infused duloxetine in ACC for 3 days only to find out its analgesic effect in CFA mice. Furthermore, we used fiber photometry to discover decreased glutamatergic excitability and enhanced serotonin concentration in ACC after intraperitoneal injection of duloxetine. Overall, this study proposed a potential mechanism for the analgesic effect of duloxetine and shed light on further studies on the mechanism of its anxiolytic effect in chronic pain-induced anxiety.

Electroacupuncture alleviates orofacial allodynia and anxiety-like behaviors by regulating synaptic plasticity of the CA1 hippocampal region in a mouse model of trigeminal neuralgia

Objective

Trigeminal neuralgia (TN), one of the most severe and debilitating chronic pain conditions, is often accompanied by mood disorders, such as anxiety and depression. Electroacupuncture (EA) is a characteristic therapy of Traditional Chinese Medicine with analgesic and anxiolytic effects. This study aimed to investigate whether EA ameliorates abnormal TN orofacial pain and anxiety-like behavior by altering synaptic plasticity in the hippocampus CA1.

Materials and methods

A mouse infraorbital nerve transection model (pT-ION) of neuropathic pain was established, and EA or sham EA was used to treat ipsilateral acupuncture points (GV20-Baihui and ST7-Xiaguan). Golgi-Cox staining and transmission electron microscopy (TEM) were administrated to observe the changes of synaptic plasticity in the hippocampus CA1.

Results

Stable and persistent orofacial allodynia and anxiety-like behaviors induced by pT-ION were related to changes in hippocampal synaptic plasticity. Golgi stainings showed a decrease in the density of dendritic spines, especially mushroom-type dendritic spines, in hippocampal CA1 neurons of pT-ION mice. TEM results showed that the density of synapses, membrane thickness of the postsynaptic density, and length of the synaptic active zone were decreased, whereas the width of the synaptic cleft was increased in pT-ION mice. EA attenuated pT-ION-induced orofacial allodynia and anxiety-like behaviors and effectively reversed the abnormal changes in dendritic spines and synapse of the hippocampal CA1 region.

Conclusion

EA modulates synaptic plasticity of hippocampal CA1 neurons, thereby reducing abnormal orofacial pain and anxiety-like behavior. This provides evidence for a TN treatment strategy.

The effects of low intensity focused ultrasound on neuronal activity in pain processing regions in a rodent model of common peroneal nerve injury

BACKGROUND

Non-invasive, external low intensity focused ultrasound (liFUS) offers promise for treating neuropathic pain when applied to the dorsal root ganglion (DRG).

OBJECTIVE

We examine how external liFUS treatment applied to the L5 DRG affects neuronal changes in single-unit activity from the primary somatosensory cortex (SI) and anterior cingulate cortex (ACC) in a common peroneal nerve injury (CPNI) rodent model.

METHODS

Male Sprague Dawley rats were divided into two cohorts: CPNI liFUS and CPNI sham liFUS. Baseline single-unit activity (SUA) recordings were taken 20 min prior to treatment and for 4 h post treatment in 20 min intervals, then analyzed for frequency and compared to baseline. Recordings from the SI and ACC were separated into pyramidal and interneurons based on waveform and principal component analysis.

RESULTS

Following CPNI surgery, all rats (n = 30) displayed a significant increase in mechanical sensitivity. In CPNI liFUS rats, there was a significant increase in pyramidal neuron spike frequency in the SI region compared to the CPNI sham liFUS animals beginning at 120 min following liFUS treatment (p < 0.05). In the ACC, liFUS significantly attenuated interneuron firing beginning at 80 min after liFUS treatment (p < 0.05).

CONCLUSION

We demonstrate that liFUS changed neuronal spiking in the SI and ACC regions 80 and 120 min after treatment, respectively, which may in part correlate with improved sensory thresholds. This may represent a mechanism of action how liFUS attenuates neuropathic pain. Understanding the impact of liFUS on pain circuits will help advance the use of liFUS as a non-invasive neuromodulation option.

