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The peripheral mu-opioid receptor (MOR) has been recognized as a potential target to provide safer analgesia with reduced central side effects. Although analgesic incompetence of the peripheral MOR in the absence of inflammation was initially identified more than a decade ago, there has been very limited investigation into the underlying signaling mechanisms. Here we identify that G protein-coupled receptor kinase 2 (GRK2) constitutively interacts with the MOR in peripheral sensory neurons to suppress peripheral MOR activity. Brief exposure to bradykinin (BK) causes uncoupling of GRK2 from the MOR and subsequent restoration of MOR functionality in dorsal root ganglion (DRG) neurons. Interestingly, prolonged BK treatment induces constitutive activation of the MOR through a mechanism that involves protein kinase C (PKC) activation. After silencing Raf kinase inhibitory protein (RKIP) by RNA interference, BK-induced constitutive MOR activation is completely abrogated, which agrees with previous findings that BK activates PKC signaling to initiate GRK2 sequestration by RKIP. Furthermore, we demonstrate that constitutive, peripheral MOR activity requires GRK2 uncoupling and that the FDA-approved SSRI paroxetine promotes this state of uncoupling. Collectively, these results indicate that GRK2 tightly regulates MOR functional states and controls constitutive MOR activity in peripheral sensory neurons, supporting the potential for targeting the kinase to provide safer analgesia.
Learn More >Delivering adeno-associated virus (AAV) vectors into the central nervous system of nonhuman primates (NHPs) via the blood or cerebral spinal fluid is associated with dorsal root ganglion (DRG) toxicity. Conventional immune-suppression regimens do not prevent this toxicity, possibly because it may be caused by high transduction rates, which can, in turn, cause cellular stress due to an overabundance of the transgene product in target cells. To test this hypothesis and develop an approach to eliminate DRG toxicity, we exploited endogenous expression of microRNA (miR) 183 complex, which is largely restricted to DRG neurons, to specifically down-regulate transgene expression in these cells. We introduced sequence targets for miR183 into the vector genome within the 3' untranslated region of the corresponding transgene messenger RNA and injected vectors into the cisterna magna of NHPs. Administration of unmodified AAV vectors resulted in robust transduction of target tissues and toxicity in DRG neurons. Consistent with the proposal that immune system activity does not mediate this neuronal toxicity, we found that steroid administration was ineffective in alleviating this pathology. However, including miR183 targets in the vectors reduced transgene expression in, and toxicity of, DRG neurons without affecting transduction elsewhere in the primate's brain. This approach might be useful in reducing DRG toxicity and the associated morbidity and should facilitate the development of AAV-based gene therapies for many central nervous system diseases.
Learn More >The neurokinin-1 receptor (NK1R; encoded by ) is expressed in spinal dorsal horn neurons and has been suggested to mediate itch in rodents. However, previous studies relied heavily on neurotoxic ablation of NK1R spinal neurons, which limited further dissection of their function in spinal itch circuitry. To address this limitation, we leveraged a newly developed mouse line to characterize the role of NK1R spinal neurons in itch. We show that pharmacological activation of spinal NK1R and chemogenetic activation of spinal neurons increases itch behavior in male and female mice, whereas pharmacological inhibition of spinal NK1R suppresses itch behavior. We use fluorescence hybridization (FISH) to characterize the endogenous expression of throughout the superficial and deeper dorsal horn (DDH), as well as the lateral spinal nucleus (LSN), of mouse and human spinal cord. Retrograde labeling studies in mice from the parabrachial nucleus (PBN) show that less than 20% of superficial dorsal horn neurons are spinal projection neurons, and thus the majority of are local interneurons. We then use a combination of hybridization and two-photon Ca imaging of the mouse spinal cord to establish that NK1R and the gastrin-releasing peptide receptor (GRPR) are coexpressed within a subpopulation of excitatory superficial dorsal horn (SDH) neurons. These findings are the first to suggest a role for NK1R interneurons in itch and extend our understanding of the complexities of spinal itch circuitry. The spinal cord is a critical hub for processing somatosensory input, yet which spinal neurons process itch input and how itch signals are encoded within the spinal cord is not fully understood. We demonstrate neurokinin-1 receptor (NK1R) spinal neurons mediate itch behavior in mice and that the majority of NK1R spinal neurons are local interneurons. These NK1R neurons comprise a subset of gastrin-releasing peptide receptor (GRPR) interneurons and are thus positioned at the center of spinal itch transmission. We show NK1R mRNA expression in human spinal cord, underscoring the translational relevance of our findings in mice. This work is the first to suggest a role for NK1R interneurons in itch and extends our understanding of the complexities of spinal itch circuitry.
