Downregulation of the spinal dorsal horn clock gene Per1 expression leads to mechanical hypersensitivity via c-jun N-terminal kinase and CCL2 production in mice

Zika Virus Infection Alters the Circadian Clock Expression in Human Neuronal Monolayer and Neurosphere Cultures

Rhythmic regulations are virtually described in all physiological processes, including central nervous system development and immunologic responses. Zika virus (ZIKV), a neurotropic arbovirus, has been recently linked to a series of birth defects and neurodevelopmental disorders. Given the well-characterized role of the intrinsic cellular circadian clock within neurogenesis, cellular metabolism, migration, and differentiation among other processes, this study aimed to characterize the influence of ZIKV infection in the circadian clock expression in human neuronal cells. For this, in vitro models of human-induced neuroprogenitor cells (hiNPCs) and neuroblastoma cell line SH-SY5Y, cultured as monolayer and neurospheres, were infected by ZIKV, followed by RNA-Seq and RT-qPCR investigation, respectively. Targeted circadian clock components presented mRNA oscillations only after exogenous synchronizing stimuli (Forskolin) in SH-SY5Y monolayer culture. Interestingly, when these cells were grown as 3D-arranged neurospheres, an intrinsic oscillatory expression pattern was observed for some core clock components without any exogenous stimulation. The ZIKV infection significantly disturbed the mRNA expression pattern of core clock components in both neuroblastoma cell culture models, which was also observed in hiNPCs infected with different strains of ZIKV. The ZIKV-mediated desynchronization of the circadian clock expression in human cells might further contribute to the virus impairment of neuronal metabolism and function observed in adults and ZIKV-induced congenital syndrome. In vitro models of Zika virus (ZIKV) neuronal infection. Human neuroprogenitor cells were cultured as monolayer and neurospheres and infected by ZIKV. Monolayer-cultured cells received forskolin (FSK) as a coupling factor for the circadian clock rhythmicity, while 3D-arranged neurospheres showed an intrinsic oscillatory pattern in the circadian clock expression. The ZIKV infection affected the mRNA expression pattern of core clock components in both cell culture models. The ZIKV-mediated desynchronization of the circadian clock machinery might contribute to the impairment of neuronal metabolism and function observed in both adults (e.g., Guillain-Barré syndrome) and ZIKV-induced congenital syndrome (microcephaly). The graphical abstract has been created with Canva at the canva.com website.

Inhibition of Nuclear Receptor Related Orphan Receptor γ Ameliorates Mechanical Hypersensitivity Through the Suppression of Spinal Microglial Activation

Microglia are crucial in induction of central sensitization under a chronic pain state. Therefore, control of microglial activity is important to ameliorate nociceptive hypersensitivity. The nuclear receptor retinoic acid related orphan receptor γ (RORγ) contributes to the regulation of inflammation-related gene transcription in some immune cells, including T cells and macrophages. Their role and function in regulation of microglial activity and nociceptive transduction have yet to be elaborated. Treatment of cultured microglia with specific RORγ inverse agonists, SR2211 or GSK2981278, significantly suppressed lipopolysaccharide (LPS)-induced mRNA expression of pronociceptive molecules interleukin-1β (IL-1β), interleukin-6 (IL-6), and tumor necrosis factor (TNF). Intrathecal treatment of naïve male mice with LPS markedly induced mechanical hypersensitivity and upregulation of ionized calcium-biding adaptor molecule (Iba1) in the spinal dorsal horn, indicating microglial activation. In addition, intrathecal treatment with LPS significantly induced mRNA upregulation of IL-1β and IL-6 in the spinal dorsal horn. These responses were prevented by intrathecal pretreatment with SR2211. In addition, intrathecal administration of SR2211 significantly ameliorated established mechanical hypersensitivity and upregulation of Iba1 immunoreactivity in the spinal dorsal horn of male mice following peripheral sciatic nerve injury. The current findings demonstrate that blockade of RORγ in spinal microglia exerts anti-inflammatory effects, and that RORγ may be an appropriate target for the treatment of chronic pain.

Single-nucleus transcriptome analysis reveals transcriptional profiles of circadian clock and pain related genes in human and mouse trigeminal ganglion

Introduction

Clinical studies have revealed the existence of circadian rhythms in pain intensity and treatment response for chronic pain, including orofacial pain. The circadian clock genes in the peripheral ganglia are involved in pain information transmission by modulating the synthesis of pain mediators. However, the expression and distribution of clock genes and pain-related genes in different cell types within the trigeminal ganglion, the primary station of orofacial sensory transmission, are not yet fully understood.