Microglial Engulfment of Spines in the Ventral Zona Incerta Regulates Anxiety-Like Behaviors in a Mouse Model of Acute Pain

Although activation of microglial cells is critical in developing brain disorders, their role in anxiety-like behaviors in pain is still vague. This study indicates that alteration of microglia’s neuronal spine engulfment capacity in ventral zona incerta (ZI_V) leads to significant pain and anxiety-like behaviors in mice 1-day post-injection of Complete Freud’s Adjuvant (CFA1D). Performing whole-cell patch-clamp recordings in GABAergic neurons in the ZI_V (ZI_V^GABA) in brain slices, we observed decreased activity in ZIv^GABA and reduced frequency of the miniature excitatory postsynaptic currents (mEPSCs) in ZI_V^GABA of CFA1D mice compared with the saline1D mice. Besides, chemogenetic activation of ZI_V^GABA significantly relieved pain and anxiety-like behaviors in CFA1D mice. Conversely, in naïve mice, chemogenetic inhibition of ZI_V^GABA induced pain and anxiety-like behaviors. Interestingly, we found changes in the density and morphology of ZI_V^Microglia and increased microglial engulfment of spines in ZI_V of CFA1D mice. Furthermore, pain sensitization and anxiety-like behaviors were reversed when the ZI_V^Microglia of CFA1D-treated mice were chemically inhibited by intra-ZI_V minocycline injection, accompanied by the recovery of decreased ZI_V^GABA excitability. Conclusively, our results provide novel insights that dysregulation of microglial engulfment capacity encodes maladaptation of ZI_V^GABA, thus promoting the development of anxiety-like behaviors in acute pain.

Optogenetic modulation of electroacupuncture analgesia in a mouse inflammatory pain model

Peripheral tissue damage and associated inflammation can trigger neuroplastic changes in somatic pain pathways, such as reduced neuronal firing thresholds and synaptic potentiation, that ultimately lead to peripheral sensitization and chronic pain. Electroacupuncture (EA) can relieve chronic inflammatory pain, but the underlying mechanisms are unknown, including the contributions of higher pain centers such as somatosensory cortex (SSC). We investigated these mechanisms using optogenetic modulation of SSC activity in a mouse inflammatory pain model. Injection of Complete Freund's Adjuvant into the hind paw reliably induced inflammation accompanied by reduced mechanical and thermal pain thresholds (hyperalgesia) within three days (mechanical: 1.54 ± 0.13 g; thermal: 3.94 ± 0.43 s). Application of EA produced significant thermal and mechanical analgesia, but these responses were reversed by optogenetic activation of SSC neurons, suggesting that EA-induced analgesia involves modulation of central pain pathways. Western blot and immunostaining revealed that EA also attenuated CaMKIIα signaling in the dorsal root ganglion, central spinal cord, SSC, and anterior cingulate cortex (ACC). In contrast, optogenetic activation of the SSC induced CaMKIIα signaling in SSC and ACC. These findings suggest that AE can relieve inflammatory pain by suppressing CaMKIIα-dependent plasticity in cortical pain pathways. The SSC and ACC CaMKIIα signaling pathways may be valuable therapeutic targets for chronic inflammatory pain treatment.

Role of anterior cingulate cortex inputs to periaqueductal gray for pain avoidance

Although pain-related excessive fear is known to be a key factor in chronic pain disability, which involves the anterior cingulate cortex (ACC), little is known about the downstream circuits of the ACC for fear avoidance in pain processing. Using behavioral experiments and functional magnetic resonance imaging with optogenetics at 15.2 T, we demonstrate that the ACC is a part of the abnormal circuit changes in chronic pain and its downstream circuits are closely related to modulating sensorimotor integration and generating active movement rather than carrying sensory information. The projection from the ACC to the dorsolateral and lateral parts of the periaqueductal gray (dl/lPAG) especially enhances both reflexive and active avoidance behavior toward pain. Collectively, our results indicate that increased signals from the ACC to the dl/lPAG might be critical for excessive fear avoidance in chronic pain disability.

