The mu-opioid receptor (MOR), a member of the G-protein-coupled receptor superfamily, is pivotal in pain modulation and analgesia. Biased agonism at MOR offers a promising avenue for developing safer opioid therapeutics by selectively engaging specific signaling pathways. This study presents a comprehensive analysis of biased agonists using a newly curated database, BiasMOR, comprising 166 unique molecules with annotated activity data for GTPγS, cAMP, and β-arrestin assays. Advanced structure-activity relationship (SAR) analyses, including network similarity graphs, maximum common substructures, and activity cliff identification, reveal critical molecular features underlying bias signaling. Modelability assessments indicate high suitability for predictive modeling, with RMODI indices exceeding 0.96 and SARI indices highlighting moderately continuous SAR landscapes for cAMP and β-arrestin assays. Interaction patterns for biased agonists are discussed, including key residues such as D, Y, and Y. Comparative studies of enantiomer-specific interactions further underscore the role of ligand-induced conformational states in modulating signaling pathways. This work underscores the potential of combining computational and experimental approaches to advance the understanding of MOR-biased signaling, paving the way for safer opioid therapies. The database provided here will serve as a starting point for designing biased mu opioid receptor ligands and will be updated as new data become available. Increasing the repertoire of biased ligands and analyzing molecules collectively, as the database described here, contributes to pinpointing structural features responsible for biased agonism that can be associated with biological effects still under debate.

Complex Regional Pain Syndrome (CRPS) presents significant diagnostic challenges due to its diverse clinical presentation. This study aims to describe the diagnostic trajectory of patients labeled with CRPS, focusing on referral patterns, application of the Budapest criteria, and accuracy of CRPS diagnosis.

Chronic pain affects millions globally, yet no universally effective treatment exists. The primary motor cortex (M1) has been a key target for chronic pain therapies, with electrical stimulation of the M1 (eMCS) showing promise. However, the mechanisms underlying M1-mediated analgesic effects are not fully understood. We investigated the role of the primary somatosensory cortex (S1) in M1-mediated analgesia using a neuropathic pain mouse model. In this model, neuropathic pain is associated with increased spontaneous activity of layer V pyramidal neurons (LV-PNs) in the S1, partly attributed to the reduced activity of somatostatin-expressing inhibitory neurons (SST INs), which normally suppress LV-PNs. While manipulation of either LV-PNs or SST INs has been shown to alleviate pain, the role of S1 in M1-mediated analgesia has not been identified. Using multichannel silicon probes, we applied eMCS to neuropathic mice and observed significant analgesia. Histological analyses revealed that eMCS activated SST INs and suppressed hyperactivity of LV-PNs in the S1, suggesting that eMCS suppresses pain by modulating S1 neuronal circuits, alongside other pain-related regions. Notably, eMCS induced long-lasting analgesia, persisting for at least two days post-stimulation. These findings implicate S1 as a critical mediator of eMCS-induced analgesia and suggest eMCS as a potential durable therapeutic strategy for chronic pain. Chronic pain is a devastating disorder that affects over 25% of the global population. The lack of universally and entirely effective treatments, combined with severe social and economic burdens posed by the side effects of current analgesics, underscores the need to explore multifaceted approaches. In this study, we applied a silicon probe to target layer 5 of the M1 region of mice and delivered electrical stimulation to a chronic constriction injury mouse model. Our findings demonstrated that eMCS induced analgesic effects on mechanical stimuli, with the effect notably persisting for at least two days after the cessation of eMCS. As a potential mechanism, we identified SST+ neuronal activation in S1, along with other previously known brain regions influenced by eMCS.

The neural mechanisms underlying sex-specific pain, in which males and females exhibit distinct responses to pain, remain poorly understood. Here we show that in a mouse model of male-specific pain hypersensitivity response to pain conditioning environments (contextual pain hypersensitivity model), elevated free-testosterone leads to hyperactivity of glutamatergic neurons in the medial preoptic area (Glu) through activation of androgen receptor signaling, which in turn induces contextual pain hypersensitivity in male mice. Although not observed in naïve female mice, this pain phenotype could be induced in females via chronic administration of testosterone propionate. In addition, Glu neurons send excitatory inputs to GABAergic neurons in the ventrolateral periaqueductal gray (GABA) that are required for contextual pain hypersensitivity. Our study thus demonstrates that testosterone/androgen receptor signaling enhances Glu→ GABA pathway activity, which drives a male-specific contextual pain hypersensitivity, providing insight into the basis of sexually dimorphic pain response.