Methods

In this study, data from the normal trigeminal ganglion in the Gene Expression Omnibus (GEO) database were used to identify cell types and neuron subtypes within the human and mouse trigeminal ganglion by single nucleus RNA sequencing analysis. In the subsequent analyses, the distribution of the core clock genes, pain-related genes, and melatonin and opioid-related genes was assessed in various cell clusters and neuron subtypes within the human and mouse trigeminal ganglion. Furthermore, the statistical analysis was used to compare the differences in the expression of pain-related genes in the neuron subtypes of trigeminal ganglion.

Results

The present study provides comprehensive transcriptional profiles of core clock genes, pain-related genes, melatonin-related genes, and opioid-related genes in different cell types and neuron subtypes within the mouse and human trigeminal ganglion. A comparative analysis of the distribution and expression of the aforementioned genes was conducted between human and mouse trigeminal ganglion to investigate species differences.

Discussion

Overall, the results of this study serve as a primary and valuable resource for exploring the molecular mechanisms underlying oral facial pain and pain rhythms.

Circadian rhythms and glial cells of the central nervous system

Glial cells are the most abundant cells in the central nervous system and play crucial roles in neural development, homeostasis, immunity, and conductivity. Over the past few decades, glial cell activity in mammals has been linked to circadian rhythms, the 24-h chronobiological clocks that regulate many physiological processes. Indeed, glial cells rhythmically express clock genes that cell-autonomously regulate glial function. In addition, recent findings in rodents have revealed that disruption of the glial molecular clock could impact the entire organism. In this review, we discuss the impact of circadian rhythms on the function of the three major glial cell types - astrocytes, microglia, and oligodendrocytes - across different locations within the central nervous system. We also review recent evidence uncovering the impact of glial cells on the body's circadian rhythm. Together, this sheds new light on the involvement of glial clock machinery in various diseases.

Targeting the chemokine ligand 2-chemokine receptor 2 axis provides the possibility of immunotherapy in chronic pain

Chronic pain affects patients' physical and psychological health and quality of life, entailing a tremendous public health challenge. Currently, drugs for chronic pain are usually associated with a large number of side effects and poor efficacy. Chemokines in the neuroimmune interface combine with their receptors to regulate inflammation or mediate neuroinflammation in the peripheral and central nervous system. Targeting chemokines and their receptor-mediated neuroinflammation is an effective means to treat chronic pain. In recent years, growing evidence has shown that the expression of chemokine ligand 2 (CCL2) and its main chemokine receptor 2 (CCR2) is involved in its occurrence, development and maintenance of chronic pain. This paper summarises the relationship between the chemokine system, CCL2/CCR2 axis, and chronic pain, and the CCL2/CCR2 axis changes under different chronic pain conditions. Targeting chemokine CCL2 and its chemokine receptor CCR2 through siRNA, blocking antibodies, or small molecule antagonists may provide new therapeutic possibilities for managing chronic pain.

The Circadian Clocks, Oscillations of Pain-Related Mediators, and Pain

The circadian clock is a biochemical oscillator that is synchronized with solar time. Normal circadian rhythms are necessary for many physiological functions. Circadian rhythms have also been linked with many physiological functions, several clinical symptoms, and diseases. Accumulating evidence suggests that the circadian clock appears to modulate the processing of nociceptive information. Many pain conditions display a circadian fluctuation pattern clinically. Thus, the aim of this review is to summarize the existing knowledge about the circadian clocks involved in diurnal rhythms of pain. Possible cellular and molecular mechanisms regarding the connection between the circadian clocks and pain are discussed.

Hypothalamic A11 Nuclei Regulate the Circadian Rhythm of Spinal Mechanonociception through Dopamine Receptors and Clock Gene Expression

