Activation of the Intrinsic Pain Inhibitory Circuit from the Midcingulate Cg2 to Zona Incerta Alleviates Neuropathic Pain

The endocannabinoid system in the brain undergoes long-lasting changes following neuropathic pain

The endocannabinoid system (ECS), which is composed of endocannabinoids (eCBs), cannabinoid receptors (CBRs), and associated signaling molecules, has been identified within the brain. In neuropathic pain animal models and patients, long-lasting alterations in the ECS have been observed. These changes of neurons and glial cells in the ECS contribute to the modulation of neuropathic pain. Intervention strategies such as the activation of CBRs, the enhancement of hydrolytic enzyme function, and the inhibition of synthetizing enzymes typically alleviate neuropathic pain through CBR-dependent mechanisms. Additionally, emotions such as fear, anxiety, and depression are frequently experienced with neuropathic pain. Exogenous cannabinoids can mitigate these mood disorders via CBR signaling pathways. Therefore, the targeting of long-lasting ECS alterations represents a potential therapeutic approach for both neuropathic pain and emotional disorders. In this review, the long-lasting variations in neurons and glial cells in the ECS related to neuropathic pain and the accompanying emotional comorbidities are elucidated. Furthermore, the cellular and molecular mechanisms underlying synaptic plasticity and neural circuit activities in the brain are reviewed.

A neural circuit from thalamic paraventricular nucleus via zona incerta to periaqueductal gray for the facilitation of neuropathic pain

Top-down projections transmit a series of signals encoding pain sensation to the ventrolateral periaqueductal gray (vlPAG), where they converge with various incoming projections to regulate pain. Clarifying the upstream regulatory hierarchy of vlPAG can enhance our understanding of the neural circuitry involved in pain modulation. Here, we show that a in a mouse model of spared nerve injury (SNI), activation of a circuit arising from posterior paraventricular thalamic nucleus CaMKIIα-positive neurons (PVPCaMKIIα) projects to gamma-aminobutyric acid neurons in the rostral zona incerta (ZIrGABA) to facilitate the development of pain hypersensitivity behaviors. In turn, these ZIrGABA neurons project to CaMKIIα-positive neurons in the vlPAG (vlPAGCaMKIIα), a well-known neuronal population involved in pain descending modulation. In vivo calcium signal recording and whole-cell electrophysiological recordings reveal that the PVPCaMKIIα→ZIrGABA→vlPAGCaMKIIα circuit is activated in SNI models of persistent pain. Inhibition of this circuit using chemogenetics or optogenetics can alleviate the mechanical pain behaviors. Our study indicates that the PVPCaMKIIα→ZIrGABA→vlPAGCaMKIIα circuit is involved in the facilitation of neuropathic pain. This previously unrecognized circuit could be explored as a potential target for neuropathic pain treatment.

Coordination between midcingulate cortex and retrosplenial cortex in pain regulation

Introduction

The cingulate cortex, with its subregions ACC, MCC, and RSC, is key in pain processing. However, the detailed interactions among these regions in modulating pain sensation have remained unclear.

Methods

In this study, chemogenetic tools were employed to selectively activate or inhibit neuronal activity in the MCC and RSC of rodents to elucidate their roles in pain regulation.Results: Our results showed that chemogenetic activation in both the RSC and MCC heightened pain sensitivity. Suppression of MCC activity disrupted the RSC's regulation of both mechanical and thermal pain, while RSC inhibition specifically affected the MCC's regulation of thermal pain.

Discussion

The findings indicate a complex interplay between the MCC and RSC, with the MCC potentially governing the RSC's pain regulatory mechanisms. The RSC, in turn, is crucial for the MCC's control over thermal sensation, revealing a collaborative mechanism in pain processing.

Conclusion

This study provides evidence for the MCC and RSC's collaborative roles in pain regulation, highlighting the importance of their interactions for thermal and mechanical pain sensitivity. Understanding these mechanisms could aid in developing targeted therapies for pain disorders.

Ventral zona incerta parvalbumin neurons modulate sensory-induced and stress-induced self-grooming via input-dependent mechanisms in mice

Self-grooming is an innate stereotyped behavior influenced by sense and emotion. It is considered an important characteristic in various disease models. However, the neural circuit mechanism underlying sensory-induced and emotion-driven self-grooming remains unclear. We found that the ventral zona incerta (Ziv) was activated during spontaneous self-grooming (SG), corn oil-induced sensory self-grooming (OG), and tail suspension-induced stress self-grooming (TG). Optogenetic excitation of Ziv parvalbumin (PV) neurons increased the duration of SG. Conversely, optogenetic inhibition of ZivPV neurons significantly reduced self-grooming in all three models. Furthermore, glutamatergic inputs from the primary sensory cortex activated the Ziv and contributed to OG. Activation of GABAergic inputs from the central amygdala to the Ziv increased SG, OG, and TG, potentially through local negative regulation of the Ziv. These findings suggest that the Ziv may play a crucial role in processing sensory and emotional information related to self-grooming, making it a potential target for regulating stereotyped behavior.

