Microglial Responses and Pain Behaviors Are Exacerbated by Chronic Sleep Deprivation in Rats with Chronic Pain Via Neuroinflammatory Pathways

Inhibition of S100A4 decreases neurotoxic astrocyte reactivity and attenuates neuropathic pain via the TLR4/NF-κB pathway in a rat model of spinal nerve ligation

S100A4 participates in inflammation and immune reactions in the central nervous system and is involved in the pathogenesis of multiple neurological disorders. It can affect the functions of astrocytes, microglia, infiltrating cells and neurons and further modulates neuronal plasticity and survival in the central nervous system. However, its impact on astrocyte phenotypes and neuropathic pain and the intrinsic mechanisms involved remain poorly understood. Here, we showed that S100A4 was markedly upregulated after spinal nerve ligation and was mainly expressed in neurons in the spinal dorsal horn. Transcriptional inhibition of S100A4 with niclosamide attenuated neuropathic pain after surgery. We found that astrocytes differentiated into C3-positive reactive populations, so-called neurotoxic (A1) astrocytes and identified differentially expressed genes and specific molecular expression signatures after ligation. Neurotoxic astrocyte reactivity is regulated by exogenous S100A4 in vitro, and targeted inhibition of S100A4 suppresses neurotoxic astrocyte proliferation in rats. Finally, we reported that TLR4/NF-κB signaling pathway is a downstream of S100A4 activation, and that specific depletion this pathway suppresses deleterious A1 astrocyte activation and further attenuates the development and maintenance of neuropathic pain after spinal nerve ligation. Thus, S100A4 in neurons plays a key role in neurotoxic astrocyte reactivity and can be targeted for treatment to prevent and alleviate neuropathic pain.

Glia: the cellular glue that binds circadian rhythms and sleep

Glia are increasingly appreciated as serving an important function in the control of sleep and circadian rhythms. Glial cells in Drosophila and mammals regulate daily rhythms of locomotor activity and sleep as well as homeostatic rebound following sleep deprivation. In addition, they contribute to proposed functions of sleep, with different functions mapping to varied glial subtypes. Here, we discuss recent findings in Drosophila and rodent models establishing a role of glia in circadian or sleep regulation of synaptic plasticity, brain metabolism, removal of cellular debris, and immune challenges. These findings underscore the relevance of glia for benefits attributed to sleep and have implications for understanding the neurobiological mechanisms underlying sleep and associated disorders.

TNF-α Enhanced Activity of Sympathetic Neurons in Superior Cervical Ganglion to Promote Chronic Sleep Deprivation-Related Hyperalgesia

The mechanisms underlying the association between sleep deprivation (SD) and hyperalgesia remain incompletely understood. In this study, the Modified Horizontal Platform Method was employed to induce chronic SD. Neuropathic pain was induced using chronic constriction injury of the sciatic nerve. Pain-like behaviors were assessed through measurements of mechanical allodynia and thermal hyperalgesia, while gait analysis was used to evaluate motor function. Immunofluorescence and western blot analyses were conducted to examine the expression of TNF-α, Iba-1, TH, neurons and c-Fos. Apoptosis was assessed using TUNEL staining. To explore anatomical connections, anterograde and retrograde tracer viruses were injected into the superior cervical ganglion (SCG) and the spinal cord, respectively. Local injection of 6-OHDA was used to ablate sympathetic neurons in the SCG, and R-7050 was administrated to block the TNF-α receptor. We found that chronic SD induced hyperalgesia in both normal and neuropathic pain model, accompanied by significant infiltration of microglia in the dorsal horn. TH expression and apoptotic cells were increased in the SCG following chronic SD. Viral tracer results demonstrated the existence of anatomical connections between the SCG and the spinal cord. Ablation of sympathetic innervation improved pain-like behaviors and reduced microglia, without affecting movement. Furthermore, chronic SD led to increased expression of TNF-α in sympathetic neurons, which was associated with heightened SCG activity. Blocking the TNF-α receptor ameliorated pain-like behaviors, decreased microglia, reduced apoptosis, lowered SCG activity. In conclusion, TNF-α enhanced the activity of sympathetic neurons in the SCG, promoting hyperalgesia related to chronic SD.

