Voltage-gated potassium channels involved in regulation of physiological function in MrgprA3-specific itch neurons

Trpv1-lineage neuron-expressing Kcnq4 channel modulates itch sensation in mice

ABSTRACT

Voltage-gated potassium channel subfamily q member 4 (Kcnq4) is predominantly expressed by hair cells and auditory neurons and regulates the neuronal excitability in the auditory pathway. Although it is further detected in myelinated large-diameter dorsal root ganglia (DRG) neurons in the periphery, the expression and function of Kcnq4 channel in nociceptors remains unknown. Here we showed that Kcnq4 is substantially expressed by unmyelinated small-diameter DRG neurons in both human and mouse. In spite of a dispensable role in acute pain and chronic skin inflammation, Kcnq4 is specifically involved in the regulation of scratching behavior through controlling action potential firing properties, evidenced by the increased neuronal excitability in small-diameter DRG neurons isolated from Kcnq4 deficient mice. Moreover, genetic ablation of Kcnq4 in Trpv1-positive neurons exacerbates both acute and chronic itch behavior in mice. Taken together, our results uncover a functional role of Trpv1-lineage neuron-expressing Kcnq4 channel in the modulation of itch-specific neuronal excitation in the periphery.

Dictamnine ameliorates chronic itch in DNFB-induced atopic dermatitis mice via inhibiting MrgprA3

Chronic itch is the most prominent feature of atopic dermatitis (AD), and antihistamine treatment is often less effective in reducing clinical pruritus severity in AD. Multiple studies have shown that histamine-independent itch pathway is thought to predominate in AD-induced chronic itch. Mas-related G-protein-coupled receptor (Mrgpr) A3⁺ sensory neurons have been identified as one of the major itch-sensing neuron populations, and transient receptor potential (TRP) channel A1 is the key downstream of MrgprA3-mediated histamine-independent itch. MrgprA3-TRPA1 signal pathway is necessary for the development of chronic itch and may be the potentially promising target of chronic itch in AD. Dictamnine is one of the main quinoline alkaloid components of Cortex Dictamni (a traditional Chinese medicine widely used in clinical treatment of skin diseases). However, the anti-inflammatory and anti-pruritic effect of dictamnine on AD have not been reported. In this study, we used the 2,4-dinitrofluorobenzene (DNFB)-induced AD mouse model to observe the scratching behavior, inflammatory manifestations, and to detect the expression of MrgprA3 and TRPA1 in skin and DRG. The data demonstrated that dictamnine effectively inhibited AD-induced chronic itch, inflammation symptoms, epidermal thickening, inflammatory cell infiltration, and downregulated the expression of MrgprA3 and TRPA1. Furthermore, dictamnine restrained the excitability of MrgprA3⁺ and TRPA1⁺ neurons. Molecular docking also indicated that dictamnine has better binding affinity with MrgprA3. These results suggest that dictamnine may inhibit chronic itch caused by AD through the MrgprA3-TRPA1 mediated histamine-independent itch pathway, and may have a potential utility in AD treatment.

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.

Paeoniflorin alleviates CFA-induced inflammatory pain by inhibiting TRPV1 and succinate/SUCNR1-HIF-1α/NLPR3 pathway

BACKGROUND

Treatment of chronic inflammatory pain remains a major goal in the clinic. It is thus of prime importance to characterize inherent pathophysiological pathways to design new therapeutic strategies and analgesics for pain management. Paeoniflorin (PF), a monoterpenoid glycoside from Paeonia lactiflora Pallas plants, possesses promising anti-nociceptive property. However, therapeutic effect and underlying mechanism of action of PF on inflammatory pain have not yet been fully elucidated. In this study, we aim to investigate the analgesic effect further and clarify its mechanism of action of PF on complete freund's adjuvant (CFA)-evoked inflammatory pain.