Secondary somatosensory cortex glutamatergic innervation of the thalamus facilitates pain

ABSTRACT

Although the secondary somatosensory cortex (SII) is known to be involved in pain perception, its role in pain modulation and neuropathic pain is yet unknown. In this study, we found that glutamatergic neurons in deep layers of the SII (SII Glu ) responded to bilateral sensory inputs by changing their firing with most being inhibited by contralateral noxious stimulation. Optical inhibition and activation of unilateral SII Glu reduced and enhanced bilateral nociceptive sensitivity, respectively, without affecting mood status. Tracing experiments revealed that SII Glu sent dense monosynaptic projections to the posterolateral nucleus (VPL) and the posterior nucleus (Po) of the thalamus. Optical inhibition and activation of projection terminals of SII Glu in the unilateral VPL and Po inhibited and facilitated pain on the contralateral side, respectively. After partial sciatic nerve ligation, SII Glu became hyperactive as evidenced by higher frequency of spontaneous firing, but the response patterns to peripheral stimulation remained. Optical inhibition of SII Glu alleviated not only bilateral mechanical allodynia and thermal hyperalgesia but also the negative affect associated with spontaneous pain. Inhibition of SII Glu terminals in the VPL and Po also relieved neuropathic pain. This study revealed that SII Glu and the circuits to the VPL and Po constitute a part of the endogenous pain modulatory network. These corticothalamic circuits became hyperactive after peripheral nerve injury, hence contributes to neuropathic pain. These results justify proper inhibition of SII Glu and associated neural circuits as a potential clinical strategy for neuropathic pain treatment.

Mimicking opioid analgesia in cortical pain circuits

The anterior cingulate cortex plays a pivotal role in the cognitive and affective aspects of pain perception. Both endogenous and exogenous opioid signaling within the cingulate mitigate cortical nociception, reducing pain unpleasantness. However, the specific functional and molecular identities of cells mediating opioid analgesia in the cingulate remain elusive. Given the complexity of pain as a sensory and emotional experience, and the richness of ethological pain-related behaviors, we developed a standardized, deep-learning platform for deconstructing the behavior dynamics associated with the affective component of pain in mice-LUPE (Light aUtomated Pain Evaluator). LUPE removes human bias in behavior quantification and accelerated analysis from weeks to hours, which we leveraged to discover that morphine altered attentional and motivational pain behaviors akin to affective analgesia in humans. Through activity-dependent genetics and single-nuclei RNA sequencing, we identified specific ensembles of nociceptive cingulate neuron-types expressing mu-opioid receptors. Tuning receptor expression in these cells bidirectionally modulated morphine analgesia. Moreover, we employed a synthetic opioid receptor promoter-driven approach for cell-type specific optical and chemical genetic viral therapies to mimic morphine's pain-relieving effects in the cingulate, without reinforcement. This approach offers a novel strategy for precision pain management by targeting a key nociceptive cortical circuit with on-demand, non-addictive, and effective analgesia.

Anterior cingulate cross-hemispheric inhibition via the claustrum resolves painful sensory conflict

The anterior cingulate cortex (ACC) responds to noxious and innocuous sensory inputs, and integrates them to coordinate appropriate behavioral reactions. However, the role of the projections of ACC neurons to subcortical areas and their influence on sensory processing are not fully investigated. Here, we identified that ACC neurons projecting to the contralateral claustrum (ACC→contraCLA) preferentially respond to contralateral mechanical sensory stimulation. These sensory responses were enhanced during attending behavior. Optogenetic activation of ACC→contraCLA neurons silenced pyramidal neurons in the contralateral ACC by recruiting local circuit fast-spiking interneuron activation via an excitatory relay in the CLA. This circuit activation suppressed withdrawal behavior to mechanical stimuli ipsilateral to the ACC→contraCLA neurons. Chemogenetic silencing showed that the cross-hemispheric circuit has an important role in the suppression of contralateral nociceptive behavior during sensory-driven attending behavior. Our findings identify a cross-hemispheric cortical-subcortical-cortical arc allowing the brain to give attentional priority to competing innocuous and noxious inputs.

