Projections from the lateral parabrachial nucleus to the lateral and ventral lateral periaqueductal gray subregions mediate the itching sensation

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.

The contribution of periaqueductal gray in the regulation of physiological and pathological behaviors

Periaqueductal gray (PAG), an integration center for neuronal signals, is located in the midbrain and regulates multiple physiological and pathological behaviors, including pain, defensive and aggressive behaviors, anxiety and depression, cardiovascular response, respiration, and sleep-wake behaviors. Due to the different neuroanatomical connections and functional characteristics of the four functional columns of PAG, different subregions of PAG synergistically regulate various instinctual behaviors. In the current review, we summarized the role and possible neurobiological mechanism of different subregions of PAG in the regulation of pain, defensive and aggressive behaviors, anxiety, and depression from the perspective of the up-down neuronal circuits of PAG. Furthermore, we proposed the potential clinical applications of PAG. Knowledge of these aspects will give us a better understanding of the key role of PAG in physiological and pathological behaviors and provide directions for future clinical treatments.

Optogenetic Neuromodulation in Inflammatory Pain

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

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.

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.

Glutamatergic Neurons in the Zona Incerta Modulate Pain and Itch Behaviors in Mice

Emerging evidence suggest that parvalbumin neurons in zona incerta (ZI) modulate pain and itch behavior in opposite manners. However, the role of ZI glutamatergic neurons, a unique incertal neuronal subpopulation residing in the caudal division, in pain and itch modulation remains unknown. In the present study, by combining chemogenetic manipulation, fiber photometry, and behavioral tests, we proved that incertal glutamatergic neurons served as an endogenous negative diencephalic modulator for both pain and itch processing. We demonstrated that ZI vesicular glutamate transporter 2 (VGluT2) neurons exhibited increased calcium signal upon hindpaw withdrawal in response to experimental mechanical and thermal stimuli. Behavioral tests further showed that pharmacogenetic activation of this specific type of neurons reduced nocifensive withdrawal responses in both naïve and inflammatory pain mice. Similar neural activity and modulatory role of ZI VGluT2 neurons were also observed upon histaminergic and non-histaminergic acute itch stimuli. Together, our study would expedite our understandings of brain mechanisms underlying somatosensory processing and modulation, and supply a novel therapeutic target for the management of chronic pain and itch disorders.

Central medial thalamic nucleus dynamically participates in acute itch sensation and chronic itch-induced anxiety-like behavior in male mice

Itch is an annoying sensation consisting of both sensory and emotional components. It is known to involve the parabrachial nucleus (PBN), but the following transmission nodes remain elusive. The present study identified that the PBN-central medial thalamic nucleus (CM)-medial prefrontal cortex (mPFC) pathway is essential for itch signal transmission at the supraspinal level in male mice. Chemogenetic inhibition of the CM-mPFC pathway attenuates scratching behavior or chronic itch-related affective responses. CM input to mPFC pyramidal neurons is enhanced in acute and chronic itch models. Specifically chronic itch stimuli also alter mPFC interneuron involvement, resulting in enhanced feedforward inhibition and a distorted excitatory/inhibitory balance in mPFC pyramidal neurons. The present work underscores CM as a transmit node of the itch signal in the thalamus, which is dynamically engaged in both the sensory and affective dimensions of itch with different stimulus salience.

Projections from the Rostral Zona Incerta to the Thalamic Paraventricular Nucleus Mediate Nociceptive Neurotransmission in Mice

Zona incerta (ZI) is an integrative subthalamic region in nociceptive neurotransmission. Previous studies demonstrated that the rostral ZI (ZIR) is an important gamma-aminobutyric acid-ergic (GABAergic) source to the thalamic paraventricular nucleus (PVT), but whether the ZIR-PVT pathway participates in nociceptive modulation is still unclear. Therefore, our investigation utilized anatomical tracing, fiber photometry, chemogenetic, optogenetic and local pharmacological approaches to investigate the roles of the ZIR^GABA+-PVT pathway in nociceptive neurotransmission in mice. We found that projections from the GABAergic neurons in ZIR to PVT were involved in nociceptive neurotransmission. Furthermore, chemogenetic and optogenetic activation of the ZIR^GABA+-PVT pathway alleviates pain, whereas inhibiting the activities of the ZIR^GABA+-PVT circuit induces mechanical hypersensitivity and partial heat hyperalgesia. Importantly, in vivo pharmacology combined with optogenetics revealed that the GABA-A receptor (GABA_AR) is crucial for GABAergic inhibition from ZIR to PVT. Our data suggest that the ZIR^GABA+-PVT pathway acts through GABA_AR-expressing glutamatergic neurons in PVT mediates nociceptive neurotransmission.

Hippocampus Insulin Receptors Regulate Episodic and Spatial Memory Through Excitatory/Inhibitory Balance

It is well known that the hippocampus is a vital brain region playing a key role in both episodic and spatial memory. Insulin receptors (InsRs) are densely distributed in the hippocampus and are important for its function. However, the effects of InsRs on the function of the specific hippocampal cell types remain elusive. In this study, hippocampal InsRs knockout mice had impaired episodic and spatial memory. GABAergic neurons and glutamatergic neurons in the hippocampus are involved in the balance between excitatory and inhibitory (E/I) states and participate in the processes of episodic and spatial memory. InsRs are located mainly at excitatory neurons in the hippocampus, whereas 8.5% of InsRs are glutamic acid decarboxylase 2 (GAD2)::Ai9-positive (GABAergic) neurons. Next, we constructed a transgenic mouse system in which InsR expression was deleted from GABAergic (glutamate decarboxylase 2::InsRfl/fl, GAD2Cre::InsRfl/fl) or glutamatergic neurons (vesicular glutamate transporter 2::InsRfl/fl,Vglut2Cre::InsRfl/fl). Our results showed that in comparison to the InsRfl/fl mice, both episodic and spatial memory were lower in GAD2Cre::InsRfl/fl and Vglut2Cre::InsRfl/fl. In addition, both GAD2Cre::InsRfl/fl and Vglut2Cre::InsRfl/fl were associated with more anxiety and lower glucose tolerance. These findings reveal that hippocampal InsRs might be crucial for episodic and spatial memory through E/I balance hippocampal regulation.

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.