ABSTRACT
Intestinal macrophages are critical for gastrointestinal (GI) homeostasis, but our understanding of their role in regulating intestinal motility is incomplete. Here, we report that CX3C chemokine receptor 1-expressing muscularis macrophages (MMs) were required to maintain normal GI motility. MMs expressed the transient receptor potential vanilloid 4 (TRPV4) channel, which senses thermal, mechanical, and chemical cues. Selective pharmacologic inhibition of TRPV4 or conditional deletion of TRPV4 from macrophages decreased intestinal motility and was sufficient to reverse the GI hypermotility that is associated with chemotherapy treatment. Mechanistically, stimulation of MMs via TRPV4 promoted the release of prostaglandin E2 and elicited colon contraction in a paracrine manner via prostaglandin E receptor signaling in intestinal smooth muscle cells without input from the enteric nervous system. Collectively, our data identify TRPV4-expressing MMs as an essential component required for maintaining normal GI motility and provide potential drug targets for GI motility disorders.
Subject(s)
Colon/physiology , Gastrointestinal Motility , Macrophages/metabolism , Myocytes, Smooth Muscle/metabolism , Signal Transduction , TRPV Cation Channels/metabolism , Animals , CX3C Chemokine Receptor 1/metabolism , Colon/physiopathology , Cyclooxygenase 1/deficiency , Cyclooxygenase 1/metabolism , Dinoprostone/analysis , Dinoprostone/metabolism , Female , Gastric Mucosa/cytology , Gene Expression , Male , Membrane Proteins/antagonists & inhibitors , Membrane Proteins/deficiency , Membrane Proteins/metabolism , Mice , Mice, Knockout , Muscle Contraction , Receptors, Prostaglandin E/antagonists & inhibitors , Receptors, Prostaglandin E/metabolism , TRPV Cation Channels/antagonists & inhibitors , TRPV Cation Channels/deficiency , TRPV Cation Channels/geneticsABSTRACT
Somatosensory autonomic reflexes allow electroacupuncture stimulation (ES) to modulate body physiology at distant sites1-6 (for example, suppressing severe systemic inflammation6-9). Since the 1970s, an emerging organizational rule about these reflexes has been the presence of body-region specificity1-6. For example, ES at the hindlimb ST36 acupoint but not the abdominal ST25 acupoint can drive the vagal-adrenal anti-inflammatory axis in mice10,11. The neuroanatomical basis of this somatotopic organization is, however, unknown. Here we show that PROKR2Cre-marked sensory neurons, which innervate the deep hindlimb fascia (for example, the periosteum) but not abdominal fascia (for example, the peritoneum), are crucial for driving the vagal-adrenal axis. Low-intensity ES at the ST36 site in mice with ablated PROKR2Cre-marked sensory neurons failed to activate hindbrain vagal efferent neurons or to drive catecholamine release from adrenal glands. As a result, ES no longer suppressed systemic inflammation induced by bacterial endotoxins. By contrast, spinal sympathetic reflexes evoked by high-intensity ES at both ST25 and ST36 sites were unaffected. We also show that optogenetic stimulation of PROKR2Cre-marked nerve terminals through the ST36 site is sufficient to drive the vagal-adrenal axis but not sympathetic reflexes. Furthermore, the distribution patterns of PROKR2Cre nerve fibres can retrospectively predict body regions at which low-intensity ES will or will not effectively produce anti-inflammatory effects. Our studies provide a neuroanatomical basis for the selectivity and specificity of acupoints in driving specific autonomic pathways.
