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Concept: Nociception

188

Little is known about the molecular mechanisms underlying mammalian touch transduction. To identify novel candidate transducers, we examined the molecular and cellular basis of touch in one of the most sensitive tactile organs in the animal kingdom, the star of the star-nosed mole. Our findings demonstrate that the trigeminal ganglia innervating the star are enriched in tactile-sensitive neurons, resulting in a higher proportion of light touch fibers and lower proportion of nociceptors compared to the dorsal root ganglia innervating the rest of the body. We exploit this difference using transcriptome analysis of the star-nosed mole sensory ganglia to identify novel candidate mammalian touch and pain transducers. The most enriched candidates are also expressed in mouse somatosesensory ganglia, suggesting they may mediate transduction in diverse species and are not unique to moles. These findings highlight the utility of examining diverse and specialized species to address fundamental questions in mammalian biology.

Concepts: Nervous system, Ganglion, Dorsal root ganglion, Sensory system, Skin, Somatosensory system, Nociception, Talpidae

167

CO(2)-laser C-fibre evoked cortical potentials (LCEPs) is a potentially useful animal model for studies of pain mechanisms. A potential confounding factor when assessing analgesic effects of systemically administered drugs using LCEP is sedation. This study aims to clarify: 1) the relation between level of anaesthesia and magnitude of LCEP, 2) the effects of a sedative and an analgesic on LCEP and dominant EEG frequency 3) the effects of a sedative and analgesic on LCEP when dominant EEG frequency is kept stable. LCEP and EEG were recorded in isoflurane/nitrous-oxide anaesthetized rats. Increasing isoflurane level gradually reduced LCEPs and lowered dominant EEG frequencies. Systemic midazolam (10 μmol/kg) profoundly reduced LCEP (19% of control) and lowered dominant EEG frequency. Similarly, morphine 1 and 3 mg/kg reduced LCEP (39%, 12% of control, respectively) and decreased EEG frequency. When keeping the dominant EEG frequency stable, midazolam caused no significant change of LCEP. Under these premises, morphine at 3 mg/kg, but not 1 mg/kg, caused a significant LCEP reduction (26% of control). In conclusion, the present data indicate that the sedative effects should be accounted for when assessing the analgesic effects of drug. Furthermore, it is suggested that LCEP, given that changes in EEG induced by sedation are compensated for, can provide information about the analgesic properties of systemically administrated drugs.

Concepts: Opioid, Pain, Somatosensory system, Nociception, Morphine, Analgesic, Codeine, Sedative

133

Animal and human studies indicate that electrical stimulation of DRG neurons may modulate neuropathic pain signals.ACCURATE, a pivotal, prospective, multi-center, randomized-comparative effectiveness trial, was conducted in 152 subjects diagnosed with complex regional pain syndrome (CRPS) or causalgia in the lower extremities. Subjects received neurostimulation of the DRG or dorsal column (SCS). The primary endpoint was a composite of safety and efficacy at 3 months and subjects were assessed through 12 months for long term outcomes and adverse events. The pre-defined primary composite endpoint of treatment success was met for subjects with a permanent implant who reported 50% or greater decrease in VAS from pre-implant baseline and who did not report any stimulation-related neurological deficits.No subjects reported stimulation-related neurological deficits. The percentage of subjects receiving ≥ 50% pain relief and treatment success was greater in the DRG arm (81.2%) versus the SCS arm (55.7%, p<0.001) at 3 months. Device-related and serious adverse events were not different between the 2 groups. DRG stimulation also demonstrated greater improvements in quality of life and psychological disposition. Finally, subjects using DRG stimulation reported less postural variation in paresthesia (p<0.001) and reduced extraneous stimulation in non-painful areas (p=0.014), indicating DRG stimulation provided more targeted therapy to painful parts of the lower extremities.As the largest prospective, randomized comparative effectiveness trial to date, the results show DRG stimulation provided a higher rate of treatment success with less postural variation in paresthesia intensity compared to SCS.