Several types of sensory perception have circadian rhythms. The spinal cord can be considered a center for controlling circadian rhythms by changing clock gene expression. However, to date, it is not known if mechanonociception itself has a circadian rhythm. The hypothalamic A11 area represents the primary source of dopamine (DA) in the spinal cord and has been found to be involved in clock gene expression and circadian rhythmicity. Here, we investigate if the paw withdrawal threshold (PWT) has a circadian rhythm, as well as the role of the dopaminergic A11 nucleus, DA, and DA receptors (DR) in the PWT circadian rhythm and if they modify clock gene expression in the lumbar spinal cord. Naïve rats showed a circadian rhythm of the PWT of almost 24 h, beginning during the night–day interphase and peaking at 14.63 h. Similarly, DA and DOPAC’s spinal contents increased at dusk and reached their maximum contents at noon. The injection of 6-hydroxydopamine (6-OHDA) into the A11 nucleus completely abolished the circadian rhythm of the PWT, reduced DA tissue content in the lumbar spinal cord, and induced tactile allodynia. Likewise, the repeated intrathecal administration of D1-like and D2-like DA receptor antagonists blunted the circadian rhythm of PWT. 6-OHDA reduced the expression of Clock and Per1 and increased Per2 gene expression during the day. In contrast, 6-OHDA diminished Clock, Bmal, Per1, Per2, Per3, Cry1, and Cry2 at night. The repeated intrathecal administration of the D1-like antagonist (SCH-23390) reduced clock genes throughout the day (Clock and Per2) and throughout the night (Clock, Per2 and Cry1), whereas it increased Bmal and Per1 throughout the day. In contrast, the intrathecal injection of the D2 receptor antagonists (L-741,626) increased the clock genes Bmal, Per2, and Per3 and decreased Per1 throughout the day. This study provides evidence that the circadian rhythm of the PWT results from the descending dopaminergic modulation of spinal clock genes induced by the differential activation of spinal DR.

Regulation of Circadian Genes Nr1d1 and Nr1d2 in Sex-Different Manners during Liver Aging

Background: Circadian rhythm is associated with the aging process and sex differences; however, how age and sex can change circadian regulation systems remains unclear. Thus, we aimed to evaluate age- and sex-related changes in gene expression and identify sex-specific target molecules that can regulate aging. Methods: Rat livers were categorized into four groups, namely, young male, old male, young female, and old female, and the expression of several genes involved in the regulation of the circadian rhythm was confirmed by in silico and in vitro studies. Results: Gene Ontology and the Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyses showed that the expression of genes related to circadian rhythms changed more in males than in females during liver aging. In addition, differentially expressed gene analysis and quantitative real-time polymerase chain reaction/western blotting analysis revealed that Nr1d1 and Nr1d2 expression was upregulated in males during liver aging. Furthermore, the expression of other circadian genes, such as Arntl, Clock, Cry1/2, Per1/2, and Rora/c, decreased in males during liver aging; however, these genes showed various gene expression patterns in females during liver aging. Conclusions: Age-related elevation of Nr1d1/2 downregulates the expression of other circadian genes in males, but not females, during liver aging. Consequently, age-related upregulation of Nr1d1/2 may play a more crucial role in the change in circadian rhythms in males than in females during liver aging.

Estrogen modulation of the pronociceptive effects of serotonin on female rat trigeminal sensory neurons is timing dependent and dosage dependent and requires estrogen receptor alpha

ABSTRACT

The role of the major estrogen estradiol (E2) on orofacial pain conditions remains controversial with studies reporting both a pronociceptive and antinociceptive role of E2. E2 modulation of peripheral serotonergic activity may be one mechanism underlying the female prevalence of orofacial pain disorders. We recently reported that female rats in proestrus and estrus exhibit greater serotonin (5HT)-evoked orofacial nocifensive behaviors compared with diestrus and male rats. Further coexpression of 5HT 2A receptor mRNA in nociceptive trigeminal sensory neurons that express transient receptor potential vanilloid 1 ion channels contributes to pain sensitization. E2 may exacerbate orofacial pain through 5HT-sensitive trigeminal nociceptors, but whether low or high E2 contributes to orofacial pain and by what mechanism remains unclear. We hypothesized that steady-state exposure to a proestrus level of E2 exacerbates 5HT-evoked orofacial nocifensive behaviors in female rats, explored the transcriptome of E2-treated female rats, and determined which E2 receptor contributes to sensitization of female trigeminal sensory neurons. We report that a diestrus level of E2 is protective against 5HT-evoked orofacial pain behaviors, which increase with increasing E2 concentrations, and that E2 differentially alters several pain genes in the trigeminal ganglia. Furthermore, E2 receptors coexpressed with 5HT 2A and transient receptor potential vanilloid 1 and enhanced capsaicin-evoked signaling in the trigeminal ganglia through estrogen receptor α. Overall, our data indicate that low, but not high, physiological levels of E2 protect against orofacial pain, and we provide evidence that estrogen receptor α receptor activation, but not others, contributes to sensitization of nociceptive signaling in trigeminal sensory neurons.