Zona incerta distributes a broad movement signal that modulates behavior

The zona incerta is a subthalamic nucleus made up mostly of GABAergic neurons. It has wide-ranging inputs and outputs and is believed to have many integrative functions that link sensory stimuli with motor responses to guide behavior. However, its role is not well established perhaps because few studies have measured the activity of zona incerta neurons in behaving animals under different conditions. To record the activity of zona incerta neurons during exploratory and cue-driven goal-directed behaviors, we used electrophysiology in head-fixed mice moving on a spherical treadmill and fiber photometry in freely moving mice. We found two groups of neurons based on their sensitivity to movement, with a minority of neurons responding to whisker stimuli. Furthermore, zona incerta GABAergic neurons robustly code the occurrence of exploratory and goal-directed movements, but not their direction. To understand the function of these activations, we performed genetically targeted lesions and optogenetic manipulations of zona incerta GABAergic neurons during exploratory and goal-directed behaviors. The results showed that the zona incerta has a role in modulating the movement associated with these behaviors, but this has little impact on overall performance. Zona incerta neurons distribute a broad corollary signal of movement occurrence to their diverse projection sites, which regulates behavior.

Projections from anteromedial thalamus nucleus to the midcingulate cortex mediate pain and anxiety-like behaviors in mice

Prior research has demonstrated the involvement of the midcingulate cortex (MCC) and its downstream pathway in pain regulation. However, the mechanism via which pain information is conveyed to the MCC remains unclear. The present study utilized immunohistochemistry, chemogenetics, optogenetics, and behavior detection methods to explore the involvement of MCC, anteromedial thalamus nucleus (AM), and AM-MCC pathway in pain and emotional regulation. Chemogenetics or optogenetics methods were employed to activate/inhibit MCCCaMKIIα, AMCaMKIIα, AMCaMKIIα-MCC pathway. This manipulation evokes/relieves mechanical and partial heat hyperalgesia, as well as anxiety-like behaviors. In the complete Freund,s adjuvant (CFA) inflammatory pain model, chemogenetic inhibition of the AMCaMKIIα-MCCCaMKIIα pathway contributed to pain relief. Notably, this study presented the first evidence implicating the AM in the regulation of nociception and negative emotions. Additionally, it was observed that the MCC primarily receives projections from the AM, highlighting the crucial role of this pathway in the transmission of pain and emotional information.

Updates on brain regions and neuronal circuits of movement disorders in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disease with a global burden that affects more often in the elderly. The basal ganglia (BG) is believed to account for movement disorders in PD. More recently, new findings in the original regions in BG involved in motor control, as well as the new circuits or new nucleuses previously not specifically considered were explored. In the present review, we provide up-to-date information related to movement disorders and modulations in PD, especially from the perspectives of brain regions and neuronal circuits. Meanwhile, there are updates in deep brain stimulation (DBS) and other factors for the motor improvement in PD. Comprehensive understandings of brain regions and neuronal circuits involved in motor control could benefit the development of novel therapeutical strategies in PD.

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.

Distinct circuits in anterior cingulate cortex encode safety assessment and mediate flexibility of fear reactions

Safety assessment and threat evaluation are crucial for animals to live and survive in the wilderness. However, neural circuits underlying safety assessment and their transformation to mediate flexibility of fear-induced defensive behaviors remain largely unknown. Here, we report that distinct neuronal populations in mouse anterior cingulate cortex (ACC) encode safety status by selectively responding under different contexts of auditory threats, with one preferably activated when an animal staysing in a self-deemed safe zone and another specifically activated in more dangerous environmental settings that led to escape behavior. The safety-responding neurons preferentially target the zona incerta (ZI), which suppresses the superior colliculus (SC) via its GABAergic projection, while the danger-responding neurons preferentially target and excite SC. These distinct corticofugal pathways antagonistically modulate SC responses to threat, resulting in context-dependent expression of fear reactions. Thus, ACC serves as a critical node to encode safety/danger assessment and mediate behavioral flexibility through differential top-down circuits.

Electroacupuncture Inhibits Pain Memory and Related Anxiety-Like Behaviors by Blockading the GABAB Receptor Function in the Midcingulate Cortex

Pain memory is commonly considered an underlying cause of chronic pain and is also responsible for a range of anxiety. Electroacupuncture (EA) has been shown to ameliorate pain memories and exert anti-anxiety effects. Previous research has indicated that GABAergic neurons and/or GABA receptors (GABARs) in the midcingulate cortex (MCC) have potential associations with chronic pain and anxiety. However, there is no known empirical research that has specifically studied the effects of EA on the GABAergic system in the MCC. Here, we used cross-injection of carrageenan to establish the pain memory rats model. Immunofluorescence were used to detect the excitability of GABAergic neurons within MCC. Von Frey filament, elevated zero maze, and open field tests were used to measure mechanical allodynia and anxiety-like behaviors, combined with chemogenetic and pharmacologic technologies. Finally, this study provides evidence that pain memories contribute to generalized negative emotions and that downregulating the activity of GABAergic neurons within MCC could block pain memories and reverse anxiety emotion. Specifically, GABABR is involved in pain memory and related anxiety-like behaviors. Activation of GABAergic neurons in the MCC did not reverse the effects of EA on pain memories and related anxiety-like behaviors, whereas these effects could be reversed by a GABABR agonist. These findings highlight the functional significance of GABABR in the EA-mediated attenuation of pain memories and related anxiety-like behaviors in rats.

