Designer receptors exclusively activated by designer drugs (DREADDs) are chemogenetic tools for remotely controlling cellular signaling, neural activity, behavior, and physiology. Using a structure-guided approach, we provide a peripherally restricted Gi-DREADD, hydroxycarboxylic acid receptor DREADD (HCAD), whose native receptor is minimally expressed in the brain, and a chemical actuator that does not cross the blood-brain barrier (BBB). This was accomplished by combined mutagenesis, analoging via an ultra-large make-on-demand library, structural determination of the designed DREADD receptor via cryoelectron microscopy (cryo-EM), and validation of HCAD function. Expression and activation of HCAD in dorsal root ganglion (DRG) neurons inhibit action potential (AP) firing and reduce both acute and tissue-injury-induced inflammatory pain. The HCAD chemogenetic system expands the possibilities for studying numerous peripheral systems with little adverse effects on the central nervous system (CNS). The structure-guided approach used to generate HCAD also has the potential to accelerate the development of emerging chemogenetic tools for basic and translational sciences.

Converging findings have established that the endocannabinoid (eCB) system serves as a possible target for the development of new treatments as a complement to opioid-based treatments. Here, we show in male and female mice that enhancing levels of the eCB, 2-arachidonoylglycerol (2-AG), through pharmacological inhibition of its catabolic enzyme, monoacylglycerol lipase (MAGL), either systemically or in the ventral tegmental area (VTA) with JZL184, leads to a substantial attenuation of the rewarding effects of opioids in mice using conditioned place preference and self-administration paradigms, without altering their analgesic properties. These effects are driven by cannabinoid receptor 1 (CB1R) within the VTA, as VTA CB1R conditional knockout counteracts JZL184’s effects. Using fiber photometry with fluorescent sensors for calcium and dopamine (DA), we find that enhancing 2-AG levels diminishes opioid reward-related nucleus accumbens (NAc) activity and DA neurotransmission. Together, these findings reveal that 2-AG diminishes the rewarding properties of opioids and provides a potential adjunctive therapeutic strategy for opioid-related analgesic treatments.

Nerve growth factor (NGF) monoclonal antibodies inhibit chronic pain yet failed to gain approval due to worsened joint damage in osteoarthritis patients. We report that neuropilin-1 (NRP1) is a co-receptor for NGF and tropomyosin-related kinase A (TrkA) pain signaling. NRP1 was coexpressed with TrkA in human and mouse nociceptors. NRP1 inhibitors suppressed NGF-stimulated excitation of human and mouse nociceptors and NGF-evoked nociception in mice. NRP1 knockdown inhibited NGF/TrkA signaling, whereas NRP1 overexpression enhanced signaling. NGF bound NRP1 with high affinity and interacted with and chaperoned TrkA from the biosynthetic pathway to the plasma membrane and endosomes, enhancing TrkA signaling. Molecular modeling suggested that C-terminal R/KXXR/K NGF motif interacts with extracellular “b” NRP1 domain within a plasma membrane NGF/TrkA/NRP1 of 2:2:2 stoichiometry. G Alpha Interacting Protein C-terminus 1 (GIPC1), which scaffolds NRP1 and TrkA to myosin VI, colocalized in nociceptors with NRP1/TrkA. GIPC1 knockdown abrogated NGF-evoked excitation of nociceptors and pain-like behavior. Thus, NRP1 is a nociceptor-enriched co-receptor that facilitates NGF/TrkA pain signaling. NRP binds NGF and chaperones TrkA to the plasma membrane and signaling endosomes via GIPC1 adaptor. NRP1 and GIPC1 antagonism in nociceptors offers a long-awaited non-opioid alternative to systemic antibody NGF sequestration for the treatment of chronic pain.

