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Even though it has been well documented that stress can lead to the development of sleep disorders and the intensification of pain, their relationships have not been fully understood. The present study was aimed at investigating the effects of predictable chronic mild stress (PCMS) on sleep-wake states and pain threshold, using the PCMS rearing conditions of mesh wire (MW) and water (W) for 21 days. Exposure to PCMS decreased the amount of non-rapid eye movement (NREM) sleep during the dark phase. Moreover, the chronicity of PCMS decreased slow-wave activity (SWA) during NREM sleep in the MW and W groups in both the light and dark phases. Mechanical and aversively hot thermal hyperalgesia were more intensified in the PCMS groups than the control. Higher plasma corticosterone levels were seen in mice subjected to PCMS, whereas TNF-α expression was found higher in the hypothalamus in the W and the trigeminal ganglion in the MW group. The W group had higher expression levels of IL-6 in the thalamus as well. The PCMS paradigm decreased SWA and may have intensified mechanical and thermal hyperalgesia. The current study also suggests that rearing under PCMS may cause impaired sleep quality and heightened pain sensation to painful mechanical and aversively hot thermal stimuli.
Learn More >Many patients with chronic pain conditions suffer from depression. The mechanisms underlying pain-induced depression are still unclear. There are critical links of medial prefrontal cortex (mPFC) synaptic function to depression, with signaling through the endocannabinoid (eCB) system as an important contributor. We hypothesized that afferent noxious inputs after injury compromise activity-dependent eCB signaling in the mPFC, resulting in depression. Depression-like behaviors were tested in male and female rats with traumatic neuropathy (spared nerve injury, SNI) and neuronal activity in the mPFC was monitored using the immediate early gene, c-Fos, and electrophysiological recordings. mPFC eCB concentrations were determined using mass spectrometry while behavioral and electrophysiological experiments were employed to evaluate role of alterations in eCB signaling in depression after pain. SNI-induced pain induced the development of depression phenotypes in both male and female rats. Pyramidal neurons in mPFC showed increased excitability followed by reduced excitability in the onset and prolonged phases of pain, respectively. Concentrations of the eCBs, 2-arachidonoylglycerol (2-AG) in the mPFC, were elevated initially after SNI and our results indicate that this resulted in loss of CB1R function on GABAergic interneurons in the mPFC. These data suggest that excessive release of 2-AG as a result of noxious stimuli triggers use-dependent loss of function of eCB signaling leading to excessive GABA release in the mPFC, with the final result being behavioral depression.Pain has both somatosensory and affective components, so the complexity of mechanisms underlying chronic pain is best represented by a biopsychosocial model includes widespread central nervous system dysfunction. Many patients with chronic pain conditions develop depression. The mechanism by which pain causes depression is unclear. Whereas manipulation of the endocannabinoid (eCB) signaling system as an avenue for providing analgesia per se has not shown much promise in previous studies. An important limitation of past research has been inadequate consideration of the dynamic nature of the connection between pain and depression as they develop. Here we show that activity dependent synthesis of eCBs during the initial onset of persistent pain is the critical link leading to depression when pain is persistent.
Learn More >Numerous physiological functions rely on distinguishing temperature through temperature-sensitive transient receptor potential channels (thermo-TRPs). Although the function of thermo-TRPs has been studied extensively, structural determination of their heat- and cold-activated states has remained a challenge. Here, we present cryo-EM structures of the nanodisc-reconstituted wild-type mouse TRPV3 in three distinct conformations: closed, heat-activated sensitized and open states. The heat-induced transformations of TRPV3 are accompanied by changes in the secondary structure of the S2-S3 linker and the N and C termini and represent a conformational wave that links these parts of the protein to a lipid occupying the vanilloid binding site. State-dependent differences in the behavior of bound lipids suggest their active role in thermo-TRP temperature-dependent gating. Our structural data, supported by physiological recordings and molecular dynamics simulations, provide an insight for understanding the molecular mechanism of temperature sensing.
