Inflammatory pain resolution by mouse serum-derived small extracellular vesicles.

Chronic pain is a significant public health issue. Current treatments have limited efficacy and significant side effects, warranting research on alternative strategies for pain management. One approach involves using small extracellular vesicles (sEVs) to transport beneficial biomolecular cargo to aid pain resolution. Exosomes are 30-150 nm sEVs that can carry RNAs, proteins, and lipid mediators to recipient cells via circulation. Exosomes can be beneficial or harmful depending on their source and contents. To investigate the short and long-term effects of mouse serum-derived sEVs in pain modulation, sEVs from naïve control or spared nerve injury (SNI) model donor mice were injected intrathecally into naïve recipient mice. Basal mechanical thresholds transiently increased in recipient mice. This effect was mediated by opioid signaling as this outcome was blocked by naltrexone. Mass Spectrometry of sEVs detected endogenous opioid peptide leu-enkephalin. A single prophylactic intrathecal injection of sEVs two weeks prior to induction of the pain model in recipient mice delayed mechanical allodynia in SNI model mice and accelerated recovery from inflammatory pain after complete Freund's adjuvant (CFA) injection. ChipCytometry of spinal cord and dorsal root ganglion (DRG) from sEV treated mice showed that prophylactic sEV treatment reduced the number of natural killer (NK) and NKT cells in spinal cord and increased CD206+ anti-inflammatory macrophages in (DRG) after CFA injection. Further characterization of sEVs showed the presence of immune markers suggesting that sEVs can exert immunomodulatory effects in recipient mice to promote the resolution of inflammatory pain. Collectively, these studies demonstrate multiple mechanisms by which sEVs can attenuate pain.

Epigenetic regulation in opioid induced hyperalgesia.

About 25 million American adults experience pain daily and one of the most commonly prescribed drugs to treat pain are opioids. Prolonged opioid usage and dose escalations can cause a paradoxical response where patients experience enhanced pain sensitivity. This opioid induced hyperalgesia (OIH) is a major hurdle when treating pain in the clinic because its underlying mechanisms are still not fully understood. OIH is also commonly overlooked and lacks guidelines to prevent its onset. Research on pain disorders and opioid usage have recognized potential epigenetic drivers of disease including DNA methylation, histone modifications, miRNA regulation, but their involvement in OIH has not been well studied. This article discusses epigenetic changes that may contribute to pathogenesis, with an emphasis on miRNA alterations in OIH. There is a crucial gap in knowledge including how multiple epigenetic modulators contribute to OIH. Elucidating the epigenetic changes underlying OIH and the crosstalk among these mechanisms could lead to the development of novel targets for the prevention and treatment of this painful phenomena.

Dopamine, Immunity, and Disease.

The neurotransmitter dopamine is a key factor in central nervous system (CNS) function, regulating many processes including reward, movement, and cognition. Dopamine also regulates critical functions in peripheral organs, such as blood pressure, renal activity, and intestinal motility. Beyond these functions, a growing body of evidence indicates that dopamine is an important immunoregulatory factor. Most types of immune cells express dopamine receptors and other dopaminergic proteins, and many immune cells take up, produce, store, and/or release dopamine, suggesting that dopaminergic immunomodulation is important for immune function. Targeting these pathways could be a promising avenue for the treatment of inflammation and disease, but despite increasing research in this area, data on the specific effects of dopamine on many immune cells and disease processes remain inconsistent and poorly understood. Therefore, this review integrates the current knowledge of the role of dopamine in immune cell function and inflammatory signaling across systems. We also discuss the current understanding of dopaminergic regulation of immune signaling in the CNS and peripheral tissues, highlighting the role of dopaminergic immunomodulation in diseases such as Parkinson's disease, several neuropsychiatric conditions, neurologic human immunodeficiency virus, inflammatory bowel disease, rheumatoid arthritis, and others. Careful consideration is given to the influence of experimental design on results, and we note a number of areas in need of further research. Overall, this review integrates our knowledge of dopaminergic immunology at the cellular, tissue, and disease level and prompts the development of therapeutics and strategies targeted toward ameliorating disease through dopaminergic regulation of immunity. SIGNIFICANCE STATEMENT: Canonically, dopamine is recognized as a neurotransmitter involved in the regulation of movement, cognition, and reward. However, dopamine also acts as an immune modulator in the central nervous system and periphery. This review comprehensively assesses the current knowledge of dopaminergic immunomodulation and the role of dopamine in disease pathogenesis at the cellular and tissue level. This will provide broad access to this information across fields, identify areas in need of further investigation, and drive the development of dopaminergic therapeutic strategies.