Mediodorsal thalamus-projecting anterior cingulate cortex neurons modulate helping behavior in mice

Many species living in groups can perform prosocial behaviors via voluntarily helping others with or without benefits for themselves. To provide a better understanding of the neural basis of such prosocial behaviors, we adapted a preference lever-switching task in which mice can prevent harm to others by switching from using a lever that causes shocks to a conspecific one that does not. We found the harm avoidance behavior was mediated by self-experience and visual and social contact but not by gender or familiarity. By combining single-unit recordings and analysis of neural trajectory decoding, we demonstrated the dynamics of anterior cingulate cortex (ACC) neural activity changes synchronously with the harm avoidance performance of mice. In addition, ACC neurons projected to the mediodorsal thalamus (MDL) to modulate the harm avoidance behavior. Optogenetic activation of the ACC-MDL circuit during non-preferred lever pressing (nPLP) and inhibition of this circuit during preferred lever pressing (PLP) both resulted in the loss of harm avoidance ability. This study revealed the ACC-MDL circuit modulates prosocial behavior to avoid harm to conspecifics and may shed light on the treatment of neuropsychiatric disorders with dysfunction of prosocial behavior.

Postsynaptic histamine H3 receptors in ventral basal forebrain cholinergic neurons modulate contextual fear memory

Overly strong fear memories can cause pathological conditions. Histamine H3 receptor (H3R) has been viewed as an optimal drug target for CNS disorders, but its role in fear memory remains elusive. We find that a selective deficit of H3R in cholinergic neurons, but not in glutamatergic neurons, enhances freezing level during contextual fear memory retrieval without affecting cued memory. Consistently, genetically knocking down H3R or chemogenetically activating cholinergic neurons in the ventral basal forebrain (vBF) mimics this enhanced fear memory, whereas the freezing augmentation is rescued by re-expressing H3R or chemogenetic inhibition of vBF cholinergic neurons. Spatiotemporal regulation of H3R by a light-sensitive rhodopsin-H3R fusion protein suggests that postsynaptic H3Rs in vBF cholinergic neurons, but not presynaptic H3Rs of cholinergic projections in the dorsal hippocampus, are responsible for modulating contextual fear memory. Therefore, precise modulation of H3R in a cell-type- and subcellular-location-specific manner should be explored for pathological fear memory.

Dorsal subthalamic deep brain stimulation improves pain in Parkinson's disease

Introduction

Inconsistent effects of subthalamic deep brain stimulation (STN DBS) on pain, a common non-motor symptom of Parkinson's disease (PD), may be due to variations in active contact location relative to some pain-reducing locus of stimulation. This study models and compares the loci of maximal effect for pain reduction and motor improvement in STN DBS.

Methods

We measured Movement Disorder Society Unified PD Rating Scale (MDS-UPDRS) Part I pain score (item-9), and MDS-UPDRS Part III motor score, preoperatively and 6-12 months after STN DBS. An ordinary least-squares regression model was used to examine active contact location as a predictor of follow-up pain score while controlling for baseline pain, age, dopaminergic medication, and motor improvement. An atlas-independent isotropic electric field model was applied to distinguish sites of maximally effective stimulation for pain and motor improvement.

Results

In 74 PD patients, mean pain score significantly improved after STN DBS (p = 0.01). In a regression model, more dorsal active contact location was the only significant predictor of pain improvement (R2 = 0.17, p = 0.03). The stimulation locus for maximal pain improvement was lateral, anterior, and dorsal to that for maximal motor improvement.

Conclusion

STN stimulation, dorsal to the site of optimal motor improvement, improves pain. This region contains the zona incerta, which is known to modulate pain in humans, and may explain this observation.

Cell type-specific dissection of sensory pathways involved in descending modulation

Decades of research have suggested that stimulation of supraspinal structures, such as the periaqueductal gray (PAG) and rostral ventromedial medulla (RVM), inhibits nocifensive responses to noxious stimulation through a process known as descending modulation. Electrical stimulation and pharmacologic manipulations of the PAG and RVM identified transmitters and neuronal firing patterns that represented distinct cell types. Advances in mouse genetics, in vivo imaging, and circuit tracing methods, in addition to chemogenetic and optogenetic approaches, allowed the characterization of the cells and circuits involved in descending modulation in further detail. Recent work has revealed the importance of PAG and RVM neuronal cell types in the descending modulation of pruriceptive as well as nociceptive behaviors, underscoring their roles in coordinating complex behavioral responses to sensory input. This review summarizes how new technical advances that enable cell type-specific manipulation and recording of neuronal activity have supported, as well as expanded, long-standing views on descending modulation.