Learn More >Among the five KCNQ channels, also known as the K7 voltage-gated potassium (K) channels, KCNQ2-KCNQ5 control neuronal excitability. Dysfunctions of KCNQ2-KCNQ5 are associated with neurological disorders such as epilepsy, deafness, and neuropathic pain. Here, we report the cryoelectron microscopy (cryo-EM) structures of human KCNQ4 and its complexes with the opener retigabine or the blocker linopirdine at overall resolutions of 2.5, 3.1, and 3.3 Å, respectively. In all structures, a phosphatidylinositol 4,5-bisphosphate (PIP) molecule inserts its head group into a cavity within each voltage-sensing domain (VSD), revealing an unobserved binding mode for PIP. Retigabine nestles in each fenestration, inducing local shifts. Instead of staying within the central pore, linopirdine resides in a cytosolic cavity underneath the inner gate. Electrophysiological analyses of various mutants corroborated the structural observations. Our studies reveal the molecular basis for the modulatory mechanism of neuronal KCNQ channels and provide a framework for structure-facilitated drug discovery targeting these important channels.
Learn More >Many primary care clinics are resistant to accept new patients taking prescription opioids for chronic pain. It is unclear how much of this practice is specific to individuals who may be perceived to have aberrant opioid use. This study sought to determine whether clinics are more or less willing to accept and prescribe opioids to patients depending on whether their history is more or less suggestive of aberrant opioids use by conducting an audit survey of primary care clinics in 9 states from May to July 2019. Simulated patients taking opioids for chronic pain called each clinic twice, giving one of two scenarios for needing a new provider: their previous physician had either 1) retired or 2) stopped prescribing opioids for unspecified reasons. Clinic willingness to continue prescribing opioids and accept the patient for general primary care were assessed. Of 452 clinics responding to both scenarios (904 calls), 193 (43%) said their providers would not prescribe opioids in either scenario, 146 (32%) said their providers might prescribe in both, and 113 (25%) responded differently to each scenario. Clinics responding differently had greater odds (OR=1.83 CI[1.23,2.76]) of willingness to prescribe when the previous doctor retired than when the doctor had stopped prescribing. These findings suggest that primary care access is limited for patients taking opioids for chronic pain, and differentially further reduced for patients whose histories are suggestive of aberrant use. This denial of care could lead to unintended harms such as worsened pain or conversion to illicit substances.
Learn More >The COVID-19 pandemic has had a tremendous impact, including on individuals with chronic pain. The social distancing policies necessary to slow the spread of SARS-CoV-2 have involved increased levels of social isolation. This cross-sectional survey study examined pain severity and interference among individuals with chronic pain during an early phase of social distancing mandates, and identified characteristics of individuals who were most impacted. Approximately 4-8 weeks after social distancing mandates commenced in the state of Massachusetts, 150 patients with fibromyalgia, chronic spine and postsurgical pain completed demographic, pain, social distancing, and validated psychosocial questionnaires. Patients self-reported an overall significant increase in pain severity and pain interference, compared to before social distancing, although both pain severity and interference were quite variable among individuals under conditions of social distancing. Several demographic, socioeconomic, and psychosocial factors were associated with greater pain severity and interference during social distancing. Multivariable linear regression demonstrated that female gender, non-white race, lower education, disability, fibromyalgia, and higher pain catastrophizing were independently associated with greater pain severity, while female gender and pain catastrophizing were independently associated greater pain interference. The findings suggest that individual differences among chronic pain patients should be considered in the planning, development, and prioritization of interventions to improve pain care and to prevent worsening of symptoms during the continuing COVID-19 pandemic.