Time-of-day dependent changes in guinea pig bladder afferent mechano-sensitivity

The voiding of urine has a clear circadian rhythm with increased voiding during active phases and decreased voiding during inactive phases. Bladder spinal afferents play a key role in the regulation of bladder storage and voiding, but it is unknown whether they exhibit themselves a potential circadian rhythm. Therefore, this study aimed to determine the mechano- and chemo- sensitivity of three major bladder afferent classes at two opposite day-night time points. Adult female guinea pigs underwent conscious voiding monitoring and bladder ex vivo single unit extracellular afferent recordings at 0300 h and 1500 h to determine day-night modulation of bladder afferent activity. All guinea pigs voided a higher amount of urine at 1500 h compared to 0300 h. This was due to an increased number of voids at 1500 h. The mechano-sensitivity of low- and high-threshold stretch-sensitive muscular-mucosal bladder afferents to mucosal stroking and stretch was significantly higher at 1500 h compared to 0300 h. Low-threshold stretch-insensitive mucosal afferent sensitivity to stroking was significantly higher at 1500 h compared to 0300 h. Further, the chemosensitivity of mucosal afferents to N-Oleoyl Dopamine (endogenous TRPV1 agonist) was also significantly increased at 1500 h compared to 0300 h. This data indicates that bladder afferents exhibit a significant time-of-day dependent variation in mechano-sensitivity which may influence urine voiding patterns. Further studies across a 24 h period are warranted to reveal potential circadian rhythm modulation of bladder afferent activity.

Systems and Circuits Linking Chronic Pain and Circadian Rhythms

Research over the last 20 years regarding the link between circadian rhythms and chronic pain pathology has suggested interconnected mechanisms that are not fully understood. Strong evidence for a bidirectional relationship between circadian function and pain has been revealed through inflammatory and immune studies as well as neuropathic ones. However, one limitation of many of these studies is a focus on only a few molecules or cell types, often within only one region of the brain or spinal cord, rather than systems-level interactions. To address this, our review will examine the circadian system as a whole, from the intracellular genetic machinery that controls its timing mechanism to its input and output circuits, and how chronic pain, whether inflammatory or neuropathic, may mediate or be driven by changes in these processes. We will investigate how rhythms of circadian clock gene expression and behavior, immune cells, cytokines, chemokines, intracellular signaling, and glial cells affect and are affected by chronic pain in animal models and human pathologies. We will also discuss key areas in both circadian rhythms and chronic pain that are sexually dimorphic. Understanding the overlapping mechanisms and complex interplay between pain and circadian mediators, the various nuclei they affect, and how they differ between sexes, will be crucial to move forward in developing treatments for chronic pain and for determining how and when they will achieve their maximum efficacy.

Can chronopharmacology improve the therapeutic management of neurological diseases?

The importance of circadian rhythm dysfunctions in the pathophysiology of neurological diseases has been highlighted recently. Chronopharmacology principles imply that tailoring the timing of treatments to the circadian rhythm of individual patients could optimize therapeutic management. According to these principles, chronopharmacology takes into account the individual differences in patients' clocks, the rhythmic changes in the organism sensitivity to therapeutic and side effects of drugs, and the predictable time variations of disease. This review examines the current literature on chronopharmacology of neurological diseases focusing its scope on epilepsy, Alzheimer and Parkinson diseases, and neuropathic pain, even if other neurological diseases could have been analyzed. While the results of the studies discussed in this review point to a potential therapeutic benefit of chronopharmacology in neurological diseases, the field is still in its infancy. Studies including a sufficiently large number of patients and measuring gold standard markers of the circadian rhythmicity are still needed to evaluate the beneficial effect of administration times over the 24-hour day but also of clock modulating drugs.