The microglial innate immune receptors TREM-1 and TREM-2 in the anterior cingulate cortex (ACC) drive visceral hypersensitivity and depressive-like behaviors following DSS-induced colitis

Inflammatory bowel disease (IBD) is a chronic condition with a high recurrence rate. To date, the clinical treatment of IBD mainly focuses on inflammation and gastrointestinal symptoms while ignoring the accompanying visceral pain, anxiety, depression, and other emotional symptoms. Evidence is accumulating that bi-directional communication between the gut and the brain is indispensable in the pathophysiology of IBD and its comorbidities. Increasing efforts have been focused on elucidating the central immune mechanisms in visceral hypersensitivity and depression following colitis. The triggering receptors expressed on myeloid cells-1/2 (TREM-1/2) are newly identified receptors that can be expressed on microglia. In particular, TREM-1 acts as an immune and inflammatory response amplifier, while TREM-2 may function as a molecule with a putative antagonist role to TREM-1. In the present study, using the dextran sulfate sodium (DSS)-induced colitis model, we found that peripheral inflammation induced microglial and glutamatergic neuronal activation in the anterior cingulate cortex (ACC). Microglial ablation mitigated visceral hypersensitivity in the inflammation phase rather than in the remission phase, subsequently preventing the emergence of depressive-like behaviors in the remission phase. Moreover, a further mechanistic study revealed that overexpression of TREM-1 and TREM-2 remarkably aggravated DSS-induced neuropathology. The improved outcome was achieved by modifying the balance of TREM-1 and TREM-2 via genetic and pharmacological means. Specifically, a deficiency of TREM-1 attenuated visceral hyperpathia in the inflammatory phase, and a TREM-2 deficiency improved depression-like symptoms in the remission phase. Taken together, our findings provide insights into mechanism-based therapy for inflammatory disorders and establish that microglial innate immune receptors TREM-1 and TREM-2 may represent a therapeutic target for the treatment of pain and psychological comorbidities associated with chronic inflammatory diseases by modulating neuroinflammatory responses.

A nociceptive neuronal ensemble in the dorsomedial prefrontal cortex underlies pain chronicity

Pain chronicity involves unpleasant experience in both somatosensory and affective aspects, accompanied with the prefrontal cortex (PFC) neuroplastic alterations. However, whether specific PFC neuronal ensembles underlie pain chronicity remains elusive. Here we identify a nociceptive neuronal ensemble in the dorsomedial prefrontal cortex (dmPFC), which shows prominent reactivity to nociceptive stimuli. We observed that this ensemble shows distinct molecular characteristics and is densely connected to pain-related regions including basolateral amygdala (BLA) and lateral parabrachial nuclei (LPB). Prolonged chemogenetic activation of this nociceptive neuronal ensemble, but not a randomly transfected subset of dmPFC neurons, induces chronic pain-like behaviors in normal mice. By contrast, silencing the nociceptive dmPFC neurons relieves both pain hypersensitivity and anxiety in mice with chronic inflammatory pain. These results suggest the presence of specific dmPFC neuronal ensembles in processing nociceptive information and regulating pain chronicity.

The caudal prethalamus: Inhibitory switchboard for behavioral control?

Prethalamic nuclei in the mammalian brain include the zona incerta, the ventral lateral geniculate nucleus, and the intergeniculate leaflet, which provide long-range inhibition to many targets in the midbrain, hindbrain, and thalamus. These nuclei in the caudal prethalamus can integrate sensory and non-sensory information, and together they exert powerful inhibitory control over a wide range of brain functions and behaviors that encompass most aspects of the behavioral repertoire of mammals, including sleep, circadian rhythms, feeding, drinking, predator avoidance, and exploration. In this perspective, we highlight the evidence for this wide-ranging control and lay out the hypothesis that one role of caudal prethalamic nuclei may be that of a behavioral switchboard that-depending on the sensory input, the behavioral context, and the state of the animal-can promote a behavioral strategy and suppress alternative, competing behaviors by modulating inhibitory drive onto diverse target areas.

Whole-Brain Connectome of GABAergic Neurons in the Mouse Zona Incerta

The zona incerta (ZI) is involved in various functions and may serve as an integrative node of the circuits for global behavioral modulation. However, the long-range connectivity of different sectors in the mouse ZI has not been comprehensively mapped. Here, we obtained whole-brain images of the input and output connections via fluorescence micro-optical sectioning tomography and viral tracing. The principal regions in the input-output circuits of ZI GABAergic neurons were topologically organized. The 3D distribution of cortical inputs showed rostro-caudal correspondence with different ZI sectors, while the projection fibers from ZI sectors were longitudinally organized in the superior colliculus. Clustering results show that the medial and lateral ZI are two different major functional compartments, and they can be further divided into more subdomains based on projection and input connectivity. This study provides a comprehensive anatomical foundation for understanding how the ZI is involved in integrating different information, conveying motivational states, and modulating global behaviors.

