P2X4 receptors induced in spinal microglia gate tactile allodynia after nerve injury

Neurotransmitter receptors on microglia

Microglia are the intrinsic immune cells of the brain and express chemokine and cytokine receptors that interact with the peripheral immune cells. Recent studies have indicated that microglia also respond to the brain's classical signalling substances, the neurotransmitters. Here, we review the evidence for the expression of neurotransmitter receptors on microglia and the consequences of this receptor activation for microglial behaviour. It is evident that neurotransmitters instruct microglia to perform distinct types of responses, such as triggering an inflammatory cascade or acquiring a neuroprotective phenotype. Understanding how microglia respond to different neurotransmitters will thus have important implications for controlling the reactivity of these cells in acute injury, as well as for treating chronic neurodegenerative diseases.

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.

Go it alone no more--P2X7 joins the society of heteromeric ATP-gated receptor channels

P2X receptors (P2XR) function as ATP-gated nonselective ion channels permeable to Na+, K+, and Ca2+, and they are expressed in a wide range of excitable, epithelial/endothelial, and immune effector cell types. The channels are trimeric complexes composed of protein subunits encoded by seven different P2XR genes expressed in mammalian and other vertebrate genomes. Current genetic, biochemical, and/or physiological evidence indicates that the extended family of functional P2X receptors includes six homomeric channels composed of P2X1, P2X2, P2X3, P2X4, P2X5, or P2X7 subunits and six heteromeric channels that involve subunit pairings of P2X1/P2X2, P2X1/P2X4, P2X1/P2X5, P2X2/P2X3, P2X2/P2X6, or P2X4/P2X6. Thus, all P2XR subtypes--with the salient exception of P2X7R--have previously been implicated in the assembly of heteromeric ATP-gated ion channels that can comprise unique pharmacological targets in different tissues. The assumed "go-it alone" function of the P2X7R has important implications because agents that target this particular receptor have been proposed as useful therapeutics in various autoinflammatory diseases or amelioration of inflammatory pain. However, this assumption and the interpretations based on it now require reevaluation in light of a new report in this issue of Molecular Pharmacology (p. 1447) that provides convincing biochemical and electrophysiological evidence for the existence of P2X4/P2X7 heteromeric receptors.

ATP and acetylcholine, equal brethren

Acetylcholine was the first neurotransmitter identified and ATP is the hitherto final compound added to the list of small molecule neurotransmitters. Despite the wealth of evidence assigning a signaling role to extracellular ATP and other nucleotides in neural and non-neural tissues, the significance of this signaling pathway was accepted very reluctantly. In view of this, this short commentary contrasts the principal molecular and functional components of the cholinergic signaling pathway with those of ATP and other nucleotides. It highlights pathways of their discovery and analyses tissue distribution, synthesis, uptake, vesicular storage, receptors, release, extracellular hydrolysis as well as pathophysiological significance. There are differences but also striking similarities. Comparable to ACh, ATP is taken up and stored in synaptic vesicles, released in a Ca(2+)-dependent manner, acts on nearby ligand-gated or metabotropic receptors and is hydrolyzed extracellularly. ATP and acetylcholine are also costored and coreleased. In addition, ATP is coreleased from biogenic amine storing nerve terminals as well as from at least subpopulations of glutamatergic and GABAergic terminals. Both ACh and ATP fulfill the criteria postulated for neurotransmitters. More recent evidence reveals that the two messengers are not confined to neural functions, exerting a considerable variety of non-neural functions in non-innervated tissues. While it has long been known that a substantial number of pathologies originate from malfunctions of the cholinergic system there is now ample evidence that numerous pathological conditions have a purinergic component.

New insights into purinergic receptor signaling in neuronal differentiation, neuroprotection, and brain disorders

Ionotropic P2X and metabotropic P2Y purinergic receptors are expressed in the central nervous system and participate in the synaptic process particularly associated with acetylcholine, GABA, and glutamate neurotransmission. As a result of activation, the P2 receptors promote the elevation of free intracellular calcium concentration as the main signaling pathway. Purinergic signaling is present in early stages of embryogenesis and is involved in processes of cell proliferation, migration, and differentiation. The use of new techniques such as knockout animals, in vitro models of neuronal differentiation, antisense oligonucleotides to induce downregulation of purinergic receptor gene expression, and the development of selective inhibitors for purinergic receptor subtypes contribute to the comprehension of the role of purinergic signaling during neurogenesis. In this review, we shall discuss the participation of purinergic receptors in developmental processes and in brain physiology, including neuron-glia interactions and pathophysiology.

Astroglial glutamate-glutamine shuttle is involved in central sensitization of nociceptive neurons in rat medullary dorsal horn

Growing evidence suggests that astroglia are involved in pain states, but no studies have tested their possible involvement in modulating the activity of nociceptive neurons per se. This study has demonstrated that the central sensitization induced in functionally identified nociceptive neurons in trigeminal subnucleus caudalis (the medullary dorsal horn) by application of an inflammatory irritant to the rat's tooth pulp can be significantly attenuated by continuous intrathecal superfusion of methionine sulfoximine (MSO; 0.1 mM), an inhibitor of the astroglial enzyme glutamine synthetase that is involved in the glutamate-glutamine shuttle. Simultaneous superfusion of MSO and glutamine (0.25 mM) restored the irritant-induced central sensitization. In control experiments, superfusion of either MSO or glutamine alone, or vehicle, did not produce any significant changes in neuronal properties. These findings suggest that the astroglial glutamate-glutamine shuttle is essential for the initiation of inflammation-induced central sensitization but that inhibition of astroglial function may not affect normal nociceptive processing.

