Tac1-Expressing Neurons in the Periaqueductal Gray Facilitate the Itch-Scratching Cycle via Descending Regulation

Neural substrates for regulating self-grooming behavior in rodents

Grooming, as an evolutionarily conserved repetitive behavior, is common in various animals, including humans, and serves essential functions including, but not limited to, hygiene maintenance, thermoregulation, de-arousal, stress reduction, and social behaviors. In rodents, grooming involves a patterned and sequenced structure, known as the syntactic chain with four phases that comprise repeated stereotyped movements happening in a cephalocaudal progression style, beginning from the nose to the face, to the head, and finally ending with body licking. The context-dependent occurrence of grooming behavior indicates its adaptive significance. This review briefly summarizes the neural substrates responsible for rodent grooming behavior and explores its relevance in rodent models of neuropsychiatric disorders and neurodegenerative diseases with aberrant grooming phenotypes. We further emphasize the utility of rodent grooming as a reliable measure of repetitive behavior in neuropsychiatric models, holding promise for translational psychiatry. Herein, we mainly focus on rodent self-grooming. Allogrooming (grooming being applied on one animal by its conspecifics via licking or carefully nibbling) and heterogrooming (a form of grooming behavior directing towards another animal, which occurs in other contexts, such as maternal, sexual, aggressive, or social behaviors) are not covered due to space constraints.

Peripheral Mechanisms of Mechanical Itch

Mechanical itch, which is defined as an itch sensation caused by innocuous mechanical force, may warn of the potential risk in the skin. The increased mechanosensitivity in sensory neurons may cause scratch-induced itch and promote the transition from acute itch to chronic itch. Recent studies have not only expanded our knowledge about the neuronal circuits in the CNS but have also highlighted the importance of the peripheral epithelia-immune-neuronal crosstalk in the development of mechanical itch. In this review, we will summarize related findings about the molecular and cellular mechanisms of mechanical itch in the skin.

A visual circuit related to the parabrachial nucleus for the antipruritic effects of bright light treatment

In addition to its role in vision, light also serves non-image-forming visual functions. Despite clinical evidence suggesting the antipruritic effects of bright light treatment, the circuit mechanisms underlying the effects of light on itch-related behaviors remain poorly understood. In this study, we demonstrate that bright light treatment reduces itch-related behaviors in mice through a visual circuit related to the lateral parabrachial nucleus (LPBN). Specifically, a subset of retinal ganglion cells (RGCs) innervates GABAergic neurons in the ventral lateral geniculate nucleus and intergeniculate leaflet (vLGN/IGL), which subsequently inhibit CaMKIIα+ neurons in the LPBN. Activation of both the vLGN/IGL-projecting RGCs and the vLGN/IGL-to-LPBN projections is sufficient to reduce itch-related behaviors induced by various pruritogens. Importantly, we demonstrate that the antipruritic effects of bright light treatment rely on the activation of the retina-vLGN/IGL-LPBN pathway. Collectively, our findings elucidate a visual circuit related to the LPBN that underlies the antipruritic effects of bright light treatment.

Skin barrier: new therapeutic targets for chronic kidney disease-associated pruritus - a narrative review

The current incidence of chronic kidney disease-associated pruritus (CKD-aP) in patients with end-stage renal disease (ESRD) is approximately 70%, especially in those receiving dialysis, which negatively affects their work and private lives. The CKD-aP pathogenesis remains unclear, but uremic toxin accumulation, histamine release, and opioid imbalance have been suggested to lead to CKD-aP. Current therapeutic approaches, such as opioid receptor modulators, antihistamines, and ultraviolet B irradiation, are associated with some limitations and adverse effects. The skin barrier is the first defense in preventing external injury to the body. Patients with chronic kidney disease often experience itch due to the damaged skin barrier and reduced secretion of sweat and secretion from sebaceous glands. Surprisingly, skin barrier-repairing agents repair the skin barrier and inhibit the release of inflammatory cytokines, maintain skin immunity, and ameliorate the micro-inflammatory status of afferent nerve fibers. Here, we summarize the epidemiology, pathogenesis, and treatment status of CKD-aP and explore the possibility of skin barrier repair in CKD-aP treatment.

A spinal circuit model with asymmetric cervical-lumbar layout controls backward locomotion and scratching in quadrupeds

Locomotion and scratching are basic motor functions which are critically important for animal survival. Although the spinal circuits governing forward locomotion have been extensively investigated, the organization of spinal circuits and neural mechanisms regulating backward locomotion and scratching remain unclear. Here, we extend a model by Danner et al. to propose a spinal circuit model with asymmetrical cervical-lumbar layout to investigate these issues. In the model, the left-right alternation within the cervical and lumbar circuits is mediated by V 0D and V 0V commissural interneurons (CINs), respectively. With different control strategies, the model closely reproduces multiple experimental data of quadrupeds in different motor behaviors. Specifically, under the supraspinal drive, walk and trot are expressed in control condition, half-bound is expressed after deletion of V 0V CINs, and bound is expressed after deletion of V0 (V 0D and V 0V) CINs; in addition, unilateral hindlimb scratching occurs in control condition and synchronous bilateral hindlimb scratching appears after deletion of V 0V CINs. Under the combined drive of afferent feedback and perineal stimulation, different coordination patterns between hindlimbs during BBS (backward-biped-spinal) locomotion are generated. The results suggest that (1) the cervical and lumbar circuits in the spinal network are asymmetrically recruited during particular rhythmic limb movements. (2) Multiple motor behaviors share a single spinal network under the reconfiguration of the spinal network by supraspinal inputs or somatosensory feedback. Our model provides new insights into the organization of motor circuits and neural control of rhythmic limb movements.

A conserved brainstem region for instinctive behaviour control: The vertebrate periaqueductal gray

Instinctive behaviours have evolved across animal phyla and ensure the survival of both the individual and species. They include behaviours that achieve defence, feeding, aggression, sexual reproduction, or parental care. Within the vertebrate subphylum, the brain circuits that support instinctive behaviour output are evolutionarily conserved, being present in the oldest group of living vertebrates, the lamprey. Here, I will provide an evolutionary and comparative perspective on the function of a conserved brainstem region central to the initiation and execution of virtually all instinctive behaviours-the periaqueductal gray. In particular, I will focus on recent advances on the neural mechanisms in the periaqueductal gray that underlie the production of different instinctive behaviours within and across species.

