BDNF from microglia causes the shift in neuronal anion gradient underlying neuropathic pain

Transformation of the output of spinal lamina I neurons after nerve injury and microglia stimulation underlying neuropathic pain

BACKGROUND

Disinhibition of neurons in the superficial spinal dorsal horn, via microglia - neuron signaling leading to disruption of chloride homeostasis, is a potential cellular substrate for neuropathic pain. But, a central unresolved question is whether this disinhibition can transform the activity and responses of spinal nociceptive output neurons to account for the symptoms of neuropathic pain.

RESULTS

Here we show that peripheral nerve injury, local spinal administration of ATP-stimulated microglia or pharmacological disruption of chloride transport change the phenotype of spinal lamina I output neurons, causing them to 1) increase the gain of nociceptive responsiveness, 2) relay innocuous mechanical input and 3) generate spontaneous bursts of activity. The changes in the electrophysiological phenotype of lamina I neurons may account for three principal components of neuropathic pain: hyperalgesia, mechanical allodynia and spontaneous pain, respectively.

CONCLUSION

The transformation of discharge activity and sensory specificity provides an aberrant signal in a primarily nociceptive ascending pathway that may serve as a basis for the symptoms of neuropathic pain.

Brain derived nerve growth factor induces spinal noradrenergic fiber sprouting and enhances clonidine analgesia following nerve injury in rats

Many treatments for neuropathic pain activate or augment norepinephrine release in the spinal cord, yet these treatments are less effective against acute nociceptive stimuli. We previously showed in mice that peripheral nerve injury results in sprouting of spinal noradrenergic fibers, possibly reflecting the substrate for this shift in drug efficacy. Here, we tested whether such sprouting also occurs in rats after nerve injury and examined one signal for such sprouting. Ligation of L5 and L6 spinal nerves unilaterally in rats resulted in hypersensitivity to tactile stimulation of the ipsilateral paw, and sprouting of noradrenergic fibers in the dorsal horn of the lumbar spinal cord. Brain derived nerve growth factor (BDNF) content increased in L4-L6 dorsal root ganglia ipsilateral to injury and in lumbar spinal cord following nerve injury, and intrathecal infusion of BDNF antiserum prevented spinal noradrenergic sprouting. This treatment also prevented the increased analgesic efficacy of intrathecal clonidine observed after nerve injury. Intraspinal injection of BDNF in non-injured rats mimicked the sprouting of spinal noradrenergic fibers seen after nerve injury. These results suggest that increased BDNF synthesis and release drives spinal noradrenergic sprouting following nerve injury, and that this sprouting may paradoxically increase the capacity for analgesia in the setting of neuropathic pain from drugs which utilize or mimic the noradrenergic pathway.

Pain mechanisms and their implication for the management of pain in farm and companion animals

Over the last two decades there has been a dramatic increase in the literature relating to the mechanisms and management of pain in domestic animals. Understanding the mechanisms of pain is crucial for its effective management. This review highlights the current understanding of the neurophysiology of nociception and the plastic changes involved in chronic pain states. Additionally, we describe a range of novel molecules and pathways that offer opportunities for the development of mechanism-based analgesic therapies. Pain management in animals is limited by pain assessment which remains highly subjective, with clinicians relying on indirect measures of pain, using rating scales and (less frequently) quantifiable physiological and behavioural parameters. The need for a systematic approach which would assess different pain components is well justified. Species-specific issues on pain assessment and management in mammalian companion and farm animals are addressed in the later part of this review.

Neuron type-specific effects of brain-derived neurotrophic factor in rat superficial dorsal horn and their relevance to 'central sensitization'

Chronic constriction injury (CCI) of the rat sciatic nerve increases the excitability of the spinal dorsal horn. This 'central sensitization' leads to pain behaviours analogous to human neuropathic pain. We have established that CCI increases excitatory synaptic drive to putative excitatory, 'delay' firing neurons in the substantia gelatinosa but attenuates that to putative inhibitory, 'tonic' firing neurons. Here, we use a defined-medium organotypic culture (DMOTC) system to investigate the long-term actions of brain-derived neurotrophic factor (BDNF) as a possible instigator of these changes. The age of the cultures and their 5-6 day exposure to BDNF paralleled the protocol used for CCI in vivo. Effects of BDNF (200 ng ml(-1)) in DMOTC were reminiscent of those seen with CCI in vivo. These included decreased synaptic drive to 'tonic' neurons and increased synaptic drive to 'delay' neurons with only small effects on their membrane excitability. Actions of BDNF on 'delay' neurons were exclusively presynaptic and involved increased mEPSC frequency and amplitude without changes in the function of postsynaptic AMPA receptors. By contrast, BDNF exerted both pre- and postsynaptic actions on 'tonic' cells; mEPSC frequency and amplitude were decreased and the decay time constant reduced by 35%. These selective and differential actions of BDNF on excitatory and inhibitory neurons contributed to a global increase in dorsal horn network excitability as assessed by the amplitude of depolarization-induced increases in intracellular Ca(2+). Such changes and their underlying cellular mechanisms are likely to contribute to CCI-induced 'central sensitization' and hence to the onset of neuropathic pain.

Complement induction in spinal cord microglia results in anaphylatoxin C5a-mediated pain hypersensitivity

Microarray expression profiles reveal substantial changes in gene expression in the ipsilateral dorsal horn of the spinal cord in response to three peripheral nerve injury models of neuropathic pain. However, only 54 of the 612 regulated genes are commonly expressed across all the neuropathic pain models. Many of the commonly regulated transcripts are immune related and include the complement components C1q, C3, and C4, which we find are expressed only by microglia. C1q and C4 are, moreover, the most strongly regulated of all 612 regulated genes. In addition, we find that the terminal complement component C5 and the C5a receptor (C5aR) are upregulated in spinal microglia after peripheral nerve injury. Mice null for C5 had reduced neuropathic pain sensitivity, excluding C3a as a pain effector. C6-deficient rats, which cannot form the membrane attack complex, have a normal neuropathic pain phenotype. However, C5a applied intrathecally produces a dose-dependent, slow-onset cold pain in naive animals. Furthermore, a C5aR peptide antagonist reduces cold allodynia in neuropathic pain models. We conclude that induction of the complement cascade in spinal cord microglia after peripheral nerve injury contributes to neuropathic pain through the release and action of the C5a anaphylatoxin peptide.

Modification of neuropathic pain sensation through microglial ATP receptors

Neuropathic pain that typically develops when peripheral nerves are damaged through surgery, bone compression in cancer, diabetes, or infection is a major factor causing impaired quality of life in millions of people worldwide. Recently, there has been a rapidly growing body of evidence indicating that spinal glia play a critical role in the pathogenesis of neuropathic pain. Accumulating findings also indicate that nucleotides play an important role in neuron-glia communication through P2 purinoceptors. Damaged neurons release or leak nucleotides including ATP and UTP to stimulate microglia through P2 purinoceptors expressing on microglia. It was shown in an animal model of neuropathic pain that microglial P2X₄ and P2X₇ receptors are crucial in pain signaling after peripheral nerve lesion. In this review, we describe the modification of neuropathic pain sensation through microglial P2X₄ and P2X₇, with the possibility of P2Y₆ and P2Y₁₂ involvement.

