Phosphorylation of extracellular signal-regulated kinases 1/2 is predominantly enhanced in the microglia of the rat spinal cord following dorsal root transection

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.

Extracellular signal-regulated kinases (ERK) 1 and 2 as a key molecule in pain research

Pain can be divided into nociceptive, inflammatory, and neuropathic pain. It is important to understanding the molecular mechanism of pain signaling in the development of pain relief therapies. Twenty years ago, extracellular signal-regulated kinases (ERK) 1 and 2, which are members of the mitogen-activated protein kinase superfamily, were identified as molecules activated in neurons by the exposure of peripheral tissues to noxious stimuli. Further studies have revealed that peripheral nerve injury induces ERK activation in glial cells, sensory neurons, and second-order neurons, albeit at different time points. Moreover, inhibition of ERK suppresses pathological pain in animals with peripheral nerve injury. Therefore, ERK is currently recognized as an important molecule in pain signaling and a potential novel target for pain treatment. This review introduces recent advances in revealing the regulation of ERK in pain research.

Possible involvement of the peripheral Mu-opioid system in antinociception induced by bergamot essential oil to allodynia after peripheral nerve injury

The essential oil of bergamot (BEO) is one of the most common essential oils and is most familiar to the general public. The aims of this study were to investigate the effect of intraplantar (i.pl.) BEO on neuropathic allodynia induced by partial sciatic nerve ligation (PSNL) in mice and the opioid receptor subtypes involved in the antiallodynic effects of BEO. Our findings showed that a single dose of i.pl. administration of BEO significantly inhibited the PSNL-induced neuropathic pain using the von Frey test. The i.pl pretreatment with naloxone methiodide, a peripherally acting μ-opioid receptor preferring antagonist, β-funaltrexamine hydrochloride (β-FNA), a selective μ-opioid receptor antagonist, and β-endorphin antiserum significantly reversed the antiallodynic effect of BEO in the von Frey test, but not by naltrindole, the nonselective δ-opioid receptor antagonist and nor-binaltorphimine, the selective κ-opioid receptor antagonist. Furthermore, in the western blotting analysis, i.pl. administration of BEO resulted in a significant blockage of spinal extracellular signal-regulated protein kinase (ERK) activation induced by PSNL. Naloxone methiodide and β-FNA significantly reversed the blockage of spinal ERK activation induced by BEO. These results suggest that i.pl. injection of BEO-induced antiallodynic effect and blockage of spinal ERK activation may be triggered by activation of peripheral μ-opioid receptors.

PD98059 Influences Immune Factors and Enhances Opioid Analgesia in Model of Neuropathy

Neuropathic pain treatment remains challenging due to ineffective therapy and resistance to opioid analgesia. Mitogen-activated protein kinase kinase (MAPKK) have been identified as the crucial regulators of pro- and antinociceptive factors. We used PD98059, an inhibitor of the MAPKK family members MEK1/2. The aim of study was to examine the influence of single and/or repeated PD98059 on nociception and opioid effectiveness in neuropathy. Moreover, we examined how PD98059 influences selected members of cellular pathways and cytokines. The PD98059 (2.5 mcg) was intrathecally preemptively administered before chronic constriction injury (CCI), and then once daily for 7 days. Additionally, at day 7 after CCI the PD98059-treated rats received a single injection of opioids. Using Western blot and qRT-PCR techniques in PD98059-treated rats we analyzed the mRNA and/or protein level of p38, ERK1/2, JNK, NF-kappaB, IL-1beta, IL-6, iNOS and IL-10 in the lumbar spinal cord. Our results indicate that PD98059 has an analgesic effects and potentiates morphine and/or buprenorphine analgesia. Parallel we observed that PD98059 inhibit upregulation of the CCI-elevated p38, ERK1/2, JNK and NF-kappaB protein levels. Moreover, PD98059 also prevented increase of pro- (IL-1beta, IL-6, and iNOS) but enhances anti-nociceptive (IL-10) factors. Summing up, PD98059 diminished pain and increased the effectiveness of opioids in neuropathy. The inhibition of MEKs might inactivate a variety of cell signaling pathways that are implicated in nociception.