A regulatory role of the medial septum in the chloroquine-induced acute itch through local GABAergic system and GABAergic pathway to the anterior cingulate cortex

Itch, a common somatic sensation, serves as a crucial protective system. Recent studies have unraveled the neural mechanisms of itch at peripheral, spinal cord as well as cerebral levels. However, a comprehensive understanding of the central mechanism governing itch transmission and regulation remains elusive. Here, we report the role of the medial septum (MS), an integral component of the basal forebrain, in modulating the acute itch processing. The increases in c-Fos+ neurons and calcium signals within the MS during acute itch processing were observed. Pharmacogenetic activation manipulation of global MS neurons suppressed the scratching behaviors induced by chloroquine or compound 48/80. Microinjection of GABA into the MS or pharmacogenetic inhibition of non-GABAergic neurons markedly suppressed chloroquine-induced scratching behaviors. Pharmacogenetic activation of the MS-ACC GABAergic pathway attenuated chloroquine-induced acute itch. Hence, our findings reveal that MS has a regulatory role in the chloroquine-induced acute itch through local increased GABA to inhibit non-GABAergic neurons and the activation of MS-ACC GABAergic pathway.

An insular cortical circuit required for itch sensation and aversion

Itch encompasses both sensory and emotional dimensions, with the two dimensions reciprocally exacerbating each other. However, whether a shared neural circuit mechanism governs both dimensions remains elusive. Here, we report that the anterior insular cortex (AIC) is activated by both histamine-dependent and -independent itch stimuli. The activation of AIC elicits aversive emotion and exacerbates pruritogen-induced itch sensation and aversion. Mechanistically, AIC excitatory neurons project to the GABAergic neurons in the dorsal bed nucleus of the stria terminalis (dBNST). Manipulating the activity of the AIC → dBNST pathway affects both itch sensation and itch-induced aversion. Our study discovers the shared neural circuit (AIC → dBNST pathway) underlying the itch sensation and aversion, highlights the critical role of the AIC as a central hub for the itch processing, and provides a framework to understand the neural mechanisms underlying the sensation and emotion interaction.

Novel aspects of signal processing in lamina I

The most superficial layer of the spinal dorsal horn, lamina I, is a key element of the nociceptive processing system. It contains different types of projection neurons (PNs) and local-circuit neurons (LCNs) whose functional roles in the signal processing are poorly understood. This article reviews recent progress in elucidating novel anatomical features and physiological properties of lamina I PNs and LCNs revealed by whole-cell recordings in ex vivo spinal cord. This article is part of the Special Issue on "Ukrainian Neuroscience".

Orexin neurons play contrasting roles in itch and pain neural processing via projecting to the periaqueductal gray

Pain and itch are recognized as antagonistically regulated sensations; pain suppresses itch, whilst pain inhibition enhances itch. The neural mechanisms at the central nervous system (CNS) underlying these pain-itch interactions still need to be explored. Here, we revealed the contrasting role of orexin-producing neurons (ORX neurons) in the lateral hypothalamus (LH), which suppresses pain while enhancing itch neural processing, by applying optogenetics to the acute pruritus and pain model. We also revealed that the circuit of ORX neurons from LH to periaqueductal gray regions served in the contrasting modulation of itch and pain processing using optogenetic terminal inhibition techniques. Additionally, by using an atopic dermatitis model, we confirmed the involvement of ORX neurons in regulating chronic itch processing, which could lead to a novel therapeutic target for persistent pruritus in clinical settings. Our findings provide new insight into the mechanism of antagonistic regulation between pain and itch in the CNS.

Role of stress in skin diseases: A neuroendocrine-immune interaction view

Psychological stress is a crucial factor in the development of many skin diseases, and the stigma caused by skin disorders may further increase the psychological burden, forming a vicious cycle of psychological stress leading to skin diseases. Therefore, understanding the relationship between stress and skin diseases is necessary. The skin, as the vital interface with the external environment, possesses its own complex immune system, and the neuroendocrine system plays a central role in the stress response of the body. Stress-induced alterations in the immune system can also disrupt the delicate balance of immune cells and inflammatory mediators in the skin, leading to immune dysregulation and increased susceptibility to various skin diseases. Stress can also affect the skin barrier function, impair wound healing, and promote the release of pro-inflammatory cytokines, thereby exacerbating existing skin diseases such as psoriasis, atopic dermatitis, acne, and urticaria. In the present review, we explored the intricate relationship between stress and skin diseases from a neuroendocrine-immune interaction perspective. We explored the occurrence and development of skin diseases in the context of stress, the stress models for skin diseases, the impact of stress on skin function and diseases, and relevant epidemiological studies and clinical trials. Understanding the relationship between stress and skin diseases from a neuroendocrine-immune interaction perspective provides a comprehensive framework for targeted interventions and new insights into the diagnosis and treatment of skin diseases.

Intrapatient comparison of atopic dermatitis skin transcriptome shows differences between tape-strips and biopsies

BACKGROUND

Our knowledge of etiopathogenesis of atopic dermatitis (AD) is largely derived from skin biopsies, which are associated with pain, scarring and infection. In contrast, tape-stripping is a minimally invasive, nonscarring technique to collect skin samples.

METHODS

To construct a global AD skin transcriptomic profile comparing tape-strips to whole-skin biopsies, we performed RNA-seq on tape-strips and biopsies taken from the lesional skin of 20 moderate-to-severe AD patients and the skin of 20 controls. Differentially expressed genes (DEGs) were defined by fold-change (FCH) ≥2.0 and false discovery rate <0.05.

RESULTS

We detected 4104 (2513 Up; 1591 Down) and 1273 (546 Up; 727 Down) DEGs in AD versus controls, in tape-strips and biopsies, respectively. Although both techniques captured dysregulation of key immune genes, tape-strips showed higher FCHs for innate immunity (IL-1B, IL-8), dendritic cell (ITGAX/CD11C, FCER1A), Th2 (IL-13, CCL17, TNFRSF4/OX40), and Th17 (CCL20, CXCL1) products, while biopsies showed higher upregulation of Th22 associated genes (IL-22, S100As) and dermal cytokines (IFN-γ, CCL26). Itch-related genes (IL-31, TRPV3) were preferentially captured by tape-strips. Epidermal barrier abnormalities were detected in both techniques, with terminal differentiation defects (FLG2, PSORS1C2) better represented by tape-strips and epidermal hyperplasia changes (KRT16, MKI67) better detected by biopsies.

CONCLUSIONS

Tape-strips and biopsies capture overlapping but distinct features of the AD molecular signature, suggesting their respective utility for monitoring specific AD-related immune, itch, and barrier abnormalities in clinical trials and longitudinal studies.