Evidence for Functional P2X4/P2X7 Heteromeric Receptors

The cytolytic ionotropic ATP receptor P2X7 has several important roles in immune cell regulation, such as cytokine release, apoptosis, and microbial killing. Although P2X7 receptors are frequently coexpressed with another subtype of P2X receptor, P2X4, they are believed not to form heteromeric assemblies but to function only as homomers. Both receptors play a role in neuropathic pain; therefore, understanding how they coordinate the cellular response to ATP is important for the development of effective pain therapies. Here, we provide biochemical and electrophysiological evidence for an association between P2X4 and P2X7 that increases the diversity of receptor currents mediated via these two subtypes. The heterologously expressed receptors were coimmunoprecipitated from human embryonic kidney (HEK) 293 cells, and the endogenous P2X4 and P2X7 receptors were similarly coimmunoprecipitated from bone marrow-derived macrophages. In HEK293 cells, the fraction of P2X4 receptors biotinylated at the plasma membrane increased 2-fold in the presence of P2X7 although there was no change in overall expression. Coexpression of a dominant-negative P2X4 mutant (C353W) with P2X7, inhibited P2X7 receptor mediated currents by greater than 2-fold, whereas a nonfunctional but non-dominant-negative mutant (S341W) did not. Coexpression of P2X4S341W with P2X7 produced a current that was potentiated by ivermectin and inhibited by 2',3'-O-(2,4,6-trinitrophenyl) adenosine 5-triphosphate (TNP-ATP), whereas expression of P2X7 alone produced a current that was insensitive to both of these compounds at the concentrations used. These results demonstrate a structural and functional interaction between P2X4 and P2X7, which suggests that they associate to form heteromeric receptors.

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.

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.

P2X currents in peritoneal macrophages of wild type and P2X4 -/- mice

In this study the ATP-induced (P2X) currents in isolated peritoneal macrophages of wild type (WT) and P2X(4) knockout (P2X(4)(-/-)) mice were studied by means of whole-cell patch clamp in order to (1) survey the P2X currents of native macrophages and (2) to investigate the expression of P2X(4)-like currents in the WT versus P2X(4)(-/-) mice. Three types of currents were observed in the isolated macrophages: (1) in approximately 10% of both WT and P2X(4)(-/-) macrophages a fast activating and inactivating P2X1-like current was recorded with low concentrations (0.1-1 microM) of ATP; (2) 85% of wild type and 100% of P2X(4)(-/-) macrophages exhibited a non-desensitizing P2X(7)-like current activated at high concentrations of ATP (10mM). The identity of the P2X(7) current was confirmed using the specific blocker A-740003; (3) 88.6% of the WT but none of the P2X(4)(-/-) macrophages showed a small P2X(4)-like current that desensitized slowly upon ATP application at intermediate concentrations (3-300 microM). Several observations indicated that the slowly desensitizing current in WT macrophages was P2X(4). The EC50 value of 5.3 microM ATP was as expected for P2X(4) and the current induced by 3-300 microM ATP was absent in P2X(4)(-/-) mice. Upon application of 3 microM ivermectin, a P2X(4)-selective modulator, the amplitude of this current was increased and the desensitization was inhibited in WT cells. In addition, this current was facilitated by 10 microM Zn(2+) but inhibited by Cu(2+) (in contrast to P2X(2)). We conclude that the P2X(4) and P2X(7) currents are functionally expressed in recruited peritoneal macrophages of WT mice and that the P2X(4)-like current is absent in P2X(4)(-/-) mice.

Evidence for suppression of electroacupuncture on spinal glial activation and behavioral hypersensitivity in a rat model of monoarthritis

Our previous study demonstrated that single intrathecal (i.t.) application of fluorocitrate, a glial metabolic inhibitor, synergized electroacupuncture (EA) antagonizing behavioral hypersensitivity in complete Freund's adjuvant (CFA)-induced monoarthritic rat. To further investigate the relationship between spinal glial activation and EA analgesia, the present study examined the effects of multiple EA on spinal glial activation evoked by monoarthritis (MA). The results showed that (1) unilateral intra-articular injection of CFA produced a robust glial activation on the spinal cord, which was associated with the development and maintenance of behavioral hypersensitivity; (2) multiple EA stimulation of ipsilateral "Huantiao" (GB30) and "Yanglingquan" (GB34) acupoints or i.t. injection of fluorocitrate (1 nmol) significantly suppressed spinal glial activation; (3) inhibitory effects of EA on spinal glial activation and behavioral hypersensitivity were significantly enhanced when EA combined with fluorocitrate, indicating that disruption of glial function may potentiate EA analgesia in inflammatory pain states. These data suggested that analgesic effects of EA might be associated with its counter-regulation to spinal glial activation, and thereby provide a potential strategy for the treatment of arthritis.

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.

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.

Reduced pain behaviors and extracellular signal-related protein kinase activation in primary sensory neurons by peripheral tissue injury in mice lacking platelet-activating factor receptor

Peripheral tissue injury causes the release of various mediators from damaged and inflammatory cells, which in turn activates and sensitizes primary sensory neurons and thereby produces persistent pain. The present study investigated the role of platelet-activating factor (PAF), a phospholipid mediator, in pain signaling using mice lacking PAF receptor (pafr-/- mice). Here we show that pafr-/- mice displayed almost normal responses to thermal and mechanical stimuli but exhibit attenuated persistent pain behaviors resulting from tissue injury by locally injecting formalin at the periphery as well as capsaicin pain and visceral inflammatory pain without any alteration in cytoarchitectural or neurochemical properties in dorsal root ganglion (DRG) neurons and a defect in motor function. However, pafr-/- mice showed no alterations in spinal pain behaviors caused by intrathecally administering agonists for N-methyl-d-aspartate (NMDA) and neurokinin(1) receptors. A PAFR agonist evoked an intracellular Ca(2+) response predominantly in capsaicin-sensitive DRG neurons, an effect was not observed in pafr-/- mice. By contrast, the PAFR agonist did not affect C- or Adelta-evoked excitatory post-synaptic currents in substantia gelatinosa neurons in the dorsal horn. Interestingly, mice lacking PAFR showed reduced phosphorylation of extracellular signal-related protein kinase (ERK), an important kinase for the sensitization of primary sensory neurons, in their DRG neurons after formalin injection. Furthermore, U0126, a specific inhibitor of the ERK pathway suppressed the persistent pain by formalin. Thus, PAFR may play an important role in both persistent pain and the sensitization of primary sensory neurons after tissue injury.