Activation profile of dorsal root ganglia Iba-1 (+) macrophages varies with the type of lesion in rats

The interactions between neurons, immune and immune-like glial cells can initiate the abnormal processes that underlie neuropathic pain. In the peripheral nervous system the resident macrophages may play an important role. In this study we investigated in experimental adult Sprague-Dawley rats how Iba-1 (ionized calcium binding adaptor molecule 1) (+) resident macrophages in the dorsal root ganglion (DRG) are activated after a spinal nerve ligation (SNL) or streptozotocin (STZ)-induced diabetes. The activation profile was defined by comparing the responses of resident macrophages against microglia in the spinal cord as they share a common origin. After SNL, the Iba-1 (+) macrophages in L5 DRG reached their activation peak 5 days later, clustered as satellite cells around large A-neurons, expressed the MHC-II marker, but did not show p-p38 and p-ERK1/2 activation and did not secrete IL-18. After STZ-induced diabetes, the Iba-1 (+) macrophages reached their activation peak 1 week later in L4 and L5 DRG, remained scattered between neurons, expressed the MHC-II marker only in L5 DRG, did not show p-p38 and p-ERK1/2 activation and did not secrete any of the investigated cytokines/chemokines. These responses suggest that depending on the type of lesion DRG Iba-1 (+) resident macrophages have different activation mechanisms, which are dissimilar to those in microglia.

The ceramide transporter and the Goodpasture antigen binding protein: one protein--one function?

The Goodpasture antigen-binding protein (GPBP) and its splice variant the ceramide transporter (CERT) are multifunctional proteins that have been found to play important roles in brain development and biology. However, the function of GPBP and CERT is controversial because of their involvement in two apparently unrelated research fields: GPBP was initially isolated as a protein associated with collagen IV in patients with the autoimmune disease Goodpasture syndrome. Subsequently, a splice variant lacking a serine-rich domain of 26 amino acids (GPBPDelta26) was found to mediate the cytosolic transport of ceramide and was therefore (re)named CERT. The two splice forms likely carry out different functions in specific sub-cellular localizations. Selective GPBP knockdown induces extensive apoptosis and tissue loss in the brain of zebrafish. GPBP/GPBPDelta26 knock-out mice die as a result of structural and functional defects in endoplasmic reticulum and mitochondria. Because both mitochondria and ceramide play an important role in many biological events that regulate neuronal differentiation, cellular senescence, proliferation and cell death, we propose that GPBP and CERT are pivotal in neurodegenerative processes. In this review, we discuss the current state of knowledge on GPBP and CERT, including the molecular and biochemical characterization of GPBP in the field of autoimmunity as well as the fundamental research on CERT in ceramide transport, biosynthesis, localization, metabolism and cell homeostasis.

MAP kinase and pain

Mitogen-activated protein kinases (MAPKs) are important for intracellular signal transduction and play critical roles in regulating neural plasticity and inflammatory responses. The MAPK family consists of three major members: extracellular signal-regulated kinases (ERK), p38, and c-Jun N-terminal kinase (JNK), which represent three separate signaling pathways. Accumulating evidence shows that all three MAPK pathways contribute to pain sensitization after tissue and nerve injury via distinct molecular and cellular mechanisms. Activation (phosphorylation) of MAPKs under different persistent pain conditions results in the induction and maintenance of pain hypersensitivity via non-transcriptional and transcriptional regulation. In particular, ERK activation in spinal cord dorsal horn neurons by nociceptive activity, via multiple neurotransmitter receptors, and using different second messenger pathways plays a critical role in central sensitization by regulating the activity of glutamate receptors and potassium channels and inducing gene transcription. ERK activation in amygdala neurons is also required for inflammatory pain sensitization. After nerve injury, ERK, p38, and JNK are differentially activated in spinal glial cells (microglia vs astrocytes), leading to the synthesis of proinflammatory/pronociceptive mediators, thereby enhancing and prolonging pain. Inhibition of all three MAPK pathways has been shown to attenuate inflammatory and neuropathic pain in different animal models. Development of specific inhibitors for MAPK pathways to target neurons and glial cells may lead to new therapies for pain management. Although it is well documented that MAPK pathways can increase pain sensitivity via peripheral mechanisms, this review will focus on central mechanisms of MAPKs, especially ERK.

