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Although most patients with chronic pain are women, the preclinical literature regarding pain processing and the pathophysiology of chronic pain has historically been derived overwhelmingly from the study of male rodents. This Review describes how the recent adoption by a number of funding agencies of policies mandating the incorporation of sex as a biological variable into preclinical research has correlated with an increase in the number of studies investigating sex differences in pain and analgesia. Trends in the field are analysed, with a focus on newly published findings of qualitative sex differences: that is, those findings that are suggestive of differential processing mechanisms in each sex. It is becoming increasingly clear that robust differences exist in the genetic, molecular, cellular and systems-level mechanisms of acute and chronic pain processing in male and female rodents and humans.Historically, preclinical pain research has been dominated by studies in male subjects. Jeffrey Mogil describes recent trends towards the inclusion of male and female subjects in research and the subsequent identification of qualitative sex differences in the mechanisms of pain processing.

Acute nociceptive pain is a key defence system that enables the detection of danger signals that threaten homeostasis and survival. However, chronic pain (such as the neuropathic pain that occurs after peripheral nerve injury) is not simply a consequence of the continuity of acute nociceptive signals but rather of maladaptive nervous system function. Over recent decades, studies have provided evidence for the necessity and sufficiency of microglia for the alterations in synaptic remodelling, connectivity and network function that underlie chronic pain and have shed light on the underlying molecular and cellular mechanisms. It is also becoming clear that microglia have active roles in brain regions important for the emotional and memory-related aspects of chronic pain. Recent advances in the development of new drugs targeting microglia and the establishment of new sources of human microglia-like cells may facilitate translation of these findings from bench to bedside.

Nociplastic pain is a mechanistic term used to describe pain that arises or is sustained by altered nociception, despite the absence of tissue damage. Although nociplastic pain has distinct pathophysiology from nociceptive and neuropathic pain, these pain mechanisms often coincide within individuals, which contributes to the intractability of chronic pain. Key symptoms of nociplastic pain include pain in multiple body regions, fatigue, sleep disturbances, cognitive dysfunction, depression and anxiety. Individuals with nociplastic pain are often diffusely tender — indicative of hyperalgesia and/or allodynia — and are often more sensitive than others to non-painful sensory stimuli such as lights, odours and noises. This Review summarizes the risk factors, clinical presentation and treatment of nociplastic pain, and describes how alterations in brain function and structure, immune processing and peripheral factors might contribute to the nociplastic pain phenotype. This article concludes with a discussion of two proposed subtypes of nociplastic pain that reflect distinct neurobiological features and treatment responsivity.Nociplastic pain arises from altered nociception despite the absence of tissue damage. In this Review, the authors summarize the risk factors and clinical presentation of nociplastic pain, and discuss its potential underlying mechanisms, including evidence of CNS, immune and peripheral contributions.

Best practices in preclinical algesiometry (pain behaviour testing) have shifted over the past decade as a result of technological advancements, the continued dearth of translational progress and the emphasis that funding institutions and journals have placed on rigour and reproducibility. Here we describe the changing trends in research methods by analysing the methods reported in preclinical pain publications from the past 40 years, with a focus on the last 5 years. We also discuss how the status quo may be hampering translational success. This discussion is centred on four fundamental decisions that apply to every pain behaviour experiment: choice of subject (model organism), choice of assay (pain-inducing injury), laboratory environment and choice of outcome measures. Finally, we discuss how human tissues, which are increasingly accessible, can be used to validate the translatability of targets and mechanisms identified in animal pain models.The translation of analgesic drug candidates to the clinic relies upon successful preclinical pain modelling. In this Review, Stucky and colleagues describe recent trends in the methods used to model pain in laboratory animals and provide recommendations for experimental designs that may increase translational success.

Pain medication plays an important role in the treatment of acute and chronic pain conditions, but some drugs, opioids in particular, have been overprescribed or prescribed without adequate safeguards, leading to an alarming rise in medication-related overdose deaths. The NIH Helping to End Addiction Long-term (HEAL) Initiative is a trans-agency effort to provide scientific solutions to stem the opioid crisis. One component of the initiative is to support biomarker discovery and rigorous validation in collaboration with industry leaders to accelerate high-quality clinical research into neurotherapeutics and pain. The use of objective biomarkers and clinical trial end points throughout the drug discovery and development process is crucial to help define pathophysiological subsets of pain, evaluate target engagement of new drugs and predict the analgesic efficacy of new drugs. In 2018, the NIH-led Discovery and Validation of Biomarkers to Develop Non-Addictive Therapeutics for Pain workshop convened scientific leaders from academia, industry, government and patient advocacy groups to discuss progress, challenges, gaps and ideas to facilitate the development of biomarkers and end points for pain. The outcomes of this workshop are outlined in this Consensus Statement.In 2018, the Discovery and Validation of Biomarkers to Develop Non-Addictive Therapeutics for Pain workshop convened to discuss strategies to facilitate the development of biomarkers and end points for pain. The outcomes of this workshop are outlined in this Consensus Statement.