Subject(s)
Adrenal Glands/physiology , Autonomic Nervous System , Electroacupuncture , Vagus Nerve/physiology , Acupuncture Points , Animals , Hindlimb/innervation , Male , Mice , Mice, Inbred C57BL , Mice, Transgenic , ReflexABSTRACT
Zinc is a transition metal that has a long history of use as an anti-inflammatory agent. It also soothes pain sensations in a number of animal models. However, the effects and mechanisms of zinc on chemotherapy-induced peripheral neuropathy remain unknown. Here we show that locally injected zinc markedly reduces neuropathic pain in male and female mice induced by paclitaxel, a chemotherapy drug, in a TRPV1-dependent manner. Extracellularly applied zinc also inhibits the function of TRPV1 expressed in HEK293 cells and mouse DRG neurons, which requires the presence of zinc-permeable TRPA1 to mediate entry of zinc into the cytoplasm. Moreover, TRPA1 is required for zinc-induced inhibition of TRPV1-mediated acute nociception. Unexpectedly, zinc transporters, but not TRPA1, are required for zinc-induced inhibition of TRPV1-dependent chronic neuropathic pain produced by paclitaxel. Together, our study demonstrates a novel mechanism underlying the analgesic effect of zinc on paclitaxel-induced neuropathic pain that relies on the function of TRPV1.SIGNIFICANCE STATEMENT The chemotherapy-induced peripheral neuropathy is a major limiting factor affecting the chemotherapy patients. There is no effective treatment available currently. We demonstrate that zinc prevents paclitaxel-induced mechanical hypersensitivity via inhibiting the TRPV1 channel, which is involved in the sensitization of peripheral nociceptors in chemotherapy. Zinc transporters in DRG neurons are required for the entry of zinc into the intracellular side, where it inhibits TRPV1. Our study provides insight into the mechanism underlying the pain-soothing effect of zinc and suggests that zinc could be developed to therapeutics for the treatment of chemotherapy-induced peripheral neuropathy.
Subject(s)
Antineoplastic Agents, Phytogenic/toxicity , Neuralgia/metabolism , Paclitaxel/toxicity , TRPV Cation Channels/antagonists & inhibitors , Zinc Acetate/pharmacology , Animals , Female , HEK293 Cells , Humans , Male , Mice , Mice, Inbred C57BL , Mice, Knockout , Neuralgia/chemically induced , TRPV Cation Channels/drug effectsABSTRACT
Emerging evidence indicates that spinal neuroinflammation contributes to the maintenance of chronic inflammatory pain. IL-36, as a novel member of the interleukin (IL)-1 super-family cytokines, plays an important role in inflammatory responses. The present study aimed to investigate the role of spinal IL-36 and IL-36 receptor (IL-36R) signaling in the pathology of chronic inflammatory pain. IL-36γ and IL-36R, but not IL-36α and IL-36ß, were persistently upregulated in the spinal cord of mice with intraplantar injections of complete Freund's adjuvant (CFA). Intrathecal administration of both IL-36R antagonist (IL-36Ra) and IL-36γ siRNA significantly attenuated CFA-induced chronic inflammatory pain behaviors. Furthermore, CFA-induced IL-36γ expression was mainly observed in spinal neurons whereas IL-36R was primarily expressed in spinal astrocytes. Additionally, the intrathecal injection of IL-36γ was sufficient to induce pain hypersensitivity and astrocyte activation in naive mice, and these effects could be inhibited by blocking c-Jun N-terminal kinase (JNK) phosphorylation. In vitro experiments also demonstrated that the IL-36γ could induce astrocytic JNK activation and inflammatory cytokines release, which was mediated by IL-36R. Finally, intrathecal injection of IL-36γ-activated astrocytes in a pJNK-dependent manner induced mechanical allodynia and thermal hyperalgesia in naive mice. Collectively, these findings reveal that the neuronal/astrocytic interaction in the spinal cord by which neuronally produced IL-36γ activates astrocytes via IL-36R-mediated JNK pathway is crucial for the maintenance of chronic inflammatory pain. Thus, IL-36γ/IL-36R-mediated astrocyte signaling may be a suitable therapeutic target for chronic inflammatory pain.