Sleep deprivation affects pain sensitivity by increasing oxidative stress and apoptosis in the medial prefrontal cortex of rats via the HDAC2-NRF2 pathway

Sleep is crucial for sustaining normal physiological functions, and sleep deprivation has been associated with increased pain sensitivity. The histone deacetylases (HDACs) are known to significantly regulate in regulating neuropathic pain, but their involvement in nociceptive hypersensitivity during sleep deprivation is still not fully understood. Utilizing a modified multi-platform water environment technique to establish a sleep deprivation model. We measured the expression levels of HDAC1/2 in the medial prefrontal cortex (mPFC) through immunoblotting and real-time quantitative PCR. The presence of pyroptosis was determined using a TUNEL assay. Suberoylanilide hydroxamic acid (SAHA), an HDAC inhibitor employed clinically, was injected into the peritoneal cavity to inhibit HDAC2 expression. Animal pain behaviors were evaluated by measuring paw withdrawal thresholds (PWTs) and paw withdrawal latencies (PWLs). Our findings indicate that sleep deprivation leads to increased nociceptive hypersensitivity, an upregulation of HDAC2 expression in the mPFC, a downregulation of the expression of nuclear factor erythroid 2-related factor 2 (NRF2), and changes in markers of oxidative stress in rats. SAHA, the HDAC inhibitor, enhanced NRF2 expression by inhibiting HDAC2, which consequently ameliorated oxidative stress and mitigated nociceptive hypersensitivity in rats. The incidence of apoptosis was found to be higher in the mPFC tissues of sleep deprivation rats, and the intraperitoneal administration of SAHA decreased this apoptosis. The co-injection of SAHA and the NRF2 inhibitor ML385 into sleep deprivation rats negated the beneficial effects of SAHA. In conclusion, HDAC2 is implicated in the induction of oxidative stress and apoptosis by suppressing NRF2 levels, thereby exacerbating nociceptive hypersensitivity in sleep deprivation rats.

Estrogen receptor α regulates the IKKs/NF-kB activity involved in the development of mechanical allodynia induced by REM sleep deprivation in rats

Several signaling pathways that converge in NF-kB activation have been linked to developing and maintaining different types of pathological pain. In addition, some mechanisms implied in the establishment of chronic pain have been demonstrated to have a sex-dependent correlation. This study aimed to determine if the IKKs/NF-kB signaling pathway is involved in establishing REM sleep deprivation (REMSD) induced mechanical allodynia in rats and its possible regulation depending on estradiol and estrogen receptors. Intrathecal administration of BMS-345541 or minocycline, two drugs that reduce the IKKs/NF-kB activity, avoided the development of mechanical allodynia in female but not in male rats subjected to 48 h of REMSD. Ovariectomy in female rats abolished the effect of BMS-345541 and minocycline. Meanwhile, the 17-β-estradiol restitution restored it. Intrathecal administration of MPP, a selective ERα antagonist, but not PHTPP, a selective ERβ antagonist, avoided the effect of BMS-345541 in female rats without hormonal manipulation. In addition, the transient run-down of ERα in female rats abolished the effect of BMS-345541. All data suggest an important role of ERα as a regulator of the IKKs/NF-kB activity. REMSD increased the ERα protein expression in the dorsal root ganglia and the dorsal spinal cord in females but not in male rats. Interestingly, ERα activation or ERα overexpression allowed the effect of BMS-345541 in male rats. Data suggest an important regulatory role of ERα in the IKKs/NF-kB activity on establishing mechanical allodynia induced by REMSD in female rats.

