Posterior Thalamic Nucleus Mediates Facial Histaminergic Itch

Tac1-expressing neurons in the central amygdala predominantly mediate histamine-induced itch by receiving inputs from parabrachial Tac1-expressing neurons

Itch is a distinct and bothersome sensation closely associated with a strong urge to scratch. Both the parabrachial nucleus (PBN) and the central amygdala (CeA) are responsive to itch stimuli and contain neurons that express tachykinin 1 (Tac1), which are known for their significant involvement in itch-induced scratching at both spinal and supraspinal levels. Significantly, the PBN neurons project their axons to form close connections with the CeA neurons. However, the role of the PBNTac1-CeATac1 pathway in modulating itch remains to be determined. We utilized immunohistochemistry, fiber photometry, chemogenetic, and behavioral techniques to investigate the role of the PBNTac1-CeATac1 pathway in itch. Our results indicate that neurons in the CeA can be more activated by acute itch than chronic itch. Notably, in response to acute itch stimuli, both CeATac1 and PBNTac1 neurons were specifically activated by histamine (His)-induced itch. Furthermore, the Tac1-positive terminals from the PBNTac1 neurons formed close connections with CeATac1 neurons. We also demonstrated that activating the PBNTac1-CeA pathway using a chemogenetic approach could increase scratching behaviors in His-induced itch, other than chloroquine (CQ)-induced itch. Conversely, inhibiting the PBNTac1-CeA pathway decreased scratching behaviors in mice with His-induced itch. Taken together, these results suggest that the PBNTac1-CeATac1 pathway may play a specific role in modulating His-induced acute itch.

Cell Type-Specific Modulation of Acute Itch Processing in the Anterior Cingulate Cortex

Despite remarkable progress in understanding the fundamental bases of itching, its cortical mechanisms remain poorly understood. Herein, the causal contributions of defined anterior cingulate cortex (ACC) neuronal populations to acute itch modulation in mice are established. Using cell type-specific manipulations, the opposing functions of ACC glutamatergic and GABAergic neurons in regulating acute itching are demonstrated. Photometry studies indicated that ACC glutamatergic neurons are activated during scratching induced by both histamine and chloroquine, whereas the activation pattern of GABAergic neurons is complicated by GABAergic subpopulations and acute itch modalities. By combining cell type- and projection-specific techniques, a thalamocortical circuit is further identified from the mediodorsal thalamus driving the itch-scratching cycle related to histaminergic and non-histaminergic itching, which is contingent on the activation of postsynaptic parvalbumin-expressing neurons in the ACC. These findings reveal a cellular and circuit signature of ACC neurons orchestrating behavioral responses to itching and may provide insights into therapies for itch-related diseases.

Atopic Dermatitis-Related Problems in Daily Life, Goals of Therapy and Deciding Factors for Systemic Therapy: A Review

Background/Objectives/Methods: Atopic dermatitis (AD) impacts various aspects of patients' lives including personal life, psychological aspects/disturbances (e.g., depression, anxiety, or even suicidal thoughts), school, and work-related activities, including career advancement. The aim of this narrative review is to present the latest information available on how to best approach AD patient management, as well as decisions regarding standard/advanced systemic therapy, by gathering evidence from the relevant medical literature (PubMed and other prominent medical databases). Results: Thus, AD patient management and decisions regarding advanced/systemic therapy are complex, requiring the consideration of multiple disease-related factors: age; disease severity; patient medical history and comorbidities; previous topical therapy use and any adverse reactions; treatment efficacy concerns; patient preferences, expectations and fears; pregnancy planning; ability and willingness to adhere to the treatment regimen; impact on related risks; and any associated psychological or psychiatric issues. Current guidelines and systematic reviews support the safety and efficacy of systemic therapy including conventional drugs (cyclosporine, methotrexate, and azathioprine), biologics (dupilumab and tralokinumab), and JAK inhibitors (baricitinib, upadacitinib, and abrocitinib) recommended for treating moderate and severe AD. Recently, additional biologics have been evaluated in clinical trials, including lebrikizumab, nemolizumab, eblasakimab, and OX40/OX40L, among others. Conclusions: The most recently suggested approach to treating AD patients suggests focusing on therapy that targets and achieves minimal disease activity (MDA), where therapy decisions are informed by both the patient and the clinician. Available data also indicate the importance of a personalized, stepwise, and multidisciplinary approach. This type of approach promotes patient compliance, satisfaction with therapy, and increased engagement, which all lead to better patient outcomes.

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 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.