Xanthotoxol relieves itch in mice via suppressing spinal GRP/GRPR signaling

Although pruritus, commonly known as itch, is a common and debilitating symptom associated with various skin conditions, there is a lack of effective therapies available. Xanthotoxol (XAN), a biologically active linear furocoumarin, shows potential in the treatment of various neurological disorders. In this study, we discovered that administering XAN either through intraperitoneal or intrathecal injections effectively reduced scratching behavior induced by compound 48/80 or chloroquine. Importantly, XAN also substantially alleviates chronic itch in dry skin and allergic contact dermatitis mice. Substantial progress has highlighted the crucial role of gastrin-releasing peptide (GRP)-gastrin-releasing peptide receptor (GRPR) signaling in the dorsal spinal cord in transmitting various types of itch. Our behavior tests revealed that XAN significantly alleviated scratching behaviors induced by intrathecal administration of GRP or GRPR agonist bombesin. Furthermore, XAN reduced the activation of neurons in the spinal cord caused by intrathecal administration of GRP in mice. Moreover, XAN attenuates the activation of spinal GRPR-positive neurons in itchy mice. These findings suggest that XAN mitigates itch in mice by suppressing spinal GRP/GRPR signaling, thereby establishing XAN as a promising therapeutic option for treating pruritus.

Whole-brain monosynaptic inputs to lateral periaqueductal gray glutamatergic neurons in mice

OBJECTIVE

The lateral periaqueductal gray (LPAG), which mainly contains glutamatergic neurons, plays an important role in social responses, pain, and offensive and defensive behaviors. Currently, the whole-brain monosynaptic inputs to LPAG glutamatergic neurons are unknown. This study aims to explore the structural framework of the underlying neural mechanisms of LPAG glutamatergic neurons.

METHODS

This study used retrograde tracing systems based on the rabies virus, Cre-LoxP technology, and immunofluorescence analysis.

RESULTS

We found that 59 nuclei projected monosynaptic inputs to the LPAG glutamatergic neurons. In addition, seven hypothalamic nuclei, namely the lateral hypothalamic area (LH), lateral preoptic area (LPO), substantia innominata (SI), medial preoptic area, ventral pallidum, posterior hypothalamic area, and lateral globus pallidus, projected most densely to the LPAG glutamatergic neurons. Notably, we discovered through further immunofluorescence analysis that the inputs to the LPAG glutamatergic neurons were colocalized with several markers related to important neurological functions associated with physiological behaviors.

CONCLUSION

The LPAG glutamatergic neurons received dense projections from the hypothalamus, especially nuclei such as LH, LPO, and SI. The input neurons were colocalized with several markers of physiological behaviors, which show the pivotal role of glutamatergic neurons in the physiological behaviors regulation by LPAG.

The Anterior Insula and its Projection to the Prelimbic Cortex are Involved in the Regulation of 5-HT-Induced Itch

Itch is an unpleasant sensation that urges people and animals to scratch. Neuroimaging studies on itch have yielded extensive correlations with diverse cortical and subcortical regions, including the insular lobe. However, the role and functional specificity of the insular cortex (IC) and its subdivisions in itch mediation remains unclear. Here, we demonstrated by immunohistochemistry and fiber photometry tests, that neurons in both the anterior insular cortex (AIC) and the posterior insular cortex (PIC) are activated during acute itch processes. Pharmacogenetic experiments revealed that nonselective inhibition of global AIC neurons, or selective inhibition of the activity of glutaminergic neurons in the AIC, reduced the scratching behaviors induced by intradermal injection of 5-hydroxytryptamine (5-HT), but not those induced by compound 48/80. However, both nonselective inhibition of global PIC neurons and selective inhibition of glutaminergic neurons in the PIC failed to affect the itching-scratching behaviors induced by either 5-HT or compound 48/80. In addition, pharmacogenetic inhibition of AIC glutaminergic neurons effectively blocked itch-associated conditioned place aversion behavior, and inhibition of AIC glutaminergic neurons projecting to the prelimbic cortex significantly suppressed 5-HT-evoked scratching. These findings provide preliminary evidence that the AIC is involved, at least partially via aversive emotion mediation, in the regulation of 5-HT-, but not compound 48/80-induced itch.

A cholinergic circuit that relieves pain despite opioid tolerance

Chronic pain is a tremendous burden for afflicted individuals and society. Although opioids effectively relieve pain, significant adverse outcomes limit their utility and efficacy. To investigate alternate pain control mechanisms, we explored cholinergic signaling in the ventrolateral periaqueductal gray (vlPAG), a critical nexus for descending pain modulation. Biosensor assays revealed that pain states decreased acetylcholine release in vlPAG. Activation of cholinergic projections from the pedunculopontine tegmentum to vlPAG relieved pain, even in opioid-tolerant conditions, through ⍺7 nicotinic acetylcholine receptors (nAChRs). Activating ⍺7 nAChRs with agonists or stimulating endogenous acetylcholine inhibited vlPAG neuronal activity through Ca2+ and peroxisome proliferator-activated receptor α (PPAR⍺)-dependent signaling. In vivo 2-photon imaging revealed that chronic pain induces aberrant excitability of vlPAG neuronal ensembles and that ⍺7 nAChR-mediated inhibition of these cells relieves pain, even after opioid tolerance. Finally, pain relief through these cholinergic mechanisms was not associated with tolerance, reward, or withdrawal symptoms, highlighting its potential clinical relevance.

An atlas of itch-associated neural dynamics in the mouse brain

The itch-scratching cycle is mediated by neural dynamics in the brain. However, our understanding of the neural dynamics during this cycle remains limited. In this study, we examine the neural dynamics of 126 mouse brain areas by measuring the calcium signal using fiber photometry. We present numerous response patterns in the mouse brain during the itch-scratching cycle. Interestingly, we find that a group of brain areas exhibit activation only at the end of histamine-induced scratching behavior. Additionally, several brain areas exhibit transient activation at the onset of scratching induced by chloroquine. Both histamine- and chloroquine-induced itch evoke diverse response patterns across the mouse brain. In summary, our study provides a comprehensive dataset for the diverse activity pattern of mouse brain during the itch-scratching cycle, paving the way for further exploration into the neural mechanisms underlying the itch-scratching cycle.

Descending dopaminergic pathway facilitates itch signal processing via activating spinal GRPR+ neurons

A11 dopaminergic neurons regulate somatosensory transduction by projecting from the diencephalon to the spinal cord, but the function of this descending projection in itch remained elusive. Here, we report that dopaminergic projection neurons from the A11 nucleus to the spinal dorsal horn (dopaminergicA11-SDH ) are activated by pruritogens. Inhibition of these neurons alleviates itch-induced scratching behaviors. Furthermore, chemogenetic inhibition of spinal dopamine receptor D1-expressing (DRD1+ ) neurons decreases acute or chronic itch-induced scratching. Mechanistically, spinal DRD1+ neurons are excitatory and mostly co-localize with gastrin-releasing peptide (GRP), an endogenous neuropeptide for itch. In addition, DRD1+ neurons form synapses with GRP receptor-expressing (GRPR+ ) neurons and activate these neurons via AMPA receptor (AMPAR). Finally, spontaneous itch and enhanced acute itch induced by activating spinal DRD1+ neurons are relieved by antagonists against AMPAR and GRPR. Thus, the descending dopaminergic pathway facilitates spinal itch transmission via activating DRD1+ neurons and releasing glutamate and GRP, which directly augments GRPR signaling. Interruption of this descending pathway may be used to treat chronic itch.