HMGB1 in the mPFC governs comorbid anxiety in neuropathic pain

Background

Whether neuroinflammation causes comorbid mood disorders in neuropathic pain remains elusive. Here we investigated the role of high mobility group box 1 protein (HMGB1), a proinflammatory cytokine, in the medial prefrontal cortex (mPFC) in anxiety comorbidity of neuropathic pain.

Methods

Neuropathic pain was induced by partial transection of the infraorbital nerve (p-IONX) or partial sciatic nerve ligation (PSL) in mice and evaluated by measuring nociceptive thresholds to mechanical and heat stimulation. Anxiety-like behaviors were assessed by elevated plus maze, light dark box and open field tests. Aversive or anti-aversive effect was detected by conditioned place preference test. Neuronal activity was evaluated by single-unit and patch clamp recordings. The contribution of mPFC pyramidal neurons to anxiety was further examined by selectively inhibiting them by optogenetics. HMGB1 expression was measured by immunohistochemistry and western blotting. Antagonism of HMGB1 was achieved by injecting anti-HMGB1 monoclonal antibody (mAb) intracerebrally or intraperitoneally.

Results

Anxiety-like behaviors were presented earlier after p-IONX than after PSL. HMGB1 expression was upregulated in the mPFC temporally in parallel to anxiety onset, rather than in other regions associated with anxiety. The upregulation of HMGB1 expression and its translocation from the nucleus to cytoplasm in the mPFC occurred predominantly in neurons and were accompanied with activation of microglia and astrocytes. Infusion of anti-HMGB1 mAb into the mPFC during the early and late phases after either p-IONX or PSL alleviated anxiety-like behaviors and aversion without changing pain sensitization, while local infusion of exogenous ds-HMGB1, the proinflammatory form of HMGB1, into the mPFC induced anxiety and aversion but not pain sensitization in naïve mice. In addition to reversing established pain sensitization and anxiety simultaneously, intraperitoneal injection of anti-HMGB1 mAb reduced HMGB1 upregulation and suppressed the hyperexcitability of layer 2/3 pyramidal neurons in the mPFC after p-IONX. Moreover, optogenetic inhibition of mPFC pyramidal neurons alleviated anxiety in p-IONX mice.

Conclusion

These results demonstrate that HMGB1 in the mPFC drives and maintains anxiety comorbidity in neuropathic pain by increasing the excitability of layer 2/3 pyramidal neurons, and justify antagonism of HMGB1, e.g., neutralization by mAb, as a promising therapeutic strategy for neuropathic pain with anxiety comorbidity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s10194-022-01475-z.

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.

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.

Parvalbumin Neurons in Zona Incerta Regulate Itch in Mice

Pain and itch are intricately entangled at both circuitry and behavioral levels. Emerging evidence indicates that parvalbumin (PV)-expressing neurons in zona incerta (ZI) are critical for promoting nocifensive behaviors. However, the role of these neurons in itch modulation remains elusive. Herein, by combining FOS immunostaining, fiber photometry, and chemogenetic manipulation, we reveal that ZI PV neurons act as an endogenous negative diencephalic modulator for itch processing. Morphological data showed that both histamine and chloroquine stimuli induced FOS expression in ZI PV neurons. The activation of these neurons was further supported by the increased calcium signal upon scratching behavior evoked by acute itch. Behavioral data further indicated that chemogenetic activation of these neurons reduced scratching behaviors related to histaminergic and non-histaminergic acute itch. Similar neural activity and modulatory role of ZI PV neurons were seen in mice with chronic itch induced by atopic dermatitis. Together, our study provides direct evidence for the role of ZI PV neurons in regulating itch, and identifies a potential target for the remedy of chronic itch.

Engaging endogenous opioid circuits in pain affective processes

The pervasive use of opioid compounds for pain relief is rooted in their utility as one of the most effective therapeutic strategies for providing analgesia. While the detrimental side effects of these compounds have significantly contributed to the current opioid epidemic, opioids still provide millions of patients with reprieve from the relentless and agonizing experience of pain. The human experience of pain has long recognized the perceived unpleasantness entangled with a unique sensation that is immediate and identifiable from the first-person subjective vantage point as "painful." From this phenomenological perspective, how is it that opioids interfere with pain perception? Evidence from human lesion, neuroimaging, and preclinical functional neuroanatomy approaches is sculpting the view that opioids predominately alleviate the affective or inferential appraisal of nociceptive neural information. Thus, opioids weaken pain-associated unpleasantness rather than modulate perceived sensory qualities. Here, we discuss the historical theories of pain to demonstrate how modern neuroscience is revisiting these ideas to deconstruct the brain mechanisms driving the emergence of aversive pain perceptions. We further detail how targeting opioidergic signaling within affective or emotional brain circuits remains a strong avenue for developing targeted pharmacological and gene-therapy analgesic treatments that might reduce the dependence on current clinical opioid options.