Subject(s)
Astrocytes/metabolism , Inflammation/metabolism , Interleukin-1/metabolism , MAP Kinase Signaling System/physiology , Pain/metabolism , Receptors, Interleukin-1/metabolism , Animals , Astrocytes/drug effects , Disease Models, Animal , Freund's Adjuvant , Hyperalgesia/metabolism , Inflammation/chemically induced , Interleukins/pharmacology , Mice , Neurons/drug effects , Neurons/metabolism , Pain/chemically induced , Pain Measurement , PhosphorylationABSTRACT
Interleukin-33 (IL-33) and its receptor ST2 contribute to spinal glial activation and chronic pain. A recent study showed that peripheral IL-33 plays a pivotal role in the pathogenesis of chronic itch induced by poison ivy. However, how IL-33/ST2 signaling in the spinal cord potentially mediates chronic itch remains elusive. Here, we determined that St2-/- substantially reduced scratching behaviors in 2,4-dinitrofluorobenzene (DNFB)-induced allergic contact dermatitis (ACD) as well as acetone and diethylether followed by water-induced dry skin in mice. Intrathecal administration of the neutralizing anti-ST2 or anti-IL-33 antibody remarkably decreased the scratching response in DNFB-induced ACD mice. Expression of spinal IL-33 and ST2 significantly increased in ACD mice, as evidenced by increased mRNA and protein levels. Immunofluorescence and in situ hybridization demonstrated that increased expression of spinal IL-33 was predominant in oligodendrocytes and astrocytes, whereas ST2 was mainly expressed in astrocytes. Further studies showed that in ACD mice, the activation of astrocytes and increased phosphorylation of signal transducer and activator of transcription 3 (STAT3) were markedly attenuated by St2-/- . Intrathecal injection of Janus Kinase 2 Inhibitor AG490 significantly alleviated scratching behaviors in ACD mice. rIL-33 pretreatment exacerbated gastrin-releasing peptide (GRP)-evoked scratching behaviors. This increased gastrin-releasing peptide receptor (GRPR) expression was abolished by St2-/- . Tnf-α upregulation was suppressed by St2-/- . Our results indicate that the spinal IL-33/ST2 signaling pathway contributes to chronic itch via astrocytic JAK2-STAT3 cascade activation, promoting TNF-α release to regulate the GRP/GRPR signaling-related itch response. Thus, these findings provide a potential therapeutic option for treating chronic pruritus.
Subject(s)
Astrocytes/metabolism , Dermatitis, Allergic Contact/metabolism , Interleukin-1 Receptor-Like 1 Protein/metabolism , Interleukin-33/metabolism , Pruritus/metabolism , Spinal Cord/metabolism , Animals , Astrocytes/pathology , Dermatitis, Allergic Contact/pathology , Disease Models, Animal , Gastrin-Releasing Peptide/metabolism , Interleukin-1 Receptor-Like 1 Protein/genetics , Janus Kinase 2/metabolism , Male , Mice, Inbred BALB C , Mice, Inbred C57BL , Mice, Knockout , Oligodendroglia/metabolism , Oligodendroglia/pathology , Pruritus/pathology , STAT3 Transcription Factor/metabolism , Signal Transduction , Spinal Cord/pathologyABSTRACT
BACKGROUND: Chronic itch is a highly debilitating symptom that underlies many medical disorders with no universally effective treatments. Although unique neuronal signaling cascades in the sensory ganglia and spinal cord have been shown to critically promote the pathogenesis of chronic itch, the role of skin-associated cells remains poorly understood. OBJECTIVE: We sought to examine the cutaneous mechanisms underlying transient receptor potential vanilloid 4 (TRPV4)-mediated allergic and nonallergic chronic itch. METHODS: Expression of TRPV4 in chronic itch and healthy control skin preparations was examined by using real-time RT-PCR. Trpv4eGFP mice were used to study the expression and function of TRPV4 in the skin by means of immunofluorescence staining, flow cytometry, calcium imaging, and patch-clamp recordings. Genetic and pharmacologic approaches were used to examine the role and underlying mechanisms of TRPV4 in mouse models of dry skin-associated chronic itch and spontaneous scratching associated with squaric acid dibutylester-induced allergic contact dermatitis. RESULTS: TRPV4 is selectively expressed by dermal macrophages and epidermal keratinocytes in mice. Lineage-specific deletion of TRPV4 in macrophages and keratinocytes reduces allergic and nonallergic chronic itch in mice, respectively. Importantly, TRPV4 expression is significantly increased in skin biopsy specimens from patients with chronic idiopathic pruritus in comparison with skin from healthy control subjects. Moreover, TRPV4-dependent chronic itch requires 5-hydroxytryptamine (5-HT) signaling secondary to activation of distinct 5-HT receptors in mice with allergic and those with nonallergic chronic itch conditions. CONCLUSION: Our study reveals previously unrecognized mechanisms by which TRPV4-expressing epithelial and immune cells in the skin critically and dynamically mediate chronic itch and unravels novel targets for therapeutics in the setting of chronic itch.