Focal nerve injuries are often associated with neuropathic pain. Preclinical research suggests altered neuroimmune signalling underlies such neuropathic pain; however, its cause remains poorly understood in humans. In this multicentre cohort study, we describe the local cellular and molecular signature of neuropathic pain at the lesion site, using Morton’s neuroma as a human model system of neuropathic pain (n = 22; 18 women) compared with nerves from participants without nerve injury (n = 11; 4 women). Immunofluorescent staining revealed demyelination and chronic infiltration of immune cells in Morton’s neuroma. RNA bulk sequencing identified 3349 differentially expressed genes between Morton’s neuroma and controls. Gene ontology enrichment analysis and weighted gene co-expression network analyses revealed modules specific for host defence and neurogenesis. Deconvolution analysis confirmed higher densities of macrophages and B cells in Morton’s neuroma than control samples. Modules associated with defence response, neurogenesis, and muscle system development as well as macrophage cell populations identified by deconvolution correlated with patients’ paroxysmal or evoked pain. Of note, we identified a consistently differentially expressed gene signature (MARCO, CD163, STAB1), indicating the presence of a specific M(GC) subset of macrophages. MARCO gene expression correlated with paroxysmal pain. Targeted immunofluorescent analyses confirmed higher densities of intraneural CD163+MARCO+ macrophage subsets in Morton’s neuroma. Our findings provide detailed insight into the local molecular signature in the context of human focal nerve injury. There is clear evidence for an ongoing role of the immune system in chronic peripheral neuropathic pain in humans, with macrophages and specifically the M(GC) MARCO+ subset implicated.

The human MRGPRD protein is a member of the Mas-related G protein-coupled receptors (MRGPRs) that is involved in the sensing of pain, itch, and other inflammatory stimuli. As with other MRGPRs, MRGPRD is a relatively understudied receptor with few known agonists. The most potent small-molecule agonist of MRGPRD reported so far is β-alanine, with an affinity in the micromole range, which largely restricts its functional study. Here, we report two MRGPRD agonists, EP-2825 and EP-3945, that are approximately 100-fold more potent than β-alanine and determine the structures of MRGPRD-Gq in complex with EP-2825 and EP-3945, respectively. The structures reveal distinct agonist binding modes of MRGPRD and large conformational plasticity of the orthosteric pocket. Collectively, the discovery of high-affinity MRGPRD agonists and their distinct binding modes will facilitate the functional study and the structure-based design of ligands targeting this understudied receptor.

A wide variety of systemic pathologies, including infectious and autoimmune diseases, are accompanied by joint pain or inflammation, often mediated by circulating immune complexes (ICs). How such stimuli access joints and trigger inflammation is unclear. Whole-mount synovial imaging revealed PV1 fenestrated capillaries at the periphery of the synovium in the lining-sublining interface. Circulating ICs extravasated from these PV1 capillaries, and nociceptor neurons and three distinct macrophage subsets formed a sentinel unit around them. Macrophages showed subset-specific responses to systemic IC challenge; LYVE1CXCR1 macrophages orchestrated neutrophil recruitment and activated calcitonin gene-related peptide (CGRP) nociceptor neurons via interleukin-1β. In contrast, major histocompatibility complex class IICD11c (MHCIICD11c) and MHCIICD11c interstitial macrophages formed tight clusters around PV1 capillaries in response to systemic immune stimuli, a feature enhanced by nociceptor-derived CGRP. Altogether, we identify the anatomical location of synovial PV1 capillaries and subset-specific macrophage-nociceptor cross-talk that forms a blood-joint barrier protecting the synovium from circulating immune challenges.