P2X3 receptor involvement in pain states

The understanding of how pain is processed at each stage in the peripheral and central nervous system is the precondition to develop new therapies for the selective treatment of pain. In the periphery, ATP can be released from various cells as a consequence of tissue injury or visceral distension and may stimulate the local nociceptors. The highly selective distribution of P2X(3) and P2X(2/3) receptors within the nociceptive system has inspired a variety of approaches to elucidate the potential role of ATP as a pain mediator. Depolarization by ATP of neurons in pain-relevant neuronal structures such as trigeminal ganglion, dorsal root ganglion, and spinal cord dorsal horn neurons are well investigated. P2X receptor-mediated afferent activation appears to have been implicated in visceral and neuropathic pain and even in migraine and cancer pain. This article reviews recently published research describing the role that ATP and P2X receptors may play in pain perception, highlighting the importance of the P2X(3) receptor in different states of pain.

An evolving cellular pathology occurs in dorsal root ganglia, peripheral nerve and spinal cord following intravenous administration of paclitaxel in the rat

Paclitaxel (Taxol) is a frontline antineoplastic agent used to treat a variety of solid tumors including breast, ovarian, or lung cancer. The major dose limiting side effect of paclitaxel is a peripheral sensory neuropathy that can last days to a lifetime. To begin to understand the cellular events that contribute to this neuropathy, we examined a marker of cell injury/regeneration (activating transcription factor 3; ATF3), macrophage hyperplasia/hypertrophy; satellite cell hypertrophy in the dorsal root ganglia (DRG) and sciatic nerve as well as astrocyte and microglial activation within the spinal cord at 1, 4, 6 and 10 days following intravenous infusion of therapeutically relevant doses of paclitaxel. At day 1 post-infusion, there was an up-regulation of ATF3 in a subpopulation of large and small DRG neurons and this up-regulation was present through day 10. In contrast, hypertrophy of DRG satellite cells, hypertrophy and hyperplasia of CD68(+) macrophages in the DRG and sciatic nerve, ATF3 expression in S100beta(+) Schwann cells and increased expression of the microglial marker (CD11b) and the astrocyte marker glial fibrillary acidic protein (GFAP) in the spinal cord were not observed until day 6 post-infusion. The present results demonstrate that using the time points and markers examined, DRG neurons show the first sign of injury which is followed days later by other neuropathological changes in the DRG, peripheral nerve and dorsal horn of the spinal cord. Understanding the cellular changes that generate and maintain this neuropathy may allow the development of mechanism-based therapies to attenuate or block this frequently painful and debilitating condition.

Role of the CX3CR1/p38 MAPK pathway in spinal microglia for the development of neuropathic pain following nerve injury-induced cleavage of fractalkine

Accumulating evidence suggests that microglial cells in the spinal cord play an important role in the development of neuropathic pain. However, it remains largely unknown how glia interact with neurons in the spinal cord after peripheral nerve injury. Recent studies suggest that the chemokine fractalkine may mediate neural/microglial interaction via its sole receptor CX3CR1. We have examined how fractalkine activates microglia in a neuropathic pain condition produced by spinal nerve ligation (SNL). SNL induced an upregulation of CX3CR1 in spinal microglia that began on day 1, peaked on day 3, and maintained on day 10. Intrathecal injection of a neutralizing antibody against CX3CR1 suppressed not only mechanical allodynia but also the activation of p38 MAPK in spinal microglia following SNL. Conversely, intrathecal infusion of fractalkine produced a marked p38 activation and mechanical allodynia. SNL also induced a dramatic reduction of the membrane-bound fractalkine in the dorsal root ganglion, suggesting a cleavage and release of this chemokine after nerve injury. Finally, application of fractalkine to spinal slices did not produce acute facilitation of excitatory synaptic transmission in lamina II dorsal horn neurons, arguing against a direct action of fractalkine on spinal neurons. Collectively, our data suggest that (a) fractalkine cleavage (release) after nerve injury may play an important role in neural-glial interaction, and (b) microglial CX3CR1/p38 MAPK pathway is critical for the development of neuropathic pain.