Brainstem Modulation of Nociception by Periaqueductal Gray Neurons Expressing the μ-Opioid Receptor in Mice

BACKGROUND

Pharmacologic manipulations directed at the periaqueductal gray have demonstrated the importance of the μ-opioid receptor in modulating reflexive responses to nociception. The authors hypothesized that a supraspinal pathway centered on neurons in the periaqueductal gray containing the μ-opioid receptor could modulate nociceptive and itch behaviors.

METHODS

The study used anatomical, optogenetic, and chemogenetic approaches in male and female mice to manipulate μ-opioid receptor neurons in the periaqueductal gray. Behavioral assays including von Frey, Hargreaves, cold plantar, chloroquine-induced itch, hotplate, formalin-induced injury, capsaicin-induced injury, and open field tests were used. In separate experiments, naloxone was administered in a postsurgical model of latent sensitization.

RESULTS

Activation of μ-opioid receptor neurons in the periaqueductal gray increased jumping (least-squares mean difference of -3.30 s; 95% CI, -6.17 to -0.44; P = 0.023; n = 7 or 8 mice per group), reduced itch responses (least-squares mean difference of 70 scratching bouts; 95% CI, 35 to 105; P < 0.001; n = 8 mice), and elicited modestly antinociceptive effects (least-squares mean difference of -0.7 g on mechanical and -10.24 s on thermal testing; 95% CI, -1.3 to -0.2 and 95% CI, -13.77 to -6.70, and P = 0.005 and P < 0.001, respectively; n = 8 mice). Last, the study uncovered the role of the periaqueductal gray in suppressing hyperalgesia after a postsurgical state of latent sensitization (least-squares mean difference comparing saline and naloxone of -12 jumps; 95% CI, -17 to -7; P < 0.001 for controls; and -2 jumps; 95% CI, -7 to 4; P = 0.706 after optogenetic stimulation; n = 7 to 9 mice per group).

CONCLUSIONS

μ-Opioid receptor neurons in the periaqueductal gray modulate distinct nocifensive behaviors: their activation reduced responses to mechanical and thermal testing, and attenuated scratching behaviors, but facilitated escape responses. The findings emphasize the role of the periaqueductal gray in the behavioral expression of nociception using reflexive and noxious paradigms.

EDITOR’S PERSPECTIVE

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.

Identification of an essential spinoparabrachial pathway for mechanical itch

The sensation of itch is a protective response that is elicited by either mechanical or chemical stimuli. The neural pathways for itch transmission in the skin and spinal cord have been characterized previously, but the ascending pathways that transmit sensory information to the brain to evoke itch perception have not been identified. Here, we show that spinoparabrachial neurons co-expressing Calcrl and Lbx1 are essential for generating scratching responses to mechanical itch stimuli. Moreover, we find that mechanical and chemical itch are transmitted by separate ascending pathways to the parabrachial nucleus, where they engage separate populations of FoxP2PBN neurons to drive scratching behavior. In addition to revealing the architecture of the itch transmission circuitry required for protective scratching in healthy animals, we identify the cellular mechanisms underlying pathological itch by showing the ascending pathways for mechanical and chemical itch function cooperatively with the FoxP2PBN neurons to drive chronic itch and hyperknesis/alloknesis.

Somatostatin Neurons from Periaqueductal Gray to Medulla Facilitate Neuropathic Pain in Male Mice

Projections from the periaqueductal gray (PAG) to the rostral ventromedial medulla (RVM) are known to engage in descending pain modulation, but how the neural substrates of the PAG-RVM projections contribute to neuropathic pain remains largely unknown. In this study, we showed somatostatin-expressing glutamatergic neurons in the lateral/ventrolateral PAG that facilitate mechanical and thermal hypersensitivity in a mouse model of chemotherapy-induced neuropathic pain. We found that these neurons form direct excitatory connections with neurons in the RVM region. Inhibition of this PAG-RVM projection alleviates mechanical and thermal hypersensitivity associated with neuropathy, whereas its activation enhances hypersensitivity in the mice. Thus, our findings revealed that somatostatin neurons within the PAG-RVM axial are crucial for descending pain facilitation and can potentially be exploited as a useful therapeutic target for neuropathic pain. PERSPECTIVE: We report the profound contribution of somatostatin neurons within the PAG-RVM projections to descending pain facilitation underlying neuropathic pain. These results may help to develop central therapeutic strategies for neuropathic pain.

Representation and control of pain and itch by distinct prefrontal neural ensembles

Pain and itch are two closely related but essentially distinct sensations that elicit different behavioral responses. However, it remains mysterious how pain and itch information is encoded in the brain to produce differential perceptions. Here, we report that nociceptive and pruriceptive signals are separately represented and processed by distinct neural ensembles in the prelimbic (PL) subdivision of the medial prefrontal cortex (mPFC) in mice. Pain- and itch-responsive cortical neural ensembles were found to significantly differ in electrophysiological properties, input-output connectivity profiles, and activity patterns to nociceptive or pruriceptive stimuli. Moreover, these two groups of cortical neural ensembles oppositely modulate pain- or itch-related sensory and emotional behaviors through their preferential projections to specific downstream regions such as the mediodorsal thalamus (MD) and basolateral amygdala (BLA). These findings uncover separate representations of pain and itch by distinct prefrontal neural ensembles and provide a new framework for understanding somatosensory information processing in the brain.

Orchestration of innate and conditioned defensive actions by the periaqueductal gray

The midbrain periaqueductal gray (PAG) has been recognized for decades as having a central role in the control of a wide variety of defensive responses. Initial discoveries relied primarily on lesions, electrical stimulation and pharmacology. Recent developments in neural activity imaging and in methods to control activity with anatomical and genetic specificity have revealed additional streams of data informing our understanding of PAG function. Here, we discuss both classic and modern studies reporting on how PAG-centered circuits influence innate as well as learned defensive actions in rodents and humans. Though early discoveries emphasized the PAG's role in rapid induction of innate defensive actions, emerging new data indicate a prominent role for the PAG in more complex processes, including representing behavioral states and influencing fear learning and memory. This article is part of the Special Issue on "Fear, Anxiety and PTSD".