Dissecting the Neural Circuitry for Pain Modulation and Chronic Pain: Insights from Optogenetics

Pain is an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage. The processing of pain involves complicated modulation at the levels of the periphery, spinal cord, and brain. The pathogenesis of chronic pain is still not fully understood, which makes the clinical treatment challenging. Optogenetics, which combines optical and genetic technologies, can precisely intervene in the activity of specific groups of neurons and elements of the related circuits. Taking advantage of optogenetics, researchers have achieved a body of new findings that shed light on the cellular and circuit mechanisms of pain transmission, pain modulation, and chronic pain both in the periphery and the central nervous system. In this review, we summarize recent findings in pain research using optogenetic approaches and discuss their significance in understanding the pathogenesis of chronic pain.

Rostral Anterior Cingulate Cortex–Ventrolateral Periaqueductal Gray Circuit Underlies Electroacupuncture to Alleviate Hyperalgesia but Not Anxiety-Like Behaviors in Mice With Spared Nerve Injury

Neuropathic pain is a common cause of chronic pain and is often accompanied by negative emotions, making it complex and difficult to treat. However, the neural circuit mechanisms underlying these symptoms remain unclear. Herein, we present a novel pathway associated with comorbid chronic pain and anxiety. Using chemogenetic methods, we found that activation of glutamatergic projections from the rostral anterior cingulate cortex (rACC^Glu) to the ventrolateral periaqueductal gray (vlPAG) induced both hyperalgesia and anxiety-like behaviors in sham mice. Inhibition of the rACC^Glu-vlPAG pathway reduced anxiety-like behaviors and hyperalgesia in the spared nerve injury (SNI) mice model; moreover, electroacupuncture (EA) effectively alleviated these symptoms. Investigation of the related mechanisms revealed that the chemogenetic activation of the rACC^Glu-vlPAG circuit effectively blocked the analgesic effect of EA in the SNI mice model but did not affect the chronic pain-induced negative emotions. This study revealed a novel pathway, the rACC^Glu-vlPAG pathway, that mediates neuropathic pain and pain-induced anxiety.

Early-life inflammation promotes depressive symptoms in adolescence via microglial engulfment of dendritic spines

Early-life inflammation increases the risk for depression in later life. Here, we demonstrate how early-life inflammation causes adolescent depressive-like symptoms: by altering the long-term neuronal spine engulfment capacity of microglia. For mice exposed to lipopolysaccharide (LPS)-induced inflammation via the Toll-like receptor 4/NF-κB signaling pathway at postnatal day (P) 14, ongoing longitudinal imaging of the living brain revealed that later stress (delivered during adolescence on P45) increases the extent of microglial engulfment around anterior cingulate cortex (ACC) glutamatergic neuronal (ACCGlu) spines. When the ACC microglia of LPS-treated mice were deleted or chemically inhibited, the mice did not exhibit depressive-like behaviors during adolescence. Moreover, we show that the fractalkine receptor CX3CR1 mediates stress-induced engulfment of ACCGlu neuronal spines. Together, our findings establish that early-life inflammation causes dysregulation of microglial engulfment capacity, which encodes long-lasting maladaptation of ACCGlu neurons to stress, thus promoting development of depression-like symptoms during adolescence.

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.