Satellite glial cells in sensory ganglia: their possible contribution to inflammatory pain

Neurons in dorsal root ganglia (DRG) are surrounded by an envelope of satellite glial cells (SGCs). Little is known about SGC physiology and their interactions with neurons. In this work, we investigated changes in mouse DRG neurons and SGC following the induction of inflammation in the hind paw by the injection of complete Freund's adjuvant (CFA). The electrophysiological properties of neurons were characterized by intracellular electrodes. Changes in coupling mediated by gap junctions between SGCs were monitored using intracellular injection of the fluorescent dye Lucifer yellow. Pain was assessed with von Frey hairs. We found that two weeks after CFA injection there was a 38% decrease in the threshold for firing an action potential in DRG neurons, consistent with neuronal hyperexcitability. Injection of Lucifer yellow into SGCs revealed that, compared with controls, coupling by gap junctions among SGCs surrounding adjacent neurons increased 2.7-, 3.2-, and 2.5-fold one week, two weeks, and one month, respectively, after CFA injection. In SGCs enveloping neurons that project into the inflamed paw this effect was more enhanced (5.4-fold). Interneuronal coupling was augmented by up to 7% after CFA injection. Pain threshold in the injected paw decreased by 13%, 16%, and 11% compared with controls at one week, two weeks, and one month, respectively, after CFA injection. Intraperitoneal injection of the gap junction blocker carbenoxolone prevented the inflammation-induced decrease in pain threshold. The results show that augmented glial coupling is one of the major events occurring in DRG following inflammation. The elevation in pain threshold after carbenoxolone administration provides indirect support for the idea that augmented intercellular coupling might contribute to chronic pain.

Involvement of EphB1 receptor/EphrinB2 ligand in neuropathic pain

STUDY DESIGN

We investigated involvement of EphB/ephrinB system in neuropathic pain.

OBJECTIVE

Using immunoblotting, immunohistochemistry, and RNA interference techniques, we examined the expression levels of EphB receptors and ephrinB ligands in neuropathic pain. We also explored the effect of ephrinB siRNA for neuropathic pain.

SUMMARY OF BACKGROUND DATA

It has been reported that EphB2 regulates the development of synaptic plasticity in the hippocampus by interacting with N-methyl-D-aspartate (NMDA) receptors. In acute pain models, it has been clear that EphB1/ephrinB2 interactions via the NMDA receptor modulates synaptic efficacy in spinal cord.

METHODS

Adult female Sprague-Dawley rats were used in this study. A crush injury model was prepared by crushing the left L5 spinal nerve distal to dorsal root ganglions (DRG) under deep anesthesia. The sham operation was subjected as control. Expression of ephrinB2 and EphB1 were examined by immunoblotting and immunohistochemical analyses with anti-EphB and anti-ephrinB antibodies. To assess involvement of ephrinB in neuropathic pain, we examined the effect of small interference RNA (siRNA) on mechanical allodynia.

RESULTS

Among EphB and ephrinB isoforms tested, ephrinB2 and EphB1 were predominant in DRG and spinal cord. Results showed that the expression of ephrinB2 was enhanced in neurons in DRG and spinal cord by the injury in a time-dependent manner. EphB1 was expressed in neurons of spinal cord. Administration of ephrinB2 siRNA reduced the expression of ephrinB2 and mechanical allodynia.

CONCLUSION

Expression of ephrinB2 is enhanced by nerve injury in neurons in DRG and spinal cord, while its receptor EphB1 is expressed in spinal cord. These results suggest that induction of ephrinB2 might activate EphB1/ephrinB2 signaling pathway to regulate synaptic plasticity and reorganization, and that ephrinB2 siRNA could be a potential therapeutic agent for neuropathic pain.

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.

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.

Intrathecal administration of ATP produces long-lasting allodynia in rats: differential mechanisms in the phase of the induction and maintenance

Several lines of evidence suggest that extracellular ATP plays a role in pain signaling through the activation of ionotropic P2X-receptors, especially homomeric P2X3- and heteromeric P2X2/3-receptors on capsaicin-sensitive and -insensitive primary afferent neurons, respectively, at peripheral and spinal sites. We investigated the mechanisms of the induction and maintenance of mechanical allodynia produced by a single intrathecal (i.t.) administration of ATP in rats. We found that i.t. administration of ATP and the P2X-receptor agonist alpha,beta-methylene-ATP produced tactile allodynia which lasted more than 1 week. The i.t. ATP- and alpha,beta-methylene-ATP-produced long-lasting allodynia remained in neonatal capsaicin-treated adult rats. I.t. administration of a P2X3/P2X2/3-receptor selective antagonist completely prevented the induction (co-administration on day 0) and partially attenuated the early phase (day 1 post-ATP administration), but not the late phase (day 7 post-ATP administration) of maintenance of allodynia. The N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 completely prevented the induction phase, but not the early and late phases of maintenance of allodynia. Immunohistochemical and immunoblotting studies for microglial and astrocytic markers revealed that i.t. ATP administration caused spinal microglial activation within 1 day, and astrocytic activation which peaked at 1-3 days after ATP administration. Furthermore, minocycline, a microglial inhibitor, attenuated the induction but not the early and late phases of maintenance, while fluorocitrate, a glial metabolic inhibitor, attenuated the induction and the early phase but not the late phase of maintenance. Taken together, these results suggest that the activation of P2X-receptors, most likely spinal P2X2/3-receptors on capsaicin-insensitive primary afferent neurons, triggers the induction of long-lasting allodynia through NMDA receptors, and the induction and early maintenance phase, but not the late phase, is mediated through the functions of spinal glial cells.

Glial-cytokine-neuronal interactions underlying the mechanisms of persistent pain