Sensory-motor integration is crucial in the processing of chronic pain. The primary motor cortex (M1) is emerging as a promising target for chronic pain treatment. However, it remains elusive how nociceptive sensory inputs influence M1 activity and how rectifying M1 defects, in turn, regulates pain processing at cellular and network levels. We show that injury/inflammation leads to hypoactivity of M1 pyramidal neurons by excitation-inhibition imbalance between the primary somatosensory cortex (S1) and the M1. The impaired M1 output further weakens inputs to excitatory parvalbumin neurons of the lateral hypothalamus (LH) and impairs the descending inhibitory system, hence exacerbating spinal nociceptive sensitivity. When rectifying M1 defects with repetitive transcranial magnetic stimulation (rTMS), the imbalance of the S1-M1 microcircuitry can be effectively reversed, which aids in restoring the ability of the M1 to trigger the descending inhibitory system, thereby alleviating nociceptive hypersensitivity. Thus, a sensory-motor-sensory loop is identified for pain-related interactions between the sensory and motor systems and can be potentially exploited for treating chronic pain.

Sex differences in the pathogenesis of a variety of diseases have drawn increasing attention. However, it remains unclear whether such differences exist in chemotherapy-induced neuropathic pain. Here, we conducted a retrospective analysis of clinical case data and found that peripheral sensory disorders occurred earlier in females than in males following bortezomib (BTZ) treatment in patients with multiple myeloma. BTZ treatment led to an early elevation of intercellular adhesion molecule-1, which triggered the infiltration of peripheral monocytes into the perivascular region of the spinal cord in female mice. The CC-chemokine ligand 1 released by infiltrating macrophages directly activated neurons or indirectly activated neurons by enhancing the astrocyte activity, ultimately leading to the earlier onset of BTZ-induced neuropathic pain in females. Together, clarifying the mechanism underlying the earlier onset of BTZ-induced neuropathic pain will contribute to the precise treatment of multiple myeloma in females.

In this issue of Neuron, Xu et al. demonstrate that activating GPR37L1, a G-protein-coupled receptor that negatively regulates astrocytes, suppresses the onset and maintenance of neuropathic pain, an intractable chronic pain caused by nerve damage, thereby serving as a therapeutic target.

Somatosensory pathways convey crucial information about pain, touch, itch and body part movement from peripheral organs to the central nervous system. Despite substantial needs to understand how these pathways assemble and to develop pain therapeutics, clinical translation remains challenging. This is probably related to species-specific features and the lack of in vitro models of the polysynaptic pathway. Here we established a human ascending somatosensory assembloid (hASA), a four-part assembloid generated from human pluripotent stem cells that integrates somatosensory, spinal, thalamic and cortical organoids to model the spinothalamic pathway. Transcriptomic profiling confirmed the presence of key cell types of this circuit. Rabies tracing and calcium imaging showed that sensory neurons connect to dorsal spinal cord neurons, which further connect to thalamic neurons. Following noxious chemical stimulation, calcium imaging of hASA demonstrated a coordinated response. In addition, extracellular recordings and imaging revealed synchronized activity across the assembloid. Notably, loss of the sodium channel Na1.7, which causes pain insensitivity, disrupted synchrony across hASA. By contrast, a gain-of-function SCN9A variant associated with extreme pain disorder induced hypersynchrony. These experiments demonstrated the ability to functionally assemble the essential components of the human sensory pathway, which could accelerate our understanding of sensory circuits and facilitate therapeutic development.

During pathological conditions, tactile stimuli can aberrantly engage nociceptive pathways leading to the perception of touch as pain, known as mechanical allodynia. The brain stem dorsal column nuclei integrate tactile inputs, yet their role in mediating tactile sensitivity and allodynia remains understudied. We found that gracile nucleus (Gr) inhibitory interneurons and thalamus-projecting neurons are differentially innervated by primary afferents and spinal inputs. Functional manipulations of these distinct Gr neuronal populations bidirectionally shifted tactile sensitivity but did not affect noxious mechanical or thermal sensitivity. During neuropathic pain, Gr neurons exhibited increased sensory-evoked activity and asynchronous excitatory drive from primary afferents. Silencing Gr projection neurons or activating Gr inhibitory neurons in neuropathic mice reduced tactile hypersensitivity, and enhancing inhibition ameliorated paw-withdrawal signatures of neuropathic pain and induced conditioned place preference. These results suggest that Gr activity contributes to tactile sensitivity and affective, pain-associated phenotypes of mechanical allodynia.