Loss of glycinergic and GABAergic inhibition in chronic pain--contributions of inflammation and microglia

Tissue trauma, inflammation and neuropathy can under unfortunate condition progress into chronic pain syndromes. It is meanwhile generally accepted that chronic pain, i.e. pain, which persists beyond the resolution of tissue traumata and inflammation, is due to plastic changes in the neuronal processing of sensory stimuli in the CNS. A loss of synaptic inhibition (i.e. dis-inhibition) in the spinal cord dorsal horn has been increasingly recognized as an important process in the development and maintenance of chronic pain of both inflammatory and neuropathic origin. Although inflammation and neuropathy involve distinct mechanisms of synaptic dis-inhibition, the production of inflammatory mediators and/or the activation of immune cells, two events that have once been thought to be normally excluded from the CNS, appear to be critical for both conditions.

Glia: they make your memories stick!

Synaptic plasticity underlies higher brain functions such as learning and memory. At glutamatergic synapses in the vertebrate central nervous system, plasticity usually requires changes in the number of postsynaptic AMPA receptors. Recently, several studies have revealed that glial cells play an important role in regulating postsynaptic AMPA receptor density. This is accomplished through the release of gliotransmitters such as D-serine, ATP and TNF-alpha. More specifically, the availability of D-serine, the endogenous co-agonist of N-methyl-D-aspartate receptors in many brain areas, governs the induction of long-term potentiation and long-term depression. Meanwhile, ATP and TNF-alpha trigger long-lasting increases in synaptic strength at glutamatergic hypothalamic and hippocampal inputs, respectively, through mechanisms that promote AMPA receptor insertion in the absence of coincident presynaptic and postsynaptic activity. These data clearly demonstrate a vital role for glia in plasticity and argue that their contributions to brain function extend well beyond their outdated role as cellular 'glue'.

Do glial cells control pain?

Management of chronic pain is a real challenge, and current treatments that focus on blocking neurotransmission in the pain pathway have resulted in limited success. Activation of glial cells has been widely implicated in neuroinflammation in the CNS, leading to neurodegeneration in conditions such as Alzheimer's disease and multiple sclerosis. The inflammatory mediators released by activated glial cells, such as tumor necrosis factor-a and interleukin-1b not only cause neurodegeneration in these disease conditions, but also cause abnormal pain by acting on spinal cord dorsal horn neurons in injury conditions. Pain can also be potentiated by growth factors such as brain-derived growth factor and basic fibroblast growth factor, which are produced by glia to protect neurons. Thus, glial cells can powerfully control pain when they are activated to produce various pain mediators. We review accumulating evidence that supports an important role for microglial cells in the spinal cord for pain control under injury conditions (e.g. nerve injury). We also discuss possible signaling mechanisms, in particular mitogen-activated protein kinase pathways that are crucial for glial-mediated control of pain.Investigating signaling mechanisms in microglia might lead to more effective management of devastating chronic pain.

BDNF-mediated modulation of GABA and glycine release in dorsal horn lamina II from postnatal rats

Recent studies show that excitatory glutamatergic transmission is potentiated by BDNF in superficial dorsal horn, both at the pre- and the postsynaptic site. The role of BDNF in modulating GABA and glycine-mediated inhibitory transmission has not been fully investigated. To determine whether the neurotrophin is effective in regulating the spontaneous release of the two neurotransmitters, we have recorded miniature inhibitory postsynaptic currents (mIPSCs) in lamina II of post-natal rats. We show that application of BDNF enhanced the spontaneous release of GABA and glycine, in presence of tetrodotoxin. The effect was blocked by the trk-receptor inhibitor k-252a. Amplitude and kinetics of mIPSCs were not altered. Evoked GABA and glycine IPSCs (eIPSCs) were depressed by BDNF and the coefficient of variation of eIPSC amplitude was significantly increased. By recording glycine eIPSCs with the paired-pulse protocol, an increase of paired-pulse ratio during BDNF application was observed. We performed parallel ultrastructural studies to unveil the circuitry involved in the effects of BDNF. These studies show that synaptic interactions between full length functional trkB receptors and GABA-containing profiles only occur at non peptidergic synaptic glomeruli of types I and II. Expression of trkB in presynaptic vesicle-containing dendrites originating from GABAergic islet cells, indicates these profiles as key structures in the modulation of inhibitory neurotransmission by the neurotrophin. Our results thus describe a yet uncharacterized effect of BDNF in lamina II, giving further strength to the notion that the neurotrophin plays an important role in pain neurotransmission.

Locomotor exercise alters expression of pro-brain-derived neurotrophic factor, brain-derived neurotrophic factor and its receptor TrkB in the spinal cord of adult rats

Previous evidence indicates that locomotor exercise is a powerful means of increasing brain-derived neurotrophic factor (BDNF) and its signal transduction receptor TrkB mRNA levels, immunolabeling intensity and number of BDNF- and TrkB-immunopositive cells in the spinal cord of adult rats but the contribution of specific cell types to changes resulting from long-term activity is unknown. As changes in BDNF protein distribution due to systemic stimuli may reflect either its in-situ synthesis or its translocation from other sources, we investigated where BDNF and TrkB mRNA are expressed in the spinal lumbar segments. We report on the cell types defined by size, BDNF mRNA levels and number of cells with TrkB transcripts in sedentary and exercised animals following 28 days of treadmill walking. In the majority of cells, exercise increased perikaryonal levels of BDNF mRNA but did not affect TrkB transcript levels. Bidirectional changes in a number of TrkB mRNA-expressing cells occurred in small groups of ventral horn neurons. An increase in BDNF transcripts was translated into changes in pro-BDNF and BDNF levels. A 7-day walking regimen increased BDNF protein levels similarly to 28-day treadmill walking. Our observations indicate that long- and short-term locomotor activity of moderate intensity produce stimuli sufficient to recruit a majority of spinal cells to increased BDNF synthesis, suggesting that continuous tuning of pro-BDNF and BDNF levels permits spinal networks to undergo trophic modulation not requiring changes in TrkB mRNA supply.

Platelet and plasma BDNF in lower respiratory tract infections of the adult

Enhanced bronchial responsiveness during and following lower respiratory tract infections is a major clinical problem, but its pathogenesis is poorly understood. Brain-derived neurotrophic factor (BDNF), which can be released by platelets and leukocytes, has been identified as a mediator of bronchial hyperresponsiveness. It is unknown whether the release of BDNF is altered during lower respiratory tract infections of the adult. In this clinical pilot study, 16 patients (35-80 years old) with the diagnosis of an acute bacterial lower respiratory tract infection and elevated serum concentrations of c-reactive protein (>100 microg/ml) and procalcitonin (>0.1 ng/ml) were examined on admission to the hospital and 1 week after antibiotic treatment. Sixteen age- and sex-matched controls were examined in the same time period. BDNF concentrations in serum and platelets, but not in plasma, were markedly reduced in patients on the day of admission (median <25% of the controls). Analysis of the platelet marker serotonin (5-HT) suggested that the decrease of platelet BDNF is part of a non-specific release of platelet-derived mediators in this condition. Clinical improvement was accompanied by a restoration of serum and platelet BDNF concentrations which returned to control levels after 1 week of treatment. Cell culture experiments revealed that bacterial lipopolysaccharide (LPS) enhanced the release of BDNF by peripheral blood mononuclear cells of the patients at both time points. In conclusion, these data suggest that lower respiratory tract infections might be associated with an augmented release of BDNF by platelets and mononuclear cells.