Learn More >Peripheral nerve injuries result in pronounced alterations in dorsal root ganglia (DRG), which can lead to the development of neuropathic pain. Although the polymodal mechanosensitive transient receptor potential ankyrin 1 ion channel (TRPA1) is emerging as a relevant target for potential analgesic therapies, preclinical studies do not provide unequivocal mechanistic insight into its relevance for neuropathic pain pathogenesis. By utilizing a transgenic mouse model with a conditional depletion of the interleukin-6 signal transducer gp130 in Nav1.8 expressing neurons (SNS-gp130-/-), we provide a mechanistic regulatory link between IL-6/gp130 and TRPA1 in the spared nerve injury model (SNI). SNI mice developed profound mechanical hypersensitivity as indicated by increased responses in the von Frey behavioral test in vivo, as well as a significant increase in mechanosensitivity of unmyelinated nociceptive primary afferents in ex vivo skin nerve recordings. In contrast to wild type and control gp130fl/fl animals, SNS-gp130-/- mice did not develop mechanical hypersensitivity after SNI and exhibited low levels of Trpa1 mRNA in sensory neurons, which were partially restored by adenoviral gp130 re-expression in vitro. Importantly, uninjured but not injured neurons developed increased responsiveness to the TRPA1 agonist cinnamon aldehyde (CA), and neurons derived from SNS-gp130-/- mice after SNI were significantly less responsive to CA. Our study shows for the first time that TRPA1 upregulation is attributed specifically to uninjured neurons in the SNI model and this depended on the IL-6 signal transducer gp130. We provide a solution to the enigma of TRPA1 regulation following nerve injury and stress its significance as an important target for neuropathic pain disorders.
Learn More >and loss-of-function mutations in Nav1.7 cause chronic pain and pain insensitivity, respectively. The preferential expression of Nav1.7 in peripheral nervous system and its role in human pain signaling make Nav1.7 a promising target for next-generation pain therapeutics. However, pharmacological agents have not fully recapitulated these pain phenotypes, and, due to the lack of subtype-selective molecular modulators, the role of Nav1.7 in the perception of pain remains poorly understood. Scorpion venom is an excellent source of bioactive peptides that modulate various ion channels, including voltage-gated sodium (Nav) channels . Here, we demonstrate that Buthus martensii Karsch scorpion venom (BV) elicits pain responses in mice through direct enhancement of Nav1.7 activity, and have identified that Makatoxin-3, an α-like toxin as a critical component for BV-mediated effects on Nav1.7. Blocking other Nav subtypes did not eliminate BV-evoked pain responses, supporting the pivotal role of Nav1.7 in BV-induced pain . Makatoxin-3 acts on the S3-S4 loop of voltage sensor domain IV (VSD4) of Nav1.7, which causes a hyperpolarizing shift in the steady-state fast inactivation and impairs inactivation kinetics. We also determined the key residues and structure-function relationships for the toxin-channel interactions, which are distinct from those of other well-studied α-toxins. This study not only reveals a new mechanism underlying BV-evoked pain, but also enriches our knowledge of key structural elements of scorpion toxins that are pivotal for toxin-Nav1.7 interaction, which facilitates the design of novel Nav1.7 selective modulators.
Learn More >A new series of propionamide derivatives was developed as dual μ-opioid receptor agonists and σ receptor antagonists. Modification of a high-throughput screening hit originated a series of piperazinylcycloalkylmethyl propionamides, which were explored to overcome the challenge of achieving balanced dual activity and convenient drug-like properties. The lead compound identified, , showed good analgesic effects in several animal models of both acute (paw pressure) and chronic (partial sciatic nerve ligation) pain, with reduced gastrointestinal effects in comparison with oxycodone.
Learn More >Group I metabotropic glutamate receptors (mGluR1 and mGluR5, mGluR1/5) have been implicated in several CNS diseases including chronic pain. It is known that activation of mGluR1/5 results in production of inositol triphosphate (IP3) and diacylglycerol (DAG) that leads to activation of extracellular signal-regulated kinases (ERK1/2) and an increase in neuronal excitability, but how mGluR1/5 mediate this process remains unclear. We previously reported that Orai1 is responsible for store-operated calcium entry (SOCE) and plays a key role in central sensitization. However, how Orai1 is activated under physiological conditions is unknown. Here, we tested the hypothesis that mGluR1/5 recruit Orai1 as part of its downstream signaling pathway in dorsal horn neurons. We demonstrate that neurotransmitter glutamate induces STIM1 puncta formation, which is not mediated by NMDA or AMPA receptors. Glutamate-induced Ca2+ entry in the presence of NMDA/AMPA receptor antagonists is eliminated in Orai1-deficient neurons. DHPG (an agonist of mGluR1/5)-induced Ca2+ entry is abolished by Orai1 deficiency, but not affected by knocking down of TRPC1 or TRPC3. DHPG-induced activation of ERK1/2 and modulation of neuronal excitability are abolished in cultured Orai1-deficient neurons. Moreover, DHPG-induced nociceptive behavior is markedly reduced in Orai1-deficient mice. Our findings reveal previously unknown functional coupling between Orai1 and mGluR1/5 and shed light on the mechanism underlying mGluR1/5-mediated neuronal plasticity.