Effects of ethanol on mechanical allodynia and dynamic weight bearing in male and female mice with spared nerve injury.

BACKGROUND: Men and women with chronic pain report increased alcohol use and are more likely to be diagnosed with alcohol use disorder. The relationship between alcohol use and pain is bidirectional. Alcohol is used as an analgesic, but chronic alcohol intake increases pain. Sex differences in the relationship between chronic pain and alcohol are reported in the clinical and preclinical literature, but due to this bidirectional relationship, it is challenging to investigate the mechanisms that contribute to these differences. Thus, animal models of chronic pain are needed to characterize the efficacy of ethanol as an analgesic in males and females. The current experiments tested the hypothesis that ethanol differentially reduces pain behaviors in male and female mice in chronic neuropathic pain. METHODS: The spared nerve injury (SNI) model was used to investigate the analgesic effects of multiple doses of ethanol (0.5, 1, 2, g/kg i.p.) in male and female mice using von Frey and dynamic weight-bearing (DWB) assays. RESULTS: In both male and female mice, SNI led to robust allodynia and shifts in dynamic weight bearing. In male SNI mice, all three doses of ethanol fully reversed mechanical allodynia and shifts in DWB. In SNI females, only the highest dose (2.0 g/kg) was fully antiallodynic in the von Frey assay, while shifts in weight bearing were reversed at the 1.0 and 2.0 g/kg doses. The differences between male and females were not due to lower blood ethanol concentrations in female mice. CONCLUSION: These data indicate that while ethanol has antiallodynic and antinociceptive effects in male and female mice, the doses and time course of these effects are distinct. Studies investigating the relationship between pain and ethanol exposure in mice should consider sex as a key variable. These data also inform reported sex differences in rodent models of chronic pain and in chronic pain patients.

Circulating microRNAs from the mouse tibia fracture model reflect the signature from patients with complex regional pain syndrome.

INTRODUCTION: Complex regional pain syndrome (CRPS) often results from an initial trauma that later produces a disproportionate amount of pain. The mechanisms underlying CRPS have been studied using a tibia fracture model (TFM) in rodents because this model closely mimics symptoms and has several molecular correlates observed in patients with CRPS. OBJECTIVE: Here, we determined whether the TFM has alterations in circulating microRNAs (miRNAs) and cytokines transported by small extracellular vesicles (sEVs) that faithfully model previously reported miRNA alterations from patients with CRPS. METHODS: We isolated and characterized serum-derived sEVs from mice 3 weeks after fracture when symptoms such as pain hypersensitivity develop. Whole-transcriptome profiling was used to determine sEV miRNAs, and Bio-Plex Pro Mouse Cytokine 23-plex assay was used to measure cytokines. Differentially expressed miRNAs from TFM were compared with previously reported circulating miRNA alterations from patients with CRPS. RESULTS: Although sEV cytokine levels were unchanged, there were significant changes in sEV miRNA profiles. Differentially expressed miRNAs from TFM sEVs significantly overlapped with those previously reported in patients with CRPS. Of the 57 sEV miRNAs dysregulated in the TFM, 30 were previously reported in patients with CRPS compared with healthy control donors both in sEVs and 23 in whole blood. CONCLUSIONS: These findings enhance the validity of TFM as a model for CRPS and suggest that specific miRNA dysregulation may be a shared feature of CRPS and the TFM. These dysregulated miRNAs could help identify mechanistic targets or serve as biomarker candidates for both diagnosis and treatment responses in clinical trials.