The emerging literature implicates a role for glia/cytokines in persistent pain. However, the mechanisms by which these non-neural elements contribute to CNS activity-dependent plasticity and pain are unclear. Using a trigeminal model of inflammatory hyperalgesia, here we provide evidence that demonstrates a mechanism by which glia interact with neurons, leading to activity-dependent plasticity and hyperalgesia. In response to masseter inflammation, there was an upregulation of glial fibrillary acidic proteins (GFAPs), a marker of astroglia, and interleukin-1beta (IL-1beta), a prototype proinflammatory cytokine, in the region of the trigeminal nucleus specifically related to the processing of deep orofacial input. The activated astroglia exhibited hypertrophy and an increased level of connexin 43, an astroglial gap junction protein. The upregulated IL-1beta was selectively localized to astrocytes but not to microglia and neurons. Local anesthesia of the masseter nerve prevented the increase in GFAP and IL-1beta after inflammation, and substance P, a prototype neurotransmitter of primary afferents, induced similar increases in GFAP and IL-1beta, which was blocked by a nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester. Injection of IL-1 receptor antagonist and fluorocitrate, a glial inhibitor, attenuated hyperalgesia and NMDA receptor phosphorylation after inflammation. In vitro application of IL-1beta induced NR1 phosphorylation, which was blocked by an IL-1 receptor antagonist, a PKC inhibitor (chelerythrine), an IP3 receptor inhibitor (2-aminoethoxydiphenylborate), and inhibitors of phospholipase C [1-[6-((17b-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl]-1H-pyrrole-2,5-dione] and phospholipase A2 (arachidonyltrifluoromethyl ketone). These findings provide evidence of astroglial activation by tissue injury, concomitant IL-1beta induction, and the coupling of NMDA receptor phosphorylation through IL-1 receptor signaling.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

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.

Expression of RhoA by inflammatory macrophages and T cells in rat experimental autoimmune neuritis

RhoA is one of the best-studied members of Rho GTPases. Experimental autoimmune neuritis (EAN), which is characterized by infiltration of T cells and macrophages into the peripheral nervous system, is an autoantigen-specific T-cell-mediated animal model of human Guillain-Barré Syndrome. In this study, RhoA expression has been investigated in the dorsal/ventral roots of EAN rats by immunohistochemistry. A significant accumulation of RhoA+ cells was observed on Day 12, with a maximum around Day 15, correlating to the clinical severity of EAN. In dorsal/ventral roots of EAN, RhoA+ cells were seen in perivascular areas but also in the parenchyma. Furthermore, double-labelling experiments showed that the major cellular sources of RhoA were reactive macrophages and T cells. In conclusion, this is the first demonstration of the presence of RhoA in the dorsal/ventral roots of EAN. The time courses and cellular sources of RhoA together with the functions of RhoA indicate that RhoA may function to facilitate macrophage and T-cell infiltration in EAN and therefore could be a potential therapeutic target.

P2X7-related modulation of pathological nociception in rats

Growing evidence supports a role for the immune system in the induction and maintenance of chronic pain. ATP is a key neurotransmitter in this process. Recent studies demonstrate that the glial ATP receptor, P2X7, contributes to the modulation of pathological pain. To further delineate the endogenous mechanisms that are involved in P2X7-related antinociception, we utilized a selective P2X7 receptor antagonist, A-438079, in a series of in vivo and in vitro experiments. Injection of A-438079 (10-300 micromol/kg, i.p.) was anti-allodynic in three different rat models of neuropathic pain and it attenuated formalin-induced nocifensive behaviors. Using in vivo electrophysiology, A-438079 (80 micromol/kg, i.v.) reduced noxious and innocuous evoked activity of different classes of spinal neurons (low threshold, nociceptive specific, wide dynamic range) in neuropathic rats. The effects of A-438079 on evoked firing were diminished or absent in sham rats. Spontaneous activity of all classes of spinal neurons was also significantly reduced by A-438079 in neuropathic but not sham rats. In vitro, A-438079 (1 microM) blocked agonist-induced (2,3-O-(4-benzoylbenzoyl)-ATP, 30 microM) current in non-neuronal cells taken from the vicinity of the dorsal root ganglia. Furthermore, A-438079 dose-dependently (0.3-3 microM) decreased the quantity of the cytokine, interleukin-1beta, released from peripheral macrophages. Thus, ATP, acting through the P2X7 receptor, exerts a wide-ranging influence on spinal neuronal activity following a chronic injury. Antagonism of the P2X7 receptor can in turn modulate central sensitization and produce antinociception in animal models of pathological pain. These effects are likely mediated through immuno-neural interactions that affect the release of endogenous cytokines.

Cysteine substitution mutants give structural insight and identify ATP binding and activation sites at P2X receptors

P2X receptors for extracellular ATP are a distinct family of ligand-gated cation channels involved in physiological processes ranging from synaptic transmission to muscle contraction. Common ATP binding motifs are absent from P2X receptors, and the extent of the agonist binding site is unclear. We used cysteine-scanning mutagenesis, radiolabeled 2-azido ATP binding, and methanethiosulfonate (MTS) compounds to identify amino acid residues involved in ATP binding and gating of the human P2X1 receptor. The pattern of MTSEA [(2-aminoethyl)methanethiosulfonate hydrobromide] biotinylation was also used to determine the accessibility of substituted cysteine residues and whether this changed on addition of ATP. Analysis of cysteine-substituted mutants of the last 44 amino acid residues (S286-I329) in the extracellular loop before the second transmembrane segment showed that N290, F291, R292, and K309 mutants had reduced ATP potency and 2-azido ATP binding. MTS reagents produced additional shifts in ATP potency at these residues, suggesting that they are directly involved in ATP binding; the effects were dependent on the charge of the MTS reagent at K309C; one explanation for this is that K309 interacts directly with the negatively charged phosphate of ATP. The remainder of the cysteine substitutions had little or no effect on ATP potency. However, at the mutants D316C, G321C, A323C, and I328C, MTS reagents did not change ATP potency but modified agonist-evoked responses, suggesting that this region may contribute to the gating of the channel.