Identification of brain-to-spinal circuits controlling the laterality and duration of mechanical allodynia in mice

Mechanical allodynia (MA) represents one prevalent symptom of chronic pain. Previously we and others have identified spinal and brain circuits that transmit or modulate the initial establishment of MA. However, brain-derived descending pathways that control the laterality and duration of MA are still poorly understood. Here we report that the contralateral brain-to-spinal circuits, from Oprm1 neurons in the lateral parabrachial nucleus (lPBNOprm1), via Pdyn neurons in the dorsal medial regions of hypothalamus (dmHPdyn), to the spinal dorsal horn (SDH), act to prevent nerve injury from inducing contralateral MA and reduce the duration of bilateral MA induced by capsaicin. Ablating/silencing dmH-projecting lPBNOprm1 neurons or SDH-projecting dmHPdyn neurons, deleting Dyn peptide from dmH, or blocking spinal κ-opioid receptors all led to long-lasting bilateral MA. Conversely, activation of dmHPdyn neurons or their axonal terminals in SDH can suppress sustained bilateral MA induced by lPBN lesion.

Analysis of a cell niche with proliferative potential at the roof of the aqueduct of Sylvius

The aqueduct of Sylvius connects the third with the fourth ventricle and is surrounded by the Periaqueductal Grey. Here, we report a novel niche of cells in the dorsal section of the aqueduct, hereby named dorsal aqueduct niche or DAN, by applying a battery of selective markers and transgenic mouse lines. The somata of DAN cells are located toward the lumen of the ventricle forming multiple layers in close association with the cerebrospinal fluid (CSF). A single process emerges from the soma and run with the blood vessels. Cells of the DAN express radial glia/stem cell markers such as GFAP, vimentin and nestin, and the glutamate transporter GLAST or the oligodendrocyte precursor/pericyte marker NG2, thereby suggesting their potential for the generation of new cells. Morphologically, DAN cells resemble tanycytes of the third ventricle, which transfer biochemical signals from the CSF to the central nervous system and display proliferative capacity. The aqueduct ependymal lining can proliferate as observed by the integration of BrdU and expression of Ki67. Thus, the dorsal section of the aqueduct of Sylvius possesses cells that may act a niche of new glial cells in the adult mouse brain.

The opposite roles of orexin neurons in pain and itch neural processing

Pain and itch are antagonistically regulated sensations; pain suppresses itch, and inhibition of pain enhances itch. Understanding the central neural circuit of antagonistic regulation between pain and itch is required to develop new therapeutics better to manage these two feelings in a clinical situation. However, evidence of the neural mechanism underlying the pain-itch interaction in the central nervous system (CNS) is still insufficient. To pave the way for this research area, our laboratory has focused on orexin (ORX) producing neurons in the hypothalamus, which is known as a master switch that induces various defense responses when animals face a stressful environment. This review article summarized the previous evidence and our latest findings to argue the neural regulation between pain and itch and the bidirectional roles of ORX neurons in processing these two sensations. i.e., pain relief and itch exacerbation. Further, we discussed the possible neural circuit mechanism for the opposite controlling of pain and itch by ORX neurons. Focusing on the roles of ORX neurons would provide a new perspective to understand the antagonistic regulation of pain and itch in CNS.

Role of GRPR in Acupuncture Intervention in the "Itch-scratch Vicious Cycle" Spinal Circuit of Chronic Pruritus

Many previous studies have shown the potential antipruritic effect of acupuncture. This paper reviews the antipruritic mechanisms of acupuncture according to these aspects: sample characteristics, detail of intervention, and effects evaluation. The majority of research on acupuncture's antipruritic effect has focused on primary afferents of the peripheral mechanism. Relatively few studies, however, have addressed the central mechanisms. Combination the latest research achievements of chronic itch, gastrin-releasing peptide receptor (GRPR) in the dorsal horn of the spinal cord may represent the first molecule identified that is dedicated to mediating the itch response and may provide an important therapeutic target for the treatment of chronic pruritic conditions. Therefore, GRPR may be a new target for acupuncture to relieve itch in the future and provide new ideas for acupuncture intervention in the mechanisms of the spinal level of the "itch-scratch vicious cycle" of chronic itch.

GABAergic neurons in the ventral lateral geniculate nucleus and intergeniculate leaflet modulate itch processing in mice

Itch and pain are two closely related sensations that receiving similar encodings at multiple levels. Accumulated evidences suggest that activation of the ventral lateral geniculate nucleus and intergeniculate leaflet (vLGN/IGL)-to-lateral and ventrolateral periaqueductal gray (l/vlPAG) projections mediates the antinociceptive effects of bright light therapy. Clinical study showed that bright light therapy may ameliorate cholestasis-induced pruritus. However, the underlying mechanism and whether this circuit participates in itch modulation remains unclear. In this study, chloroquine and histamine were utilized to induce acute itch models in mice. Neuronal activities in vLGN/IGL nucleus were evaluated with c-fos immunostaining as well as fiber photometry. Optogenetic manipulations were performed to activate or inhibit GABAergic neurons in the vLGN/IGL nucleus. Our results showed that the expressions of c-fos in vLGN/IGL were significantly increased upon both chloroquine- and histamine-induced acute itch stimuli. GABAergic neurons in vLGN/IGL were activated during histamine and chloroquine-induced scratching. Optogenetic activation of the vLGN/IGL GABAergic neurons exerts antipruritic effect, while inhibiting these neurons exerts pruritic effect. Our results provide evidence that GABAergic neurons in vLGN/IGL nucleus might play a crucial role in modulating itch, which may provide clue for application of bright light as an antipruritic treatment in clinic.

An Anterior Cingulate Cortex-to-Midbrain Projection Controls Chronic Itch in Mice

Itch is an unpleasant sensation that provokes the desire to scratch. While acute itch serves as a protective system to warn the body of external irritating agents, chronic itch is a debilitating but poorly-treated clinical disease leading to repetitive scratching and skin lesions. However, the neural mechanisms underlying the pathophysiology of chronic itch remain mysterious. Here, we identified a cell type-dependent role of the anterior cingulate cortex (ACC) in controlling chronic itch-related excessive scratching behaviors in mice. Moreover, we delineated a neural circuit originating from excitatory neurons of the ACC to the ventral tegmental area (VTA) that was critically involved in chronic itch. Furthermore, we demonstrate that the ACC→VTA circuit also selectively modulated histaminergic acute itch. Finally, the ACC neurons were shown to predominantly innervate the non-dopaminergic neurons of the VTA. Taken together, our findings uncover a cortex–midbrain circuit for chronic itch-evoked scratching behaviors and shed novel insights on therapeutic intervention.

The prelimbic cortex regulates itch processing by controlling attentional bias

Itch is a complex and unpleasant sensory experience. Recent studies have begun to investigate the neural mechanisms underlying the modulation of sensory and emotional components of itch in the brain. However, the key brain regions and neural mechanism involved in modulating the attentional processing of itch remain elusive. Here, we showed that the prelimbic cortex (PrL) is associated with itch processing and that the manipulation of itch-responsive neurons in the PrL significantly disrupted itch-induced scratching. Interestingly, we found that increasing attentional bias toward a distracting stimulus could disturb itch processing. We also demonstrated the existence of a population of attention-related neurons in the PrL that drive attentional bias to regulate itch processing. Importantly, itch-responsive neurons and attention-related neurons significantly overlapped in the PrL and were mutually interchangeable in the regulation of itch processing at the cellular activity level. Our results revealed that the PrL regulates itch processing by controlling attentional bias.