MicroRNA-19b predicts widespread pain and posttraumatic stress symptom risk in a sex-dependent manner following trauma exposure

Posttraumatic widespread pain (PTWP) and posttraumatic stress symptoms (PTSS) are frequent comorbid sequelae of trauma that occur at different rates in women and men. We sought to identify microRNA (miRNA) that may contribute to sex-dependent differences in vulnerability to these outcomes. Monte Carlo simulations (x10,000) identified miRNA in which predicted targeting of PTWP or PTSS genes was most enriched. Expression of the leading candidate miRNA to target PTWP/PTSS-related genes, miR-19b, has been shown to be influenced by estrogen and stress exposure. We evaluated whether peritraumatic miR-19b blood expression levels predicted PTWP and PTSS development in women and men experiencing trauma of motor vehicle collision (n = 179) and in women experiencing sexual assault trauma (n = 74). A sex-dependent relationship was observed between miR-19b expression levels and both PTWP (β = -2.41, P = 0.034) and PTSS (β = -3.01, P = 0.008) development 6 months after motor vehicle collision. The relationship between miR-19b and PTSS (but not PTWP) was validated in sexual assault survivors (β = -0.91, P = 0.013). Sex-dependent expression of miR-19b was also observed in blood and nervous tissue from 2 relevant animal models. Furthermore, in support of increasing evidence indicating a role for the circadian rhythm (CR) in PTWP and PTSS pathogenesis, miR-19b targets were enriched in CR gene transcripts. Human cohort and in vitro analyses assessing miR-19b regulation of key CR transcripts, CLOCK and RORA, supported the potential importance of miR-19b to regulating the CR pathway. Together, these results highlight the potential role that sex-dependent expression of miR-19b might play in PTWP and PTSS development after trauma/stress exposure.

Microarray analyses of the dorsal root ganglia support a role for innate neuro-immune pathways in persistent pain in experimental osteoarthritis

OBJECTIVE

Following destabilization of the medial meniscus (DMM), mice develop experimental osteoarthritis (OA) and associated pain behaviors that are dependent on the stage of disease. We aimed to describe changes in gene expression in knee-innervating dorsal root ganglia (DRG) after surgery, in order to identify molecular pathways associated with three pre-defined pain phenotypes: "post-surgical pain", "early-stage OA pain", and "persistent OA pain".

DESIGN

We performed DMM or sham surgery in 10-week old male C57BL/6 mice and harvested L3-L5 DRG 4, 8, and 16 weeks after surgery or from age-matched naïve mice (n = 3/group). RNA was extracted and an Affymetrix Mouse Transcriptome Array 1.0 was performed. Three pain phenotypes were defined: "post-surgical pain" (sham and DMM 4-week vs 14-week old naïve), "early OA pain" (DMM 4-week vs sham 4-week), and "persistent OA pain" (DMM 8- and 16-week vs naïve and sham 8- and 16-week). 'Top hit' genes were defined as P < 0.001. Pathway analysis (Ingenuity Pathway Analysis) was conducted using differentially expressed genes defined as P < 0.05. In addition, we performed qPCR for Ngf and immunohistochemistry for F4/80+ macrophages in the DRG.

RESULTS

For each phenotype, top hit genes identified a small number of differentially expressed genes, some of which have been previously associated with pain (7/67 for "post-surgical pain"; 2/14 for "early OA pain"; 8/37 for "persistent OA pain"). Overlap between groups was limited, with 8 genes differentially regulated (P < 0.05) in all three phenotypes. Pathway analysis showed that in the persistent OA pain phase many of the functions of differentially regulated genes are related to immune cell recruitment and activation. Genes previously linked to OA pain (CX3CL1, CCL2, TLR1, and NGF) were upregulated in this phenotype and contributed to activation of the neuroinflammation canonical pathway. In separate sets of mice, we confirmed that Ngf was elevated in the DRG 8 weeks after DMM (P = 0.03), and numbers of F4/80+ macrophages were increased 16 weeks after DMM (P = 0.002 vs Sham).

CONCLUSION

These transcriptomics findings support the idea that distinct molecular pathways discriminate early from persistent OA pain. Pathway analysis suggests neuroimmune interactions in the DRG contribute to initiation and maintenance of pain in OA.