Differential contribution of spinal mitogen-activated protein kinases to the phase of long-lasting allodynia evoked by intrathecal administration of ATP in rats

Several lines of evidence suggest that activation of spinal mitogen-activated protein kinases (MAPKs), including extracellular signal-regulated kinase (ERK) and p38 MAPK, contributes to the induction and maintenance of chronic pain. We recently reported that an intrathecal (i.t.) administration of ATP evoked tactile allodynia, which lasted more than 1 week in rats. The long-lasting allodynia was induced by activation of spinal P2X 2/3-receptors, and the induction and early phase of maintenance, but not the late phase, was mediated, at least in part, by the activation of spinal glial cells. In this study, we examined the involvement of spinal ERK and p38 MAPK in each phase of i.t. ATP-evoked long-lasting allodynia. I.t. administration of ATP (100 nmol) markedly increased phosphorylated ERK, which peaked at 1-8 h before gradually decreasing to a level that was sustained until 7 d after administration. In contrast, only a slight increase in phosphorylated p38 MAPK was observed. Consistent with the increased phosphorylation of MAPKs, the ERK kinase MEK inhibitor, U0126 (3 nmol), attenuated the induction phase (co-administration with ATP) and early maintenance phase (1-d post-ATP administration) of the i.t. ATP-evoked allodynia, but not the late maintenance phase (7-d post-ATP administration), while the p38 MAPK inhibitor, SB203580 (10 microg), had little effect. These results suggest that the induction phase and early maintenance phase, but not the late maintenance phase of long-lasting allodynia is mediated by the activation of ERK, rather than by the activation of p38 MAPK, in the spinal cord. These findings are informative for elucidating the mechanisms underlying the pathogenesis of chronic pain.

Activation of dorsal horn microglia contributes to diabetes-induced tactile allodynia via extracellular signal-regulated protein kinase signaling

Painful neuropathy is one of the most common complications of diabetes, one hallmark of which is tactile allodynia (pain hypersensitivity to innocuous stimulation). The underlying mechanisms of tactile allodynia are, however, poorly understood. Emerging evidence indicates that, following nerve injury, activated microglia in the spinal cord play a crucial role in tactile allodynia. However, it remains unknown whether spinal microglia are activated under diabetic conditions and whether they contribute to diabetes-induced tactile allodynia. In the present study, using streptozotocin (STZ)-induced diabetic rats that displayed tactile allodynia, we found several morphological changes of activated microglia in the dorsal horn. These included increases in Iba1 and OX-42 labeling (markers of microglia), hypertrophic morphology, the thickness and the retraction of processes, and in the number of activated microglia cells. Furthermore, in the dorsal horn of STZ diabetic rats, extracellular signal-regulated protein kinase (ERK) and an upstream kinase, Src-family kinase (SFK), both of which are implicated in microglial functions, were activated exclusively in microglia. Moreover, inhibition of ERK phosphorylation in the dorsal horn by intrathecal administration of U0126, an inhibitor of ERK activation, produced a striking alleviation of existing, long-term tactile allodynia of diabetic rats. We also found that a single administration of U0126 reduced the expression of allodynia. Together, these results suggest that activated dorsal horn microglia may be a crucial component of diabetes-induced tactile allodynia, mediated, in part, by the ERK signaling pathway. Thus, inhibiting microglia activation in the dorsal horn may represent a therapeutic strategy for treating diabetic tactile allodynia.

An optimized immunohistochemical method for detection of phosphorylated mitogen-activated protein kinases

Study of the in vivo spatio-temporal localization of modified proteins is likely to become a major focus of proteomics research in the near future. In this study we optimized and tested an immunohistochemical procedure for detecting unstable phosphorylated mitogen-activated protein (MAP) kinases. Using our method, phosphorylated MAP kinases can be sensitively and reproducibly localized in the developing white matter of murine spinal cord on embryonic day 15. Our method is simple and effective, and so may be useful in future proteomics research.

Region- or state-related differences in expression and activation of extracellular signal-regulated kinases (ERKs) in naïve and pain-experiencing rats

BACKGROUND

Extracellular signal-regulated kinase (ERK), one member of the mitogen-activated protein kinase (MAPK) family, has been suggested to regulate a diverse array of cellular functions, including cell growth, differentiation, survival, as well as neuronal plasticity. Recent evidence indicates a role for ERKs in nociceptive processing in both dorsal root ganglion and spinal cord. However, little literature has been reported to examine the differential distribution and activation of ERK isoforms, ERK1 and ERK2, at different levels of pain-related pathways under both normal and pain states. In the present study, quantitative blot immunolabeling technique was used to determine the spatial and temporal expression of ERK1 and ERK2, as well as their activated forms, in the spinal cord, primary somatosensory cortex (SI area of cortex), and hippocampus under normal, transient pain and persistent pain states.

