Neuron-restrictive silencer factor in periaqueductal gray contributes to remifentanil-induced postoperative hyperalgesia via repression of the mu-opioid receptor

How do opioids control pain circuits in the brainstem during opioid-induced disorders and in chronic pain? Implications for the treatment of chronic pain

ABSTRACT

Brainstem areas involved in descending pain modulation are crucial for the analgesic actions of opioids. However, the role of opioids in these areas during tolerance, opioid-induced hyperalgesia (OIH), and in chronic pain settings remains underappreciated. We conducted a revision of the recent studies performed in the main brainstem areas devoted to descending pain modulation with a special focus on the medullary dorsal reticular nucleus (DRt), as a distinctive pain facilitatory area and a key player in the diffuse noxious inhibitory control paradigm. We show that maladaptive processes within the signaling of the µ-opioid receptor (MOR), which entail desensitization and a switch to excitatory signaling, occur in the brainstem, contributing to tolerance and OIH. In the context of chronic pain, the alterations found are complex and depend on the area and model of chronic pain. For example, the downregulation of MOR and δ-opioid receptor (DOR) in some areas, including the DRt, during neuropathic pain likely contributes to the inefficacy of opioids. However, the upregulation of MOR and DOR, at the rostral ventromedial medulla, in inflammatory pain models, suggests therapeutic avenues to explore. Mechanistically, the rationale for the diversity and complexity of alterations in the brainstem is likely provided by the alternative splicing of opioid receptors and the heteromerization of MOR. In conclusion, this review emphasizes how important it is to consider the effects of opioids at these circuits when using opioids for the treatment of chronic pain and for the development of safer and effective opioids.

LncRNA MRAK159688 facilitates morphine tolerance by promoting REST-mediated inhibition of mu opioid receptor in rats

Morphine tolerance (MT) caused by the long-term use of morphine is a major medical problem. The molecular mechanism of morphine tolerance remains elusive. Here, we established a morphine tolerance model in rats and verified whether the long noncoding RNA (lncRNA) MRAK159688 is involved in morphine tolerance and its specific molecular mechanism. We show the significant upregulation of MRAK159688 expression in the spinal cord of morphine-tolerant rats. Overexpression of MRAK159688 by a lentivirus reduces the analgesic efficacy of morphine and induces pain behavior. Downregulation of MRAK159688 using a small interfering RNA (siRNA) attenuates the formation of morphine tolerance, partially reverses the development of morphine tolerance and alleviates morphine-induced hyperalgesia. MRAK159688 is located in the nucleus and cytoplasm of neurons, and it colocalizes with repressor element-1 silencing transcription factor (REST) in the nucleus. MRAK159688 potentiates the expression and function of REST, thereby inhibiting the expression of mu opioid receptor (MOR) and subsequently inducing morphine tolerance. Moreover, REST overexpression blocks the effects of MRAK159688 siRNA on relieving morphine tolerance. In general, chronic morphine administration-mediated upregulation of MRAK159688 in the spinal cord contributes to morphine tolerance and hyperalgesia by promoting REST-mediated inhibition of MOR. MRAK159688 downregulation may represent a novel RNA-based therapy for morphine tolerance.

Pleiotrophin Potentiates Sevoflurane Anesthesia-induced Learning Deficits in Mice

Postoperative cognitive dysfunction (POCD) is a major postoperative neurological complication in children and the elderly. However, the detailed mechanisms underlying this process remain unclear. This study aims to investigate the effect of pleiotrophin on sevoflurane-induced neuroinflammation and cognitive impairment. The novel object recognition test was performed to evaluate the cognitive and motor function of aged C57BL/6 (wild-type, WT) and pleiotrophin-knockout mice treated with sevoflurane. Small molecule inhibitors targeting receptor protein tyrosine phosphatase (RPTP) β/ζ, a pleiotrophin receptor, were used to ameliorate cognitive dysfunction. Sevoflurane treatment induced cognitive dysfunction and motor impairment in aged WT mice. Sevoflurane anesthesia induced the upregulation of certain inflammatory cytokines. Pleiotrophin knockout ameliorated the sevoflurane-induced cognitive dysfunction and motor impairment in vivo. Treatment with small molecule inhibitors targeting RPTP β/ζ inhibited sevoflurane-induced neuroinflammation. In summary, pleiotrophin was shown to potentiate sevoflurane anesthesia-induced cognitive dysfunction and learning deficits in mice.