Learn More >Our knowledge of the prevalence, impact, and outcomes of chronic pain in the general population is predominantly based on studies over relatively short periods of time. The aim of this study was to identify and describe trajectories of chronic pain status over a period of 21 years. Self-reported population data (n=1858) from five timepoints were analyzed. Pain was categorized by: no chronic pain (NCP); chronic regional pain (CRP); and chronic widespread pain (CWP). Latent Class Growth Analysis was carried out for identification of trajectories, and logistic regression analysis for identification of predictors for pain prognosis. Five trajectories were identified: 1) Persistent NCP (57%); 2) Migrating from NCP to CRP or CWP (5%), 3) Persistent CRP or Migration between CRP and NCP (22%); 4) Migration from CRP to CWP (10%); 5) Persistent CWP (6%). Age, sleeping problems, poor vitality and physical function at baseline were associated with pain progression from NCP. Female gender, seeking care for pain, lack of social support, poor physical function, vitality, and mental health predicted poor pain prognosis among those with CRP. In conclusion chronic pain was common in the population including 6 % reporting persistent CWP, although the majority persistently reported no chronic pain. Most people had stable pain status, but some had ongoing change in pain status over time including people who improved from chronic pain. It was possible to identify clinically relevant factors, characterizing trajectories of chronic pain development, that can be useful for identifying individuals at risk and potential targets for intervention.
Learn More >The Revised Short McGill Pain Questionnaire Version-2 (SF-MPQ-2) is a multidimensional outcome measure designed to capture, evaluate and discriminate pain from neuropathic and non-neuropathic sources. A recent systematic review found insufficient psychometric data with respect to musculoskeletal (MSK) health conditions. This study aimed to describe the reproducibility (test-retest reliability and agreement) and internal consistency of the SF-MPQ-2 for use among patients with musculoskeletal shoulder pain.
Learn More >Central neuropathic extremity pain (CNEP) is the most frequent type of pain in multiple sclerosis (MS). The aim of the present study was to evaluate sensory and pain modulation profiles in MS patients with CNEP. In a single-centre observational study, a group of 56 CNEP MS patients was compared with 63 pain-free MS patients and with a sex- and age-adjusted control group. Standardized quantitative sensory testing (QST) and dynamic QST (dQST) protocols comprising temporal summation and conditioned pain modulation tests were used to compare sensory profiles. Loss-type QST abnormalities in both thermal and mechanical QST modalities prevailed in both MS subgroups and correlated significantly with higher degree of disability expressed as Expanded Disability Status Scale (EDSS). Comparison of sensory phenotypes disclosed a higher frequency of the "sensory loss" prototypic sensory phenotype in the CNEP subgroup (30%) compared with pain-free MS patients (6%; p = 0.003). The role of aging process and higher lesion load in the spinothalamocortical pathway might be possible explanation for pain development in this particular "deafferentation" subtype of central neuropathic pain in MS. We were unable to support the role of central sensitization or endogenous facilitatory and inhibitory mechanisms in the development of CNEP in MS.
Learn More >The "funnel effect" of Fibromyalgia (FM) assumes that as patients access healthcare services, they present greater severity and a more complex clinical situation than individuals with FM from the general population, but the studies comparing patients treated in different levels of healthcare are scarce. The aim of this study is to analyse the "funnel effect" hypothesis by comparing patients from secondary and tertiary healthcare services.
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