丝裂原活化的蛋白激酶:一个治疗病理性痛潜在的药物新靶点

病理性痛(pathological pain)是临床上常见的疼痛,通常分为炎性痛(inflammatory pain)和神经病理性痛(neuro-pathic pain),具有痛觉过敏和痛觉异常等疼痛神经可塑性特点;丝裂原活化的蛋白激酶(mitogen-activated proteins kinases,MAPKs)作为一种关键细胞信号分子,包括细胞外信号调节激酶(ERK)、p38和c-Jun氨基末端激酶(JNK)3条主要激活通路,在病理性痛觉信号转导和神经可塑性调控中起重要作用。在不同动物模型中,抑制MAPKs信号通路,可明显减轻炎症痛和神经病理性痛,这为MAPKs抑制剂开发成为治疗病理性痛的药物提供了可能,MAPKs已成为治疗病理性痛药物作用的一个重要的潜在的靶点。     

The ERK/MAPK pathway,as a target for the treatment of neuropathic pain

Peripheral nerve injury produces neuropathic pain as well as phosphorylation of mitogen activated protein kinase (MAPK) family in dorsal root ganglia (DRG) and dorsal horn. Following nerve injury, phosphorylation of extracellular signal-regulated protein kinase (ERK), an important member of this family, is sequentially increased in neurons, microglia and astrocytes of the dorsal horn and gracile nucleus, and in injured large DRG neurons. Nerve injury-induced phosphorylation of ERK occurs early and is long-lasting. In several animal models of neuropathic pain, MEK inhibitors, known to suppress the synthesis of ERK, have proven effective to alleviate pain at various time points. Thus, the regulation of ERK/MAPK can be considered as a promising therapeutic target for the treatment of neuropathic pain.     

The ERK/MAPK pathway, as a target for the treatment of neuropathic pain

Peripheral nerve injury produces neuropathic pain as well as phosphorylation of mitogen activated protein kinase (MAPK) family in dorsal root ganglia (DRG) and dorsal horn. Following nerve injury, phosphorylation of extracellular signal-regulated protein kinase (ERK), an important member of this family, is sequentially increased in neurons, microglia and astrocytes of the dorsal horn and gracile nucleus, and in injured large DRG neurons. Nerve injury-induced phosphorylation of ERK occurs early and is long-lasting. In several animal models of neuropathic pain, MEK inhibitors, known to suppress the synthesis of ERK, have proven effective to alleviate pain at various time points. Thus, the regulation of ERK/MAPK can be considered as a promising therapeutic target for the treatment of neuropathic pain.     

基于炎症信号通路的生物碱治疗神经病理性疼痛的研究进展

神经病理性疼痛是由神经系统原发性损害和功能障碍所激发或引起的疼痛,炎症作为神经病理性疼痛发展的重要病机之一,是生物体对组织损伤做出的一种正常生理反应。炎症介导的神经病理性疼痛发展机制与外周神经敏化、中枢神经敏化息息相关,包括神经炎症反应、氧化应激反应、离子通道改变、胶质细胞的活化。常见的中药成分马钱子碱、小檗碱、去氢紫堇鳞茎碱、川芎嗪、氧化苦参碱、青藤碱均可缓解神经病理性疼痛。生物碱可通过多条途径影响神经病理性疼痛,其发挥的抗炎作用影响着外周神经敏化和中枢神经敏化,是治疗神经病理性疼痛的重要机制之一。因此生物碱介导的炎症反应具有良好的抗神经元损伤作用,对神经病理性疼痛产生一定的治疗作用。总结了炎症参与神经病理性疼痛和生物碱抗炎镇痛的机制,拟从分子层面阐释生物碱发挥抗神经病理性疼痛的作用机制。     

Pathobiology of neuropathic pain.