Stereological and somatotopic analysis of the spinal microglial response to peripheral nerve injury

The involvement of glia, and glia-neuronal signalling in enhancing nociceptive transmission has become an area of intense scientific interest. In particular, a role has emerged for activated microglia in the development and maintenance of neuropathic pain following peripheral nerve injury. Following activation, spinal microglia proliferate and release many substances which are capable of modulating neuronal excitability within the spinal cord. Here, we the investigated the response of spinal microglia to a unilateral spared nerve injury (SNI) in terms of the quantitative increase in cell number and the spatial distribution of the increase. Design-based stereological techniques were combined with iba-1 immunohistochemistry to estimate the total number of microglia in the spinal dorsal horn in naïve and peripheral nerve-injured adult rats. In addition, by mapping the central terminals of hindlimb nerves, the somatotopic distribution of the microglial response was mapped. Following SNI there was a marked increase in the number of spinal microglia: The total number of microglia (mean+/-SD) in the dorsal horn sciatic territory of the naïve rat was estimated to be 28,591+/-2715. Following SNI the number of microglia was 82,034+/-8828. While the pattern of microglial activation generally followed somatotopic boundaries, with the majority of microglia within the territory occupied by peripherally axotomised primary afferents, some spread was seen into regions occupied by intact, 'spared' central projections of the sural nerve. This study provides a reproducible method of assaying spinal microglial dynamics following peripheral nerve injury both quantitatively and spatially.

Differential regulation of microglial P2X4 and P2X7 ATP receptors following LPS-induced activation

Activation of microglia has been implicated in many neurological conditions including Alzheimer's disease and neuropathic pain. Recent studies provide evidence that P2X ATP receptors on the surface of microglia play a crucial role in initiation of inflammatory cascades. We investigated changes in surface P2X receptors in BV-2 murine microglial cells following their activation by pro-inflammatory bacterial lipopolysaccharides (LPS). mRNA analysis using RT-PCR confirmed the presence of P2X4 and P2X7 as the main P2X subunits. Application of ATP at low (< or =100 microM) and high (> or =1 mM) concentrations, as well as BzATP, activated inward currents in BV-2 cells. Current responses of P2X4 and P2X7 subtypes could be distinguished based on their respective sensitivity to the positive modulator ivermectin and to the antagonist Brilliant Blue G. Treatment of BV-2 cells with LPS leads to a transient increase in ivermectin-sensitive P2X4 currents, while dominant P2X7 currents remain largely unaffected. This increase in P2X4 function was concomitant with higher receptor protein expression, itself related to an upregulation of P2X4 mRNA levels that peaked at 48 h post-LPS treatment. Our data demonstrate that although LPS activation has a minor impact on P2X7 receptors that remain the major ionotropic ATP receptors in microglia, it specifically enhances responses to low ATP concentrations mediated by P2X4 receptors, highlighting the significant contribution of both subtypes to neuroinflammatory mechanisms and pathologies.

Dissociation and trafficking of rat GABAB receptor heterodimer upon chronic capsaicin stimulation

Gamma-aminobutyric acid type B receptors (GABAB) are G-protein-coupled receptors that mediate GABAergic inhibition in the brain. Their functional expression is dependent upon the formation of heterodimers between GABAB1 and GABAB2 subunits, a process that occurs within the endoplasmic reticulum. However, the mechanisms that regulate GABAB receptor oligomerization at the plasma membrane remain largely unknown. We first characterized the functional cytoarchitecture of an organotypic co-culture model of rat dorsal root ganglia and spinal cord. Subsequently, we studied the interactions between GABAB subunits after chronic stimulation of sensory fibres with capsaicin. Surface labelling of recombinant proteins showed a decrease in subunit co-localization and GABAB2 labelling, after capsaicin treatment. In these conditions, fluorescence lifetime imaging measurements further demonstrated a loss of interactions between green fluorescent protein-GABAB1b and t-dimer discosoma sp red fluorescent protein-GABAB2 subunits. Finally, we established that the GABAB receptor undergoes clathrin-dependent internalization and rapid recycling to the plasma membrane following activation with baclofen, a GABAB agonist. However, in cultures chronically stimulated with capsaicin, the agonist-induced endocytosis was decreased, reflecting changes in the dimeric state of the receptor. Taken together, our results indicate that the chronic stimulation of sensory fibres can dissociate the GABAB heterodimer and alters its responsiveness to the endogenous ligand. Chronic stimulation thus modulates receptor oligomerization, providing additional levels of control of signalling.

Endogenous TrkB ligands suppress functional mechanosensory plasticity in the deafferented spinal cord

Dorsal root injury (DRI) disrupts the flow of sensory information to the spinal cord. Although primary afferents do not regenerate to their original targets, spontaneous recovery can, by unknown mechanisms, occur after DRI. Here, we show that brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), but not nerve growth factor or neurotrophin-4, are upregulated in the spinal gray matter after DRI. Because endogenous BDNF and NT-3 have well established roles in synaptic and axonal plasticity, we hypothesized that they contributed to spontaneous recovery after DRI. We first developed a model of DRI-induced mechanosensory dysfunction: rat C7/8 DRI produced a deficit in low-threshold cutaneous mechanosensation that spontaneously improved within 10 d but did not recover completely. To determine the effects of endogenous BDNF and NT-3, we administered TrkB-Fc or TrkC-Fc fusion proteins throughout the recovery period. To our surprise, TrkB-Fc stimulated complete recovery of mechanosensation by 6 d after DRI. It also stimulated mechanosensory axon sprouting but prevented deafferentation-induced serotonergic sprouting. TrkC-Fc had no effect on low-threshold mechanosensory behavior or axonal plasticity. There was no mechanosensory improvement with single-bolus TrkB-Fc infusions at 10 d after DRI (despite significantly reducing rhizotomy-induced cold pain), indicating that neuromodulatory effects of BDNF did not underlie mechanosensory recovery. Continuous infusion of the pan-neurotrophin antagonist K252a also stimulated behavioral and anatomical plasticity, indicating that these effects of TrkB-Fc treatment occurred independent of signaling by other neurotrophins. These results illustrate a novel, plasticity-suppressing effect of endogenous TrkB ligands on mechanosensation and mechanosensory primary afferent axons after spinal deafferentation.

Neuronal mechanism for neuropathic pain

Among different forms of persistent pain, neuropathic pain presents as a most difficult task for basic researchers and clinicians. Despite recent rapid development of neuroscience and modern techniques related to drug discovery, effective drugs based on clear basic mechanisms are still lacking. Here, I will review the basic neuronal mechanisms that maybe involved in neuropathic pain. I will present the problem of neuropathic pain as a rather difficult task for neuroscientists, and we may have to wait for a long time before we fully understand how brain encode, store, and retrieve painful information after the injury. I propose that neuropathic pain as a major brain disease, rather being a clinic problem due to peripheral injury.