Sex-Specific Abnormalities and Treatment-Related Plasticity of Subgenual Anterior Cingulate Cortex Functional Connectivity in Chronic Pain

The subgenual anterior cingulate cortex (sgACC) is a key node of the descending antinociceptive system with sex differences in its functional connectivity (FC). We previously reported that, in a male-prevalent chronic pain condition, sgACC FC is abnormal in women but not in men. This raises the possibility that, within a sex, sgACC FC may be either protective or represent a vulnerability to develop a sex-dominant chronic pain condition. The aim of this study was to characterize sgACC FC in a female-dominant chronic pain condition, carpal tunnel syndrome (CTS), to investigate whether sgACC abnormalities are a common feature in women with chronic pain or unique to individuals with pain conditions that are more prevalent in the opposite sex. We used fMRI to determine the resting state FC of the sgACC in healthy controls (HCs, n = 25, 18 women; 7 men) and people with CTS before (n = 25, 18 women; 7 men) and after (n = 17, 13 women; 4 men) successful surgical treatment. We found reduced sgACC FC with the medial pre-frontal cortex (mPFC) and temporal lobe in CTS compared with HCs. The group-level sgACC-mPFC FC abnormality was driven by men with CTS, while women with CTS did not have sgACC FC abnormalities compared with healthy women. We also found that age and sex influenced sgACC FC in both CTS and HCs, with women showing greater FC with bilateral frontal poles and men showing greater FC with the parietal operculum. After surgery, there was reduced sgACC FC with the orbitofrontal cortex, striatum, and premotor areas and increased FC with the posterior insula and precuneus compared with pre-op scans. Abnormally reduced sgACC-mPFC FC in men but not women with a female-prevalent chronic pain condition suggests pain-related sgACC abnormalities may not be specific to women but rather to individuals who develop chronic pain conditions that are more dominant in the opposite sex. Our data suggest the sgACC plays a role in chronic pain in a sex-specific manner, and its communication with other regions of the dynamic pain connectome undergoes plasticity following pain-relieving treatment, supporting it as a potential therapeutic target for neuromodulation in chronic pain.

Neural Plasticity in the Brain during Neuropathic Pain

Neuropathic pain is an intractable chronic pain, caused by damage to the somatosensory nervous system. To date, treatment for neuropathic pain has limited effects. For the development of efficient therapeutic methods, it is essential to fully understand the pathological mechanisms of neuropathic pain. Besides abnormal sensitization in the periphery and spinal cord, accumulating evidence suggests that neural plasticity in the brain is also critical for the development and maintenance of this pain. Recent technological advances in the measurement and manipulation of neuronal activity allow us to understand maladaptive plastic changes in the brain during neuropathic pain more precisely and modulate brain activity to reverse pain states at the preclinical and clinical levels. In this review paper, we discuss the current understanding of pathological neural plasticity in the four pain-related brain areas: the primary somatosensory cortex, the anterior cingulate cortex, the periaqueductal gray, and the basal ganglia. We also discuss potential treatments for neuropathic pain based on the modulation of neural plasticity in these brain areas.

HSV-1 H129-Derived Anterograde Neural Circuit Tracers: Improvements, Production, and Applications

Anterograde viral tracers are powerful and essential tools for dissecting the output targets of a brain region of interest. They have been developed from herpes simplex virus 1 (HSV-1) strain H129 (H129), and have been successfully applied to map diverse neural circuits. Initially, the anterograde polysynaptic tracer H129-G4 was used by many groups. We then developed the first monosynaptic tracer, H129-dTK-tdT, which was highly successful, yet improvements are needed. Now, by inserting another tdTomato expression cassette into the H129-dTK-tdT genome, we have created H129-dTK-T2, an updated version of H129-dTK-tdT that has improved labeling intensity. To help scientists produce and apply our H129-derived viral tracers, here we provide the protocol describing our detailed and standardized procedures. Commonly-encountered technical problems and their solutions are also discussed in detail. Broadly, the dissemination of this protocol will greatly support scientists to apply these viral tracers on a large scale.

Pain: Integration of Sensory and Affective Aspects of Pain

Painful stimuli are detected by peripheral nociceptors, and the brain processes this nociceptive input into an unpleasant sensation. A new study identifies a brain circuit that integrates sensory and affective aspects of inflammatory and neuropathic pain.