This review deals with physiological and biological mechanisms of neuropathic pain, that is, pain induced by injury or disease of the nervous system. Animal models of neuropathic pain mostly use injury to a peripheral nerve, therefore, our focus is on results from nerve injury models. To make sure that the nerve injury models are related to pain, the behavior was assessed of animals following nerve injury, i.e. partial/total nerve transection/ligation or chronic nerve constriction. The following behaviors observed in such animals are considered to indicate pain: (a) autotomy, i.e. self-attack, assessed by counting the number of wounds implied, (b) hyperalgesia, i.e. strong withdrawal responses to a moderate heat stimulus, (c) allodynia, i.e. withdrawal in response to non-noxious tactile or cold stimuli. These behavioral parameters have been exploited to study the pharmacology and modulation of neuropathic pain. Nerve fibers develop abnormal ectopic excitability at or near the site of nerve injury. The mechanisms include unusual distributions of Na(+) channels, as well as abnormal responses to endogenous pain producing substances and cytokines such as tumor necrosis factor alpha (TNF-alpha). Persistent abnormal excitability of sensory nerve endings in a neuroma is considered a mechanism of stump pain after amputation. Any local nerve injury tends to spread to distant parts of the peripheral and central nervous system. This includes erratic mechano-sensitivity along the injured nerve including the cell bodies in the dorsal root ganglion (DRG) as well as ongoing activity in the dorsal horn. The spread of pathophysiology includes upregulation of nitric oxide synthase (NOS) in axotomized neurons, deafferentation hypersensitivity of spinal neurons following afferent cell death, long-term potentiation (LTP) of spinal synaptic transmission and attenuation of central pain inhibitory mechanisms. In particular, the efficacy of opioids at the spinal level is much decreased following nerve injury. Repeated or prolonged noxious stimulation and the persistent abnormal input following nerve injury activate a number of intracellular second messenger systems, implying phosphorylation by protein kinases, particularly protein kinase C (PKC). Intracellular signal cascades result in immediate early gene (IEG) induction which is considered as the overture of a widespread change in protein synthesis, a general basis for nervous system plasticity. Although these processes of increasing nervous system excitability may be considered as a strategy to compensate functional deficits following nerve injury, its by-product is widespread nervous system sensitization resulting in pain and hyperalgesia. An important sequela of nerve injury and other nervous system diseases such as virus attack is apoptosis of neurons in the peripheral and central nervous system. Apoptosis seems to induce neuronal sensitization and loss of inhibitory systems, and these irreversible processes might be in common to nervous system damage by brain trauma or ischemia as well as neuropathic pain. The cellular pathobiology including apoptosis suggests future strategies against neuropathic pain that emphasize preventive aspects.     

Tissue Injury and Related Mediators of Pain Exacerbation

Tissue injury and inflammation result in release of various mediators that promote ongoing pain or pain hypersensitivity against mechanical, thermal and chemical stimuli. Pro-nociceptive mediators activate primary afferent neurons directly or indirectly to enhance nociceptive signal transmission to the central nervous system. Excitation of primary afferents by peripherally originating mediators, so-called “peripheral sensitization”, is a hallmark of tissue injury-related pain. Many kinds of pro-nociceptive mediators, including ATP, glutamate, kinins, cytokines and tropic factors, synthesized at the damaged tissue, contribute to the development of peripheral sensitization. In the present review we will discuss the molecular mechanisms of peripheral sensitization following tissue injury.     

Assessing the relevance of in vitro studies in nanotoxicology by examining correlations between in vitro and in vivo data

Anti-cancer drugs such as vincristine, paclitaxel, oxaliplatin, cisplatin and bortezomib are well reported to exert direct and indirect effects on sensory nerves to alter the amplitude of action potential, conduction velocity and induce pain. It results in patient suffering and also limits the treatment with potentially useful anticancer drugs. The different scientists have worked in this area to explore the mechanisms responsible for its pathogenesis. Anti-cancer agents activate plasma membrane localized ion channels on dorsal root ganglia and dorsal horn neurons including sodium, calcium, potassium, glutamate activated NMDA receptors to alter cytosolic ionic mileu particularly intracellular calcium that trigger secondary changes to induce neuropathic pain. These may include opening of mPTP pore on mitochondria to induce intracellular calcium release; activation of protein kinase C; phosphorylation of TRPV; activation of calpases/calpains; generation of nitric oxide and free radicals to induce cytotoxicity to axons and neuronal cell bodies. Furthermore, the inflammatory process initiated in glial cells and macrophages also trigger changes in the sensory neurons to alter nociceptive processing. The present review elaborates the role of all these individual targets in the pathogenesis of anticancer agents-induced neuropathic pain to develop effective therapeutic modalities for pain management.     