Ventrolateral Periaqueductal Gray Astrocytes Regulate Nociceptive Sensation and Emotional Motivation in Diabetic Neuropathic Pain

Diabetic neuropathic pain (DNP) is a diabetes complication experienced by many patients. Ventrolateral periaqueductal gray (vlPAG) neurons are essential mediators of the descending pain modulation system, yet the role of vlPAG astrocytes in DNP remains unclear. The present study applied a multidimensional approach to elucidate the role of these astrocytes in DNP. We verified the activation of astrocytes in different regions of the PAG in male DNP-model rats. We found that only astrocytes in the vlPAG exhibited increased growth. Furthermore, we described differences in vlPAG astrocyte activity at different time points during DNP progression. After the 14th day of modeling, vlPAG astrocytes exhibited obvious activation and morphologic changes. Furthermore, activation of Gq-designer receptors exclusively activated by a designer drug (Gq-DREADDs) in vlPAG astrocytes in naive male rats induced neuropathic pain-like symptoms and pain-related aversion, whereas activation of Gi-DREADDs in vlPAG astrocytes in male DNP-model rats alleviated sensations of pain and promoted pain-related preference behavior. Thus, bidirectional manipulation of vlPAG astrocytes revealed their potential to regulate pain. Surprisingly, activation of Gi-DREADDs in vlPAG astrocytes also mitigated anxiety-like behavior induced by DNP. Thus, our results provide direct support for the hypothesis that vlPAG astrocytes regulate diabetes-associated neuropathic pain and concomitant anxiety-like behavior.SIGNIFICANCE STATEMENT Many studies examined the association between the ventrolateral periaqueductal gray (vlPAG) and neuropathic pain. However, few studies have focused on the role of vlPAG astrocytes in diabetic neuropathic pain (DNP) and DNP-related emotional changes. This work confirmed the role of vlPAG astrocytes in DNP by applying a more direct and robust approach. We used chemogenetics to bidirectionally manipulate the activity of vlPAG astrocytes and revealed that vlPAG astrocytes regulate DNP and pain-related behavior. In addition, we discovered that activation of Gi-designer receptors exclusively activated by a designer drug in vlPAG astrocytes alleviated anxiety-like behavior induced by DNP. Together, these findings provide new insights into DNP and concomitant anxiety-like behavior and supply new therapeutic targets for treating DNP.

Crossing nerve transfer drives sensory input-dependent plasticity for motor recovery after brain injury

Restoring limb movements after central nervous system injury remains a substantial challenge. Recent studies proved that crossing nerve transfer surgery could rebuild physiological connectivity between the contralesional cortex and the paralyzed arm to compensate for the lost function after brain injury. However, the neural mechanism by which this surgery mediates motor recovery remains still unclear. Here, using a clinical mouse model, we showed that this surgery can restore skilled forelimb function in adult mice with unilateral cortical lesion by inducing cortical remapping and promoting corticospinal tract sprouting. After reestablishing the ipsilateral descending pathway, resecting of the artificially rebuilt peripheral nerve did not affect motor improvements. Furthermore, retaining the sensory afferent, but not the motor efferent, of the transferred nerve was sufficient for inducing brain remapping and facilitating motor restoration. Thus, our results demonstrate that surgically rebuilt sensory input triggers neural plasticity for accelerating motor recovery, which provides an approach for treating central nervous system injuries.

Excitatory SST neurons in the medial paralemniscal nucleus control repetitive self-grooming and encode reward

The use of body-focused repetitive behaviors (BFRBs) is conceptualized as a means of coping with stress. However, the neurological mechanism by which repetitive behaviors affect anxiety regulation is unclear. Here, we identify that the excitatory somatostatin-positive neurons in the medial paralemniscal nucleus (MPL^SST neurons) in mice promote self-grooming and encode reward. MPL^SST neurons display prominent grooming-related neuronal activity. Loss of function of MPL^SST neurons impairs both self-grooming and post-stress anxiety alleviation. Activation of MPL^SST neurons is rewarding and sufficient to drive reinforcement by activating dopamine (DA) neurons in the ventral tegmental area (VTA) and eliciting dopamine release. The neuropeptide SST facilitates the rewarding impact of MPL^SST neurons. MPL^SST neuron-mediated self-grooming is triggered by the input from the central amygdala (CeA). Our study reveals a dual role of CeA-MPL^SST-VTA^DA circuit in self-grooming and post-stress anxiety regulation and conceptualizes MPL^SST neurons as an interface linking the stress and reward systems in mice.

Hypothalamic orexinergic neurons modulate pain and itch in an opposite way: pain relief and itch exacerbation

Pain and itch are recognized as antagonistic sensations; pain suppresses itch and inhibition of pain generates itch. There is still a lack of evidence about the neural mechanism of the interaction between pain and itch in the central nervous system. In this study, we focused on the orexin (ORX) neurons in the lateral hypothalamus (LH), which mediate various "defense responses" when animals confront stressors. We found that the scratching behaviors induced by the pruritogen were significantly suppressed in ORX-neuron-ablated (ORX-abl) mice. The exaggerated pain behavior and attenuated itch behavior observed in ORX-abl mice indicated that ORX neurons modulate pain and itch in an opposite way, i.e., pain relief and itch exacerbation. In addition, most of the ORX neurons responded to both pain and itch input. Our results suggest that ORX neurons inversely regulate pain- and itch-related behaviors, which could be understood as a defense response to cope with stress environment.

Inhibition of itch by neurokinin 1 receptor (Tacr1) -expressing ON cells in the rostral ventromedial medulla in mice

The rostral ventromedial medulla (RVM) is important in descending modulation of spinal nociceptive transmission, but it is unclear if the RVM also modulates spinal pruriceptive transmission. RVM ON cells are activated by noxious algesic and pruritic stimuli and are pronociceptive. Many RVM-spinal projection neurons express the neurokinin-1 receptor (Tacr1), and ON-cells are excited by local administration of substance P (SP). We hypothesized that Tacr1-expressing RVM ON cells exert an inhibitory effect on itch opposite to their pronociceptive action. Intramedullary microinjection of SP significantly potentiated RVM ON cells and reduced pruritogen-evoked scratching while producing mild mechanical sensitization. Chemogenetic activation of RVM Tacr1-expressing RVM neurons also reduced acute pruritogen-evoked scratching. Optotagging experiments confirmed RVM Tacr1-expressing neurons to be ON cells. We conclude that Tacr1-expressing ON cells in RVM play a significant role in the modulation of pruriceptive transmission.