Concepts: Nervous system, Dorsal root ganglion, Neurology, Pain, Nociception, Suffering, Complex regional pain syndrome, Peripheral neuropathy

130

A hallmark feature of chronic pain is its ability to impact other sensory and affective experiences. It is notably associated with hypersensitivity at the site of tissue injury. It is less clear, however, if chronic pain can also induce a generalized site-nonspecific enhancement in the aversive response to nociceptive inputs. Here, we showed that chronic pain in one limb in rats increased the aversive response to acute pain stimuli in the opposite limb, as assessed by conditioned place aversion. Interestingly, neural activities in the anterior cingulate cortex (ACC) correlated with noxious intensities, and optogenetic modulation of ACC neurons showed bidirectional control of the aversive response to acute pain. Chronic pain, however, altered acute pain intensity representation in the ACC to increase the aversive response to noxious stimuli at anatomically unrelated sites. Thus, chronic pain can disrupt cortical circuitry to enhance the aversive experience in a generalized anatomically nonspecific manner.

Concepts: Cerebrum, Pain, Nociception, Nociceptor, Brodmann area 24, Anterior cingulate cortex, Cingulate cortex, Insular cortex

115

Primary pain disorders (formerly “functional pain syndromes”) are common, under-diagnosed and under-treated in children and teenagers. This manuscript reviews key aspects which support understanding the development of pediatric chronic pain, points to the current pediatric chronic pain terminology, addresses effective treatment strategies, and discusses the evidence-based use of pharmacology. Common symptoms of an underlying pain vulnerability present in the three most common chronic pain disorders in pediatrics: primary headaches, centrally mediated abdominal pain syndromes, and/or chronic/recurrent musculoskeletal and joint pain. A significant number of children with repeated acute nociceptive pain episodes develop chronic pain in addition to or as a result of their underlying medical condition “chronic-on-acute pain.” We provide description of the structure and process of our interdisciplinary, rehabilitative pain clinic in Minneapolis, Minnesota, USA with accompanying data in the treatment of chronic pain symptoms that persist beyond the expected time of healing. An interdisciplinary approach combining (1) rehabilitation; (2) integrative medicine/active mind-body techniques; (3) psychology; and (4) normalizing daily school attendance, sports, social life and sleep will be presented. As a result of restored function, pain improves and commonly resolves. Opioids are not indicated for primary pain disorders, and other medications, with few exceptions, are usually not first-line therapy.

Concepts: Medicine, Physician, Abdominal pain, Pain, Nociception, Medical school, Abdomen, Chronic pain

61

The EGFR belongs to the well-studied ErbB family of receptor tyrosine kinases. EGFR is activated by numerous endogenous ligands that promote cellular growth, proliferation, and tissue regeneration. In the present study, we have demonstrated a role for EGFR and its natural ligand, epiregulin (EREG), in pain processing. We show that inhibition of EGFR with clinically available compounds strongly reduced nocifensive behavior in mouse models of inflammatory and chronic pain. EREG-mediated activation of EGFR enhanced nociception through a mechanism involving the PI3K/AKT/mTOR pathway and matrix metalloproteinase-9. Moreover, EREG application potentiated capsaicin-induced calcium influx in a subset of sensory neurons. Both the EGFR and EREG genes displayed a genetic association with the development of chronic pain in several clinical cohorts of temporomandibular disorder. Thus, EGFR and EREG may be suitable therapeutic targets for persistent pain conditions.

Concepts: Signal transduction, Developmental biology, Receptor tyrosine kinase, Protein kinase, Pain, Sensory system, Nociception, Nociceptor

60

The treatment of chronic pain could benefit from additional non-opioid interventions. Virtual reality (VR) has been shown to be effective in decreasing pain for procedural or acute pain but to date there have been few studies on its use in chronic pain. The present study was an investigation of the impact of a virtual reality application for chronic pain. Thirty (30) participants with various chronic pain conditions were offered a five-minute session using a virtual reality application called Cool! Participants were asked about their pain using a 0-10 visual analog scale rating before the VR session, during the session and immediately after the session. They were also asked about immersion into the VR world and about possible side effects. Pain was reduced from pre-session to post-session by 33%. Pain was reduced from pre-session during the VR session by 60%. These changes were both statistically significant at the p < .001 level. Three participants (10%) reported no change between pre and post pain ratings. Ten participants (33%) reported complete pain relief while doing the virtual reality session. All participants (100%) reported a decrease in pain to some degree between pre-session pain and during-session pain. The virtual reality experience was found here to provide a significant amount of pain relief. A head mounted display (HMD) was used with all subjects and no discomfort was experienced. Only one participant noted any side effects. VR seems to have promise as a non-opioid treatment for chronic pain and further investigation is warranted.