Chemokine receptor CXCR2 in primary sensory neurons of trigeminal ganglion mediates orofacial itch

The CXCR2 chemokine receptor is known to have a significant impact on the initiation and control of inflammatory processes. However, its specific involvement in the sensation of itch is not yet fully understood. In this study, we aimed to elucidate the function of CXCR2 in the trigeminal ganglion (TG) by utilizing orofacial itch models induced by incision, chloroquine (CQ), and histamine. Our results revealed a significant up-regulation of CXCR2 mRNA and protein expressions in the primary sensory neurons of TG in response to itch stimuli. The CXCR2 inhibitor SB225002 resulted in notable decrease in CXCR2 protein expression and reduction in scratch behaviors. Distal infraorbital nerve (DION) microinjection of a specific shRNA virus inhibited CXCR2 expression in TG neurons and reversed itch behaviors. Additionally, the administration of the PI3K inhibitor LY294002 resulted in a decrease in the expressions of p-Akt, Akt, and CXCR2 in TG neurons, thereby mitigating pruritic behaviors. Collectively, we report that CXCR2 in the primary sensory neurons of trigeminal ganglion contributes to orofacial itch through the PI3K/Akt signaling pathway. These observations highlight the potential of molecules involved in the regulation of CXCR2 as viable therapeutic targets for the treatment of itch.

Health-Related Behaviors and Psychological Status of Adolescent Patients with Atopic Dermatitis: The 2019 Korea Youth Risk Behavior Web-Based Survey

OBJECTIVE

Atopic dermatitis (AD) is one of the common chronic diseases that occur in children and adolescents as a chronic relapsing pruritic inflammatory skin disease. This study investigated how AD is associated with stress and depressive symptoms in a large representative sample of adolescents in South Korea.

METHODS

The Korea Youth Risk Behavior Web-based Survey 2019 was used in this study (n = 57,069, weighted national estimates = 2,672,170). Multivariate logistic regression was used to determine significant associations between AD and mental health, measured by stress and depressive symptoms. Sub-group analysis was also conducted using various socio-economic variables.

RESULTS

Among the present sample, 6.5% of adolescents (n = 173,909) were diagnosed with AD in the past 12 months. After adjusting for other variables, AD diagnosis was associated with significantly higher odds of experiencing stress (OR = 1.43) and depressive symptoms (OR = 1.32) as compared to adolescents without AD. A similar trend is found in subgroup model analysis using socio-economic variables (ie, education levels, parent's income levels, and residence area). Specifically, female adolescents with AD, adolescents of low socio-economic status, those reporting smoking and drinking experience, and who do not participate in regular physical activity are more vulnerable to stress and depressive symptoms.

CONCLUSION

This is a noteworthy finding because it denotes that AD may lead to negative outcomes, like depressive symptoms or stress, which could be prevented if suspected early.

The anatomy of head pain

Pain-sensitive structures in the head and neck, including the scalp, periosteum, meninges, and blood vessels, are innervated predominantly by the trigeminal and upper cervical nerves. The trigeminal nerve supplies most of the sensation to the head and face, with the ophthalmic division (V1) providing innervation to much of the supratentorial dura mater and vessels. This creates referral patterns for pain that may be misleading to clinicians and patients, as described by studies involving awake craniotomies and stimulation with electrical and mechanical stimuli. Most brain parenchyma and supratentorial vessels refer pain to the ipsilateral V1 territory, and less commonly the V2 or V3 region. The upper cervical nerves provide innervation to the posterior scalp, while the periauricular region and posterior fossa are territories with shared innervation. Afferent fibers that innervate the head and neck send nociceptive input to the trigeminocervical complex, which then projects to additional pain processing areas in the brainstem, thalamus, hypothalamus, and cortex. This chapter discusses the pain-sensitive structures in the head and neck, including pain referral patterns for many of these structures. It also provides an overview of peripheral and central nervous system structures responsible for transmitting and interpreting these nociceptive signals.

Peripheral itch sensitization in atopic dermatitis

Atopic dermatitis is a skin disorder caused by skin dryness and barrier dysfunction, resulting in skin inflammation and chronic itch (or pruritus). The pathogenesis of atopic dermatitis is thought to be initiated by a lowering of the itch threshold due to dry skin. This lowering of the itch threshold is at least partially due to the increase in intraepidermal nerve fibers and sensitization of sensory nerves by interleukin (IL)-33 produced and secreted by keratinocytes. Such skin is easily prone to itch due to mechanical stimuli, such as rubbing of clothing and chemical stimuli from itch mediators. In patients with atopic dermatitis, once itch occurs, further itch is induced by scratching, and the associated scratching breaks down the skin barrier. Disruption of the skin barrier allows entry into the epidermis of external foreign substances, such as allergens derived from house dust mites, leading to an increased induction of type 2 inflammatory responses. As a result, type 2 cytokines IL-4, IL-13, and IL-31 are mainly secreted by Th2 cells, and their action on sensory nerve fibers causes further itch sensitization. These sequences of events are thought to occur simultaneously in patients with atopic dermatitis, leading to a vicious itch-scratch cycle. This vicious cycle becomes a negative spiral that leads to disease burden. Therefore, controlling itch is essential for the treatment of atopic dermatitis. In this review, we summarize and discuss advances in the mechanisms of peripheral itch sensitization in atopic dermatitis, focusing on skin barrier-neuro-immune triadic connectivity.