Subject(s)
Dermatitis, Allergic Contact/immunology , Dermis/immunology , Gene Expression Regulation/immunology , Keratinocytes/immunology , Macrophages/immunology , Pruritus/immunology , TRPV Cation Channels/immunology , Animals , Chronic Disease , Dermatitis, Allergic Contact/genetics , Dermatitis, Allergic Contact/pathology , Dermis/pathology , Female , Gene Expression Regulation/genetics , Humans , Keratinocytes/pathology , Macrophages/pathology , Male , Mice , Mice, Knockout , Pruritus/genetics , Pruritus/pathology , TRPV Cation Channels/geneticsABSTRACT
Somatosensory neurons mediate our sense of touch. They are critically involved in transducing pain and itch sensations under physiological and pathological conditions, along with other skin-resident cells. Tissue damage and inflammation can produce a localized or systemic sensitization of our senses of pain and itch, which can facilitate our detection of threats in the environment. Although acute pain and itch protect us from further damage, persistent pain and itch are debilitating. Recent exciting discoveries have significantly advanced our knowledge of the roles of membrane-bound G protein-coupled receptors and ion channels in the encoding of information leading to pain and itch sensations. This review focuses on molecular and cellular events that are important in early stages of the biological processing that culminates in our senses of pain and itch.
Subject(s)
Keratinocytes/metabolism , Models, Neurological , Pain/physiopathology , Pruritus/physiopathology , Receptors, G-Protein-Coupled/metabolism , Sensory Receptor Cells/physiology , Signal Transduction/physiology , Dendritic Cells/metabolism , Humans , Macrophages/metabolism , Mast Cells/metabolism , Neutrophils/metabolism , Receptors, Opioid/metabolism , TRPM Cation Channels/metabolismABSTRACT
Clinical usage of opioids in pain relief is dampened by analgesic tolerance after chronic exposure, which is related to opioid-associated neuroinflammation. In the current study, which is based on a chronic morphine tolerance rat model and sustained morphine treatment on primary neuron culture, it was observed that Akt phosphorylation, cleaved-Caspase-1-dependent NALP1 inflammasome activation and IL-1ß maturation in spinal cord neurons were significantly enhanced by morphine. Moreover, treatment with LY294002, a specific inhibitor of PI3k/Akt signaling, significantly reduced Caspase-1 cleavage, NALP1 inflammasome activation and attenuated morphine tolerance. Tail-flick tests demonstrated that pharmacological inhibition on Caspase-1 activation or antagonizing IL-1ß dramatically blocked the development of morphine tolerance. The administration of an exogenous analogue of lipoxin, Aspirin-triggered Lipoxin (ATL), caused a decline in Caspase-1 cleavage, inflammasome activation and mature IL-1ß production and thus attenuated the development of morphine tolerance by inhibiting upstream Akt phosphorylation. Additionally, treatment with DAMGO, a selective µ-opioid receptor peptide, significantly induced Akt phosphorylation, Caspase-1 cleavage and anti-nociception tolerance, all of which were attenuated by ATL treatment. Taken together, the present study revealed the involvement of spinal NALP1 inflammasome activation in the development of morphine tolerance and the role of the µ-receptor/PI3k-Akt signaling/NALP1 inflammasome cascade in this process. By inhibiting this signaling cascade, ATL blocked the development of morphine tolerance.