Distinct neural networks derived from galanin-containing nociceptors and neurotensin-expressing pruriceptors

Pain and itch are distinct sensations arousing evasion and compulsive desire for scratching, respectively. It's unclear whether they could invoke different neural networks in the brain. Here, we use the type 1 herpes simplex virus H129 strain to trace the neural networks derived from two types of dorsal root ganglia (DRG) neurons: one kind of polymodal nociceptors containing galanin (Gal) and one type of pruriceptors expressing neurotensin (Nts). The DRG microinjection and immunosuppression were performed in transgenic mice to achieve a successful tracing from specific types of DRG neurons to the primary sensory cortex. About one-third of nuclei in the brain were labeled. More than half of them were differentially labeled in two networks. For the ascending pathways, the spinothalamic tract was absent in the network derived from Nts-expressing pruriceptors, and the two networks shared the spinobulbar projections but occupied different subnuclei. As to the motor systems, more neurons in the primary motor cortex and red nucleus of the somatic motor system participated in the Gal-containing nociceptor-derived network, while more neurons in the nucleus of the solitary tract (NST) and the dorsal motor nucleus of vagus nerve (DMX) of the emotional motor system was found in the Nts-expressing pruriceptor-derived network. Functional validation of differentially labeled nuclei by c-Fos test and chemogenetic inhibition suggested the red nucleus in facilitating the response to noxious heat and the NST/DMX in regulating the histamine-induced scratching. Thus, we reveal the organization of neural networks in a DRG neuron type-dependent manner for processing pain and itch.

Medullary kappa-opioid receptor neurons inhibit pain and itch through a descending circuit

In perilous and stressful situations, the ability to suppress pain can be critical for survival. The rostral ventromedial medulla contains neurons that robustly inhibit nocioception at the level of the spinal cord through a top-down modulatory pathway. Although much is known about the role of the rostral ventromedial medulla in the inhibition of pain, the precise ability to directly manipulate pain-inhibitory neurons in the rostral ventromedial medulla has never been achieved. We now expose a cellular circuit that inhibits nocioception and itch in mice. Through a combination of molecular, tracing and behavioural approaches, we found that rostral ventromedial medulla neurons containing the kappa-opioid receptor inhibit itch and nocioception. With chemogenetic inhibition, we uncovered that these neurons are required for stress-induced analgesia. Using intersectional chemogenetic and pharmacological approaches, we determined that rostral ventromedial medulla kappa-opioid receptor neurons inhibit nocioception and itch through a descending circuit. Lastly, we identified a dynorphinergic pathway arising from the periaqueductal grey that modulates nociception within the rostral ventromedial medulla. These discoveries highlight a distinct population of rostral ventromedial medulla neurons capable of broadly and robustly inhibiting itch and nocioception.

Itch-specific neurons in the ventrolateral orbital cortex selectively modulate the itch processing

Itch is a cutaneous sensation that is critical in driving scratching behavior. The long-standing question of whether there are specific neurons for itch modulation inside the brain remains unanswered. Here, we report a subpopulation of itch-specific neurons in the ventrolateral orbital cortex (VLO) that is distinct from the pain-related neurons. Using a Tet-Off cellular labeling system, we showed that local inhibition or activation of these itch-specific neurons in the VLO significantly suppressed or enhanced itch-induced scratching, respectively, whereas the intervention did not significantly affect pain. Conversely, suppression or activation of pain-specific neurons in the VLO significantly affected pain but not itch. Moreover, fiber photometry and immunofluorescence verified that these itch- and pain-specific neurons are distinct in their functional activity and histological location. In addition, the downstream targets of itch- and pain-specific neurons were different. Together, the present study uncovers an important subpopulation of neurons in the VLO that specifically modulates itch processing.

Spinal ascending pathways for somatosensory information processing

The somatosensory system processes diverse types of information including mechanical, thermal, and chemical signals. It has an essential role in sensory perception and body movement and, thus, is crucial for organism survival. The neural network for processing somatosensory information comprises multiple key nodes. Spinal projection neurons represent the key node for transmitting somatosensory information from the periphery to the brain. Although the anatomy of spinal ascending pathways has been characterized, the mechanisms underlying somatosensory information processing by spinal ascending pathways are incompletely understood. Recent studies have begun to reveal the diversity of spinal ascending pathways and their functional roles in somatosensory information processing. Here, we review the anatomic, molecular, and functional characteristics of spinal ascending pathways.

Sparse genetically defined neurons refine the canonical role of periaqueductal gray columnar organization

During threat exposure, survival depends on defensive reactions. Prior works linked large glutamatergic populations in the midbrain periaqueductal gray (PAG) to defensive freezing and flight, and established that the overarching functional organization axis of the PAG is along anatomically-defined columns. Accordingly, broad activation of the dorsolateral column induces flight, while activation of the lateral or ventrolateral (l and vl) columns induces freezing. However, the PAG contains diverse cell types that vary in neurochemistry. How these cell types contribute to defense remains unknown, indicating that targeting sparse, genetically-defined populations may reveal how the PAG generates diverse behaviors. Though prior works showed that broad excitation of the lPAG or vlPAG causes freezing, we found in mice that activation of lateral and ventrolateral PAG (l/vlPAG) cholecystokinin-expressing (CCK) cells selectively caused flight to safer regions within an environment. Furthermore, inhibition of l/vlPAG-CCK cells reduced predator avoidance without altering other defensive behaviors like freezing. Lastly, l/vlPAG-CCK activity decreased when approaching threat and increased during movement to safer locations. These results suggest CCK cells drive threat avoidance states, which are epochs during which mice increase distance from threat and perform evasive escape. Conversely, l/vlPAG pan-neuronal activation promoted freezing, and these cells were activated near threat. Thus, CCK l/vlPAG cells have opposing function and neural activation motifs compared to the broader local ensemble defined solely by columnar boundaries. In addition to the anatomical columnar architecture of the PAG, the molecular identity of PAG cells may confer an additional axis of functional organization, revealing unexplored functional heterogeneity.

Strategies for Targeting Neural Circuits: How to Manipulate Neurons Using Virus Vehicles

Viral strategies are the leading methods for mapping neural circuits. Viral vehicles combined with genetic tools provide the possibility to visualize entire functional neural networks and monitor and manipulate neural circuit functions by high-resolution cell type- and projection-specific targeting. Optogenetics and chemogenetics drive brain research forward by exploring causal relationships among different brain regions. Viral strategies offer a fresh perspective for the analysis of the structure-function relationship of the neural circuitry. In this review, we summarize current and emerging viral strategies for targeting neural circuits and focus on adeno-associated virus (AAV) vectors.

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.