Calcium imaging for analgesic drug discovery

Somatosensation and pain are complex phenomena involving a rangeofspecialised cell types forming different circuits within the peripheral and central nervous systems. In recent decades, advances in the investigation of these networks, as well as their function in sensation, resulted from the constant evolution of electrophysiology and imaging techniques to allow the observation of cellular activity at the population level both in vitro and in vivo. Genetically encoded indicators of neuronal activity, combined with recent advances in DNA engineering and modern microscopy, offer powerful tools to dissect and visualise the activity of specific neuronal subpopulations with high spatial and temporal resolution. In recent years various groups developed in vivo imaging techniques to image calcium transients in the dorsal root ganglia, the spinal cord and the brain of anesthetised and awake, behaving animals to address fundamental questions in both the physiology and pathophysiology of somatosensation and pain. This approach, besides giving unprecedented details on the circuitry of innocuous and painful sensation, can be a very powerful tool for pharmacological research, from the characterisation of new potential drugs to the discovery of new, druggable targets within specific neuronal subpopulations. Here we summarise recent developments in calcium imaging for pain research, discuss technical challenges and advances, and examine the potential positive impact of this technique in early preclinical phases of the analgesic drug discovery process.

Inactivation of Zona Incerta Blocks Social Conditioned Place Aversion and Modulates Post-traumatic Stress Disorder-Like Behaviors in Mice

Zona incerta (ZI), a largely inhibitory subthalamic region connected with many brain areas, has been suggested to serve as an integrative node for modulation of behaviors and physiological states, such as fear memory conditioning and aversion responses. It is, however, unclear whether ZI regulated the repeated social defeat stress (RSDS)-induced social conditioned place aversion (CPA) and post-traumatic stress disorder (PTSD)-like behaviors. In this study, the function of ZI was silenced via bilateral injection of tetanus toxin light chain (Tet-tox), a neurotoxin that completely blocks the evoked synaptic transmissions, expressing adeno-associated viruses (AAVs). We found ZI silencing: (1) significantly blocked the expression of RSDS-induced social CPA with no effect on the innate preference; (2) significantly enhanced the anxiety level in mice experienced RSDS with no effect on the locomotion activity; (3) altered the PTSD-associated behaviors, including the promotion of spatial cognitive impairment and the preventions of PPI deficit and social avoidance behavior. These effects were not observed on non-stressed mice. In summary, our results suggest the important role of ZI in modulating RSDS-induced social CPA and PTSD-like behaviors.

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.

Aberrant functional and effective connectivity of the frontostriatal network in unilateral acute tinnitus patients with hearing loss

PURPOSE

The present study combined resting-state functional connectivity (FC) and Granger causality analysis (GCA) to explore frontostriatal network dysfunction in unilateral acute tinnitus (AT) patients with hearing loss.

METHODS

The participants included 42 AT patients and 43 healthy control (HC) subjects who underwent resting-state functional magnetic resonance imaging (fMRI) scans. Based on the seed regions in the frontostriatal network, FC and GCA were conducted between the AT patients and HC subjects. Correlation analyses were used to examine correlations among altered FC values, GCA values, and clinical features in AT patients.

RESULTS

Compared with HCs, AT patients showed a general reduction in FC between the seed regions in the frontostriatal network and nonauditory areas, including the frontal cortices, midcingulate cortex (MCC), supramarginal gyrus, and postcentral gyrus (PoCG). Using the GCA algorithm, we detected abnormal effective connectivity (EC) in the inferior occipital gyrus, MCC, Cerebelum_Crus1, and PoCG. Furthermore, correlations between disrupted FC/EC and clinical characteristics, especially tinnitus distress-related characteristics, were found in AT patients.

CONCLUSIONS

Our work demonstrated abnormal FC and EC between the frontostriatal network and several nonauditory regions in AT patients with hearing loss, suggesting that multiple large-scale network dysfunctions and interactions are involved in the perception of tinnitus. These findings not only enhance the current understanding of the frontostriatal network in tinnitus but also serve as a reminder of the importance of focusing on tinnitus at an early stage.

Stimulation of zona incerta selectively modulates pain in humans

Stimulation of zona incerta in rodent models has been shown to modulate behavioral reactions to noxious stimuli. Sensory changes observed in Parkinsonian patients with subthalamic deep brain stimulation suggest that this effect is translatable to humans. Here, we utilized the serendipitous placement of subthalamic deep brain stimulation leads in 6 + 5 Parkinsonian patients to directly investigate the effects of zona incerta stimulation on human pain perception. We found that stimulation at 20 Hz, the physiological firing frequency of zona incerta, reduces experimental heat pain by a modest but significant amount, achieving a 30% reduction in one fifth of implants. Stimulation at higher frequencies did not modulate heat pain. Modulation was selective for heat pain and was not observed for warmth perception or pressure pain. These findings provide a mechanistic explanation of sensory changes seen in subthalamic deep brain stimulation patients and identify zona incerta as a potential target for neuromodulation of pain.