Nociplastic pain mechanisms and toll-like receptors as promising targets for its management

ABSTRACT

Nociplastic pain, characterized by abnormal pain processing without an identifiable organic cause, affects a significant portion of the global population. Unfortunately, current pharmacological treatments for this condition often prove ineffective, prompting the need to explore new potential targets for inducing analgesic effects in patients with nociplastic pain. In this context, toll-like receptors (TLRs), known for their role in the immune response to infections, represent promising opportunities for pharmacological intervention because they play a relevant role in both the development and maintenance of pain. Although TLRs have been extensively studied in neuropathic and inflammatory pain, their specific contributions to nociplastic pain remain less clear, demanding further investigation. This review consolidates current evidence on the connection between TLRs and nociplastic pain, with a specific focus on prevalent conditions like fibromyalgia, stress-induced pain, sleep deprivation-related pain, and irritable bowel syndrome. In addition, we explore the association between nociplastic pain and psychiatric comorbidities, proposing that modulating TLRs can potentially alleviate both pain syndromes and related psychiatric disorders. Finally, we discuss the potential sex differences in TLR signaling, considering the higher prevalence of nociplastic pain among women. Altogether, this review aims to shed light on nociplastic pain, its underlying mechanisms, and its intriguing relationship with TLR signaling pathways, ultimately framing the potential therapeutic role of TLRs in addressing this challenging condition.

A Narrative Review of the Reciprocal Relationship Between Sleep Deprivation and Chronic Pain: The Role of Oxidative Stress

Sleep is crucial for human health, insufficient sleep or poor sleep quality may negatively affect sleep function and lead to a state of sleep deprivation. Sleep deprivation can result in various health problems, including chronic pain. The intricate relationship between sleep and pain is complex and intertwined, with daytime pain affecting sleep quality and poor sleep increasing pain intensity. The article first describes the influence of sleep on the onset and development of pain, and then explores the impact of daytime pain intensity on nighttime sleep quality and subsequent pain thresholds. However, the primary emphasis is placed on the pivotal role of oxidative stress in this bidirectional relationship. Although the exact mechanisms underlying sleep and chronic pain are unclear, this review focuses on the role of oxidative stress. Numerous studies on sleep deprivation have demonstrated that it can lead to varying degrees of increased pain sensitivity, while chronic pain leads to sleep deprivation and further exacerbates pain. Further research on the role of oxidative stress in the mechanism of sleep deprivation-induced pain sensitization seems reasonable. This article comprehensively reviews the current research on the interrelationship between sleep deprivation, pain and the crucial role of oxidative stress.

Repeated closed-head mild traumatic brain injury-induced inflammation is associated with nociceptive sensitization

BACKGROUND

Individuals who have experienced mild traumatic brain injuries (mTBIs) suffer from several comorbidities, including chronic pain. Despite extensive studies investigating the underlying mechanisms of mTBI-associated chronic pain, the role of inflammation in long-term pain after mTBIs is not fully elucidated. Given the shifting dynamics of inflammation, it is important to understand the spatial-longitudinal changes in inflammatory processes following mTBIs and their effects on TBI-related pain.

METHODS

We utilized a recently developed transgenic caspase-1 luciferase reporter mouse model to monitor caspase-1 activation through a thinned skull window in the in vivo setting following three closed-head mTBI events. Organotypic coronal brain slice cultures and acutely dissociated dorsal root ganglion (DRG) cells provided tissue-relevant context of inflammation signal. Mechanical allodynia was assessed by mechanical withdrawal threshold to von Frey and thermal hyperalgesia withdrawal latency to radiant heat. Mouse grimace scale (MGS) was used to detect spontaneous or non-evoked pain. In some experiments, mice were prophylactically treated with MCC950, a potent small molecule inhibitor of NLRP3 inflammasome assembly to inhibit injury-induced inflammatory signaling. Bioluminescence spatiotemporal dynamics were quantified in the head and hind paws, and caspase-1 activation was confirmed by immunoblot. Immunofluorescence staining was used to monitor the progression of astrogliosis and microglial activation in ex vivo brain tissue following repetitive closed-head mTBIs.

RESULTS

Mice with repetitive closed-head mTBIs exhibited significant increases of the bioluminescence signals within the brain and paws in vivo for at least one week after each injury. Consistently, immunoblotting and immunofluorescence experiments confirmed that mTBIs led to caspase-1 activation, astrogliosis, and microgliosis. Persistent changes in MGS and hind paw withdrawal thresholds, indicative of pain states, were observed post-injury in the same mTBI animals in vivo. We also observed enhanced inflammatory responses in ex vivo brain slice preparations and DRG for at least 3 days following mTBIs. In vivo treatment with MCC950 significantly reduced caspase-1 activation-associated bioluminescent signals in vivo and decreased stimulus-evoked and non-stimulus evoked nociception.

