Improving pain assessment in mice and rats with advanced videography and computational approaches

Accurately measuring pain in humans and rodents is essential to unravel the neurobiology of pain and discover effective pain therapeutics. However, given its inherently subjective nature, pain is nearly impossible to objectively assess. In the clinic, patients can articulate their pain experience using questionnaires and pain scales but self-reporting can be unreliable due to various psychological and social influences or difficulties for some patients to verbalize their experience (eg, infants, toddlers, and those with neurodevelopmental disorders). At the bench, these challenges are even more daunting as researchers rely on the behaviors of rodents to measure pain or pain relief. Given this, there is a growing realization among pain researchers, clinicians, and funding entities that these traditional approaches of assessing pain in rodents may be flawed. Importantly, these flaws may have contributed to several failed drugs that initially showed promise as analgesics and point toward inconsistencies in our understanding of basic pain neurobiology

The genetics of neuropathic pain from model organisms to clinical application

Neuropathic pain (NeuP) arises due to injury of the somatosensory nervous system and is both common and disabling, rendering an urgent need for non-addictive, effective new therapies. Given the high evolutionary conservation of pain, investigative approaches from Drosophila mutagenesis to human Mendelian genetics have aided our understanding of the maladaptive plasticity underlying NeuP. Successes include the identification of ion channel variants causing hyper-excitability and the importance of neuro-immune signaling. Recent developments encompass improved sensory phenotyping in animal models and patients, brain imaging, and electrophysiology-based pain biomarkers, the collection of large well-phenotyped population cohorts, neurons derived from patient stem cells, and high-precision CRISPR generated genetic editing. We will discuss how to harness these resources to understand the pathophysiological drivers of NeuP, define its relationship with comorbidities such as anxiety, depression, and sleep disorders, and explore how to apply these findings to the prediction, diagnosis, and treatment of NeuP in the clinic

Microglial Refinement of A-Fiber Projections in the Postnatal Spinal Cord Dorsal Horn Is Required for Normal Maturation of Dynamic Touch

Sensory systems are shaped in postnatal life by the refinement of synaptic connectivity. In the dorsal horn of the spinal cord, somatosensory circuits undergo postnatal activity-dependent reorganisation, including the refinement of primary afferent A-fibre terminals from superficial to deeper spinal dorsal horn laminae which is accompanied by decreases in cutaneous sensitivity. Here we show in the mouse that microglia, the resident immune cells in the CNS, phagocytose A-fibre terminals in superficial laminae in the first weeks of life. Genetic perturbation of microglial engulfment during the initial postnatal period in either sex prevents the normal process of A-fibre refinement and elimination, resulting in altered sensitivity of dorsal horn cells to dynamic tactile cutaneous stimulation, and behavioural hypersensitivity to dynamic touch. Thus, functional microglia are necessary for the normal postnatal development of dorsal horn sensory circuits. In the absence of microglial engulfment, superfluous A-fibre projections remain in the dorsal horn and the balance of sensory connectivity is disrupted, leading to lifelong hypersensitivity to dynamic touch.Significance statement Dynamic touch is the sensation of movement across the skin, transmitted by mechanosensory A-fibres, the myelinated primary afferents that respond to innocuous mechanical stimulation. The central terminals of these fibres are located in the deep laminae of the sensory spinal cord dorsal horn in the adult. However, in early life they are widespread and retract from the superficial laminae of the dorsal horn during normal postnatal development. The underlying mechanisms remain unknown. We found that microglia phagocytose superfluous A-fibres and furthermore, disruption of this process leads to long-term aberrant dynamic touch processing and behaviour. Microglia mediated refinement of A-fibres during the early postnatal period is therefore critical to both normal dorsal horn development and appropriate spatial encoding of dynamic touch

Gene Expression Profiling of Cutaneous Injured and Non-Injured Nociceptors in SNI Animal Model of Neuropathic Pain.

Nociceptors are a particular subtype of dorsal root ganglion (DRG) neurons that detect noxious stimuli and elicit pain. Although recent efforts have been made to reveal the molecular profile of nociceptors in normal conditions, little is known about how this profile changes in pathological conditions. In this study we exploited laser capture microdissection to specifically collect individual injured and non-injured nociceptive DRG neurons and to define their gene profiling in rat spared nerve injury (SNI) model of neuropathic pain. We found minimal transcriptional changes in non-injured neurons at 7 days after SNI. In contrast, several novel transcripts were altered in injured nociceptors, and the global signature of these LCM-captured neurons differed markedly from that the gene expression patterns found previously using whole DRG tissue following SNI. Pathway analysis of the transcriptomic profile of the injured nociceptors revealed oxidative stress as a key biological process. We validated the increase of caspase-6 (CASP6) in small-sized DRG neurons and its functional role in SNI- and paclitaxel-induced neuropathic pain. Our results demonstrate that the identification of gene regulation in a specific population of DRG neurons (e.g., nociceptors) is an effective strategy to reveal new mechanisms and therapeutic targets for neuropathic pain from different origins