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Purpose of Review Chronic pain is a distressing condition that is ineffectively treated at present. In order to develop novel, more efficacious analgesics for chronic pain, a better understanding of the underlying mechanisms is required. Despite chronic pain initially being considered as a neurocentric process, the role of communication between immune cells and neurons has been shown to be essential to the modulation of chronic pain. In the spinal cord, chemokine-mediated communication between microglia and neurons has been shown to play a crucial mechanistic role in preclinical chronic pain. Recent Findings Here, we present convincing evidence specifically for the role of the neuronal chemokine, fractalkine and its receptor CX3CR1, which is expressed by microglia, in mediating neuronal/microglia crosstalk in the spinal cord in the context of preclinical pain behaviour. Summary In light of the compelling preclinical evidence and emerging clinical evidence, we consider the promising therapeutic potential of manipulating this signalling partnership for the treatment of chronic pain.

Highlights • Vincristine (VCR) induced nociception can be dose-limiting. • VCR increases permeability of the blood-spinal cord barrier. • Increased permeability results in infiltration of monocytes into the spinal cord. • Infiltrating monocytes express the pronociceptive enzyme Cathepsin S (CatS). • A centrally-penetrant CatS antagonist reduces VCR-induced nociception., Chemotherapy-induced neuropathic pain is a dose-limiting side effect of many cancer therapies due to their propensity to accumulate in peripheral nerves, which is facilitated by the permeability of the blood-nerve barrier. Preclinically, the chemotherapy agent vincristine (VCR) activates endothelial cells in the murine peripheral nervous system and in doing so allows the infiltration of monocytes into nerve tissue where they orchestrate the development of VCR-induced nociceptive hypersensitivity. In this study we demonstrate that VCR also activates endothelial cells in the murine central nervous system, increases paracellular permeability and decreases trans endothelial resistance. In in vivo imaging studies in mice, VCR administration results in trafficking of inflammatory monocytes through the endothelium. Indeed, VCR treatment affects the integrity of the blood-spinal cord-barrier as indicated by Evans Blue extravasation, disrupts tight junction coupling and is accompanied by the presence of monocytes in the spinal cord. Such inflammatory monocytes (Iba-1+ CCR2+ Ly6C+ TMEM119- cells) that infiltrate the spinal cord also express the pro-nociceptive cysteine protease Cathepsin S. Systemic treatment with a CNS-penetrant, but not a peripherally-restricted, inhibitor of Cathepsin S prevents the development of VCR-induced hypersensitivity, suggesting that infiltrating monocytes play a functional role in sensitising spinal cord nociceptive neurons. Our findings guide us towards a better understanding of central mechanisms of pain associated with VCR treatment and thus pave the way for the development of innovative antinociceptive strategies.

The circadian clock is based on a transcriptional feedback loop with an essential time delay before feedback inhibition. Previous work has shown that PERIOD (PER) proteins generate circadian time cues through rhythmic nuclear accumulation of the inhibitor complex and subsequent interaction with the activator complex in the feedback loop. Although this temporal manifestation of the feedback inhibition is the direct consequence of PER's cytoplasmic trafficking before nuclear entry, how this spatial regulation of the pacemaker affects circadian timing has been largely unexplored. Here we show that circadian rhythms, including wake-sleep cycles, are lengthened and severely unstable if the cytoplasmic trafficking of PER is disrupted by any disease condition that leads to increased congestion in the cytoplasm. Furthermore, we found that the time delay and robustness in the circadian clock are seamlessly generated by delayed and collective phosphorylation of PER molecules, followed by synchronous nuclear entry. These results provide clear mechanistic insight into why circadian and sleep disorders arise in such clinical conditions as metabolic and neurodegenerative diseases and aging, in which the cytoplasm is congested.