Inhibition of spinal microglial cathepsin S for the reversal of neuropathic pain

A recent major conceptual advance has been the recognition of the importance of immune system-neuronal interactions in the modulation of brain function, one example of which is spinal pain processing in neuropathic states. Here, we report that in peripheral nerve-injured rats, the lysosomal cysteine protease cathepsin S (CatS) is critical for the maintenance of neuropathic pain and spinal microglia activation. After injury, CatS was exclusively expressed by activated microglia in the ipsilateral dorsal horn, where expression peaked at day 7, remaining high on day 14. Intrathecal delivery of an irreversible CatS inhibitor, morpholinurea-leucine-homophenylalanine-vinyl phenyl sulfone (LHVS), was antihyperalgesic and antiallodynic in neuropathic rats and attenuated spinal microglia activation. Consistent with a pronociceptive role of endogenous CatS, spinal intrathecal delivery of rat recombinant CatS (rrCatS) induced hyperalgesia and allodynia in naïve rats and activated p38 mitogen-activated protein kinase (MAPK) in spinal cord microglia. A bioinformatics approach revealed that the transmembrane chemokine fractalkine (FKN) is a potential substrate for CatS cleavage. We show that rrCatS incubation reduced the levels of cell-associated FKN in cultured sensory neurons and that a neutralizing antibody against FKN prevented both FKN- and CatS-induced allodynia, hyperalgesia, and p38 MAPK activation. Furthermore, rrCatS induced allodynia in wild-type but not CX3CR1-knockout mice. We suggest that under conditions of increased nociception, microglial CatS is responsible for the liberation of neuronal FKN, which stimulates p38 MAPK phosphorylation in microglia, thereby activating neurons via the release of pronociceptive mediators.

BDNF contributes to animal model neuropathic pain after peripheral nerve transection

The outcome of peripheral nerve injury is often impaired by neuropathic pain, which is resistant to most analgesics and presents a serious clinical problem. The mechanisms underlying post-traumatic neuropathic pain remain unclear, but they are likely associated with the regeneration processes. Brain-derived neurotrophic factor (BDNF) is known to enhance peripheral nerve regeneration and is also considered to be an endogenous modulator of nociceptive responses following spinal cord lesion. The aim of this work was to examine the local effect of BDNF in a neuropathic pain model. Sciatic nerves of adult male rats were transected and supplied with connective tissue chambers filled with (1) fibrin only, (2) fibrin with BDNF, or (3) fibrin with antibodies against BDNF. In control animals the nerve was transected and no chamber was applied. During follow-up, autotomy behavior was assessed. Seven weeks after the operation, the number of surviving and regenerating neurons in dorsal root ganglia was counted and the neuroma incidence was examined. We found that local inactivation of BDNF decreased the incidence as well as severity of autotomy and neuroma formation, but did not influence neuron regeneration into the chambers. These results indicate that BDNF plays a locally crucial role in neuropathic pain development after peripheral nerve injury.

Roles of extracellular signal-regulated protein kinases 5 in spinal microglia and primary sensory neurons for neuropathic pain

Neuropathic pain that occurs after peripheral nerve injury is poorly controlled by current therapies. Increasing evidence shows that mitogen-activated protein kinase (MAPK) play an important role in the induction and maintenance of neuropathic pain. Here we show that activation of extracellular signal-regulated protein kinases 5 (ERK5), also known as big MAPK1, participates in pain hypersensitivity caused by nerve injury. Nerve injury increased ERK5 phosphorylation in spinal microglia and in both damaged and undamaged dorsal root ganglion (DRG) neurons. Antisense knockdown of ERK5 suppressed nerve injury-induced neuropathic pain and decreased microglial activation. Furthermore, inhibition of ERK5 blocked the induction of transient receptor potential channels and brain-derived neurotrophic factor expression in DRG neurons. Our results show that ERK5 activated in spinal microglia and DRG neurons contributes to the development of neuropathic pain. Thus, blocking ERK5 signaling in the spinal cord and primary afferents has potential for preventing pain after nerve damage.

Quantification of the rat spinal microglial response to peripheral nerve injury as revealed by immunohistochemical image analysis and flow cytometry

Microgliosis is implicated in the pathophysiology of several neurological disorders, including neuropathic pain. Consequently, perturbation of microgliosis is a mechanistic and drug development target in neuropathic pain, which highlights the requirement for specific, sensitive and reproducible methods of microgliosis measurement. In this study, we used the spinal microgliosis associated with L5 spinal nerve transection and minocycline-induced attenuation thereof to: (1) evaluate novel software based semi-quantitative image analysis paradigms for the assessment of immunohistochemical images. Microgliosis was revealed by immunoreactivity to OX42. Several image analysis paradigms were assessed and compared to a previously validated subjective categorical rating scale. This comparison revealed that grey scale measurement of the proportion of a defined area of spinal cord occupied by OX42 immunoreactive cells is a robust image analysis paradigm. (2) Develop and validate a flow cytometric approach for quantification of spinal microgliosis. The flow cytometric technique reliably quantified microgliosis in spinal cord cell suspensions, using OX42 and ED9 immunoreactivity to identify microglia.

The results suggest that image analysis of immunohistochemical revelation of microgliosis reliably detects the spinal microgliosis in response to peripheral nerve injury and pharmacological attenuation thereof. In addition, flow cytometry provides an alternative approach for quantitative analysis of spinal microgliosis elicited by nerve injury.

A meta-analysis of the analgesic effects of omega-3 polyunsaturated fatty acid supplementation for inflammatory joint pain

Between 40% and 60% of Americans use complementary and alternative medicine to manage medical conditions, prevent disease, and promote health and well-being. Omega-3 polyunsaturated fatty acids (omega-3 PUFAs) have been used to treat joint pain associated with several inflammatory conditions. We conducted a meta-analysis of 17 randomized, controlled trials assessing the pain relieving effects of omega-3 PUFAs in patients with rheumatoid arthritis or joint pain secondary to inflammatory bowel disease and dysmenorrhea. Meta-analysis was conducted with Cochrane Review Manager 4.2.8. for six separate outcomes using standardized mean differences (SMDs) as a measure of effect size: (1) patient assessed pain, (2) physician assessed pain, (3) duration of morning stiffness, (4) number of painful and/or tender joints, (5) Ritchie articular index, and (6) nonselective nonsteroidal anti-inflammatory drug consumption. Supplementation with omega-3 PUFAs for 3-4 months reduces patient reported joint pain intensity (SMD: -0.26; 95% CI: -0.49 to -0.03, p=0.03), minutes of morning stiffness (SMD: -0.43; 95% CI: -0.72 to -0.15, p=0.003), number of painful and/or tender joints (SMD: -0.29; 95% CI: -0.48 to -0.10, p=0.003), and NSAID consumption (SMD: -0.40; 95% CI: -0.72 to -0.08, p=0.01). Significant effects were not detected for physician assessed pain (SMD: -0.14; 95% CI: -0.49 to 0.22, p=0.45) or Ritchie articular index (SMD: 0.15; 95% CI: -0.19 to 0.49, p=0.40) at 3-4 months. The results suggest that omega-3 PUFAs are an attractive adjunctive treatment for joint pain associated with rheumatoid arthritis, inflammatory bowel disease, and dysmenorrhea.

Cell-cell signaling in the neurovascular unit

Historically, the neuron has been the conceptual focus for almost all of neuroscience research. In recent years, however, the concept of the neurovascular unit has emerged as a new paradigm for investigating both physiology and pathology in the CNS. This concept proposes that a purely neurocentric focus is not sufficient, and emphasizes that all cell types in the brain including neuronal, glial and vascular components, must be examined in an integrated context. Cell-cell signaling and coupling between these different compartments form the basis for normal function. Disordered signaling and perturbed coupling form the basis for dysfunction and disease. In this mini-review, we will survey four examples of this phenomenon: hemodynamic neurovascular coupling linking blood flow to brain activity; cellular communications that evoke the blood-brain barrier phenotype; parallel systems that underlie both neurogenesis and angiogenesis in the CNS; and finally, the potential exchange of trophic factors that may link neuronal, glial and vascular homeostasis.