Over 60% of women with endometriosis experience abdominopelvic pain and broader pain manifestations, including chronic back pain, fibromyalgia, chronic fatigue, vulvodynia, and migraine. Although the imbalance of proinflammatory mediators, including the complement component C5a, is associated with endometriosis-related pain, the mechanisms causing widespread pain and the C5a role remain unclear. Female mice and women with endometriosis exhibit increased plasma C5a levels and pain. We hypothesize the Schwann cells involvement in endometriotic pain. Here, we show that silencing the C5a receptor (C5aR1) in Schwann cells blocks the C5a-induced activation of the NLRP1 inflammasome and subsequent release of interleukin-1β (IL-1β). Macrophages, recruited to sciatic/trigeminal nerves by IL-1β from Schwann cells, increase oxidative stress, which activates the proalgesic TRPA1 pathway, resulting in widespread pain. These findings reveal a pathway involving Schwann cell C5aR1, NLRP1/IL-1β activation, macrophage recruitment, oxidative stress, and TRPA1 engagement, contributing to pain in a mouse model of endometriosis.

Dorsal root ganglia (DRGs) play a crucial role in processing sensory information, making it essential to understand their development. Here, we construct a single-cell spatiotemporal transcriptomic atlas of human embryonic DRG. This atlas reveals the diversity of cell types and highlights the extrinsic signaling cascades and intrinsic regulatory hierarchies that guide cell fate decisions, including neuronal/glial lineage restriction, sensory neuron differentiation and specification, and the formation of neuron-satellite glial cell (SGC) units. Additionally, we identify a human-enriched NTRK3/DCC nociceptor subtype, which is involved in multimodal nociceptive processing. Mimicking the programmed activation of signaling pathways in vivo, we successfully establish functional human DRG organoids and underscore the critical roles of transcriptional regulators in the fate commitment of unspecialized sensory neurons (uSNs). Overall, our research elucidates the multilevel signaling pathways and transcription factor (TF) regulatory hierarchies that underpin the diversity of somatosensory neurons, emphasizing the phenotypic distinctions in human nociceptor subtypes.

Navigating the duality of opioids’ potent analgesia and side effects, including tolerance and hyperalgesia, is a significant challenge in chronic pain management, often prompting hazardous dose escalation to maintain analgesic effects. The peripheral mu-opioid receptor (MOR) is known to mediate these contradictory effects. Here, we show that the fms-like tyrosine kinase receptor 3 (FLT3) in peripheral somatosensory neurons drives morphine tolerance and hyperalgesia in a male rodent model. We found that chronic morphine treatment increases FLT3 and MOR co-expression, and that inhibiting FLT3 represses MOR-induced hyperactivation of the cyclic adenosine monophosphate (cAMP) signaling pathway, mitigating maladaptive excitatory processes engaged after chronic morphine treatment. Furthermore, in postsurgical or inflammatory models of chronic pain, co-administering morphine with a FLT3-specific inhibitor not only prevents or suppresses tolerance and hyperalgesia but also potentiates the analgesic efficacy of morphine, without aggravating other morphine-induced adverse effects. Our findings suggest that pairing morphine with FLT3 inhibitors could become a promising avenue for chronic pain management to safely harness the power of opioids, without the risk of dose escalation. By enhancing morphine analgesic potency through FLT3 inhibition, this approach could minimize opioid dosage, thereby curtailing the risk of addiction and other opioid-related side effects.

Immune cells are involved in the pathogenesis of pain by directly activating or sensitizing nociceptor sensory neurons. However, because the immune system also has the capacity to self-regulate through anti-inflammatory mechanisms that drive the resolution of inflammation, it might promote pain resolution and prevention. Here, we describe how immune cell-derived cytokines can act directly on sensory neurons to inhibit pain hypersensitivity and how immune-derived endogenous opioids promote analgesia. We also discuss how immune cells support healthy tissue innervation by clearing debris after nerve injury, protecting against axon retraction from target tissues and enhancing regeneration, preventing the development of chronic neuropathic pain. Finally, we review the accumulating evidence that manipulating immune activity positively alters somatosensation, albeit with currently unclear molecular and cellular mechanisms. Exploration of immune-mediated analgesia and pain prevention could, therefore, be important for the development of novel immune therapies for the treatment of clinical pain states.