METHODS

Twenty-four male mice were divided into 3 groups: sham, CFA, and CFA + PF groups (n = 8/group). Mice were treated with normal saline or PF (30 mg/kg) for 11 days. Footpad swelling (n = 8/group), mechanical (n = 8/group) and thermal hypersensitivity (n = 8/group) were measured to evaluate the analgesic effect of PF on CFA-injected mice. At the end of the animal experiment, blood and L4-L6 dorsal root ganglion neurons were collected to assess the therapeutic effect of PF on CFA-induced inflammatory pain. Next, hematoxylin and eosin, quantitative realtime PCR, ELISA, capsaicin and dimethyl succinate induced pain test (n = 8/group), motor coordination test (n = 8/group), tail flicking test (n = 8/group), pyruvate and succinate dehydrogenase assay (n = 6/group), immunohistochemical staining, were performed to clarify the action mechanism of PF on CFA-evoked inflammatory pain. Besides, the effect of PF on TRPV1 was evaluated by whole-cell patch clamp recording on primary neurons (n = 7). Finally, molecular docking further performed to evaluate the binding ability of PF to TRPV1.

RESULTS

PF significantly relieved inflammatory pain (P < 0.001) and paw edema (P < 0.001) on a complete Freund adjuvant (CFA)-induced peripheral inflammatory pain model. Furthermore, PF inhibited neutrophil infiltration (P < 0.01), IL-1β increase (P < 0.01), and pain-related peptide substance P release (P < 0.001). Intriguingly, CFA-induced succinate aggregation was notably reversed by PF via modulating pyruvate and SDH activity (P < 0.01). In addition, PF dampened the high expression of subsequent succinate receptor SUCNR1 (P < 0.01), HIF-1α (P < 0.05), as well as the activation of NLPR3 inflammasome (P < 0.05) and TRPV1 (P < 0.05). More importantly, both capsaicin and dimethyl succinate supplementation obviously counteracted the pain-relieving effect of PF and TRPV1 (P < 0.01 or P < 0.001).

CONCLUSION

Our findings suggest that PF can significantly relieve CFA-induced paw swelling, as well as mechanical and thermal hyperalgesia. PF alleviated inflammatory pain partly through inhibiting the activation of TRPV1 and succinate/SUCNR1-HIF-1α/NLPR3 pathway. Furthermore, we found that PF exerted its analgesic effect without affecting motor coordination and pain-related cold ion-channels. In summary, this study may provide valuable evidence for the potential application of PF as therapeutic strategy for inflammatory pain treatment.

Pacific-Ciguatoxin-2 and Brevetoxin-1 Induce the Sensitization of Sensory Receptors Mediating Pain and Pruritus in Sensory Neurons

Ciguatera fish poisoning (CFP) and neurotoxic shellfish poisoning syndromes are induced by the consumption of seafood contaminated by ciguatoxins and brevetoxins. Both toxins cause sensory symptoms such as paresthesia, cold dysesthesia and painful disorders. An intense pruritus, which may become chronic, occurs also in CFP. No curative treatment is available and the pathophysiology is not fully elucidated. Here we conducted single-cell calcium video-imaging experiments in sensory neurons from newborn rats to study in vitro the ability of Pacific-ciguatoxin-2 (P-CTX-2) and brevetoxin-1 (PbTx-1) to sensitize receptors and ion channels, (i.e., to increase the percentage of responding cells and/or the response amplitude to their pharmacological agonists). In addition, we studied the neurotrophin release in sensory neurons co-cultured with keratinocytes after exposure to P-CTX-2. Our results show that P-CTX-2 induced the sensitization of TRPA1, TRPV4, PAR2, MrgprC, MrgprA and TTX-r NaV channels in sensory neurons. P-CTX-2 increased the release of nerve growth factor and brain-derived neurotrophic factor in the co-culture supernatant, suggesting that those neurotrophins could contribute to the sensitization of the aforementioned receptors and channels. Our results suggest the potential role of sensitization of sensory receptors/ion channels in the induction or persistence of sensory disturbances in CFP syndrome.

