Andrew J. Mannes

Peripheral sensitization is a key process in the development of painful inflammatory conditions, driven in part by immune-cell mediator release following tissue injury. The G protein–coupled receptor, GPR183, predominantly expressed on immune cells, regulates their migration, positioning, and mediator production. However, its role in peripheral sensitization and the specific immune cells involved remains insufficiently understood. In rats, intraplantar injection of 7α,25-dihydroxycholesterol (7α,25-OHC), the most potent endogenous GPR183 ligand, produced long-lasting nociception that was prevented by the selective GPR183 antagonist SAE-14. As GPR183 activates extracellular signal-regulated kinase signaling, which influences pain pathways including nitric oxide synthase (NOS) activity and NO formation, we used NOS inhibitors and knockout animals to test the contribution of inducible and neuronal NOS isoforms to 7α,25-OHC-induced sensitization. We found that both isoforms influence this response, independent of cyclooxygenases. In a well-characterized rat incisional injury model, GPR183 protein expression increased in injured paw tissue, and SAE-14 reversed hypersensitivity. Meta-analysis of human postsurgical skin samples similarly showed elevated GPR183 expression and transcriptional changes favoring 7α,25-OHC production after injury. We identified macrophages, Langerhans cells (LCs), and T cells as the principal GPR183 -expressing cell types in human skin. Ablation of LCs and removal of mature B and T cells revealed that 7α,25-OHC-evoked hypersensitivity does not depend on these cell types, leaving GPR183 ⁺ macrophages as the most likely drivers of GPR183-induced hypersensitivity. Overall, our findings define the cellular and molecular pathways linking GPR183 to peripheral sensitization and highlight GPR183 antagonism as a promising strategy for pain management.

Background Fatigue, a prevalent symptom of chronic illness, impacts quality of life. This proof-of-concept, randomized, double-blind, crossover trial assessed the anti-fatigue effects of ketamine (0.5 mg/kg) versus midazolam (0.045 mg/kg), the active comparator. Methods Ten participants, who were cancer survivors, with fibromyalgia, myalgic encephalomyelitis/chronic fatigue syndrome, or systemic lupus erythematosus, were randomized into Arm 1 (n = 4, Period 1: ketamine to Period 2: midazolam) or Arm 2 (n = 6, Period 1: midazolam to Period 2: ketamine). Results The two periods were separately analyzed because of carryover effects with baseline fatigue scores, assessed by the fatigue visual analog scale (VAS), between the study periods (p = 0.03). Looking at changes in fatigue VAS scores from baseline (pre-infusion) to 3 days post-infusion, the ketamine group had a 21.0% decrease in Period 1 and 10.9% in Period 2, while the midazolam group showed a 17.7% decrease in Period 1 and 12.6% in Period 2. We did not observe a statistically significant difference in both periods. The largest fatigue score reduction for the ketamine group was at 1 day post-infusion, at − 38.7% in Period 1. Conclusion Despite no statistical significance, a reduction in real-time fatigue scores was observed, which exceeded the 20% efficacy threshold, the primary outcome, in the ketamine arm from pre-infusion to 3 days post-infusion. The carryover effects and the peak reduction in fatigue at 24 hours after ketamine administration suggest that future trials may need to consider a study design without cross-over and an optimal active placebo alternative.

Peripheral sensitization is a key process in the development of painful inflammatory conditions, driven in part by immune-cell mediator release following tissue injury. The G protein–coupled receptor, GPR183, predominantly expressed on immune cells, regulates their migration, positioning, and mediator production. Yet its role in peripheral sensitization and the specific immune cells involved remains insufficiently understood. In rats, intraplantar injection of 7α,25-dihydroxycholesterol (7α,25-OHC), the most potent endogenous GPR183 ligand, produced long-lasting nociception that was prevented by the selective GPR183 antagonist SAE-14. Because GPR183 activates ERK signaling, which influences pain pathways including nitric oxide synthase (NOS) activity and NO formation, we used NOS inhibitors and knockout animals to test the contribution of inducible and neuronal NOS isoforms to 7α,25-OHC-induced sensitization. We found that both isoforms influence this response, independent of cyclooxygenases. In a well-characterized rat incisional injury model, GPR183 protein expression increased in injured paw tissue, and SAE-14 reversed hypersensitivity. Meta-analysis of human post-surgical skin samples similarly showed elevated GPR183 expression and transcriptional changes favoring 7α,25-OHC production after injury. We identified macrophages and Langerhans cells (LCs) as the principal GPR183-expressing cell types in human skin. LC ablation studies revealed that 7α,25-OHC-evoked hypersensitivity does not depend on LCs, implicating GPR183 ⁺ macrophages as predominant drivers of GPR3-induced hypersensitivity. Overall, our findings define the cellular and molecular pathways linking GPR183 to peripheral sensitization and highlight GPR183 antagonism as a promising strategy for pain management.