Understanding LTP in pain pathways

Long-term potentiation (LTP) at synapses of nociceptive nerve fibres is a proposed cellular mechanism underlying some forms of hyperalgesia. In this review fundamental properties of LTP in nociceptive pathways are described. The following topics are specifically addressed: A concise definition of LTP is given and a differentiation is made between LTP and "central sensitisation". How to (and how not to) measure and how to induce LTP in pain pathways is specified. The signal transduction pathways leading to LTP at C-fibre synapses are highlighted and means of how to pre-empt and how to reverse LTP are delineated. The potential functional roles of LTP are evaluated at the cellular level and at the behavioural level in experimental animals. Finally, the impact of LTP on the perception of pain in human subjects is discussed.

Interferon-gamma induced disruption of GABAergic inhibition in the spinal dorsal horn in vivo

The proinflammatory cytokine interferon-gamma (IFN-gamma), which can be present in elevated levels in the central nervous system during pathological conditions, may be involved in the generation of persistent pain states by inducing neuronal hyperexcitability. The aim of the present study was to examine whether loss of dorsal horn GABAergic inhibition may underlie this IFN-gamma-mediated neuronal hyperexcitability. Repetitive intrathecal injections of recombinant rat IFN-gamma (1000 U) or control buffer were administered to rats every second day for eight days. Electrophysiological recordings from lumbar dorsal horn neurons (n=46) were performed under halothane anaesthesia. Cellular responses were recorded before, during and after microiontophoretic application of the GABA antagonist bicuculline. In control animals, all cellular responses studied were significantly enhanced in the presence of bicuculline, including increased spontaneous activity, enhanced responses to innocuous and noxious mechanical stimulation and reduced paired-pulse depression. In contrast, in IFN-gamma-treated animals, bicuculline ejection had little or no facilitating effect on neuronal responses and instead a significant proportion of neurons displayed reduced responses. Seventy-four percent of cells from IFN-gamma treated animals showed a reduction in the response to noxious stimulation and 47% of the cells showed increased rather than reduced paired-pulse depression in the presence of bicuculline, thus suggesting IFN-gamma-induced excitatory actions by GABA. These findings show that the prolonged presence of increased levels of IFN-gamma in the central nervous system may contribute to the generation of central sensitization and persistent pain by reducing inhibitory tone in the dorsal horn. This implies a potential link between disinhibition and cytokine action in the spinal cord.

Spinal microglia and neuropathic pain in young rats

Neuropathic pain behaviour is not observed in neonatal rats and tactile allodynia does not develop in the spared nerve injury (SNI) model until rats are 4 weeks of age at the time of surgery. Since activated spinal microglia are known to play a key role in neuropathic pain, we have investigated whether the microglial response to nerve injury in young rats differs from that in adults. Here we show that dorsal horn microglial activation, visualised with IBA-1 immunostaining, is significantly less in postnatal day (P) 10 rat pups than in adults, 7 days after SNI. This was confirmed by qPCR analysis of IBA-1 mRNA and mRNA of other microglial markers, integrin-alpha M, MHC-II DMalpha and MHC-II DMbeta. Dorsal horn IBA-1+ve microglia could be activated, however, by intraspinal injections of lipopolysaccharide (LPS) or N-methyl-d-aspartate (NMDA) at P10, although the increase in the levels of mRNA for all microglial markers was less than in the adult rat. In addition, P10 rats developed a small but significant mechanical allodynia in response to intrathecal LPS. Intrathecal injection of cultured ATP-activated microglia, known to cause mechanical allodynia in adult rats, had no behavioural effect at P10 and only began to cause allodynia if injections were performed at P16. The results clearly demonstrate immaturity of the microglial response triggered by nerve injury in the first postnatal weeks which may explain the absence of tactile allodynia following peripheral nerve injury in young rats.

Extracellular signal-regulated kinase-regulated microglia-neuron signaling by prostaglandin E2 contributes to pain after spinal cord injury

Many patients with traumatic spinal cord injury (SCI) report pain that persists indefinitely and is resistant to available therapeutic approaches. We recently showed that microglia become activated after experimental SCI and dynamically maintain hyperresponsiveness of spinal cord nociceptive neurons and pain-related behaviors. Mechanisms of signaling between microglia and neurons that help to maintain abnormal pain processing are unknown. In this study, adult male Sprague Dawley rats underwent T9 spinal cord contusion injury. Four weeks after injury when lumbar dorsal horn multireceptive neurons became hyperresponsive and when behavioral nociceptive thresholds to mechanical and thermal stimuli were decreased, we tested the hypothesis that prostaglandin E2 (PGE2) contributes to signaling between microglia and neurons. Immunohistochemical data showed specific localization of phosphorylated extracellular signal-regulated kinase 1/2 (pERK1/2), an upstream regulator of PGE2 release, to microglial cells and a neuronal localization of the PGE2 receptor E-prostanoid 2 (EP2). Enzyme immunoassay analysis showed that PGE2 release was dependent on microglial activation and ERK1/2 phosphorylation. Pharmacological antagonism of PGE2 release was achieved with the mitogen-activated protein kinase kinase 1/2 (MEK1/2) inhibitor PD98059 [2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one] and the microglial inhibitor minocycline. Cyclooxygenase-2 expression in microglia was similarly reduced by MEK1/2 inhibition. PD98059 and EP2 receptor blockade with AH6809 (6-isopropoxy-9-oxoxanthene-2-carboxylic acid) resulted in a decrease in hyperresponsiveness of dorsal horn neurons and partial restoration of behavioral nociceptive thresholds. Selective targeting of dorsal horn microglia with the Mac-1-SAP immunotoxin, a chemical conjugate of mouse monoclonal antibody to CD11b and the ribosome-inactivating protein saporin, resulted in reduced microglia staining, reduction in PGE2 levels, and reversed pain-related behaviors [corrected]. On the basis of these observations, we propose a PGE2-dependent, ERK1/2-regulated microglia-neuron signaling pathway that mediates the microglial component of pain maintenance after injury to the spinal cord.

Genetic manipulation of CD74 in mouse strains of different backgrounds can result in opposite responses to central nervous system injury

The ability to recover from CNS injuries is strain dependent. Transgenic mice that weakly express the p41 CD74 isoform (an integral membrane protein functioning as a MHC class II chaperone) on an I-A(b) genetic background have normal CD4(+) T cell populations and normal surface expression of MHC class II, but their B cell development is arrested while the cells are still immature. After a CNS injury, these mice recover better than their matched wild-type controls. We generated p41-transgenic mice on an I-A(d) background (p41-I-A(d) mice), and found that their recovery from CNS injuries was worse than that of controls. A correlative inverse effect was seen with respect to the kinetics of T cell and B cell recruitment to the injured CNS and the expression of insulin-like growth factor at the lesion site. These results, besides verifying previous findings that B cells function in the damaged CNS, demonstrate that the outcome of a particular genetic manipulation may be strain dependent.