Subject(s)
Analgesics/administration & dosage , Drug Tolerance , Lipoxins/administration & dosage , Morphine/administration & dosage , Nociception/drug effects , Signal Transduction/drug effects , Animals , Astrocytes/drug effects , Astrocytes/metabolism , Caspase 1/metabolism , Cells, Cultured , Chromones/pharmacology , Inflammasomes/drug effects , Inflammasomes/metabolism , Inflammation/metabolism , Interleukin-1beta/metabolism , Male , Mice , Mice, Inbred C57BL , Morpholines/pharmacology , Nerve Tissue Proteins/metabolism , Neurons/drug effects , Neurons/metabolism , Nociception/physiology , Phosphatidylinositol 3-Kinases/metabolism , Phosphoinositide-3 Kinase Inhibitors , Proto-Oncogene Proteins c-akt/metabolism , Rats , Rats, Sprague-Dawley , Spinal Cord/drug effects , Spinal Cord/metabolismABSTRACT
BACKGROUND: Emerging evidence indicates that nerve damage-initiated neuroinflammation and immune responses, which are evidenced by the up-regulation of proinflammatory cytokines, contribute to the development of neuropathic pain. This study investigated the role of spinal interleukin (IL)-33 and its receptor ST2 in spared nerve injury (SNI)-induced neuropathic pain. METHODS: The von Frey test and acetone test were performed to evaluate neuropathic pain behaviors (n = 8 to 12), and Western blot (n = 4 to 6), immunohistochemistry, real-time polymerase chain reaction (n = 5), and Bio-Plex (n = 5) assays were performed to understand the molecular mechanisms. RESULTS: Intrathecal administration of ST2-neutralizing antibody or ST2 gene knockout (ST2) significantly attenuated the SNI-induced mechanical and cold allodynia. On the 7th day after SNI, the expression of spinal IL-33 and ST2 was increased by 255.8 ± 27.3% and 266.4 ± 83.5% (mean ± SD), respectively. Mechanistic studies showed that the increased expression of the spinal N-methyl-D-aspartate (NMDA) receptor subunit 1 after SNI was reduced by ST2 antibody administration or ST2. The induction of nociceptive behaviors in naive mice due to recombinant IL-33 was reversed by the noncompetitive NMDA antagonist MK-801. ST2 antibody administration or ST2 markedly inhibited the increased activation of the astroglial janus kinase 2 (JAK2)-signal transducer and activator of transcription 3 (STAT3) cascade and the neuronal calcium-calmodulin-dependent kinase II (CaMKII)-cyclic adenosine monophosphate response element-binding protein (CREB) cascade after SNI. Moreover, intrathecal pretreatment with the CaMKII inhibitor KN-93 or the JAK2-STAT3 cascade inhibitor AG490 attenuated recombinant IL-33-induced nociceptive behaviors and NMDA subunit 1 up-regulation in naive mice. CONCLUSION: Spinal IL-33/ST2 signaling contributes to neuropathic pain by activating the astroglial JAK2-STAT3 cascade and the neuronal CaMKII-CREB cascade.
Subject(s)
Calcium-Calmodulin-Dependent Protein Kinase Type 2/metabolism , Cyclic AMP Response Element-Binding Protein/metabolism , Interleukin-33/metabolism , Janus Kinase 2/metabolism , Neuralgia/metabolism , Receptors, Interleukin/metabolism , STAT3 Transcription Factor/metabolism , Animals , Astrocytes/metabolism , Astrocytes/pathology , Interleukin-1 Receptor-Like 1 Protein , Male , Mice , Mice, Inbred BALB C , Mice, Knockout , Neuralgia/pathology , Neurons/metabolism , Neurons/pathology , Receptors, Interleukin/deficiency , Signal Transduction/physiologyABSTRACT
ABSTRACT: Cold allodynia is a common complaint of patients suffering from neuropathic pain initiated by peripheral nerve injury. However, the mechanisms that drive neuropathic cold pain remain elusive. In this study, we show that the interleukin (IL)-33/ST2 signaling in the dorsal root ganglion (DRG) is a critical contributor to neuropathic cold pain by interacting with the cold sensor transient receptor potential melastatin 8 (TRPM8). By using the St2-/- mice, we demonstrate that ST2 is required for the generation of nociceptor hyperexcitability and cold allodynia in a mouse model of spared nerve injury (SNI). Moreover, the selective elimination of ST2 function from the Nav1.8-expressing nociceptor markedly suppresses SNI-induced cold allodynia. Consistent with the loss-of-function studies, intraplantar injection of recombinant IL-33 (rIL-33) is sufficient to induce cold allodynia. Mechanistically, ST2 is co-expressed with TRPM8 in both mouse and human DRG neurons and rIL-33-induced Ca2+ influx in mouse DRG neurons through TRPM8. Co-immunoprecipitation assays further reveal that ST2 interacts with TRPM8 in DRG neurons. Importantly, rIL-33-induced cold allodynia is abolished by pharmacological inhibition of TRPM8 and genetic ablation of the TRPM8-expressing neurons. Thus, our findings suggest that the IL-33/ST2 signaling mediates neuropathic cold pain through downstream cold-sensitive TRPM8 channels, thereby identifying a potential analgesic target for the treatment of neuropathic cold pain.