Post-surgical pain remains a widespread problem reducing quality of life. The present study investigates the initial molecular changes underlying nociceptive sensitization through longitudinal, temporal sampling at the surgical wound edge. Using RNA-Seq and multiplex fluorescence in situ hybridization, we examined the most significant genes induced by tissue injury including those coding for the secreted factors interleukin 6, oncostatin M and leukemia inhibitory factor, and localized these induction events to several cutaneous structures including the epidermis, vascular endothelia, hair follicles, and sweat glands. Our data also demonstrate the receptors for these key secreted factors are expressed by dorsal root ganglion neurons, indicating long-range signaling from damaged skin to spinal cord, thereby leading to pain. This study provides a novel understanding of tissue structures and the molecular interactome activated following tissue injury by elucidating the inflammatory and tissue repair transcriptional milieu induced by surgery in human skin excision biopsies.

Adenosine receptors are a family of purinergic G protein–coupled receptors that are widely distributed in bodily organs and in the peripheral and central nervous systems. Recently, antihyperalgesic actions have been suggested for the adenosine A₃ receptor, and its agonists have been proposed as new neuropathic pain treatments. We hypothesized that these receptors may be expressed in nociceptive primary afferent neurons. However, RNA sequencing across species, eg, rat, mouse, dog, and human, suggests that dorsal root ganglion (DRG) expression of ADORA3 is inconsistent. In rat and mouse, Adora3 shows very weak to no expression in DRG, whereas it is well expressed in human DRG. However, the cell types in human DRG that express ADORA3 have not been delineated. An examination of DRG cell types using in situ hybridization clearly detected ADORA3 transcripts in peripheral macrophages that are in close apposition to the neuronal perikarya but not in peripheral sensory neurons. By contrast, ADORA1 was found primarily in neurons, where it is broadly expressed at low levels. These results suggest that a more complex or indirect mechanism involving modulation of macrophage and/or microglial cells may underlie the potential analgesic action of adenosine A₃ receptor agonism.

Genetic modifications leading to pain insensitivity phenotypes, while rare, provide invaluable insights into the molecular biology of pain and reveal targets for analgesic drugs. Pain insensitivity typically results from Mendelian loss-of-function mutations in genes expressed in nociceptive (pain-sensing) dorsal root ganglion (DRG) neurons that connect the body to the spinal cord. We document a pain insensitivity mechanism arising from gene overexpression in individuals with the rare 7q11.23 duplication syndrome (Dup7), who have 3 copies of the approximately 1.5-megabase Williams syndrome (WS) critical region. Based on parental accounts and pain ratings, people with Dup7, mainly children in this study, are pain insensitive following serious injury to skin, bones, teeth, or viscera. In contrast, diploid siblings (2 copies of the WS critical region) and individuals with WS (1 copy) show standard reactions to painful events. A converging series of human assessments and cross-species cell biological and transcriptomic studies identified 1 likely candidate in the WS critical region, STX1A , as underlying the pain insensitivity phenotype. STX1A codes for the synaptic vesicle fusion protein syntaxin1A. Excess syntaxin1A was demonstrated to compromise neuropeptide exocytosis from nociceptive DRG neurons. Taken together, these data indicate a mechanism for producing “genetic analgesia” in Dup7 and offer previously untargeted routes to pain control.