Cortical reorganization in NT3-treated experimental spinal cord injury: Functional magnetic resonance imaging

Functional magnetic resonance imaging (fMRI) studies were performed for visualizing ongoing brain plasticity in Neurotrophin-3 (NT3)-treated experimental spinal cord injury (SCI). In response to the electrical stimulation of the forepaw, the NT3-treated animals showed extensive activation of brain structures that included contralateral cortex, thalamus, caudate putamen, hippocampus, and periaqueductal gray. Quantitative analysis of the fMRI data indicated significant changes both in the volume and center of activations in NT3-treated animals relative to saline-treated controls. A strong activation in both ipsi- and contralateral periaqueductal gray and thalamus was observed in NT3-treated animals. These studies indicate ongoing brain reorganization in the SCI animals. The fMRI results also suggest that NT3 may influence nociceptive pathways.

Placing pain on the sensory map: Classic papers by Ed Perl and colleagues

This essay looks at two papers published by Ed Perl and co-workers that identified specifically nociceptive neurons in the periphery and superficial dorsal horn.

Bessou P and Perl ER. Response of cutaneous sensory units with unmyelinated fibers to noxious stimuli. J Neurophysiol 32: 1025–1043 1969.

Christensen BN and Perl ER. Spinal neurons specifically excited by noxious or thermal stimuli: marginal zone of the dorsal horn. J Neurophysiol 33: 293–307 1970.

Distribution of RET immunoreactivity in the rodent spinal cord and changes after nerve injury

RET (for "rearranged during transfection") is a transmembrane tyrosine kinase signaling receptor for members of the glial cell line-derived neurotrophic factor (GDNF) family of ligands. We used RET immunohistochemistry (IHC), double-labeling immunofluorescence (IF), and in situ hybridization (ISH) in adult naïve and nerve-injured rats to study the distribution of RET in the spinal cord. In the dorsal horn, strong RET-immunoreactive (-ir) fibers were abundant in lamina II-inner (II(i)), although this labeling was preferentially observed after an antigen-unmasking procedure. After dorsal rhizotomy, RET-ir fibers in lamina II(i) completely disappeared from the dorsal horn, indicating that they were all primary afferents. After peripheral axotomy, RET-ir in primary afferents decreased in lamina II(i) and appeared to increase slightly in laminae III and IV. RET-ir was also observed in neurons and dendrites throughout the dorsal horn. Some RET-ir neurons in lamina I had the morphological appearance of nociceptive projection neurons, which was confirmed by the finding that 53% of RET-ir neurons in lamina I colocalized with neurokinin-1. GDNF-ir terminals were in close proximity to RET-ir neurons in the superficial dorsal horn. In the ventral horn, RET-ir was strongly expressed by motoneurons, with the strongest staining in small, presumably gamma-motoneurons. Increased RET expression following peripheral axotomy was most pronounced in alpha-motoneurons. The expression and regulation pattern of RET in the spinal cord are in line with its involvement in regenerative processes following nerve injury. The presence of RET in dorsal horn neurons, including nociceptive projection neurons, suggests that RET also has a role in signal transduction at the spinal level. This role may include mediating the effects of GDNF released from nociceptive afferent fibers.

Tinnitus behavior and hearing function correlate with the reciprocal expression patterns of BDNF and Arg3.1/arc in auditory neurons following acoustic trauma

The molecular changes following sensory trauma and the subsequent response of the CNS are poorly understood. We focused on finding a molecular tool for monitoring the features of excitability which occur following acoustic trauma to the auditory system. Of particular interest are genes that alter their expression pattern during activity-induced changes in synaptic efficacy and plasticity. The expression of brain-derived neurotrophic factor (BDNF), the activity-dependent cytoskeletal protein (Arg3.1/arc), and the immediate early gene c-Fos were monitored in the peripheral and central auditory system hours and days following a traumatic acoustic stimulus that induced not only hearing loss but also phantom auditory perception (tinnitus), as shown in rodent animal behavior models. A reciprocal responsiveness of activity-dependent genes became evident between the periphery and the primary auditory cortex (AI): as c-Fos and BDNF exon IV expression was increased in spiral ganglion neurons, Arg3.1/arc and (later on) BDNF exon IV expression was reduced in AI. In line with studies indicating increased spontaneous spike activity at the level of the inferior colliculus (IC), an increase in BDNF and GABA-positive neurons was seen in the IC. The data clearly indicate the usefulness of Arg3.1/arc and BDNF for monitoring trauma-induced activity changes and the associated putative plasticity responses in the auditory system.

Spinal mechanisms of NPY analgesia

We review previously published data, and present some new data, indicating that spinal application of neuropeptide Y (NPY) reduces behavioral and neurophysiological signs of acute and chronic pain. In models of acute pain, early behavioral studies showed that spinal (intrathecal) administration of NPY and Y2 receptor agonists decrease thermal nociception. Subsequent neurophysiological studies indicated that Y2-mediated inhibition of excitatory neurotransmitter release from primary afferent terminals in the substantia gelatinosa may contribute to the antinociceptive actions of NPY. As with acute pain, NPY reduced behavioral signs of inflammatory pain such as mechanical allodynia and thermal hyperalgesia; however, receptor antagonist studies indicate an important contribution of spinal Y1 rather than Y2 receptors. Interestingly, Y1 agonists suppress inhibitory synaptic events in dorsal horn neurons (indeed, well known mu-opioid analgesic drugs produce similar cellular actions). To resolve the behavioral and neurophysiological data, we propose that NPY/Y1 inhibits the spinal release of inhibitory neurotransmitters (GABA and glycine) onto inhibitory neurons, e.g. disinhibition of pain inhibition, resulting in hyporeflexia. The above mechanisms of Y1- and Y2-mediated analgesia may also operate in the setting of peripheral nerve injury, and new data indicate that NPY dose-dependently inhibits behavioral signs of neuropathic pain. Indeed, neurophysiological studies indicate that Y2-mediated inhibition of Ca(2+) channel currents in dorsal root ganglion neurons is actually increased after axotomy. We conclude that spinal delivery of Y1 agonists may be of use in the treatment of chronic inflammatory pain, and that the use of Y1 and Y2 agonists in neuropathic pain warrants further consideration.

Role of activity-dependent regulation of neuronal chloride homeostasis in development

The polarity of neurotransmission mediated by the gamma-amino butyric acid (GABA) type A receptor depends crucially on intracellular chloride concentration, which is largely determined by the expression and function of cation/chloride co-transporters. Recent evidence shows how both activity and neurotrophic factors can affect GABAergic transmission in the mammalian central nervous system through their effects on the neuron-specific chloride-extruding transporter KCC2. In particular, GABAergic neurotransmission early in development, sustained neuronal activity in mature networks and brain-derived neurotrophic factor each modulate the expression or function of KCC2. The resulting changes in intracellular chloride concentration alter the nature or strength of fast GABAergic neurotransmission, profoundly affecting the development and function of neuronal networks.

Neurotrophins and peripheral neuropathies

Peripheral neuropathy is a common disorder seen in general neurology and neuromuscular specialty clinics. Treatment options directed at the underlying cause can only be offered in a handful of conditions, such as those with possible autoimmune etiology. The remainder fall into the idiopathic or genetic category with no known treatment. This review surveys the evidence supporting the rationale for the therapeutic use of neurotrophins and other neurotrophic factors in these disorders in relationship to the underlying pathobiological process. Previous clinical trials are assessed, and increasingly better understood and appreciated therapeutic potential of neurotrophins is emphasized.