ABSTRACT
The system physiology approaches that emerge in western countries in recent years echo the holistic view of ancient Traditional Chinese Medicine (TCM) practices that deal with the root, rather than only the symptoms of diseases. Particularly, TCM practices, including acupuncture, emphasize the mobilization of self-healing mechanisms to bring back body homeostasis. Acupuncture has been practiced for over two thousand years to modulate body physiology via stimulation at specific body regions (acupoints). With the development of various research on acupuncture therapy, its regulatory effect on the immune system has been gradually recognized, especially on immunological diseases, including infectious and allergic diseases. In this study, we reviewed the immunomodulatory mechanism of acupuncture and systematically integrates existing research to respectively elucidate the modulatory mechanisms of acupuncture on the innate immune system, adaptive immune system, and well-known neuroanatomical mechanisms, including intact somatosensory-autonomic reflex pathway. With the advances made in recent systems physiology studies, we now have a great opportunity to gain insight into how acupuncture modulates immunity, and subsequently improves its efficacy.
Subject(s)
Acupuncture Therapy , Acupuncture , Hypersensitivity , Humans , Medicine, Chinese Traditional , Acupuncture Points , Hypersensitivity/therapyABSTRACT
Microglia, resident macrophages of the CNS, are essential to brain development, homeostasis, and disease. Microglial activation and proliferation are hallmarks of many CNS diseases, including neuropathic pain. However, molecular mechanisms that govern the spinal neuroimmune axis in the setting of neuropathic pain remain incompletely understood. Here, we show that genetic ablation or pharmacological blockade of transient receptor potential vanilloid type 4 (TRPV4) markedly attenuated neuropathic pain-like behaviors in a mouse model of spared nerve injury. Mechanistically, microglia-expressed TRPV4 mediated microglial activation and proliferation and promoted functional and structural plasticity of excitatory spinal neurons through release of lipocalin-2. Our results suggest that microglial TRPV4 channels reside at the center of the neuroimmune axis in the spinal cord, which transforms peripheral nerve injury into central sensitization and neuropathic pain, thereby identifying TRPV4 as a potential new target for the treatment of chronic pain.
Subject(s)
Neuralgia , Neuroimmunomodulation , Mice , Animals , TRPV Cation Channels/genetics , Spinal Cord , Neuralgia/genetics , MicrogliaABSTRACT
Morphine and other opiates are highly effective for treating moderate to severe pain. However, morphine-induced hyperalgesia and analgesic tolerance prevent durable efficacy in patients. Here, we investigated the underlying molecular mechanisms of this phenomenon. We found that repeated subcutaneous injections of morphine in mice increased the abundance of the cytokine interleukin-33 (IL-33) primarily in oligodendrocytes and astrocytes and that of its receptor ST2 mainly in astrocytes. Pharmacological inhibition or knockdown of IL-33 or ST2 in the spinal cord attenuated morphine-induced hyperalgesia and analgesic tolerance in mice, as did global knockout of either Il33 or St2, which also reduced morphine-enhanced astroglial activation and excitatory synaptic transmission. Furthermore, a pathway mediated by tumor necrosis factor receptorassociated factor 6 (TRAF6) and the kinase JNK in astrocytes was required for IL-33mediated hyperalgesia and tolerance through promoting the production of the chemokine CXCL12 in the spinal cord. The findings suggest that targeting IL-33ST2 signaling could enable opioids to produce sustained analgesic effects in chronic pain management.