Analgesic Efficacy of a Combination of Fentanyl and a Japanese Herbal Medicine "Yokukansan" in Rats with Acute Inflammatory Pain

Background: Fentanyl can induce acute opioid tolerance and postoperative hyperalgesia when administered at a single high dose; thus, this study examined the analgesic efficacy of a combination of fentanyl and Yokukansan (YKS). Methods: Rats were divided into control, formalin-injected (FOR), YKS-treated+FOR (YKS), fentanyl-treated+FOR (FEN), and YKS+FEN+FOR (YKS+FEN) groups. Acute pain was induced via subcutaneous injection of formalin into the paw. The time engaged in pain-related behavior was measured. Results: In the early (0–10 min) and intermediate (10–20 min) phases, pain-related behavior in the YKS+FEN group was significantly inhibited compared with the FOR group. In the late phase (20–60 min), pain-related behavior in the FEN group was the longest and significantly increased compared with the YKS group. We explored the influence on the extracellular signal-regulated kinase (ERK) pathway in the spinal cord, and YKS suppressed the phosphorylated ERK expression, which may be related to the analgesic effect of YKS in the late phase. Conclusions: These findings suggest that YKS could reduce the use of fentanyl and combined use of YKS and fentanyl is considered clinically useful.

Dezocine attenuates the remifentanil-induced postoperative hyperalgesia by inhibition of phosphorylation of CaMKⅡα

Remifentanil, ultra-short-acting μ-opioid receptor agonist, has the greatest advantage in analgesia but could increase postoperative pain scores and induces postoperative hyperalgesia. Dezocine is a mixed opioid receptor partial agonist/antagonist and has been used for postoperative hyperalgesia management in clinical patients,but the potential molecular mechanism is still unclear. Ca²⁺/calmodulin-dependent protein kinase Ⅱ(CaMKⅡ) has been reported involved in remifentanil-induced hyperalgesia (RIH) in previous studies, but the relationship between CaMKⅡ and dezocine in RIH is still unclear. To investigate the mechanism of dezocine in RIH, we used a remifentanil induced postoperative hyperalgesia (RIPH) in incisional pain model of mouse. We subcutaneously infused remifentanil (40 μg/kg) to induce postoperative hyperalgesia. Dezocine (1.5 mg/kg, 3.0 mg/kg, and 6.0 mg/kg) was infused subcutaneously with remifentanil using the apparatus pump for 30 min. Paw withdrawal thermal latency (PWTL) and paw withdrawal mechanical threshold (PWMT) were used to assess thermal hyperalgesia and mechanical allodynia. Western blotting analysis and immunohistochemistry analysis were used to assess the expression of phosphorylated CaMKⅡα (p-CaMKⅡα) in somatosensory cortex, hippocampus and spinal cord. Subcutaneous infusion of remifentanil enhanced postoperative pain induced by surgical incision and increased PWTL and PWMT. Dezocine dose-dependently decreased the PWTL and PWMT in RIPH model. Correlating with behavioral effects, dezocine inhibited remifentanil-induced up-regulation of p-CaMKⅡα expression in somatosensory cortex, hippocampus and spinal cord. Dezocine could attenuate RIPH by suppressing p-CaMKⅡα.