RESULTS

In naïve rats, we detected regional differences in total expression of ERK1 and ERK2 across different areas. In the spinal cord, ERK1 was expressed more abundantly than ERK2, while in the SI area of cortex and hippocampus, there was a larger amount of ERK2 than ERK1. Moreover, phosphorylated ERK2 (pERK2), not phosphorylated ERK1 (pERK1), was normally expressed with a high level in the SI area and hippocampus, but both pERK1 and pERK2 were barely detectable in normal spinal cord. Intraplantar saline or bee venom injection, mimicking transient or persistent pain respectively, can equally initiate an intense and long-lasting activation of ERKs in all three areas examined. However, isoform-dependent differences existed among these areas, that is, pERK2 exhibited stronger response than pERK1 in the spinal cord, whereas ERK1 was more remarkably activated than ERK2 in the S1 area and hippocampus.

CONCLUSION

Taken these results together, we conclude that: (1) under normal state, while ERK immunoreactivity is broadly distributed in the rat central nervous system in general, the relative abundance of ERK1 and ERK2 differs greatly among specific regions; (2) under pain state, either ERK1 or ERK2 can be effectively phosphorylated with a long-term duration by both transient and persistent pain, but their response patterns differ from each other across distinct regions; (3) The long-lasting ERKs activation induced by bee venom injection is highly correlated with our previous behavioral, electrophysiological, morphological and pharmacological observations, lending further support to the functional importance of ERKs-mediated signaling pathways in the processing of negative consequences of pain associated with sensory, emotional and cognitive dimensions.

Tissue type plasminogen activator induced in rat dorsal horn astrocytes contributes to mechanical hypersensitivity following dorsal root injury

Dorsal root injury is known to induce alteration of the extracellular environment in the spinal cord and synaptic reorganization with degradation of injured primary afferent and sprouting of spared terminal. These changes affect behavioral sensitivity and sometimes lead to neuropathic pain. We have hypothesized that changes in extracellular proteolysis in the dorsal horn is involved in neuroplastic changes in the dorsal horn after nerve injury. Tissue type plasminogen activator (tPA) is a well-known extracellular serine protease and is involved in the modification of the extracellular matrix, which leads to neuroplastic changes such as long-term potentiation in the hippocampus. In the present study, we found a marked induction of tPA in activated astrocytes following L4/5 root injury and a resultant increase of proteolytic enzymatic activity in the dorsal horn. We also examined the involvement of tPA activity on mechanical hypersensitivity using a root ligation model which has been used for investigating radiculopathy pain behavior. Intrathecal and continuous administration of tPA inhibitor, tPA-STOP, suppressed root ligation-induced mechanical allodynia in a dose-dependent manner during an early stage of injury (0-4 days). In contrast, the delayed administration of tPA-STOP during the chronic stage of injury (10 days) did not affect pain behavior. These data suggest an important contribution of astrocytes in the dorsal horn to the pathophysiology of radiculopathy pain, and astrocyte-derived tPA and the proteolytic activity in the dorsal horn may be one of the essential factors involved in pain following root injury.

ERK is sequentially activated in neurons, microglia, and astrocytes by spinal nerve ligation and contributes to mechanical allodynia in this neuropathic pain model

Activation of extracellular signal-regulated kinase (ERK), a mitogen activated-protein kinase (MAPK), in dorsal horn neurons contributes to inflammatory pain by transcription-dependent and -independent means. We have now investigated if ERK is activated in the spinal cord after a spinal nerve ligation (SNL) and if this contributes to the neuropathic pain-like behavior generated in this model. An L5 SNL induces an immediate (<10 min) but transient (<6 h) induction of phosphoERK (pERK) restricted to neurons in the superficial dorsal horn. This is followed by a widespread induction of pERK in spinal microglia that peaks between 1 and 3 days post-surgery. On Day 10, pERK is expressed both in astrocytes and microglia, but by Day 21 predominantly in astrocytes in the dorsal horn. In the L5 DRG SNL transiently induces pERK in neurons at 10 min, and in satellite cells on Day 10 and 21. Intrathecal injection of the MEK (ERK kinase) inhibitor PD98059 on Day 2, 10 or 21 reduces SNL-induced mechanical allodynia. Our results suggest that ERK activation in the dorsal horn, as well as in the DRG, mediates pain through different mechanisms operating in different cells at different times. The sequential activation of ERK in dorsal horn microglia and then in astrocytes might reflect distinct roles for these two subtypes of glia in the temporal evolution of neuropathic pain.