Electrophysiological Properties of Substantia Gelatinosa Neurons in the Preparation of a Slice of Middle-Aged Rat Spinal Cord

A patch-clamp recording in slices generated from the brain or the spinal cord has facilitated the exploration of neuronal circuits and the molecular mechanisms underlying neurological disorders. However, the rodents that are used to generate the spinal cord slices in previous studies involving a patch-clamp recording have been limited to those in the juvenile or adolescent stage. Here, we applied an N-methyl-D-glucamine HCl (NMDG-HCl) solution that enabled the patch-clamp recordings to be performed on the superficial dorsal horn neurons in the slices derived from middle-aged rats. The success rate of stable recordings from substantia gelatinosa (SG) neurons was 34.6% (90/260). When stimulated with long current pulses, 43.3% (39/90) of the neurons presented a tonic-firing pattern, which was considered to represent γ-aminobutyric acid-ergic (GABAergic) signals. Presumptive glutamatergic neurons presented 38.9% (35/90) delayed and 8.3% (7/90) single-spike patterns. The intrinsic membrane properties of both the neuron types were similar but delayed (glutamatergic) neurons appeared to be more excitable as indicated by the decreased latency and rheobase values of the action potential compared with those of tonic (GABAergic) neurons. Furthermore, the glutamatergic neurons were integrated, which receive more excitatory synaptic transmission. We demonstrated that the NMDG-HCl cutting solution could be used to prepare the spinal cord slices of middle-aged rodents for the patch-clamp recording. In combination with other techniques, this preparation method might permit the further study of the functions of the spinal cord in the pathological processes that occur in aging-associated diseases.

Switching of delta opioid receptor subtypes in central amygdala microcircuits is associated with anxiety states in pain

Anxiety is often comorbid with pain. Delta opioid receptors (DORs) are promising targets for the treatment of pain and mental disorders with little addictive potential. However, their roles in anxiety symptoms at different stages of pain are unclear. In the current study, mice with inflammatory pain at the fourth hour following complete Freund’s adjuvant (CFA) injection displayed significant anxiety-like behavior, which disappeared at the seventh day. Combining electrophysiology, optogenetics, and pharmacology, we found that activation of delta opioid receptor 1 (DOR1) in the central nucleus amygdala (CeA) inhibited both the anxiolytic excitatory input from the basolateral amygdala (BLA) and the anxiogenic excitatory input from the parabrachial nucleus (PBN). In contrast, activation of delta opioid receptor 2 (DOR2) did not affect CeA excitatory synaptic transmission in normal and 4-h CFA mice but inhibited the excitatory projection from the PBN rather than the BLA in 7-day CFA mice. Furthermore, the function of both DOR1 and DOR2 was downregulated to the point of not being detectable in the CeA of mice at the 21st day following CFA injection. Taken together, these results suggest that functional switching of DOR1 and DOR2 is associated with anxiety states at different stages of pain via modulating the activity of specific pathways (BLA-CeA and PBN-CeA).

MeCP2 mediates transgenerational transmission of chronic pain

Pain symptoms can be transmitted across generations, but the mechanisms underlying these outcomes remain poorly understood. Here, we identified an essential role for primary somatosensory cortical (S1) glutamate neuronal DNA methyl-CpG binding protein 2 (MeCP2) in the transgenerational transmission of pain. In a female mouse chronic pain model, the offspring displayed significant pain sensitization. In these mice, MeCP2 expression was increased in S1 glutamate (Glu^S1) neurons, correlating with increased neuronal activity. Downregulation of Glu^S1 neuronal MeCP2 in maternal mice with pain abolished offspring pain sensitization, whereas overexpression of MeCP2 in naïve maternal mice induced pain sensitization in offspring. Notably, single-cell sequencing and chromatin immunoprecipitation analysis showed that the expression of a wide range of genes was changed in offspring and maternal Glu^S1 neurons, some of which were regulated by MeCP2. These results collectively demonstrate the putative importance of MeCP2 as a key regulator in pain transgenerational transmission through actions on Glu^S1 neuronal maladaptation.