The Effects of General Anesthetics on P2X7 and P2Y Receptors in a Rat Microglial Cell Line

BACKGROUND

Microglial cells play important roles in coordinating the inflammatory brain responses to hypoxia and trauma. Ionotropic P2X receptors and metabotropic P2Y receptors (P2YRs) expressed in microglia can be activated by extracellular adenosine triphosphate (ATP) derived from damaged cells or astrocytes, and participate in the signaling pathways evoked in brain insult. Although several inhaled and IV anesthetics produce neuroprotective effects through neuronal mechanisms, little is known about how general anesthetics modulate microglial responses in the pathological state. We examined the effects of various general anesthetics on purinergic responses in a rat microglial cell line.

METHODS

Currents were consistently activated by applications of ATP via a U-tube system under the whole-cell configuration. ATP-induced nondesensitizing currents observed after several applications of ATP exhibited characteristics of P2X7 receptors. The P2YRs-mediated mobilization of intracellular Ca2+ was measured using a Ca2+-sensitive fluorescent dye (fura-2).

RESULTS

Inhaled anesthetics (sevoflurane, isoflurane, and halothane) at doses three times as high as minimum alveolar concentrations had no effect on the P2X7Rs-mediated currents. IV anesthetics (ketamine, propofol, and thiopental) enhanced the P2X7Rs-mediated currents reversibly. The potencies for activation of P2X7Rs were not correlated with the octanol/buffer partition coefficients. Thiopental, at low concentrations, slightly inhibited the P2X7Rs-mediated currents, suggesting its dual actions on P2X7Rs. The P2YRs-mediated mobilization of intracellular Ca2+ was not affected by any of the general anesthetics tested.

CONCLUSIONS

Our results suggest that IV anesthetics, particularly thiopental and propofol, may modulate microglial functions through P2X7Rs in pathological conditions.

Discovery of P2X7 receptor-selective antagonists offers new insights into P2X7 receptor function and indicates a role in chronic pain states

ATP-sensitive P2X(7) receptors are localized on cells of immunological origin including peripheral macrophages and glial cells in the CNS. Activation of P2X(7) receptors leads to rapid changes in intracellular calcium concentrations, release of the proinflammatory cytokine interleukin-1beta and following prolonged agonist exposure, the formation of cytolytic pores in plasma membranes. Both the localization and functional consequences of P2X(7) receptor activation indicate a role in inflammatory processes. The phenotype of P2X(7) receptor gene-disrupted mice also indicates that P2X(7) receptor activation contributes to ongoing inflammation. More recently, P2X(7) receptor knockout data has also suggested a specific role in inflammatory and neuropathic pain states. The recent discovery of potent and highly selective antagonists for P2X(7) receptors has helped to further clarify P2X receptor pharmacology, expanded understanding of P2X(7) receptor signaling, and offers new evidence that P2X(7) receptors play a specific role in nociceptive signaling in chronic pain states. In this review, we incorporate the recent discoveries of novel P2X(7) receptor-selective antagonists with a brief update on P2X(7) receptor pharmacology and its therapeutic potential.

Involvement of P2X4 and P2Y12 receptors in ATP-induced microglial chemotaxis

We previously reported that extracellular ATP induces membrane ruffling and chemotaxis of microglia and suggested that their induction is mediated by the Gi/o-protein coupled P2Y(12) receptor (P2Y(12)R). Here we report discovering that the P2X(4) receptor (P2X(4)R) is also involved in ATP-induced microglial chemotaxis. To understand the intracellular signaling pathway downstream of P2Y(12)R that underlies microglial chemotaxis, we examined the effect of two phosphatidylinositol 3'-kinase (PI3K) inhibitors, wortmannin, and LY294002, on chemotaxis in a Dunn chemotaxis chamber. The PI3K inhibitors significantly suppressed chemotaxis without affecting ATP-induced membrane ruffling. ATP stimulation increased Akt phosphorylation in the microglia, and the increase was reduced by the PI3K inhibitors and a P2Y(12)R antagonist. These results indicate that P2Y(12)R-mediated activation of the PI3K pathway is required for microglial chemotaxis in response to ATP. We also found that the Akt phosphorylation was reduced when extracellular calcium was chelated, suggesting that ionotropic P2X receptors are involved in microglial chemotaxis by affecting the PI3K pathway. We therefore tested the effect of various P2X(4)R antagonists on the chemotaxis, and the results showed that pharmacological blockade of P2X(4)R significantly inhibited it. Knockdown of the P2X(4) receptor in microglia by RNA interference through the lentivirus vector system also suppressed the microglial chemotaxis. These results indicate that P2X(4)R as well as P2Y(12)R is involved in ATP-induced microglial chemotaxis.

Lysophosphatidylcholine potentiates Ca2+ influx, pore formation and p44/42 MAP kinase phosphorylation mediated by P2X7 receptor activation in mouse microglial cells

The P2X7 receptor (P2X7R) is an ATP-gated ion channel highly expressed in microglia. P2X7R plays important roles in inflammatory responses in the brain. However, little is known about the mechanisms regulating its functions in microglia. Lysophosphatidylcholine (LPC), an inflammatory phospholipid that promotes microglial activation, may have some relevance to P2X7R signaling in terms of microglial function. In this study, we examined its effects on P2X7R signaling in a mouse microglial cell line (MG6) and primary microglia. LPC facilitated the sustained increase in the intracellular Ca(2+) concentration ([Ca(2+)](i)) through P2X7R channels activated by ATP or BzATP. The potentiated increase in [Ca(2+)](i) was actually inhibited by P2X7R antagonists, brilliant blue G and oxidized ATP. The potentiating effect of LPC was not observed with P2Y receptor systems, which are also expressed in MG6 cells. G2A, a receptor for LPC, was expressed in MG6 cells, but not involved in the facilitating effect of LPC on the P2X7R-mediated change in [Ca(2+)](i). Furthermore, LPC enhanced the P2X7R-associated formation of membrane pores and the activation of p44/42 mitogen-activated protein kinase. These results suggest that LPC may regulate microglial functions in the brain by enhancing the sensitivity of P2X7R.