CONCLUSIONS

Our findings suggest that the inflammatory states in the brain and peripheral nervous system following repeated mTBIs are coincidental with the development of nociceptive sensitization, and that these events can be significantly reduced by inhibition of NLRP3 inflammasome activation.

The Underlying Mechanisms of Sleep Deprivation Exacerbating Neuropathic Pain

Pain disrupts sleep, and sleep deprivation or interference can alter pain perception in animals and humans, for example by increasing sensitivity to pain. However, the mechanism by which sleep affects neuropathic pain remains unclear. In this review, we discuss the available evidence from the epidemiologic, clinical, and human, as well as laboratory studies. In previous studies, we have found that sleep deprivation affects various injurious systems, including opioids, dopaminergic, immune, orexins, hypothalamic-pituitary-adrenal axis, and adenosine. At the same time, these systems play a crucial role in neuropathic pain regulation. In the complex interactions between these neurobiological systems, there may be potential regulatory pathways through which sleep deprivation amplifies neuropathic pain. Because of the impact sleep problems and neuropathic pain can have on the patients' quality of life, studying the link between sleep and neuropathic pain is important for neuropathic pain prevention and public health.

8-O-acetyl shanzhiside methylester protects against sleep deprivation-induced cognitive deficits and anxiety-like behaviors by regulating NLRP3 and Nrf2 pathways in mice

Sleep deprivation (SD) is prevalent throughout the world, which has negative effects on cognitive abilities, and causing mood alterations. 8-O-acetyl shanzhiside methylester (8-OaS), a chief component in Lamiophlomis rotata (L. rotata) Kudo, possesses potent neuroprotective properties and analgesic effects. Here, we evaluated the alleviative effects of 8-OaS on memory impairment and anxiety in mice subjected to SD (for 72-h). Our results demonstrated that 8-OaS (0.2, 2, 20 mg/kg) administration dose-dependently ameliorated behavioral abnormalities in SD mice, accompanied with restored synaptic plasticity and reduced shrinkage and loss of hippocampal neurons. 8-OaS reduced the inflammatory response and oxidative stress injury in hippocampus caused by SD, which may be related to inhibition of NLRP3 inflammasome-mediated inflammatory process and activation of the Nrf2/HO-1 pathway. SD also led to increases in the expressions of TLR-4/MyD88, active NF-κB, pro-IL-1β, TNFα and MDA, as well as a decrease in the level of SOD in mice hippocampus, which were reversed by 8-OaS administration. Moreover, our molecular docking analyses showed that 8-OaS also has good affinity for NLRP3 and Nrf2 signaling pathways. These results suggested that 8-OaS could be used as a novel herbal medicine for the treatment of sleep loss and for use as a structural base for developing new drugs.

Effect of sleep loss on pain-New conceptual and mechanistic avenues

Introduction

Sleep disturbances increase pain sensitivity in clinical and preclinical settings, but the precise mechanisms are unknown. This represents a major public health issue because of the growing sleep deficiency epidemic fueled by modern lifestyle. To understand the neural pathways at the intersection between sleep and pain processes, it is critical to determine the precise nature of the sleep disruptions that increase pain and the specific component of the pain response that is targeted.

Methods

We performed a review of the literature about sleep disturbances and pain sensitivity in humans and rodents by taking into consideration the targeted sleep stage (REMS, non-NREMS, or both), the amount of sleep lost, and the different types of sleep disruptions (partial or total sleep loss, duration, sleep fragmentation or interruptions), and how these differences might affect distinct components of the pain response.

Results

We find that the effects of sleep disturbances on pain are highly conserved among species. The major driver for pain hypersensitivity appears to be the total amount of sleep lost, while REMS loss by itself does not seem to have a direct effect on pain sensitivity. Sleep loss caused by extended wakefulness preferentially increases pain perception, whereas interrupted and limited sleep strongly dysregulates descending controls such as DNIC, especially in women.

Discussion

We discuss the possible mechanisms involved, including an increase in inflammatory processes, a loss of nociceptive inhibitory pathways, and a defect in the cognitive processing of noxious input.