Concepts: Vitamin D, Pain, Sensory system, Nociception, Suffering, Chronic pain, Virtual reality, Pain management

53

Cerebral processes contribute to pain beyond the level of nociceptive input and mediate psychological and behavioural influences. However, cerebral contributions beyond nociception are not yet well characterized, leading to a predominant focus on nociception when studying pain and developing interventions. Here we use functional magnetic resonance imaging combined with machine learning to develop a multivariate pattern signature-termed the stimulus intensity independent pain signature-1 (SIIPS1)-that predicts pain above and beyond nociceptive input in four training data sets (Studies 1-4, N=137). The SIIPS1 includes patterns of activity in nucleus accumbens, lateral prefrontal and parahippocampal cortices, and other regions. In cross-validated analyses of Studies 1-4 and in two independent test data sets (Studies 5-6, N=46), SIIPS1 responses explain variation in trial-by-trial pain ratings not captured by a previous fMRI-based marker for nociceptive pain. In addition, SIIPS1 responses mediate the pain-modulating effects of three psychological manipulations of expectations and perceived control. The SIIPS1 provides an extensible characterization of cerebral contributions to pain and specific brain targets for interventions.

Concepts: Psychology, Brain, Magnetic resonance imaging, Pain, Nociception, Nociceptor, Learning, Limbic system

43

Here, we describe a targeted reverse genetic screen for thermal nociception genes in Drosophila larvae. Using laser capture microdissection and microarray analyses of nociceptive and non-nociceptive neurons, we identified 275 nociceptor-enriched genes. We then tested the function of the enriched genes with nociceptor-specific RNAi and thermal nociception assays. Tissue-specific RNAi targeted against 14 genes caused insensitive thermal nociception while targeting of 22 genes caused hypersensitive thermal nociception. Previously uncategorized genes were named for heat resistance (i.e., boilerman, fire dancer, oven mitt, trivet, thawb, and bunker gear) or heat sensitivity (firelighter, black match, eucalyptus, primacord, jet fuel, detonator, gasoline, smoke alarm, and jetboil). Insensitive nociception phenotypes were often associated with severely reduced branching of nociceptor neurites and hyperbranched dendrites were seen in two of the hypersensitive cases. Many genes that we identified are conserved in mammals.

Concepts: Gene, RNA, Classical genetics, Heat, Pain, Nociception, Nociceptor, Molecular genetics

42

Polypeptide toxins have played a central part in understanding physiological and physiopathological functions of ion channels. In the field of pain, they led to important advances in basic research and even to clinical applications. Acid-sensing ion channels (ASICs) are generally considered principal players in the pain pathway, including in humans. A snake toxin activating peripheral ASICs in nociceptive neurons has been recently shown to evoke pain. Here we show that a new class of three-finger peptides from another snake, the black mamba, is able to abolish pain through inhibition of ASICs expressed either in central or peripheral neurons. These peptides, which we call mambalgins, are not toxic in mice but show a potent analgesic effect upon central and peripheral injection that can be as strong as morphine. This effect is, however, resistant to naloxone, and mambalgins cause much less tolerance than morphine and no respiratory distress. Pharmacological inhibition by mambalgins combined with the use of knockdown and knockout animals indicates that blockade of heteromeric channels made of ASIC1a and ASIC2a subunits in central neurons and of ASIC1b-containing channels in nociceptors is involved in the analgesic effect of mambalgins. These findings identify new potential therapeutic targets for pain and introduce natural peptides that block them to produce a potent analgesia.

Concepts: Protein, Opioid, Pain, Nociception, Nociceptor, Morphine, Buprenorphine, Venom