UDP acting at P2Y6 receptors is a mediator of microglial phagocytosis

Microglia, brain immune cells, engage in the clearance of dead cells or dangerous debris, which is crucial to the maintenance of brain functions. When a neighbouring cell is injured, microglia move rapidly towards it or extend a process to engulf the injured cell. Because cells release or leak ATP when they are stimulated or injured, extracellular nucleotides are thought to be involved in these events. In fact, ATP triggers a dynamic change in the motility of microglia in vitro and in vivo, a previously unrecognized mechanism underlying microglial chemotaxis; in contrast, microglial phagocytosis has received only limited attention. Here we show that microglia express the metabotropic P2Y6 receptor whose activation by endogenous agonist UDP triggers microglial phagocytosis. UDP facilitated the uptake of microspheres in a P2Y6-receptor-dependent manner, which was mimicked by the leakage of endogenous UDP when hippocampal neurons were damaged by kainic acid in vivo and in vitro. In addition, systemic administration of kainic acid in rats resulted in neuronal cell death in the hippocampal CA1 and CA3 regions, where increases in messenger RNA encoding P2Y6 receptors that colocalized with activated microglia were observed. Thus, the P2Y6 receptor is upregulated when neurons are damaged, and could function as a sensor for phagocytosis by sensing diffusible UDP signals, which is a previously unknown pathophysiological function of P2 receptors in microglia.

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.

A critical role of toll-like receptor 2 in nerve injury-induced spinal cord glial cell activation and pain hypersensitivity

The activation of spinal cord glial cells has been implicated in the development of neuropathic pain upon peripheral nerve injury. The molecular mechanisms underlying glial cell activation, however, have not been clearly elucidated. In this study, we found that damaged sensory neurons induce the expression of tumor necrosis factor-alpha, interleukin-1beta, interleukin-6, and inducible nitric-oxide synthase genes in spinal cord glial cells, which is implicated in the development of neuropathic pain. Studies using primary glial cells isolated from toll-like receptor 2 knock-out mice indicate that damaged sensory neurons activate glial cells via toll-like receptor 2. In addition, behavioral studies using toll-like receptor 2 knock-out mice demonstrate that the expression of toll-like receptor 2 is required for the induction of mechanical allodynia and thermal hyperalgesia due to spinal nerve axotomy. The nerve injury-induced spinal cord microglia and astrocyte activation is reduced in the toll-like receptor 2 knock-out mice. Similarly, the nerve injury-induced pro-inflammatory gene expression in the spinal cord is also reduced in the toll-like receptor 2 knock-out mice. These data demonstrate that toll-like receptor 2 contributes to the nerve injury-induced spinal cord glial cell activation and subsequent pain hypersensitivity.

Behavioural and morphological evidence for the involvement of glial cell activation in delta opioid receptor function: implications for the development of opioid tolerance

Previous studies have demonstrated that prolonged morphine treatment in vivo induces the translocation of delta opioid receptors (δORs) from intracellular compartments to neuronal plasma membranes and this trafficking event is correlated with an increased functional competence of the receptor. The mechanism underlying this phenomenon is unknown; however chronic morphine treatment has been shown to involve the activation and hypertrophy of spinal glial cells. In the present study we have examined whether activated glia may be associated with the enhanced δOR-mediated antinociception observed following prolonged morphine treatment. Accordingly, animals were treated with morphine with or without concomitant administration of propentofylline, an inhibitor of glial activation that was previously shown to block the development of morphine antinociceptive tolerance. The morphine regimen previously demonstrated to initiate δOR trafficking induced the activation of both astrocytes and microglia in the dorsal spinal cord as indicated by a significant increase in cell volume and cell surface area. Consistent with previous data, morphine-treated rats displayed a significant augmentation in δOR-mediated antinociception. Concomitant spinal administration of propentofylline with morphine significantly attenuated the spinal immune response as well as the morphine-induced enhancement of δOR-mediated effects. These results complement previous reports that glial activation contributes to a state of opioid analgesic tolerance, and also suggest that neuro-glial communication is likely responsible in part for the altered functional competence in δOR-mediated effects following morphine treatment.

Is functional state of spinal microglia involved in the anti-allodynic and anti-hyperalgesic effects of electroacupuncture in rat model of monoarthritis?

Spinal microglia play a key role for creating exaggerated pain following tissues inflammation or injury. Electroacupuncture (EA) can effectively control the exaggerated pain both in humans with inflammatory disease and animals with experimental inflammatory pain. However, little is known about the relationship between spinal glial activation and EA analgesia. Using immunohistochemistry, RT-PCR analysis, and behavioral testing, the present study demonstrated that (1) Unilateral intra-articular injection of CFA produced a robust microglial activation and the up-regulation of the tumor necrosis factor (TNF)-alpha, interleukin (IL-1beta), and IL-6 mRNA levels in the spinal cord; (2) Repeated intrathecal (i.t.) injection of minocycline (100 microg), a microglial inhibitor, or EA stimulation of ipsilateral "Huantiao"(GB30) and "Yanglingquan" (GB34) acupoints significantly suppressed CFA-induced nociceptive behavioral hypersensitivity and spinal microglial activation; (3) Combination of EA with minocycline significantly enhanced the inhibitory effects of EA on allodynia and hyperalgesia. For the first time, these data provide direct evidence for the involvement of spinal microglial functional state in anti-nociception of EA. Thus, anti-neuroinflammatory effect of EA might be considered as one of the mechanisms of its anti-arthritic pain effects, and thereby a multidisciplinary integrated approach to treating symptoms related to arthritis might be raised.