P2 receptors and chronic pain

There is abundant evidence that extracellular ATP and other nucleotides have an important role in pain signaling at both the periphery and in the CNS. The focus of attention now is on the possibility that endogenous ATP and its receptor system might be activated in chronic pathological pain states, particularly in neuropathic and inflammatory pain. Neuropathic pain is often a consequence of nerve injury through surgery, bone compression, diabetes or infection. This type of pain can be so severe that even light touching can be intensely painful; unfortunately, this state is generally resistant to currently available treatments. In this review, we summarize the role of ATP receptors, particularly the P2X₄, P2X₃ and P2X₇ receptors, in neuropathic and inflammatory pain. The expression of P2X₄ receptors in the spinal cord is enhanced in spinal microglia after peripheral nerve injury, and blocking pharmacologically and suppressing molecularly P2X₄ receptors produce a reduction of the neuropathic pain behaviour. Understanding the key roles of these ATP receptors may lead to new strategies for the management of intractable chronic pain.

Blocking the glial function suppresses subcutaneous formalin-induced nociceptive behavior in the rat

This study examined whether glial cells in the trigeminal nucleus caudalis (Sp5C) were necessary for orofacial nociception and nociceptive processing induced by subcutaneously (s.c.) injection of 5% formalin into left mystacial vibrissae. The immunohistochemical, immunoelectron microscopical methods and behavior assessment were used in this study. Two hours after administration of carbenoxolone (CBX, a gap junction blocker) or fluorocistrate (FCA, a glail metabolic inhibitor) into the cerebellomedullary cistern, the nociceptive behavior and scratching-cumulative time reduced significantly (P<0.01). FCA attenuated obviously the expression of Fos/NeuN-immunoreactive (-IR) neurons (mean+/-S.E.M.=29+/-2.5) and Fos/glial fibrillary acidic protein (GFAP)-IR astrocytes (7.2+/-2.2) in Sp5C. CBX decreased the number of Fos/NeuN-IR neurons (25+/-1.7), but did not affect Fos/GFAP-IR astrocytes (16.2+/-5.4), compared with vehicle-preadministered rats (Fos/NeuN-IR neurons 135+/-4.2, and Fos/GFAP-IR astrocytes 25.8+/-4). Immunoelectron microscopy established that Cx32/Cx43 heterotypic gap junctions (HGJs) were present on junction areas between astrocytes and neurons within Sp5C. The number of HGJs increased significantly following formalin s.c. injection. It suggests that the Sp5C astrocytes may play an active regulating role in orofacial nociception via Cx32/Cx43 HGJs between astrocytes and neurons of Sp5C.

Arthritis and pain. Future targets to control osteoarthritis pain

Clinical presentation of osteoarthritis (OA) is dominated by pain during joint use and at rest. OA pain is caused by aberrant functioning of a pathologically altered nervous system with key mechanistic drivers from peripheral nerves and central pain pathways. This review focuses on symptomatic pain therapy exemplified by molecular targets that alter sensitization and hyperexcitability of the nervous system, for example, opioids and cannabinoids. We highlight opportunities for targeting inflammatory mediators and their key receptors (for example, prostanoids, kinins, cytokines and chemokines), ion channels (for example, NaV1.8, NaV1.7 and CaV2.2) and neurotrophins (for example, nerve growth factor), noting evidence that relates to their participation in OA etiology and treatment. Future neurological treatments of pain appear optimistic but will require the systematic evaluation of emerging opportunities.

Glutamate-stimulated ATP release from spinal cord astrocytes is potentiated by substance P

ATP has recently emerged as a key molecule mediating pathological pain. The aim of this study was to examine whether spinal cord astrocytes could be a source of ATP in response to the nociceptive neurotransmitters glutamate and substance P. Glutamate stimulated ATP release from these astrocytes and this release was greatly potentiated by substance P, even though substance P alone did not elicit ATP release. Substance P also potentiated glutamate-induced inward currents, but did not cause such currents alone. When glutamate was applied alone it acted exclusively through alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionate receptors to stimulate Ca(2+) influx-dependent ATP release. However, when substance P was co-applied with glutamate, ATP release could be elicited by activation of NMDA and metabotropic glutamate receptors. Activation of neurokinin receptor subtypes, protein kinase C and phospholipases A(2), C and D were needed for substance P to bring about its effects. These results suggest that astrocytes may be a major source of ATP in the spinal cord on activation of nerve fibres that release substance P and glutamate.

Spinal administration of a delta opioid receptor agonist attenuates hyperalgesia and allodynia in a rat model of neuropathic pain

Neuropathic (NP) pain is a debilitating chronic pain disorder considered by some to be inherently resistant to therapy with traditional analgesics. Indeed, micro opioid receptor (OR) agonists show reduced therapeutic benefit and their long term use is hindered by the high incidence of adverse effects. However, pharmacological and physiological evidence increasingly suggests a role for deltaOR agonists in modulating NP pain symptoms. In this study, we examined the antihyperalgesic and antiallodynic effects of the spinally administered deltaOR agonist, d-[Ala(2), Glu(4)]deltorphin II (deltorphin II), as well as the changes in deltaOR expression, in rats following chronic constriction injury (CCI) of the sciatic nerve. Rats with CCI exhibited cold hyperalgesia and mechanical allodynia over a 14-day testing period. Intrathecal administration of deltorphin II reversed cold hyperalgesia on day 14 and dose-dependently attenuated mechanical allodynia. The effects of deltorphin II were mediated via activation of the deltaOR as the effect was antagonized by co-treatment with the delta-selective antagonist, naltrindole. Western blotting experiments revealed no changes in deltaOR protein in the dorsal spinal cord following CCI. Taken together, these data demonstrate the antihyperalgesic and antiallodynic effectiveness of a spinally administered deltaOR agonist following peripheral nerve injury and support further investigation of deltaORs as potential therapeutic targets in the treatment of NP pain.

Acute but not chronic stimulation of glial cells in rat spinal cord by systemic injection of lipopolysaccharide is associated with hyperalgesia

We have analyzed development of mechanical hyperalgesia after repeated systemic lipopolysaccharide (LPS) injections and correlated these findings with stimulation of astrocytes and microglia in spinal cord. Male Lewis rats received a single or seven intraperitoneal injections of LPS. Mechanical hyperalgesia was measured as rat hindpaw withdrawal thresholds (PWTs). We observed that a single LPS injection elicited a specific change of PWTs while stimulated spinal glial activation was identified by immunoreactivities of specific markers, ED1, P2X4 receptor, endothelial monocyte activating polypeptide II (EMAP II) and glial fibrillary acidic protein (GFAP), respectively; multiple LPS treatments induced tolerance to mechanical hyperalgesia, whereas expression of ED1 and GFAP were further increased. In conclusion, we have demonstrated that the number of activated spinal glial cells was increased as an acute effect of LPS correlating with increased sensitivity to mechanical stimulation. However chronic exposure to LPS can develop a tolerance to mechanical hyperalgesia despite ongoing signs of CNS glial activation.