Paralleling the activation of dorsal horn microglia after peripheral nerve injury is a significant expansion and proliferation of macrophages around injured sensory neurons in dorsal root ganglia (DRG). Here we demonstrate a critical contribution of DRG macrophages, but not those at the nerve injury site, to both the initiation and maintenance of the mechanical hypersensitivity that characterizes the neuropathic pain phenotype. In contrast to the reported sexual dimorphism in the microglial contribution to neuropathic pain, depletion of DRG macrophages reduces nerve injury-induced mechanical hypersensitivity and expansion of DRG macrophages in both male and female mice. However, fewer macrophages are induced in the female mice and deletion of colony-stimulating factor 1 from sensory neurons, which prevents nerve injury-induced microglial activation and proliferation, only reduces macrophage expansion in male mice. Finally, we demonstrate molecular cross-talk between axotomized sensory neurons and macrophages, revealing potential peripheral DRG targets for neuropathic pain management. Interactions among spinal dorsal horn neurons and microglia contribute to the induction and maintenance of neuropathic pain after peripheral nerve injury. The authors show that depletion of macrophages in the dorsal root ganglia prevents and reverses ongoing nerve injury-induced hypersensitivity.

Key Points The anterior cingulate cortex (ACC) plays an important part in chronic pain states. NMDA-receptor-dependent postsynaptic long-term potentiation (LTP) in the ACC sustains the affective component of the pain state. Kainate-receptor-dependent presynaptic LTP in the ACC contributes to pain-related anxiety. The mechanism for neuropathic pain is linked to the expression of LTP in the ACC. Upregulation of GluN2B-containing NMDA receptors is found in chronic neuropathic pain conditions. Calcium-stimulated adenylyl cyclase 1 is a potential target for future treatment of chronic pain and anxiety. Evidence suggests that activity in the anterior cingulate cortex (ACC) contributes to acute and chronic pain. In this article, Zhuo and colleagues review the different types of synaptic plasticity observed in the ACC and the implications of these forms of plasticity for pain processing. The anterior cingulate cortex (ACC) is activated in both acute and chronic pain. In this Review, we discuss increasing evidence from rodent studies that ACC activation contributes to chronic pain states and describe several forms of synaptic plasticity that may underlie this effect. In particular, one form of long-term potentiation (LTP) in the ACC, which is triggered by the activation of NMDA receptors and expressed by an increase in AMPA-receptor function, sustains the affective component of the pain state. Another form of LTP in the ACC, which is triggered by the activation of kainate receptors and expressed by an increase in glutamate release, may contribute to pain-related anxiety.

Key Points The transition from acute to chronic pain is associated with structural plasticity or circuit reorganisation at various points in somatosensory pain circuits. Peripheral sensory neurons and axons exhibit diverse types of structural changes that are specific to different types of chronic pain. Structural remodelling of synaptic contacts on spinal dorsal horn neurons is causally associated with nociceptive hypersensitivity. Neuropathic pain is associated with the reorganization of cortical sensory maps. Chronic pain is characterized by increased pain-related activation of emotional networks in the brain. Pain disrupts structural and functional brain connectivity, which can be restored with effective treatment. The mechanisms that underlie the transition of acute pain to a chronic intractable disorder are not well understood. In this Review, Kuner and Flor discuss how structural plasticity and reorganisation in somatosensory and emotional networks can contribute to chronic pain, integrating information available from animal models and human patients. Chronic pain is not simply a temporal continuum of acute pain. Studies on functional plasticity in neural circuits of pain have provided mechanistic insights and linked various modulatory factors to a change in perception and behaviour. However, plasticity also occurs in the context of structural remodelling and reorganisation of synapses, cells and circuits, potentially contributing to the long-term nature of chronic pain. This Review discusses maladaptive structural plasticity in neural circuits of pain, spanning multiple anatomical and spatial scales in animal models and human patients, and addresses key questions on structure–function relationships.

The central mechanisms underlying pain chronicity remain elusive. Here, we identify a reciprocal neuronal circuit in mice between the anterior cingulate cortex (ACC) and the ventral tegmental area (VTA) that mediates mutual exacerbation between hyperalgesia and allodynia and their emotional consequences and, thereby, the chronicity of neuropathic pain. ACC glutamatergic neurons (ACC Glu ) projecting to the VTA indirectly inhibit dopaminergic neurons (VTA DA ) by activating local GABAergic interneurons (VTA GABA ), and this effect is reinforced after nerve injury. VTA DA neurons in turn project to the ACC and synapse to the initial ACC Glu neurons to convey feedback information from emotional changes. Thus, an ACC Glu –VTA GABA –VTA DA –ACC Glu positive-feedback loop mediates the progression to and maintenance of persistent pain and comorbid anxiodepressive-like behavior. Disruption of this feedback loop relieves hyperalgesia and anxiodepressive-like behavior in a mouse model of neuropathic pain, both acutely and in the long term. This study identifies a positive-feedback loop between the ACC and the VTA that mediates the mutual exacerbation between hyperalgesia and comorbid anxiodepressive-like behaviors and, thereby, the chronicity of neuropathic pain.

The sensory, associative and limbic neocortical structures play a critical role in shaping incoming noxious inputs to generate variable pain perceptions. Technological advances in tracing circuitry and interrogation of pathways and complex behaviours are now yielding critical knowledge of neocortical circuits, cellular contributions and causal relationships between pain perception and its abnormalities in chronic pain. Emerging insights into neocortical pain processing suggest the existence of neocortical causality and specificity for pain at the level of subdomains, circuits and cellular entities and the activity patterns they encode. These mechanisms provide opportunities for therapeutic intervention for improved pain management.Neocortical circuits imparting specificity and causality to pain are not well understood. In this Review, Kuner and Tan discuss new insights into the contributions of diverse cerebral domains, their connectivity and their plasticity to the sensory and emotional aspects of pain.