Geraniol enhances inhibitory inputs to the paraventricular thalamic nucleus and induces sedation in mice

BACKGROUND

Plant extracts with sedative effects have a long history of clinical use for treating insomnia and epilepsy. Geraniol (GE), a plant-derived acyclic monoterpene, reduces locomotion and prolongs barbiturate-induced anesthesia in rats. However, the mechanisms of GE in sedation remain elusive.

PURPOSE

This study aimed to investigate the mechanisms of GE in sedation in mice.

METHODS

GE was administered systemically by nebulization and intraperitoneal injection. Open field tests, acute seizure tests, and electroencephalogram (EEG) recordings were performed to examine the sedative effects of GE in mice. The time of loss of the righting reflex and return of the righting reflex were recorded in anesthesia experiments to examine the effect of GE on anesthesia. In vitro c-Fos staining and in vivo fiber photometry recordings were performed to detect the activity change of the paraventricular thalamic nucleus (PVT). Microinjection of GE into PVT and related behavioral tests were performed to confirm that PVT was a critical target for GE. Whole-cell recordings were performed to dissect the effects of GE on PVT neurons via GABAA receptors. Molecular docking was performed to examine the interaction between GE and GABAA receptor subunits.

RESULTS

We found that GE reduced locomotion, relieved acute seizures, altered the EEG, and facilitated general anesthesia in mice. Next, we found that GE decreased c-Fos expression and suppressed the calcium activity in PVT. Microinjection of GE into PVT reduced locomotion and facilitated anesthesia. Furthermore, electrophysiology results showed that GE induced dramatic membrane hyperpolarization and suppressed the activity of PVT neurons, mainly by prolonging spontaneous inhibitory postsynaptic currents and inducing tonic inhibitory currents. Molecular docking results indicated that the β3 subunit might be a potential target for GE.

CONCLUSION

By combined using behavioral tests, immunohistochemistry, calcium recording, and electrophysiology, we systematically revealed that GE inhibits PVT and induces sedation in mice. Essential oils have long been considered part of traditional medicine, and they are playing a critical role in aromatherapy. Since GE has a comparatively ideal safety property and multiple delivery methods, GE has great application potential in aromatherapy. Our study also provides a potential candidate for further development of sedatives and anaesthetics.

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.

PBN-PVT projections modulate negative affective states in mice

Long-lasting negative affections dampen enthusiasm for life, and dealing with negative affective states is essential for individual survival. The parabrachial nucleus (PBN) and thalamic paraventricular nucleus (PVT) are critical for modulating affective states in mice. However, the functional roles of PBN-PVT projections in modulating affective states remain elusive. Here, we show that PBN neurons send dense projection fibers to the PVT and form direct excitatory synapses with PVT neurons. Activation of the PBN-PVT pathway induces robust behaviors associated with negative affective states without affecting nociceptive behaviors. Inhibition of the PBN-PVT pathway reduces aversion-like and fear-like behaviors. Furthermore, the PVT neurons innervated by the PBN are activated by aversive stimulation, and activation of PBN-PVT projections enhances the neuronal activity of PVT neurons in response to the aversive stimulus. Consistently, activation of PVT neurons that received PBN-PVT projections induces anxiety-like behaviors. Thus, our study indicates that PBN-PVT projections modulate negative affective states in mice.

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

Circuit Mechanisms of Itch in the Brain

Itch is an unpleasant somatic sensation with the desire to scratch, and it consists of sensory, affective, and motivational components. Acute itch serves as a critical protective mechanism because an itch-evoked scratching response will help to remove harmful substances invading the skin. Recently, exciting progress has been made in deciphering the mechanisms of itch at both the peripheral nervous system and the CNS levels. Key neuronal subtypes and circuits have been revealed for ascending transmission and the descending modulation of itch. In this review, we mainly summarize the current understanding of the central circuit mechanisms of itch in the brain.

Daily Lifestyle and Inflammatory Skin Diseases

Throughout life, it is necessary to adapt to the Earth’s environment in order to survive. A typical example of this is that the daily Earth cycle is different from the circadian rhythm in human beings; however, the ability to adapt to the Earth cycle has contributed to the development of human evolution. In addition, humans can consume and digest Earth-derived foods and use luxury materials for nutrition and enrichment of their lives, as an adaptation to the Earth’s environment. Recent studies have shown that daily lifestyles are closely related to human health; however, less attention has been paid to the fact that obesity due to excessive energy intake, smoking, and alcohol consumption contributes to the development of inflammatory skin diseases. Gluten or wheat protein, smoking and alcohol, sleep disturbance, and obesity drive the helper T (Th)1/Th2/Th17 immune response, whereas dietary fiber and omega-3 fatty acids negatively regulate inflammatory cytokine production. In this review, we have focused on daily lifestyles and the mechanisms involved in the pathogenesis of inflammatory skin diseases.