Spinal prostaglandin E receptors of the EP2 subtype and the glycine receptor α3 subunit, which mediate central inflammatory hyperalgesia, do not contribute to pain after peripheral nerve injury or formalin injection

Inflammation, peripheral nerve injury and chemical irritants can cause central sensitization in pain pathways. Prostaglandins produced in the CNS induce central sensitization during inflammation mainly by relieving nociceptive neurons from glycinergic inhibition. We have recently identified spinal prostaglandin E receptors of the EP2 subtype (EP2 receptors) and the glycine receptor alpha3 subunit (GlyR alpha3) as signal transduction elements involved in the generation of central inflammatory hyperalgesia. It is however still unknown to what extent inhibition of glycine receptors by PGE2 contributes to neuropathic or chemically induced pain. To address this question, we have analyzed mice deficient in the EP2 receptor (EP2-/- mice) or in the GlyR alpha3 subunit (GlyR alpha3-/- mice) using the chronic constriction injury (CCI) model of neuropathic pain and the formalin test. We found that EP2-/- mice and GlyR alpha3-/- mice develop thermal and mechanical hyperalgesia in the CCI model indistinguishable from that seen in wild-type mice. In the formalin test, EP2-/- mice, but not GlyR alpha3-/- mice, exhibited reduced nocifensive behavior. The lack of a phenotype in GlyR alpha3-/- mice together with the absence of a facilitating effect of intrathecal PGE2 on formalin-induced nociception in wild-type mice suggests that peripheral rather than spinal EP2 receptors are involved. These results indicate that inhibition of glycinergic neurotransmission by EP2 receptor activation does not contribute to pain following peripheral nerve injury or chemical irritation with formalin. Our results thus provide further evidence that inflammatory hyperalgesia and neuropathic pain involve different mechanisms of central sensitization.

Analgesic action of nicotine on tibial nerve transection (TNT)-induced mechanical allodynia through enhancement of the glycinergic inhibitory system in spinal cord

The activation of cholinergic pathways by nicotine elicits various physiological and pharmacological effects in mammals. For example, the stimulation of nicotinic acetylcholine receptors (nAChRs) leads to an antinociceptive effect. However, it remains to be elucidated which subtypes of nAChR are involved in the antinociceptive effect of nicotine on nerve injury-induced allodynia and the underlying cascades of the nAChR-mediated antiallodynic effect. In this study, we attempted to characterize the actions of nicotine at the spinal level against mechanical allodynia in an animal model of neuropathic pain, tibial nerve transection (TNT) in rats. It was found that the intrathecal injection of nicotine, RJR-2403, a selective alpha4beta2 nAChR agonist, and choline, a selective alpha7 nAChR agonist, produced an antinociceptive effect on the TNT-induced allodynia. The actions of nicotine were almost completely suppressed by pretreatment with mecamylamine, a non-selective nicotinic antagonist, or dihydro-beta-erythroidine, a selective alpha4beta2 nAChR antagonist, and partially reversed by pretreatment with methyllycaconitine, a selective alpha7 nAChR antagonist. Furthermore, pretreatment with strychnine, a glycine receptor antagonist, blocked the antinociception induced by nicotine, RJR-2403, and choline. On the other hand, the GABAA antagonist bicuculline did not reverse the antiallodynic effect of nicotine. Together, these results indicate that the alpha4beta2 and alpha7 nAChR system, by enhancing the activities of glycinergic neurons at the spinal level, exerts a suppressive effect on the nociceptive transduction in neuropathic pain.

Pain signalling pathways: from cytokines to ion channels

Pain is the major reason patients seek medical care. The treatment of pain, particularly chronic pain associated with cancer and damage to the nervous system, is at present inadequate. Lack of effective analgesics is partly due to the fact that pain signalling mechanisms are still not fully understood. Over the recent years, many channels, receptors, and regulatory proteins involved in pain pathways have bee identified, and novel pain signalling mechanisms and pathways at peripheral and spinal levels have been discovered. It is anticipated that increased understanding of the molecular mechanisms of pain would provide a hope for the future development of effective pain killers. This review examines the currently available information on the molecular aspects of pain signalling pathways, and discusses novel and promising therapeutic targets for the treatment of pain in humans.

The P2Y12 receptor regulates microglial activation by extracellular nucleotides

Microglia are primary immune sentinels of the CNS. Following injury, these cells migrate or extend processes toward sites of tissue damage. CNS injury is accompanied by release of nucleotides, serving as signals for microglial activation or chemotaxis. Microglia express several purinoceptors, including a G(i)-coupled subtype that has been implicated in ATP- and ADP-mediated migration in vitro. Here we show that microglia from mice lacking G(i)-coupled P2Y(12) receptors exhibit normal baseline motility but are unable to polarize, migrate or extend processes toward nucleotides in vitro or in vivo. Microglia in P2ry(12)(-/-) mice show significantly diminished directional branch extension toward sites of cortical damage in the living mouse. Moreover, P2Y(12) expression is robust in the 'resting' state, but dramatically reduced after microglial activation. These results imply that P2Y(12) is a primary site at which nucleotides act to induce microglial chemotaxis at early stages of the response to local CNS injury.

Successful treatment of childhood intramedullary spinal cord astrocytomas with irinotecan and cisplatin

Childhood spinal cord astrocytomas are rare diseases, and their management is controversial. We report here our successful experience using irinotecan and cisplatin in three consecutive infants with progressing intramedullary astrocytomas. The first patient was a 16-month-old girl who presented with a grade III intramedullary astrocytoma that rapidly progressed after surgery and adjuvant chemotherapy. Weekly irinotecan (50 mg/m(2)) and cisplatin (30 mg/m(2)) for four consecutive weeks (one cycle) for a total of four cycles (I/C regimen) was used in order to avoid or delay radiotherapy. Radiological complete remission was achieved 10 months after completion of therapy, and 3.5 years after diagnosis the patient remains disease free. The second patient was a 19-month-old boy with a C3-T4 grade II intramedullary astrocytoma who received up-front vincristine and carboplatin for two months but remained clinically symptomatic. A followup MRI showed a larger tumor, and the patient was switched to the I/C regimen. A marked clinical improvement occurred after the first cycle, and MRI showed a very good partial remission at the end of therapy. At 16 months after diagnosis, the patient remains disease free. The third patient was a 10-month-old girl with a C2-T3 grade II intramedullary astrocytoma. She presented with severe pain that became steroid dependent during the month she was treated with the vincristine-carboplatin regimen. When she was switched to the I/C regimen, the clinical symptoms responded within days. MRI at the end of therapy showed a significant reduction in tumor size, and one year after diagnosis the patient remains symptom free. Using this I/C regimen for childhood intramedullary astrocytoma, we obtained remarkable clinicoradiological responses while avoiding the use of radiotherapy.

Microglia and inflammation: impact on developmental brain injuries

Inflammation during the perinatal period has become a recognized risk factor for developmental brain injuries over the past decade or more. To fully understand the relationship between inflammation and brain development, a comprehensive knowledge about the immune system within the brain is essential. Microglia are resident immune cells within the central nervous system and play a critical role in the development of an inflammatory response within the brain. Microglia are critically involved with both the innate and adaptive immune system, regulating inflammation and cell damage within the brain via activation of Toll-like receptors, production of cytokines, and a myriad of other intracellular and intercellular processes. In this article, microglial physiology is reviewed along with the role of microglia in developmental brain injuries in humans and animal models. Last, microglial functions within the innate and adaptive immune system will be summarized. Understanding the processes of inflammation and microglial activation is critical for formulating effective preventative and therapeutic strategies for developmental brain injuries.