Functional disruption of cortical cingulate activity attenuates visceral hypersensitivity and anxiety induced by acute experimental colitis

Visceral pain is a highly complex experience and is the most common pathological feature in patients suffering from inflammatory gastrointestinal disorders. Whilst it is increasingly recognized that aberrant neural processing within the gut-brain axis plays a key role in development of neurological symptoms, the underlying mechanisms remain largely unknown. Here, we investigated the cortical activation patterns and effects of non-invasive chemogenetic suppression of cortical activity on visceral hypersensitivity and anxiety-related phenotypes in a well-characterized mouse model of acute colitis induced by dextran sulfate sodium (DSS). We found that within the widespread cortical network, the mid-cingulate cortex (MCC) was consistently highly activated in response to innocuous and noxious mechanical stimulation of the colon. Furthermore, during acute experimental colitis, impairing the activity of the MCC successfully alleviated visceral hypersensitivity, anxiety-like behaviors and visceromotor responses to colorectal distensions (CRDs) via downregulating the excitability of the posterior insula (PI), somatosensory and the rostral anterior cingulate cortices (rACC), but not the prefrontal or anterior insula cortices. These results provide a mechanistic insight into the central cortical circuits underlying painful visceral manifestations and implicate MCC plasticity as a putative target in cingulate-mediated therapies for bowel disorders.

Impaired effective functional connectivity of the sensorimotor network in interictal episodic migraineurs without aura

Background

Resting-state functional magnetic resonance imaging (Rs-fMRI) has confirmed sensorimotor network (SMN) dysfunction in migraine without aura (MwoA). However, the underlying mechanisms of SMN effective functional connectivity in MwoA remain unclear. We aimed to explore the association between clinical characteristics and effective functional connectivity in SMN, in interictal patients who have MwoA.

Methods

We used Rs-fMRI to acquire imaging data in 40 episodic patients with MwoA in the interictal phase and 34 healthy controls (HCs). Independent component analysis was used to profile the distribution of SMN and calculate the different SMN activity between the two groups. Subsequently, Granger causality analysis was used to analyze the effective functional connectivity between the SMN and other brain regions.

Results

Compared to the HCs, MwoA patients showed higher activity in the bilateral postcentral gyri (PoCG), but lower activity in the left midcingulate cortex (MCC). Moreover, MwoA patients showed decreased effective functional connectivity from the SMN to left middle temporal gyrus, right putamen, left insula and bilateral precuneus, but increased effective functional connectivity to the right paracentral lobule. There was also significant effective functional connectivity from the primary visual cortex, right cuneus and right putamen to the SMN. In the interictal period, there was positive correlation between the activity of the right PoCG and the frequency of headache. The disease duration was positively correlated with abnormal effective functional connectivity from the left PoCG to right precuneus. In addition, the headache impact scores were negatively correlated with abnormal effective functional connectivity from the left MCC to right paracentral lobule, as well as from the right precuneus to left PoCG.

Conclusions

These differential, resting-state functional activities of the SMN in episodic MwoA may contribute to the understanding of migraine-related intra- and internetwork imbalances associated with nociceptive regulation and chronification.

Where is Cingulate Cortex? A Cross-Species View

To compare findings across species, neuroscience relies on cross-species homologies, particularly in terms of brain areas. For cingulate cortex, a structure implicated in behavioural adaptation and control, a homologous definition across mammals is available - but currently not employed by most rodent researchers. The standard partitioning of rodent cingulate cortex is inconsistent with that in any other model species, including humans. Reviewing the existing literature, we show that the homologous definition better aligns results of rodent studies with those of other species, and reveals a clearer structural and functional organisation within rodent cingulate cortex itself. Based on these insights, we call for widespread adoption of the homologous nomenclature, and reinterpretation of previous studies originally based on the nonhomologous partitioning of rodent cingulate cortex.

Zona Incerta: An Integrative Node for Global Behavioral Modulation

Zona incerta (ZI) is a largely inhibitory subthalamic region connecting with many brain areas. Early studies have suggested involvement of ZI in various functions such as visceral activities, arousal, attention, and locomotion, but the specific roles of different ZI subdomains or cell types have not been well examined. Recent studies combining optogenetics, behavioral assays, neural tracing, and neural activity-recording reveal novel functional roles of ZI depending on specific input-output connectivity patterns. Here, we review these studies and summarize functions of ZI into four categories: sensory integration, behavioral output control, motivational drive, and neural plasticity. In view of these new findings, we propose that ZI serves as an integrative node for global modulation of behaviors and physiological states.