The Spino-Parabrachial Pathway for Itch

Itch-induced scratching is an evolutionarily conserved behavioral response that protects organisms from potential parasites/irritants in their immediate vicinity. How the exposure to a pruritogen is translated to the perception of itch and how that perception drives scratching directed towards the site of exposure remains poorly understood. In this review, we focus on the recent findings that shed light on the neural pathways in the brain that underlie itch-induced scratching. We compare the molecularly defined itch pathways with the known pain circuits as they have anatomical and functional overlap. We review the roles played by the neurons in the spinoparabrachial pathway-comprising of the neurons in the spinal cord and the parabrachial nucleus (PBN), which acts as a hub for transmitting itch information across the brain. Lastly, we deliberate on scratching as a behavioral measure of the intensity of itch and its implication in unraveling the underlying supraspinal mechanisms. In summary, we provide a resource on the recent advances and discuss a path forward on our understanding of the neural circuits for itch.

Modulation of itch and pain signals processing in ventrobasal thalamus by thalamic reticular nucleus

Thalamic reticular nucleus (TRN) is known to be crucial for dynamically modulating sensory processing. Recently, the functional role of TRN in itch and pain sensation processing has drawn much attention. We found that ventrobasal thalamus (VB) neurons exhibited scratching behavior-related and nociceptive behavior-related neuronal activity changes, and most of VB neurons responsive to pruritic stimulus were also activated by nociceptive stimulus. Inhibition of VB could relieve itch-induced scratching behaviors and pathological pain without affecting basal nociceptive thresholds, and activation of VB could facilitate scratching behaviors. Tracing and electrophysiology recording results showed that VB mainly received inhibitory inputs from ventral TRN. Furthermore, optogenetic activation of TRN-VB projections suppressed scratching behaviors, and ablation of TRN enhanced scratching behaviors. In addition, activation of TRN-VB projections relieved the pathological pain without affecting basal nociceptive thresholds. Thus, our study indicates that TRN modulates itch and pain signals processing via TRN-VB inhibitory projections.

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VB is involved in both itch and pain signals processing

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Manipulation of VB or TRN-VB inhibitory projections modulates both itch and pain

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Enhancing the inhibitory tone might be a strategy for treating itch and pain

A descending pathway emanating from the periaqueductal gray mediates the development of cough-like hypersensitivity

Chronic cough is a common refractory symptom of various respiratory diseases. However, the neural mechanisms that modulate the cough sensitivity and mediate chronic cough remain elusive. Here, we report that GABAergic neurons in the lateral/ventrolateral periaqueductal gray (l/vlPAG) suppress cough processing via a descending pathway. We found that l/vlPAG neurons are activated by coughing-like behaviors and that tussive agent-evoked coughing-like behaviors are impaired after activation of l/vlPAG neurons. In addition, we showed that l/vlPAG neurons form inhibitory synapses with the nucleus of the solitary tract (NTS) neurons. The synaptic strength of these inhibitory projections is weaker in cough hypersensitivity model mice than in naïve mice. Important, activation of l/vlPAG GABAergic neurons projecting to the NTS decreases coughing-like behaviors. In contrast, suppressing these neurons enhances cough sensitivity. These results support the notion that l/vlPAG GABAergic neurons play important roles in cough hypersensitivity and chronic cough through disinhibition of cough processing at the medullary level.

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GABAergic neurons in the l/vlPAG inhibit coughing-like behaviors

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The l/vlPAG sends predominately inhibitory projections to the NTS

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l/vlPAG GABAergic neurons modulate coughing-like behaviors via descending projections

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l/vlPAG-NTS projections mediate cough hypersensitivity via disinhibitory mechanisms

Social touch-like tactile stimulation activates a tachykinin 1-oxytocin pathway to promote social interactions

It is well known that affective and pleasant touch promotes individual well-being and facilitates affiliative social communication, although the neural circuit that mediates this process is largely unknown. Here, we show that social-touch-like tactile stimulation (ST) enhances firing of oxytocin neurons in the mouse paraventricular hypothalamus (PVH) and promotes social interactions and positively reinforcing place preference. These results link pleasant somatosensory stimulation to increased social interactions and positive affective valence. We further show that tachykinin 1 (Tac1⁺) neurons in the lateral and ventrolateral periaqueductal gray (l/vlPAG) send monosynaptic excitatory projections to PVH oxytocin neurons. Functionally, activation of PVH-projecting Tac1⁺ neurons increases firing of oxytocin neurons, promotes social interactions, and increases preference for the social touch context, whereas reducing activity of Tac1⁺ neurons abolishes ST-induced oxytocin neuronal firing. Together, these results identify a dipeptidergic pathway from l/vlPAG Tac1⁺ neurons to PVH oxytocin neurons, through which pleasant sensory experience promotes social behavior.

Selective activation of TRPA1 ion channels by nitrobenzene skin sensitizers DNFB and DNCB

2, 4-dinitrofluorobenzene (DNFB) and 2, 4-dinitrochlorobenzene (DNCB) are well known as skin sensitizers that can cause dermatitis. DNFB has shown to more potently sensitize skin; however, how DNFB and DNCB cause skin inflammation at a molecular level and why this difference in their sensitization ability is observed remains unknown. In this study, we aimed to identify the molecular targets and mechanisms on which DNFB and DNCB act. We used a fluorescent calcium imaging plate reader in an initial screening assay before patch-clamp recordings for validation. Molecular docking in combination with site-directed mutagenesis was then carried out to investigate DNFB and DNCB binding sites in the TRPA1 ion channel that may be selectively activated by these tow sensitizers. We found that DNFB and DNCB selectively activated TRPA1 channel with EC₅₀ values of 2.3 ± 0.7 μM μM and 42.4 ± 20.9 μM, respectively. Single-channel recordings revealed that DNFB and DNCB increase the probability of channel opening and acts on three residues (C621, E625 and Y658) critical for TRPA1 activation. Our findings may not only help explain the molecular mechanism underlying the dermatitis and pruritus caused by chemicals like DNFB and DNCB, but also provide a molecular tool 7.5-fold more potent than the current TRPA1 activator allyl isothiocyanate (AITC) used for investigating TRPA1 channel pharmacology and pathology.

A neuropeptide code for itch

Itch is one of the most primal sensations, being both ubiquitous and important for the well-being of animals. For more than a century, a desire to understand how itch is encoded by the nervous system has prompted the advancement of many theories. Within the past 15 years, our understanding of the molecular and neural mechanisms of itch has undergone a major transformation, and this remarkable progress continues today without any sign of abating. Here I describe accumulating evidence that indicates that itch is distinguished from pain through the actions of itch-specific neuropeptides that relay itch information to the spinal cord. According to this model, classical neurotransmitters transmit, inhibit and modulate itch information in a context-, space- and time-dependent manner but do not encode itch specificity. Gastrin-releasing peptide (GRP) is proposed to be a key itch-specific neuropeptide, with spinal neurons expressing GRP receptor (GRPR) functioning as a key part of a convergent circuit for the conveyance of peripheral itch information to the brain.