Ketamine reduces remifentanil-induced postoperative hyperalgesia mediated by CaMKII-NMDAR in the primary somatosensory cerebral cortex region in mice

Remifentanil is commonly used clinically for perioperative pain relief, but it may induce postoperative hyperalgesia. Low doses of ketamine have remained a common choice in clinical practice, but the mechanisms of ketamine have not yet been fully elucidated. In this study, we examined the possible effects of ketamine on calcium/calmodulin-dependent protein kinase II α (CaMKIIα) and N-methyl-d-aspartate receptor (NMDAR) subunit NR2B in a mouse model of remifentanil-induced postoperative hyperalgesia (RIPH) in the primary somatosensory cerebral cortex (SI) region. The paw withdrawal mechanical threshold (PWMT) and paw withdrawal thermal latency (PWTL) were used to assess mechanical allodynia and thermal hyperalgesia, respectively, before and after intraoperative remifentanil administration. Before surgery, mice received intrathecal injections of the following drugs: ketamine, NMDA, BayK8644 (CaMKII activator), and KN93 (CaMKII inhibitor). Immunofluorescence was performed to determine the anatomical location and expression of activated CaMKIIα, phosphorylated CaMKIIα (p-CaMKIIα). Additionally, western blotting was performed to assess p-CaMKIIα and NMDAR expression levels in the SI region. Remifentanil decreased the PWMT and PWTL at 0.5 h, 2 h, and 5 h and increased p-CaMKIIα expression in the SI region. Ketamine increased the PWMT and PWTL and reversed the p-CaMKIIα upregulation. Both BayK8644 and NMDA reversed the effect of ketamine, decreased the PWMT and PWTL, and upregulated p-CaMKIIα expression. In contrast, KN93 enhanced the effect of ketamine by reducing hyperalgesia and downregulating p-CaMKIIα expression. These results suggested that ketamine reversed RIPH by inhibiting the phosphorylation of CaMKIIα and the NMDA receptor in the SI region in mice.

Opioid-induced hyperalgesia in clinical anesthesia practice: what has remained from theoretical concepts and experimental studies?

PURPOSE OF REVIEW

This article reviews the phenomenon of opioid-induced hyperalgesia (OIH) and its implications for clinical anesthesia. The goal of this review is to give an update on perioperative prevention and treatment strategies, based on findings in preclinical and clinical research.

RECENT FINDINGS

Several systems have been suggested to be involved in the pathophysiology of OIH with a focus on the glutaminergic system. Very recently preclinical data revealed that peripheral μ-opioid receptors (MORs) are key players in the development of OIH and acute opioid tolerance (AOT). Peripheral MOR antagonists could, thus, become a new prevention/treatment option of OIH in the perioperative setting. Although the impact of OIH on postoperative pain seems to be moderate, recent evidence suggests that increased hyperalgesia following opioid treatment correlates with the risk of developing persistent pain after surgery. In clinical practice, distinction among OIH, AOT and acute opioid withdrawal remains difficult, especially because a specific quantitative sensory test to diagnose OIH has not been validated yet.

SUMMARY

Since the immediate postoperative period is not ideal to initiate long-term treatment for OIH, the best strategy is to prevent its occurrence. A multimodal approach, including choice of opioid, dose limitations and addition of nonopioid analgesics, is recommended.

Remifentanil-induced postoperative hyperalgesia: current perspectives on mechanisms and therapeutic strategies

The use of remifentanil in clinical practice offers several advantages and it is used for a wide range of procedures, ranging from day-surgery anesthesia to more complex procedures. Nonetheless, remifentanil has been consistently linked with development of opioid-induced hyperalgesia (OIH), which is described as a paradoxical increase in sensitivity to painful stimuli that develops after exposure to opioid treatment. The development of OIH may cause several issues, delaying recovery after surgery and preventing timely patient's discharge. Moreover, it causes patient's discomfort with higher pain scores, greater use of analgesics, and associated side effects. Remifentanil is the opioid most convincingly associated with OIH, and hereby we provide a review of remifentanil-induced hyperalgesia, describing both the underlying mechanisms involved and the available studies investigating experimental and clinical pharmacologic approaches aiming at reducing its incidence and degree.