Norman Cousins Lecture. Glia as the "bad guys": implications for improving clinical pain control and the clinical utility of opioids

Within the past decade, there has been increasing recognition that glia are far more than simply "housekeepers" for neurons. This review explores two recently recognized roles of glia (microglia and astrocytes) in: (a) creating and maintaining enhanced pain states such as neuropathic pain, and (b) compromising the efficacy of morphine and other opioids for pain control. While glia have little-to-no role in pain under basal conditions, pain is amplified when glia become activated, inducing the release of proinflammatory products, especially proinflammatory cytokines. How glia are triggered to become activated is a key issue, and appears to involve a number of neuron-to-glia signals including neuronal chemokines, neurotransmitters, and substances released by damaged, dying and dead neurons. In addition, glia become increasingly activated in response to repeated administration of opioids. Products of activated glia increase neuronal excitability via numerous mechanisms, including direct receptor-mediated actions, upregulation of excitatory amino acid receptor function, downregulation of GABA receptor function, and so on. These downstream effects of glial activation amplify pain, suppress acute opioid analgesia, contribute to the apparent loss of opioid analgesia upon repeated opioid administration (tolerance), and contribute to the development of opioid dependence. The potential implications of such glial regulation of pain and opioid actions are vast, suggestive that targeting glia and their proinflammatory products may provide a novel and effective therapy for controlling clinical pain syndromes and increasing the clinical utility of analgesic drugs.

Multiple P2X receptors are involved in the modulation of apoptosis in human mesangial cells: evidence for a role of P2X4

Apoptosis, a normal event in renal tissue homeostasis, has been considered as a major mechanism for either resolution of glomerular hypercellularity in glomerulonephritis or loss of cellularity and progression to glomerulosclerosis in chronic renal disease. This study was aimed at investigating the role of extracellular ATP (eATP) in mediating apoptosis in human mesangial cells (HMC) and identifying the subtype(s) of purinergic receptors involved. eATP, but not uridin-5'-triphosphate (UTP), caused dose-dependent modifications of cellular morphology, as assessed by contrast-phase microscopy, and late apoptosis, as measured by Annexin V/propidium iodide-based flow cytometry and caspase-3 activation. Both phenomena were prevented by the P2X antagonist oxidized-ATP. 2', 3'-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate (BzATP) was less effective than ATP, whereas 1[N,O-bis (5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl] -4-phenylpiperazine (KN62), a selective inhibitor of human P2X(7), prevented morphological changes but potentiated apoptosis induced by BzATP. P2X(7) was barely expressed in HMC and showed a relatively scarce functional activity, as assessed by monitoring nucleotide-induced intracellular calcium surge and plasma membrane depolarization by Fura-2/AM and bis[1,3-diethylthiobarbiturate]trimethineoxonal uptake, respectively. These data indicated a negligible role of P2X(7) in eATP-mediated apoptosis and pointed to the involvement of other P2X receptor(s). Molecular and inhibitor studies suggested a main role for P2X(4) receptor in nucleotide-induced apoptosis in HMC, indicating a relevant role for purinergic signaling in regulating death rate in these cells.

Protein kinases as potential targets for the treatment of pathological pain

Pathological pain or clinical pain refers to tissue injury-induced inflammatory pain and nerve injury-induced neuropathic pain and is often chronic. Pathological pain is an expression of neural plasticity that occurs both in the peripheral nervous system (e.g., primary sensory nociceptors), termed peripheral sensitization, and in the central nervous system (e.g., dorsal horn and brain neurons), termed central sensitization. Our insufficient understanding of mechanisms underlying the induction and maintenance of injury-induced neuronal plasticity hinders successful treatment for pathological pain. The human genome encodes 518 protein kinases, representing one of the largest protein families. There is growing interest in developing protein kinase inhibitors for the treatment of a number of diseases. Although protein kinases were not favored as targets for analgesics, studies in the last decade have demonstrated important roles of these kinases in regulating neuronal plasticity and pain sensitization. Multiple protein kinases have been implicated in peripheral and central sensitization following intense noxious stimuli and injuries. In particular, mitogen-activated protein kinases (MAPKs), consisting of extracellular signal-regulated kinase (ERK), p38, and c-Jun N-terminal kinase (JNK), are downstream to many kinases and are activated in primary sensory and dorsal horn neurons by nociceptive activity, growth factors and inflammatory mediators, contributing to the induction and maintenance of pain sensitization via posttranslational, translational, and transcriptional regulation. MAPKs are also activated in spinal glial cells (microglia and astrocytes) after injuries, leading to the synthesis of inflammatory mediators/neuroactive substances that act on nociceptive neurons, enhancing and prolonging pain sensitization. Inhibition of multiple kinases has been shown to attenuate inflammatory and neuropathic pain in different animal models. Development of specific inhibitors for protein kinases to target neurons and glial cells will shed light on the development of new therapies for debilitating chronic pain.

Adenosine and ATP receptors

Adenosine and ATP, via P1 and P2 receptors respectively, can modulate pain transmission under physiological, inflammatory, and neuropathic pain conditions. Such influences reflect peripheral and central actions and effects on neurons as well as other cell types. In general, adenosine A1 receptors produce inhibitory effects on pain in a number of preclinical models and are a focus of attention. In humans, i.v. infusions of adenosine reduce some aspects of neuropathic pain and can reduce postoperative pain. For P2X receptors, there is a significant body of information indicating that inhibition of P2X3 receptors may be useful for relieving inflammatory and neuropathic pain. More recently, data have begun to emerge implicating P2X4, P2X7 and P2Y receptors in aspects of pain transmission. Both P1 and P2 receptors may represent novel targets for pain relief.

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.