Supplemental Digital Content is Available in the Text. In inflammatory arthritis, we show an imbalance of proresolving bioactive lipid mediators in dorsal root ganglia, under persistent nociceptive states, and antinociceptive effects of maresin-1 administration., Rheumatoid arthritis-associated pain is poorly managed, often persisting when joint inflammation is pharmacologically controlled. Comparably, in the mouse K/BxN serum-transfer model of inflammatory arthritis, hind paw nociceptive hypersensitivity occurs with ankle joint swelling (5 days after immunisation) persisting after swelling has resolved (25 days after immunisation). In this study, lipid mediator (LM) profiling of lumbar dorsal root ganglia (DRG), the site of sensory neuron cell bodies innervating the ankle joints, 5 days and 25 days after serum transfer demonstrated a shift in specialised proresolving LM profiles. Persistent nociception without joint swelling was associated with low concentrations of the specialised proresolving LM Maresin 1 (MaR1) and high macrophage numbers in DRG. MaR1 application to cultured DRG neurons inhibited both capsaicin-induced increase of intracellular calcium ions and release of calcitonin gene-related peptide in a dose-dependent manner. Furthermore, in peritoneal macrophages challenged with lipopolysaccharide, MaR1 reduced proinflammatory cytokine expression. Systemic MaR1 administration caused sustained reversal of nociceptive hypersensitivity and reduced inflammatory macrophage numbers in DRG. Unlike gabapentin, which was used as positive control, systemic MaR1 did not display acute antihyperalgesic action. Therefore, these data suggest that MaR1 effects observed after K/BxN serum transfer relate to modulation of macrophage recruitment, more likely than to direct actions on sensory neurons. Our study highlights that, in DRG, aberrant proresolution mechanisms play a key role in arthritis joint pain dissociated from joint swelling, opening novel approaches for rheumatoid arthritis pain treatment.

Despite being a life-restricting condition, chronic pain remains poorly treated. A better understanding of the underlying mechanisms of chronic pain and thence development of innovative targets is therefore essential. Recently we have started to elucidate the importance of the role of microRNAs (miRs) in preclinical chronic pain. miRs are small, non-coding RNAs that regulate genes including those involved in nociceptive signalling. MiRs can exert their effects both intracellularly and extracellularly, the latter of which requires that they are released either as naked species or packaged in exosomes. Here we discuss changes in miR expression that occur in the dorsal root ganglia in murine models of chronic pain. We consider the downstream targets of changes in miR expression, including voltage-gated ion channels, as well as discuss extracellular consequences such as changes in macrophage phenotype that constitute of means by which neuron-immune cell crosstalk occurs. Such miR-mediated intracellular communication could provide a novel target for the treatment of chronic pain, which would be most effective if tailored to the specific cause of pain. Indeed, we conclude by reviewing evidence for the involvement of miRs in clinical cases of chronic pain, supporting the notion that tailored, miR-targeted therapies could prove to be an effective new strategy for the treatment of chronic pain clinically.

A combination of both in vivo and in vitro techniques is invaluable for studying semaphorin signaling in the avian central nervous system. Here we describe how both types of approaches can be used to compliment each other in order to unravel the role that semaphorins play during embryonic development and elucidate the functional consequences of semaphorin knockdown using RNA interference vectors. We describe and discuss specifically the use of in ovo electroporation and primary oculomotor neuron culture to identify the role of semaphorins in oculomotor neuron migration and assess functional consequences of semaphorin disruption in this system.

Chronic pain is a distressing condition, which is experienced even when the painful stimulus, whether surgery or disease related, has subsided. Current treatments for chronic pain show limited efficacy and come with a host of undesirable side‐effects, and thus there is a need for new, more effective therapies to be developed. The mechanisms underlying chronic pain are not fully understood at present, although pre‐clinical models have facilitated the progress of this understanding considerably in the last decade. The mechanisms underlying chronic pain were initially thought to be neurocentric. However, we now appreciate that non‐neuronal cells play a significant role in nociceptive signalling through their communication with neurons. One of the major signalling pathways, which mediates neuron/non‐neuronal communication, is chemokine signalling. In this review, we discuss selected chemokines that have been reported to play a pivotal role in the mechanisms underlying chronic pain in a variety of pre‐clinical models. Approaches that target each of the chemokines discussed in this review come with their advantages and disadvantages; however, the inhibition of chemokine actions is emerging as an innovative therapeutic strategy, which is now reaching the clinic, with the chemokine Fractalkine and its CX 3 CR 1 receptor leading the way. This article is part of the special article series “Pain”.