Effects of circadian rhythm disorder on the hippocampus of SHR and WKY rats

The present study investigated the effects of circadian rhythm disorder (CRD) on the hippocampus of SHR and WKY rats. Male SHR rats (n = 27) and WKY rats (n = 27) were randomly divided into six groups: SHR and WKY normal (N)CR, SHR and WKY CRD 16/8 (CRD16/8), and SHR and WKY CRD 12/12 (CRD12/12). Activity patterns were adjusted using different photoperiods over 90 days and any changes were recorded. Rats were tested in the Morris water maze and in a novel object recognition experiment; serologic analysis, magnetic resonance imaging (diffusion tensor imaging + arterial spin labeling), hippocampal Nissl staining, Fluoro-Jade B staining, and immunohistochemistry were also performed. The results showed that both types of inverted photoperiod reduced CR amplitude and prolonged the circadian period. CRD and hypertension reduced memory performance and novel object recognition and preference. The decreases in memory and preference indices were greater in rats in the CRD12/12 group compared to the CRD16/8 group. CRD and hypertension decreased fractional anisotropy values, the number of neurons and astrocytes in the hippocampus, and the expression of brain-derived neurotrophic factor and synapsin 1; it also enhanced the degeneration of neurons and microglia and reduced blood flow in the hippocampus, and increased nuclear factor κB, caspase, neuron-specific enolase, and interleukin-6 levels. These findings reveal a biological basis for the link between CRD and cognitive decline, which has implications for CRD caused by shift work and other factors.

Network and pathway-based analysis of microRNA role in neuropathic pain in rat models

The molecular mechanisms underlying neuropathic pain (NP) remain poorly understood. Emerging evidence has suggested the role of microRNAs (miRNAs) in the initiation and development of NP, but the specific effects of miRNAs in NP are largely unknown. Here, we use network- and pathway-based methods to investigate NP-induced miRNA changes and their biological functions by conducting a systematic search through multiple electronic databases. Thirty-seven articles meet the inclusion criteria. Venn analysis and target gene forecasting are performed and the results indicate that 167 overlapping target genes are co-regulated by five down-regulated miRNAs (rno-miR-183, rno-miR-96, rno-miR-30b, rno-miR-150 and rno-miR-206). Protein-protein interaction network analysis shows that 77 genes exhibit interactions, with cyclic adenosine monophosphate (cAMP)-dependent protein kinase catalytic subunit beta (degree = 11) and cAMP-response element binding protein 1 (degree = 10) having the highest connectivity degree. Gene ontology analysis shows that these target genes are enriched in neuron part, neuron projection, somatodendritic compartment and nervous system development. Moreover, analysis of Kyoto Encyclopedia of Genes and Genomes reveals that three pathways, namely, axon guidance, circadian entrainment and insulin secretion, are significantly enriched. In addition, rno-miR-183, rno-miR-96, rno-miR-30b, rno-miR-150 and rno-miR-206 are consistently down-regulated in the NP models, thus constituting the potential biomarkers of this disease. Characterizing these miRNAs and their target genes paves way for their future use in clinical practice.

Stimulation of nuclear receptor REV-ERBs suppresses production of pronociceptive molecules in cultured spinal astrocytes and ameliorates mechanical hypersensitivity of inflammatory and neuropathic pain of mice

The orphan nuclear receptors REV-ERBα and REV-ERBβ (REV-ERBs) are crucial in the regulation of inflammatory-related gene transcription in astroglioma cells, but their role in nociceptive transduction has yet to be elaborated. Spinal dorsal horn astrocytes contribute to the maintenance of chronic pain. Treatment of cultured spinal astrocytes with specific REV-ERBs agonists SR9009 or GSK4112 significantly prevented lipopolysaccharide (LPS)-induced mRNA upregulation of pronociceptive molecules interleukin-1β (IL-1β) mRNA, interleukin-6 (IL-6) mRNA and matrix metalloprotease-9 (MMP-9) mRNA, but not CCL2 mRNA expression. Treatment with SR9009 also blocked tumor necrosis factor-induced IL-1β mRNA, IL-6 mRNA and MMP-9 mRNA. In addition, treatment with SR9009 significantly blocked LPS-induced upregulation of IL-1β protein, IL-6 protein and MMP-9 activity. The inhibitory effects of SR9009 on LPS-induced expression of pronociceptive molecules were blocked by knockdown of REV-ERBs expression with short interference RNA, confirming that SR9009 exerts its effect through REV-ERBs. Intrathecal LPS treatment in male mice induces hind paw mechanical hypersensitivity, and upregulation of IL-1β mRNA, IL-6 mRNA and glial fibrillary acidic protein (GFAP) expression in spinal dorsal horn. Intrathecal pretreatment of SR9009 prevented the onset of LPS-induced mechanical hypersensitivity, cytokine expression and GFAP expression. Intrathecal injection of SR9009 also ameliorated mechanical hypersensitivity during the maintenance phase of complete Freund's adjuvant-induced inflammatory pain and partial sciatic nerve ligation-, paclitaxel-, and streptozotocin-induced neuropathy in mice. The current findings suggest that spinal astrocytic REV-ERBs could be critical in the regulation of nociceptive transduction through downregulation of pronociceptive molecule expression. Thus, spinal REV-ERBs could be an effective therapeutic target in the treatment of chronic pain.