Subject(s)
Hyperalgesia , Morphine , Animals , Hyperalgesia/chemically induced , Interleukin-33 , Morphine/adverse effects , Rats , Rats, Sprague-Dawley , Receptors, Interleukin-1 , Spinal CordABSTRACT
The neuroanatomical basis behind acupuncture practice is still poorly understood. Here, we used intersectional genetic strategy to ablate NPY+ noradrenergic neurons and/or adrenal chromaffin cells. Using endotoxin-induced systemic inflammation as a model, we found that electroacupuncture stimulation (ES) drives sympathetic pathways in somatotopy- and intensity-dependent manners. Low-intensity ES at hindlimb regions drives the vagal-adrenal axis, producing anti-inflammatory effects that depend on NPY+ adrenal chromaffin cells. High-intensity ES at the abdomen activates NPY+ splenic noradrenergic neurons via the spinal-sympathetic axis; these neurons engage incoherent feedforward regulatory loops via activation of distinct adrenergic receptors (ARs), and their ES-evoked activation produces either anti- or pro-inflammatory effects due to disease-state-dependent changes in AR profiles. The revelation of somatotopic organization and intensity dependency in driving distinct autonomic pathways could form a road map for optimizing stimulation parameters to improve both efficacy and safety in using acupuncture as a therapeutic modality.
Subject(s)
Electroacupuncture , Neurons/physiology , Neuropeptide Y/metabolism , Sympathetic Nervous System/physiology , Animals , Inflammation/metabolism , Male , Mice , Mice, Inbred C57BLABSTRACT
The chronic neuropathic pain-associated psychiatric disorders have seriously disturbed the quality of patients' life, such as depression and anxiety. Neuroinflammation in the hippocampus plays an important role in the neuropathic pain-associated depressive and anxiety disorders, but the underlying mechanism has not been thoroughly elucidated to date. The Nod-like receptor protein (NLRP)-1 inflammasome, which controls the production of pro-inflammatory cytokines, was broadly involved in the neuroinflammation-related diseases. In the present study, we show that the NLRP1 inflammasome is significantly activated in the hippocampus of rats subjected to the chronic constriction injury (CCI)-induced neuropathic pain. Inhibiting the product of NLRP1 inflammasome not only attenuated the depression-like behaviors but also suppressed the production of mature IL-1ß in the hippocampus of CCI rats. The double-stranded RNA-dependent protein kinase (PKR, also known as EIF2AK2) has been recently shown to be a pivotal regulator for the activation of inflammasome. In the rats subjected to CCI neuropathic pain, the PKR was simultaneously activated in hippocampus. Functional inhibition of PKR suppressed the NLRP1 inflammasome activation and effectively attenuated the CCI-induced depression-like behaviors. These results indicate that the hippocampal PKR/NLRP1 inflammasome pathway play an important role in the development of the depressive behaviors after chronic neuropathic pain. Thus, interrupting this pathway might provide a novel therapeutic strategy for neuropathic pain-associated depressive disorders.
Subject(s)
Depression/metabolism , Hippocampus/metabolism , Inflammasomes/metabolism , Nerve Tissue Proteins/metabolism , Neuralgia/metabolism , eIF-2 Kinase/metabolism , 2-Aminopurine/pharmacology , Animals , Behavior, Animal/drug effects , Behavior, Animal/physiology , Hippocampus/drug effects , Male , Neuralgia/etiology , Neurons/drug effects , Neurons/metabolism , Peripheral Nerve Injuries/complications , Peripheral Nerve Injuries/metabolism , Rats , Rats, Sprague-Dawley , Sciatic Nerve/injuries , Signal Transduction/drug effects , Signal Transduction/physiologyABSTRACT
The gate control theory proposes that Aß mechanoreceptor inputs to spinal pain transmission T neurons are gated via feedforward inhibition, but it remains unclear how monosynaptic excitation is gated by disynaptic inhibitory inputs that arrive later. Here we report that Aß-evoked, non-NMDAR-dependent EPSPs in T neurons are subthreshold, allowing time for inhibitory inputs to prevent action potential firing that requires slow-onset NMDAR activation. Potassium channel activities-including IA, whose sizes are established constitutively by PreprodynorphinCre-derived inhibitory neurons-either completely filter away Aß inputs or make them subthreshold, thereby creating a permissive condition to achieve gate control. Capsaicin-activated nociceptor inputs reduce IA and sensitize the T neurons, allowing Aß inputs to cause firing before inhibitory inputs arrive. Thus, distinct kinetics of glutamate receptors and electric filtering by potassium channels solve the timing problem underlying the gating by feedforward inhibition, and their modulation offers a way to bypass the gate control.