Learn More >Dietary interventions are promising approaches to treat pain associated with metabolic changes because they impact both metabolic and neural components contributing to painful neuropathy. Here, we tested whether consumption of a ketogenic diet could affect sensation, pain, and epidermal innervation loss in type 1 diabetic mice. C57Bl/6 mice were rendered diabetic using streptozotocin and administered a ketogenic diet at either three weeks (prevention) or nine weeks (reversal) of uncontrolled diabetes. We quantified changes in metabolic biomarkers, sensory thresholds, and epidermal innervation to assess impact on neuropathy parameters. Diabetic mice consuming ketogenic diet had normalized weight gain, reduced blood glucose, elevated blood ketones, and reduced hemoglobin-A1C levels. These metabolic biomarkers were also improved following nine weeks of diabetes followed by four weeks of a ketogenic diet. Diabetic mice fed a control chow diet developed rapid mechanical allodynia of the hind paw that was reversed within a week of consumption of a ketogenic diet in both prevention and reversal studies. Loss of thermal sensation was also improved by consumption of a ketogenic diet via normalized thermal thresholds. Finally, diabetic mice consuming a ketogenic diet had normalized epidermal innervation, including after nine weeks of uncontrolled diabetes and four weeks of consumption of the ketogenic diet. These results suggest that, in mice, a ketogenic diet can prevent and reverse changes in key metabolic biomarkers, altered sensation, pain and axon innervation of the skin. These results identify a ketogenic diet as a potential therapeutic intervention for patients with painful diabetic neuropathy and/or epidermal axon loss.
Learn More >Systemic treatment with resiniferatoxin (RTX) induces small-fiber sensory neuropathy by damaging TRPV1-expressing primary sensory neurons and causes distinct thermal sensory impairment and tactile allodynia, which resemble the unique clinical features of postherpetic neuralgia. However, the synaptic plasticity associated with RTX-induced tactile allodynia remains unknown. In this study, we found that RTX-induced neuropathy is associated with α2δ-1 upregulation in the dorsal root ganglion (DRG) and increased physical interaction between α2δ-1 and GluN1 in the spinal cord synaptosomes. RNAscope hybridization showed that RTX treatment significantly increased α2δ-1 expression in DRG neurons labeled with calcitonin gene-related peptide, isolectin B4, NF200, and tyrosine hydroxylase. Electrophysiological recordings revealed that RTX treatment augmented the frequency of miniature excitatory postsynaptic currents (mEPSCs) and the amplitude of evoked EPSCs in spinal dorsal horn neurons, and these effects were reversed by blocking NMDA receptors with AP-5. Inhibiting α2δ-1 with gabapentin, genetically ablating α2δ-1, or targeting α2δ-1-bound NMDA receptors with α2δ-1Tat peptide largely normalized the baseline frequency of mEPSCs and the amplitude of evoked EPSCs potentiated by RTX treatment. Furthermore, systemic treatment with memantine or gabapentin and intrathecal injection of AP-5 or Tat-fused α2δ-1 C terminus peptide reversed allodynia in RTX-treated rats and mice. In addition, RTX-induced tactile allodynia was attenuated in α2δ-1 knock-out mice and in mice in which GluN1 was conditionally knocked out in DRG neurons. Collectively, our findings indicate that α2δ-1-bound NMDA receptors at presynaptic terminals of sprouting myelinated afferent nerves contribute to RTX-induced potentiation of nociceptive input to the spinal cord and tactile allodynia. Postherpetic neuralgia (PHN), associated with shingles, is a distinct form of neuropathic pain commonly seen in elderly and immunocompromised patients. The synaptic plasticity underlying touch-induced pain hypersensitivity in PHN remains unclear. Using a nonviral animal model of PHN, we found that glutamatergic input from primary sensory nerves to the spinal cord is increased via tonic activation of glutamate NMDA receptors. Also, we showed that α2δ-1 (encoded by ), originally considered a calcium channel subunit, serves as an auxiliary protein that promotes activation of presynaptic NMDA receptors and pain hypersensitivity. This new information advances our understanding of the molecular mechanism underlying PHN and suggests new strategies for treating this painful condition.
Learn More >Currently, limited information regarding the role of calcitonin gene-related peptide (CGRP) in neuropathic pain is available. Intracerebroventricular administrations of an anti-CGRP antibody were performed in rats with infraorbital nerve ligation. Anti-CGRP antibody administration attenuated mechanical and heat hypersensitivities induced by nerve ligation and decreased the phosphorylated extracellular signal-regulated kinase expression levels in the trigeminal spinal subnucleus caudalis (Vc) following mechanical or heat stimulation. An increased CGRP immunoreactivity in the Vc appeared after nerve ligation. A decreased CGRP immunoreactivity resulted from anti-CGRP antibody administration. Our findings suggest that anti-CGRP antibody administration attenuates the symptoms of trigeminal neuropathic pain by acting on CGRP in the Vc.
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