Emerging relevance of circadian rhythms in headaches and neuropathic pain

Circadian rhythms of physiology are the keys to health and fitness, as dysregulation, by genetic mutations or environmental factors, increases disease risk and aggravates progression. Molecular and physiological studies have shed important light on an intrinsic clock that drives circadian rhythms and serves essential roles in metabolic homoeostasis, organ physiology and brain functions. One exciting new area in circadian research is pain, including headache and neuropathic pain for which new mechanistic insights have recently emerged. For example, cluster headache is an intermittent pain disorder with an exceedingly precise circadian timing, and preliminary evidence is emerging linking several circadian components (eg, Clock and Nr1d1) with the disease. In this review, we first discuss the broad metabolic and physiological relevance of the circadian timing system. We then provide a detailed review of the circadian relevance in pain disease and physiology, including cluster headache, migraine, hypnic headache and neuropathic pain. Finally, we describe potential therapeutic implications, including existing pain medicines and novel clock-modulating compounds. The physiological basis for the circadian rhythms in pain is an exciting new area of research with profound basic and translational impact.

Circadian control of pain and neuroinflammation

The importance of a neuroinflammatory response to the development and maintenance of inflammatory and neuropathic pain have been highlighted in recent years. Inflammatory cells contributing to this response include circulating immune cells such as monocytes, T and B lymphocytes, and neutrophils, as well as microglia in the central nervous system. Pain signals are transmitted via sensory neurons in the peripheral nervous system, which express various receptors and channels that respond to mediators secreted from these inflammatory cells. Chronobiological rhythms, which include the 24-hr circadian cycle, have recently been shown to regulate both nervous and immune cell activity and function. This review examines the current literature on chronobiological control of neuroinflammatory processes, with a focus on inflammatory and neuropathic pain states. While the majority of this work has stemmed from observational studies in humans, recent advances in using animal models have highlighted distinct mechanisms underlying these interactions. Better understanding interactions between the circadian and neuroimmune systems can help guide the development of new treatments and provide improved care for patients suffering from acute and chronic pain.

Stimulation of spinal dorsal horn β2-adrenergic receptor ameliorates neuropathic mechanical hypersensitivity through a reduction of phosphorylation of microglial p38 MAP kinase and astrocytic c-jun N-terminal kinase

The noradrenaline-adrenergic system has a crucial role in controlling nociceptive transduction at the spinal level. While α-adrenergic receptors are known to regulate nociceptive neurotransmitter release at the spinal presynaptic level, it is not entirely clear whether β-adrenergic receptors are involved in controlling pain transduction at the spinal level as well. The current study elucidated a role of β-adrenergic receptors in neuropathic pain in mice following a partial sciatic nerve ligation (PSNL). In addition, the cellular and intracellular signaling cascade induced by β-adrenergic receptors in neuropathic mice was elaborated. Intrathecal injection of isoproterenol (1 nmol), a nonselective β-adrenergic receptor agonist, briefly ameliorated hind paw mechanical hypersensitivity of PSNL mice. Isoproterenol's antinociceptive effect was mediated through β2-adrenergic receptors since pretreatment with ICI118551, a selective β2-adrenergic receptor antagonist, but not with CGP20712A, a selective β1-adrenergic receptor antagonist, significantly attenuated isoproterenol's effect. Furthermore, intrathecal treatment with a selective β2-adrenergic receptor agonist, terbutaline, but not a selective β1-adrenergic receptor agonist, dobutamine, also significantly ameliorated neuropathic pain. Fourteen days after PSNL, increased phosphorylation of both p38 Mitogen-activated protein kinase (MAPK) in microglia and c-jun N-terminal kinase (JNK) in astrocytes of ipsilateral spinal dorsal horn were observed. Phosphorylation of both microglial p38 MAPK and astrocytic JNK were downregulated by stimulation of the β2-adrenergic receptor. Together, these results suggest that spinal β2-adrenergic receptor have an inhibitory role in neuropathic nociceptive transduction at the spinal level through a downregulation of glial activity, perhaps through modulation of MAP kinases phosphorylation. Thus, targeting of β2-adrenergic receptors could be an effective therapeutic strategy in treating neuropathic pain.