Subject(s)
Ion Channel Gating/physiology , Neural Inhibition/physiology , Spinal Cord/physiology , Animals , Excitatory Amino Acid Antagonists/pharmacology , Ion Channel Gating/drug effects , Mice , Mice, 129 Strain , Mice, Transgenic , Neural Inhibition/drug effects , Organ Culture Techniques , Receptors, N-Methyl-D-Aspartate/agonists , Receptors, N-Methyl-D-Aspartate/antagonists & inhibitors , Receptors, N-Methyl-D-Aspartate/physiology , Spinal Cord/cytology , Spinal Cord/drug effects , Time FactorsABSTRACT
Although both persistent itch and inflammation are commonly associated with allergic contact dermatitis (ACD), it is not known if they are mediated by shared or distinct signaling pathways. Here we show that both TRPA1 and TRPV1 channels are required for generating spontaneous scratching in a mouse model of ACD induced by squaric acid dibutylester (SADBE), a small molecule hapten, through directly promoting the excitability of pruriceptors. TRPV1 but not TRPA1 channels protect the skin inflammation, as genetic ablation of TRPV1 function or pharmacological ablation of TRPV1-positive sensory nerves promotes cutaneous inflammation in the SADBE-induced ACD. Our results demonstrate that persistent itch and inflammation are mediated by distinct cellular and molecular mechanisms in a mouse model of ACD. Identification of distinct roles of TRPA1 and TRPV1 in regulating itch and inflammation may provide new insights into the pathophysiology and treatment of chronic itch and inflammation in ACD patients.
Subject(s)
Dermatitis, Allergic Contact/immunology , Pruritus/immunology , Skin/immunology , TRPA1 Cation Channel/immunology , TRPV Cation Channels/immunology , Animals , Dermatitis, Allergic Contact/genetics , Disease Models, Animal , Female , Humans , Male , Mice , Mice, Inbred C57BL , Pruritus/genetics , TRPA1 Cation Channel/genetics , TRPV Cation Channels/geneticsABSTRACT
BACKGROUND AND PURPOSE: TMEM16A, also known as anoctamin 1 channel, is a member of the Ca(2+)-activated chloride channels family and serves as a heat sensor in the primary nociceptors. Eact is a recently discovered small molecule activator of the TMEM16A channel. Here, we asked if Eact produces pain- and itch-related responses in vivo and investigated the cellular and molecular basis of Eact-elicited responses in dorsal root ganglia (DRG) neurons. EXPERIMENTAL APPROACH: We employed behavioural testing combined with pharmacological inhibition and genetic ablation approaches to identify transient receptor potential vanilloid 1 (TRPV1) as the prominent mediator for Eact-evoked itch- or pain-related responses. We investigated the effects of Eact on TRPV1 and TMEM16A channels expressed in HEK293T cells and in DRG neurons isolated from wild type and Trpv1(-/-) mice using Ca(2+) imaging and patch-clamp recordings. We also used site-directed mutagenesis to determine the molecular basis of Eact activation of TRPV1. KEY RESULTS: Administration of Eact elicited both itch- and pain-related behaviours. Unexpectedly, the Eact-elicited behavioural responses were dependent on the function of TRPV1, as shown by pharmacological inhibition and genetic ablation studies. Eact activated membrane currents and increased intracellular free Ca(2+) in both TRPV1-expressing HEK293T cells and isolated DRG neurons in a TRPV1-dependent manner. Eact activation of the TRPV1 channel was severely attenuated by mutations disrupting the capsaicin-binding sites. CONCLUSIONS AND IMPLICATIONS: Our results suggest that Eact activates primary sensory nociceptors and produces both pain and itch responses mainly through direct activation of TRPV1 channels.