CaMKIIα may modulate fentanyl-induced hyperalgesia via a CeLC-PAG-RVM-spinal cord descending facilitative pain pathway in rats

Each of the lateral capsular division of central nucleus of amygdala(CeLC), periaqueductal gray (PAG), rostral ventromedial medulla(RVM) and spinal cord has been proved to contribute to the development of opioid-induced hyperalgesia(OIH). Especially, Ca²⁺/calmodulin-dependent protein kinase IIα (CaMKIIα) in CeLC and spinal cord seems to play a key role in OIH modulation. However, the pain pathway through which CaMKIIα modulates OIH is not clear. The pathway from CeLC to spinal cord for this modulation was explored in the present study. Mechanical and thermal hyperalgesia were tested by von Frey test or Hargreaves test, respectively. CaMKIIα activity (phospho-CaMKIIα, p-CaMKIIα) was evaluated by western blot analysis. CaMKIIα antagonist (KN93) was micro-infused into CeLC, spinal cord or PAG, respectively, to evaluate its effect on behavioral hyperalgesia and p-CaMKIIα expression in CeLC, PAG, RVM and spinal cord. Then the underlying synaptic mechanism was explored by recording miniature excitatory postsynaptic currents (mEPSCs) on PAG slices using whole-cell voltage-clamp methods. Results showed that inhibition of CeLC, PAG or spinal CaMKIIα activity respectively by KN93, reversed both mechanical and thermal hyperalgesia. Microinjection of KN93 into CeLC decreased p-CaMKIIα expression in CeLC, PAG, RVM and spinal cord; while intrathecal KN93 can only block spinal but not CeLC CaMKIIα activity. KN93 injected into PAG just decreased p-CaMKIIα expression in PAG, RVM and spinal cord, but not in the CeLC. Similarly, whole-cell voltage-clamp recording found the frequency and amplitude of mEPSCs in PAG cells were decreased by KN93 added in PAG slice or micro-infused into CeLC in vivo. These results together with previous findings suggest that CaMKIIα may modulate OIH via a CeLC-PAG-RVM-spinal cord descending facilitative pain pathway.

Endogenous Opiates and Behavior: 2015

This paper is the thirty-eighth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2015 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.

Activation of the Extracellular Signal-Regulated Kinase in the Amygdale Modulates Fentanyl-Induced Hypersensitivity in Rats

Opioid-induced hyperalgesia (OIH) is one of the major problems associated with use of opioids in perioperative and chronic pain management. The mechanism underlying this paradoxical phenomenon needs to be fully elucidated. Laterocapsular division of the central nucleus of amygdale (CeLC) has emerged as an important brain center for pain modulation, so we hypothesize that the activation of extracellular signal-regulated kinase (ERK) in CeLC may modulate OIH through strengthening synaptic transmission between neurons in the CeLC. Phospho-ERK in CeLC was first found to be increased significantly in OIH rats induced by repeated subcutaneous injection of fentanyl. Blockade of this fentanyl-induced ERK activation by microinjection of U0126, an ERK inhibitor, into the CeLC reversed the behavioral hypersensitivity in a dose-dependent manner. In vitro whole-cell recordings evaluating the change in synaptic transmission found that the frequency as well as amplitude of miniature excitatory postsynaptic currents recorded on CeLC neurons from OIH rats were fundamentally increased and were completely reversed by acutely applied U0126 (10 μM in the recording well). In vivo microinjection of U0126 into the CeLC reversed the spinal long-term potentiation in OIH rats. These results showed that fentanyl-induced hypersensitivity may occur partly through the mechanism of ERK activation and followed by the strengthening of synaptic transmission in CeLC neurons.

PERSPECTIVE

This study provides evidence that ERK in the laterocapsular division of the CeLC is a key contributor to the development of fentanyl-induced hypersensitivity. Targeting the superspinal central CeLC can inhibit spinal long-term potentiation and alleviate behavioral hyperreflexia induced by fentanyl.