Glycine receptors: a new therapeutic target in pain pathways

Although glycine receptor Cl- channels (GlyRs) have long been known to mediate inhibitory neurotransmission onto spinal nociceptive neurons, their therapeutic potential for peripheral analgesia has received little attention. However, it has been shown that alpha3-subunit-containing GlyRs are concentrated into regions of the spinal cord dorsal horn where nociceptive afferents terminate. Furthermore, inflammatory mediators specifically inhibit alpha3-containing GlyRs, and deletion of the murine alpha3 gene confers insensitivity to chronic inflammatory pain. This strongly implicates GlyRs in the inflammation-mediated disinhibition of centrally projecting nociceptive neurons. Future therapies aimed at specifically increasing current flux through alpha3-containing GlyRs may prove effective in providing analgesia.     

Targeting cell surface trafficking of pain-facilitating receptors to treat chronic pain conditions

Introduction: Chronic pain conditions are serious clinical concerns. Its genesis is closely associated with sensitization of nociceptive primary sensory neurons (nociceptors) and dorsal horn neurons by various pain mediators produced during inflammation and tissue injury. Growing evidence showed that increasing cell surface trafficking of pain-facilitating receptors is an important mechanism underlying the peripheral and central sensitization.

Areas covered: We summarized the progress of this area over the past decade by showing that inflammation, tissue damage or pain mediators facilitate cell surface trafficking of pain-facilitating receptors such as transient receptor potential vanilloid-1, transient receptor potential ankyrin-1, voltage-gated sodium channel 1.8, P2X3 and EP4 in primary sensory neurons, GluR1 and GluR2 of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors, NR1 and NR2 of N-methyl-d-aspartate receptors and acid-sensing ion channels 1 in dorsal horn neurons and P2X4 in spinal microglia. The anti-allodynic effects of gabapentin was mediated by blocking surface trafficking of α2δ1 and α2δ2 subunits of voltage-gated calcium channels in primary sensory and dorsal horn neurons.

Expert opinion: Pain mediators stimulate forward surface trafficking of their own and/or other pain-facilitating receptors to amplify pain intensity and duration. Enhancing surface abundance of pain-facilitating receptors in nociceptors and dorsal horn neurons is an important mechanism underpinning chronic pain states. Targeting specific trafficking events of pain-facilitating receptors may open a novel therapeutic avenue to more efficiently treat chronic pain conditions.     

Leukotrienes in nociceptive pathway and neuropathic/inflammatory pain

The purpose of this review is to summarize the recent studies examining the expression of leukotrienes (LTs) and their receptors in nociceptive pathways, and their crucial roles in pathological pain conditions. LTs belong to a large family of lipid mediators, termed eicosanoids, which are derived from arachidonic acids and released from the cell membrane by phospholipases. LTs are known to be important factors in a variety of local and systemic diseases and allergic/inflammatory diseases. We examined whether LTs were implicated in neuropathic pain following peripheral nerve injury. Using the SNI model in rats, we investigated the expression of LT synthases and receptors mRNAs in the spinal cord and the roles on the pain behaviors. We found the expression of 5-lipoxygenase (5-LO), FLAP and the cysteinyl leukotrienes (CysLT1) mRNAs in spinal microglia, LTA4h and LTC4s mRNAs in both spinal neurons and microglia, and BLT1 mRNA in spinal neurons. Administration of the 5-LO inhibitor or the receptor antagonists suppressed mechanical allodynia. Our findings suggest that the increase of LT synthesis in spinal microglia produced via p38 mitogen-activated protein kinase (MAPK) plays a role in the generation of neuropathic pain. We also examined the expression and roles on pain behaviors of LT receptors in the dorsal root ganglion (DRG) using a peripheral inflammation model. The data indicate CysLT2 expressed in DRG neurons may play a role as a modulator of P2X3, and contribute to the potentiation of the neuronal activity following peripheral inflammation. This review summarizes the hypothesis that LTs might work in the spinal cord and primary afferent in pathological pain conditions.     