A core structural and functional motif of the vertebrate central nervous system is discrete clusters of neurons or ‘nuclei’. Yet the developmental mechanisms underlying this fundamental mode of organisation are largely unknown. We have previously shown that the assembly of motor neurons into nuclei depends on cadherin-mediated adhesion. Here, we demonstrate that the emergence of mature topography among motor nuclei involves a novel interplay between spontaneous activity, cadherin expression and gap junction communication. We report that nuclei display spontaneous calcium transients, and that changes in the activity patterns coincide with the course of nucleogenesis. We also find that these activity patterns are disrupted by manipulating cadherin or gap junction expression. Furthermore, inhibition of activity disrupts nucleogenesis, suggesting that activity feeds back to maintain integrity among motor neurons within a nucleus. Our study suggests that a network of interactions between cadherins, gap junctions and spontaneous activity governs neuron assembly, presaging circuit formation.

Interactions between the ventral premotor (PMv) and the primary motor cortex (M1) are crucial for transforming an object's geometrical properties, such as its size and shape, into a motor command suitable for grasp of the object. Recently, we showed that PMv interacts with M1 in a specific fashion, depending on the hand posture. However, the functional connectivity between PMv and M1 during the preparation of an actual grasp is still unknown. To address this issue, PMv–M1 interactions were tested while subjects were preparing to grasp different visible objects requiring either a precision grip or a whole hand grasp. A conditioning–test transcranial magnetic stimulation (TMS) paradigm was used: a test stimulus was applied over M1 either in isolation or after a conditioning stimulus delivered, at different delays, over the ipsilateral PMv. Motor evoked potentials (MEPs) were recorded in the first dorsal interosseus and abductor digiti minimi muscles, which show highly differentiated activity according to grasp. While subjects prepared to grasp, delivering a conditioning PMv pulse 6 or 8 msec before a test pulse over M1 strikingly facilitated MEPs in the specific muscles that were used in the upcoming grasp. This degree of facilitation correlated with the amount of muscle activity used later in the trial to grasp the objects. The present results demonstrate that, during grasp preparation, the PMv–M1 interactions are muscle-specific. PMv appears to process the object geometrical properties relevant for the upcoming grasp, and transmits this information to M1, which in turn generates a motor command appropriate for the grasp. We also reveal that the grasp-specific facilitation resulting from PMv–M1 interactions is differently related to the upcoming grasp muscle activity than is that from paired-pulse stimulation over M1, suggesting that these two TMS paradigms assess the excitability of cortico-cortical pathways devoted to the control of grasp at two different levels.

Spinal and bulbar muscular atrophy is a degenerative motor neuron disease caused by CAG repeat expansion in the androgen receptor gene. Montague et al. reveal an early increase in endoplasmic reticulum stress in a mouse model, and suggest that this pathway may be a therapeutic target for polyglutamine diseases., Spinal and bulbar muscular atrophy is an X-linked degenerative motor neuron disease caused by an abnormal expansion in the polyglutamine encoding CAG repeat of the androgen receptor gene. There is evidence implicating endoplasmic reticulum stress in the development and progression of neurodegenerative disease, including polyglutamine disorders such as Huntington’s disease and in motor neuron disease, where cellular stress disrupts functioning of the endoplasmic reticulum, leading to induction of the unfolded protein response. We examined whether endoplasmic reticulum stress is also involved in the pathogenesis of spinal and bulbar muscular atrophy. Spinal and bulbar muscular atrophy mice that carry 100 pathogenic polyglutamine repeats in the androgen receptor, and develop a late-onset neuromuscular phenotype with motor neuron degeneration, were studied. We observed a disturbance in endoplasmic reticulum-associated calcium homeostasis in cultured embryonic motor neurons from spinal and bulbar muscular atrophy mice, which was accompanied by increased endoplasmic reticulum stress. Furthermore, pharmacological inhibition of endoplasmic reticulum stress reduced the endoplasmic reticulum-associated cell death pathway. Examination of spinal cord motor neurons of pathogenic mice at different disease stages revealed elevated expression of markers for endoplasmic reticulum stress, confirming an increase in this stress response in vivo. Importantly, the most significant increase was detected presymptomatically, suggesting that endoplasmic reticulum stress may play an early and possibly causal role in disease pathogenesis. Our results therefore indicate that the endoplasmic reticulum stress pathway could potentially be a therapeutic target for spinal and bulbar muscular atrophy and related polyglutamine diseases.