Are Epigenetic Factors Implicated in Chronic Widespread Pain?

BACKGROUND

Chronic widespread musculoskeletal pain (CWP) is the cardinal symptom of fibromyalgia and affects about 12% of the general population. Familial aggregation of CWP has been repeatedly demonstrated with estimated heritabilities of around 50%, indicating a genetic susceptibility. The objective of the study was to explore genome-wide disease-differentially methylated positions (DMPs) for chronic widespread pain (CWP) in a sample of unrelated individuals and a subsample of discordant monozygotic (MZ) twins.

METHODOLOGY/PRINCIPLE FINDINGS

A total of N = 281 twin individuals from the TwinsUK registry, including N = 33 MZ twins discordant for self-reported CWP, were part of the discovery sample. The replication sample included 729 men and 756 women from a subsample of the KORA S4 survey-an independent population-based cohort from Southern Germany. Epigenome-wide analysis of DNA methylation was conducted using the Illumina Infinium HumanMethylation 450 DNA BeadChip in both the discovery and replication sample. Of our 40 main loci that were carried forward for replication, three CPGs reached significant p-values in the replication sample, including malate dehydrogenase 2 (MDH2; p-value 0.017), tetranectin (CLEC3B; p-value 0.039), and heat shock protein beta-6 (HSPB6; p-value 0.016). The associations between the collagen type I, alpha 2 chain (COL1A2) and monoamine oxidase B (MAOB) observed in the discovery sample-both of which have been previously reported to be biological candidates for pain-could not be replicated.

CONCLUSION/SIGNIFICANCE

Our results may serve as a starting point to encourage further investigation in large and independent population-based cohorts of DNA methylation and other epigenetic changes as possible disease mechanisms in CWP. Ultimately, understanding the key mechanisms underlying CWP may lead to new treatments and inform clinical practice.

Opioid-induced hyperalgesia: Cellular and molecular mechanisms

Opioids produce strong analgesia but their use is limited by a paradoxical hypersensitivity named opioid-induced hyperalgesia (OIH) that may be associated to analgesic tolerance. In the last decades, a significant number of preclinical studies have investigated the factors that modulate OIH development as well as the cellular and molecular mechanisms underlying OIH. Several factors have been shown to influence OIH including the genetic background and sex differences of experimental animals as well as the opioid regimen. Mu opioid receptor (MOR) variants and interactions of MOR with different proteins were shown important. Furthermore, at the cellular level, both neurons and glia play a major role in OIH development. Several neuronal processes contribute to OIH, like activation of neuroexcitatory mechanisms, long-term potentiation (LTP) and descending pain facilitation. Increased nociception is also mediated by neuroinflammation induced by the activation of microglia and astrocytes. Neurons and glial cells exert synergistic effects, which contribute to OIH. The molecular actors identified include the Toll-like receptor 4 and the anti-opioid systems as well as some other excitatory molecules, receptors, channels, chemokines, pro-inflammatory cytokines or lipids. This review summarizes the intracellular and intercellular pathways involved in OIH and highlights some mechanisms that may be challenged to limit OIH in the future.

Selective repression of gene expression in neuropathic pain by the neuron-restrictive silencing factor/repressor element-1 silencing transcription (NRSF/REST)

Neuropathic pain often develops following nerve injury as a result of maladaptive changes that occur in the injured nerve and along the nociceptive pathways of the peripheral and central nervous systems. Multiple cellular and molecular mechanisms likely account for these changes; however, the exact nature of these mechanisms remain largely unknown. A growing number of studies suggest that alteration in gene expression is an important step in the progression from acute to chronic pain states and epigenetic regulation has been proposed to drive this change in gene expression. In this review, we discuss recent evidence that the DNA-binding protein neuron-restrictive silencing factor/repressor element-1 silencing transcription factor (NRSF/REST) is an important component in the development and maintenance of neuropathic pain through its role as a transcriptional regulator for a select subset of genes that it normally represses during development.