A disaccharide derived from chondroitin sulphate proteoglycan promotes central nervous system repair in rats and mice

Chondroitin sulphate proteoglycan (CSPG) inhibits axonal regeneration in the central nervous system (CNS) and its local degradation promotes repair. We postulated that the enzymatic degradation of CSPG generates reparative products. Here we show that an enzymatic degradation product of CSPG, a specific disaccharide (CSPG-DS), promoted CNS recovery by modulating both neuronal and microglial behaviour. In neurons, acting via a mechanism that involves the PKCalpha and PYK2 intracellular signalling pathways, CSPG-DS induced neurite outgrowth and protected against neuronal toxicity and axonal collapse in vitro. In microglia, via a mechanism that involves ERK1/2 and PYK2, CSPG-DS evoked a response that allowed these cells to manifest a neuroprotective phenotype ex vivo. In vivo, systemically or locally injected CSPG-DS protected neurons in mice subjected to glutamate or aggregated beta-amyloid intoxication. Our results suggest that treatment with CSPG-DS might provide a way to promote post-traumatic recovery, via multiple cellular targets.

Metastasis-associated S100A4 (Mts1) protein is expressed in subpopulations of sensory and autonomic neurons and in Schwann cells of the adult rat

S100A4 (Mts1) is a member of a family of calcium-binding proteins of the EF-hand type, which are widely expressed in the nervous system, where they appear to be involved in the regulation of neuron survival, plasticity, and response to injury or disease. S100A4 has previously been demonstrated in astrocytes of the white matter and rostral migratory stream of the adult rat. After injury, S100A4 is markedly up-regulated in affected central nervous white matter areas as well as in the periventricular area and rostral migratory stream. Here, we show that S100A4 is expressed in a subpopulation of dorsal root, trigeminal, geniculate, and nodose ganglion cells; in a subpopulation of postganglionic sympathetic and parasympathetic neurons; in chromaffin cells of the adrenal medulla; and in satellite and Schwann cells. In dorsal root ganglia, S100A4-positive cells appear to constitute a subpopulation of small ganglion neurons, a few of which coexpressed calcitonin gene-related peptide (CGRP) and Griffonia simplicifolia agglutinin (GSA) isolectin B4 (B4). S100A4 protein appears to be transported from dorsal root ganglia to the spinal cord, where it is deposited in the tract of Lissauer. After peripheral nerve or dorsal root injury, a few S100A4-positive cells coexpress CGRP, GSA, or galanin. Peripheral nerve or dorsal root injury induces a marked up-regulation of S100A4 expression in satellite cells in the ganglion and in Schwann cells at the injury site and in the distal stump. This pattern of distribution partially overlaps that of the previously studied S100B and S100A6 proteins, indicating a possible functional cooperation between these proteins. The presence of S100A4 in sensory neurons, including their processes in the central nervous system, suggests that S100A4 is involved in propagation of sensory impulses in specific fiber types.

MAPK activation in nociceptive neurons and pain hypersensitivity

Extracellular signal-regulated protein kinase (ERK) is a mitogen-activated protein kinase (MAPK) that mediates intracellular signal transduction in response to a variety of stimuli. ERK is involved in cell proliferation and differentiation and in neuronal plasticity, including long-term potentiation, learning, and memory. Here, we present recently accumulating data about the roles of MAPK pathways in mediating the neuronal plasticity that contributes to pain hypersensitivity. The phosphorylation of ERK in the dorsal root ganglion (DRG) and dorsal horn neurons occurs in response to noxious stimulation of the peripheral tissue or electrical stimulation to the peripheral nerve, i.e., activity-dependent activation of ERK in nociceptive neurons. In addition, the activation of ERK occurs in these nociceptive neurons after peripheral inflammation and axotomy and contributes to persistent inflammatory and neuropathic pain, via transcriptional regulation of key gene products. On the other hand, peripheral inflammation and axotomy also induces p38 MAPK activation in DRG neurons. Taken together, these findings indicate that activation of MAPK in nociceptive neurons may participate in generating pain hypersensitivity through transcription-dependent and -independent means. Thus, inhibition of MAPK signaling in the primary afferents, as well as in the spinal cord, may provide a fruitful strategy for the development of novel analgesics.