Differential Coding of Itch and Pain by a Subpopulation of Primary Afferent Neurons

Itch and pain are distinct unpleasant sensations that can be triggered from the same receptive fields in the skin, raising the question of how pruriception and nociception are coded and discriminated. Here, we tested the multimodal capacity of peripheral first-order neurons, focusing on the genetically defined subpopulation of mouse C-fibers that express the chloroquine receptor MrgprA3. Using optogenetics, chemogenetics, and pharmacology, we assessed the behavioral effects of their selective stimulation in a wide variety of conditions. We show that metabotropic Gq-linked stimulation of these C-afferents, through activation of native MrgprA3 receptors or DREADDs, evokes stereotypical pruriceptive rather than nocifensive behaviors. In contrast, fast ionotropic stimulation of these same neurons through light-gated cation channels or native ATP-gated P2X3 channels predominantly evokes nocifensive rather than pruriceptive responses. We conclude that C-afferents display intrinsic multimodality, and we provide evidence that optogenetic and chemogenetic interventions on the same neuronal populations can drive distinct behavioral outputs.

Brucine alleviates neuropathic pain in mice via reducing the current of the sodium channel

ETHNOPHARMACOLOGICAL RELEVANCE

Strychnos nux-vomica L. (Loganiaceae) is grown extensively in South Asian. The dried seed of this plant, nux vomica, has been clinically used in Chinese medicine for relieving rheumatic pain, reducing swelling and treating cancer. Brucine, the second abundant alkaloid constituent of nux vomica, shows excellent clinical therapeutic effect, especially in relieving pain, but mechanism of brucine in relieving pain is still unclear.

AIM OF THE STUDY

Explore the analgesic effect of brucine, reveal the molecular mechanism of brucine analgesia.

MATERIALS AND METHODS

Antinociceptive effects of brucine were assessed in acute and chronic pain mice model. Electrophysiological experiments were used to evaluate the effects of brucine on neuronal activity and sodium channel function.

RESULTS

In acute pain models, brucine significantly inhibits response induced by nociceptive heat and mechanical stimulation. Furthermore, thermal hypersensitivity and mechanical allodynia were also alleviated by brucine treatment in a chronic constriction injury (CCI) mouse model. Sodium channel plays a crucial role in neuropathic pain. Electrophysiological results show that brucine inhibits the excitability of DRG neurons directly, the number of action potential (AP) was significantly reduced after brucine treatment, and this kind of inhibition is due to brucine inhibits both tetrodotoxin-sensitive (TTXs) and tetrodotoxin-resistant (TTXr) sodium channel.

CONCLUSIONS

Taken together, brucine is a novel drug candidate in treating acute and chronic pain diseases, which might be attributed to inhibition the excitability of sodium channel directly.

Circular RNAs and hereditary bone diseases

Circular RNA (circRNA) is a non-linear form of RNA derived from exonic, intronic, and exon-intron gene regions. circRNAs are characterized by covalent closed loops, highly stable nuclease resistance, and specific expression in species and developmental stages. CircRNA molecules have been identified as playing roles in the regulation of cell transcription, transcriptional expression after translation, interactions with microRNAs, and protein coding. A high stability and tissue- and disease-specific expression allow circRNAs to serve as potential biomarkers both for diseases and prognosis. CircRNAs function in bone remodeling by directly participating in bone-related signaling pathways and by forming the circRNA-miRNA-mRNA axis. Studies have seldom reported on the low incidence of circRNAs in genetic bone disorders. The current study reviews the characteristics of circRNAs and recent research on their role in rare hereditary bone diseases.

Morphological and functional diversity of first-order somatosensory neurons

First-order somatosensory neurons transduce and convey information about the external or internal environment of the body to the central nervous system. They are pseudo unipolar neurons with cell bodies residing in one of several ganglia located near the central nervous system, with the short branch of the axon connecting to the spinal cord or the brain stem and the long branch extending towards the peripheral organ they innervate. Besides their sensory transducer and conductive role, somatosensory neurons also have trophic functions in the tissue they innervate and participate in local reflexes in the periphery. The cell bodies of these neurons are remarkably diverse in terms of size, molecular constitution, and electrophysiological properties. These parameters have provided criteria for classification that have proved useful to establish and study their functions. In this review, we discuss ways to measure and classify populations of neurons based on their size and action potential firing pattern. We also discuss attempts to relate the different populations to specific sensory modalities.