GABA and central neuropathic pain following spinal cord injury

Spinal cord injury induces maladaptive synaptic transmission in the somatosensory system that results in chronic central neuropathic pain. Recent literature suggests that glial-neuronal interactions are important modulators in synaptic transmission following spinal cord injury. Neuronal hyperexcitability is one of the predominant phenomenon caused by maladaptive synaptic transmission via altered glial-neuronal interactions after spinal cord injury. In the somatosensory system, spinal inhibitory neurons counter balance the enhanced synaptic transmission from peripheral input. For a decade, the literature suggests that hypofunction of GABAergic inhibitory tone is an important factor in the enhanced synaptic transmission that often results in neuronal hyperexcitability in dorsal horn neurons following spinal cord injury. Neurons and glial cells synergistically control intracellular chloride ion gradients via modulation of chloride transporters, extracellular glutamate and GABA concentrations via uptake mechanisms. Thus, the intracellular “GABA-glutamate-glutamine cycle” is maintained for normal physiological homeostasis. However, hyperexcitable neurons and glial activation after spinal cord injury disrupts the balance of chloride ions, glutamate and GABA distribution in the spinal dorsal horn and results in chronic neuropathic pain. In this review, we address spinal cord injury induced mechanisms in hypofunction of GABAergic tone that results in chronic central neuropathic pain.This article is part of a Special Issue entitled ‘Synaptic Plasticity & Interneurons’.     

Peripheral and central mechanisms of pain generation

Pain research has uncovered important neuronal mechanisms that underlie clinically relevant pain states such as inflammatory and neuropathic pain. Importantly, both the peripheral and the central nociceptive system contribute significantly to the generation of pain upon inflammation and nerve injury. Peripheral nociceptors are sensitized during inflammation, and peripheral nerve fibres develop ectopic discharges upon nerve injury or disease. As a consequence a complex neuronal response is evoked in the spinal cord where neurons become hyperexcitable, and a new balance is set between excitation and inhibition. The spinal processes are significantly influenced by brain stem circuits that inhibit or facilitate spinal nociceptive processing. Numerous mechanisms are involved in peripheral and central nociceptive processes including rapid functional changes of signalling and long-term regulatory changes such as up-regulation of mediator/receptor systems. Conscious pain is generated by thalamocortical networks that produce both sensory discriminative and affective components of the pain response.     

HCN2 ion channels: an emerging role as the pacemakers of pain

Acute nociceptive pain is caused by the direct action of a noxious stimulus on pain-sensitive nerve endings, whereas inflammatory pain (both acute and chronic) arises from the actions of a wide range of inflammatory mediators released following tissue injury. Neuropathic pain, which is triggered by nerve damage, is often considered to be very different in its origins, and is particularly difficult to treat effectively. Here we review recent evidence showing that members of the hyperpolarization-activated cyclic nucleotide-modulated (HCN) ion channel family - better known for their role in the pacemaker potential of the heart - play important roles in both inflammatory and neuropathic pain. Deletion of the HCN2 isoform from nociceptive neurons abolishes heat-evoked inflammatory pain and all aspects of neuropathic pain, but acute pain sensation is unaffected. This work shows that inflammatory and neuropathic pain have much in common, and suggests that selective blockers of HCN2 may have value as analgesics in the treatment of pain.     

Block of peripheral nerve sodium channels selectively inhibits features of neuropathic pain in rats

Several sodium channel blockers are used clinically to treat neuropathic pain. However, many patients fail to achieve adequate pain relief from these highly brain-penetrant drugs because of dose-limiting central nervous system side effects. Here, we describe the functional properties of trans-N-{[2'-(aminosulfonyl)biphenyl-4-yl]methyl}-N-methyl-N'-[4-(trifluoromethoxy)benzyl]cyclopentane-1,2-dicarboxamide (CDA54), a peripherally acting sodium channel blocker. In whole-cell electrophysiological assays, CDA54 blocked the inactivated states of hNa(V)1.7 and hNa(V)1.8, two channels of the peripheral nervous system implicated in nociceptive transmission, with affinities of 0.25 and 0.18 microM, respectively. CDA54 displayed similar affinities for the tetrodotoxin-resistant Na+ current in small-diameter mouse dorsal root ganglion neurons. Peripheral nerve injury causes spontaneous electrical activity in normally silent sensory neurons. CDA54 inhibited these injury-induced spontaneous action potentials at concentrations 10-fold lower than those required to block normal A- and C-fiber conduction. Consistent with the selective inhibition of injury-induced firing, CDA54 (10 mg/kg p.o.) significantly reduced behavioral signs of neuropathic pain in two nerve injury models, whereas the same dose of CDA54 did not affect acute nociception or motor coordination. In anesthetized dogs, CDA54, at plasma concentrations of 6.7 microM, had no effect on cardiac electrophysiological parameters including conduction. Thus, the peripheral nerve sodium channel blocker CDA54 selectively inhibits sensory nerve signaling associated with neuropathic pain.     

GABAergic disinhibition induced pain hypersensitivity by upregulating NMDA receptor functions in spinal dorsal horn

Intense noxious stimuli impair GABAergic inhibition in spinal dorsal horn, which has been proposed as a critical contributor to pathological pain. However, how the reduced inhibition exacerbates the transfer of nociceptive information at excitatory glutamatergic synapses is still poorly understood. The present study demonstrated that one of the striking consequences of GABAergic disinhibition was to enhance the function of N-methyl-D-aspartate subtype glutamate receptors (NMDARs), a well-characterized player in central sensitization. We found that intrathecal application of bicuculline, a GABA_A receptor antagonist, to remove the inhibition readily elicited mechanical allodynia in naive mice, which could be dose-dependently attenuated by NMDARs antagonist D-APV. Biochemical analysis demonstrated that bicuculline did not affect the total expression levels of the obligatory NMDARs subunit NR1 and the regulatory subunit NR2A and NR2B. However, bicuculline promoted NR1 phosphorylation at Serine 897 (NR1-S897) by cAMP-dependent protein kinase (PKA). This PKA-mediated phosphorylation incorporated NR1 along with NR2B into synapses. When PKA inhibitor H-89 was intrathecally applied, it totally eliminated bicuculline-induced NMDARs phosphorylation, synaptic redistribution as well as pain sensitization. Importantly, the reduced inhibition also operated to enhance NMDARs functions after peripheral inflammation, because spinal injection of diazepam to rescue the inhibition in inflamed mice greatly depressed PKA phosphorylation of NR1-S897, reduced the synaptic concentration of NR1/NR2B and meanwhile, alleviated the inflammatory pain. These data suggested that removal of GABAergic inhibition allowed for PKA-mediated NMDARs phosphorylation and synaptic accumulation, thus exaggerating NMDARs-dependent nociceptive transmission and behavioral sensitization.     

ERK-CREB信号通路参与慢性疼痛中枢敏化的形成

慢性疼痛是临床上常见的难题,给人们的生活及工作带来了极大困扰。它是由组织损伤或潜在的组织损伤所引起,发生机制主要有中枢敏化和外周敏化两方面,研究表明中枢敏化过程在慢性疼痛的形成过程中起重要作用。cAMP反应元件结合蛋白（cAMP response element-binding protein,CREB）是一种细胞核内转录因子,通过自身磷酸化激活,对细胞内的信号通路及突触可塑性产生较大的影响,在慢性疼痛中枢敏化形成过程中起重要作用。CREB重要的上游信号分子细胞外信号调节激酶（extracellular signal regulated kinase,ERK）能将细胞外的各种刺激转化为细胞内的不同反应,参与细胞增殖、分化和神经突触可塑性。近年来研究表明ERK-CREB信号通路通过痛觉基因的调控、突触可塑性改变参与中枢敏化的形成。总结了关于ERK-CREB信号通路参与中枢敏化的研究进展、详细阐述了慢性疼痛的基本特点,ERK-CREB信号通路的一般特性及参与中枢敏化的形成,并总结目前ERK-CREB信号通路参与中枢敏化的研究局限,提出研究展望。     

Generation of acute pain: central mechanisms.

Pain can either be 'nociceptor-mediated', produced as a consequence of the activation of high threshold nociceptors, or 'A-fibre mediated', resulting from the activation of low threshold A beta afferent fibres. Under normal circumstances nociceptor mediated pain only occurs in response to high intensity noxious stimuli. Following peripheral tissue injury the inflammatory reaction generates a complex set of chemical signals that alter the transduction properties of nociceptors such that they can be activated by low intensity stimuli, the phenomenon of peripheral sensitization. Pain in this circumstance is still nociceptor mediated but can be generated by low intensity or innocuous stimuli. The nociceptive input to the spinal cord in these circumstances however produces activity-dependent alterations in the response properties of neurones in the dorsal horn. This means that they begin to respond to normal inputs, including that generated by A beta low threshold afferents, in an abnormal and exaggerated way. This is the phenomenon of central sensitization. Because afferent inputs can provoke prolonged alterations within the central nervous system, optimal treatment of acute pain states should be directed both at abolishing peripheral sensitization and to preventing the establishment of central sensitization. The latter involves the strategy of pre-emptive analgesia.