Glutamate-induced currents reveal three functionally distinct NMDA receptor populations in rat dorsal horn - effects of peripheral nerve lesion and inflammation

Selective Cell-Surface Expression of Triheteromeric NMDA Receptors

NMDA-type ionotropic glutamate receptors are critically involved in many brain functions and are implicated in a variety of brain disorders. Seven NMDA receptor subunits exist (GluN1, GluN2A-D, and GluN3A-B) that assemble into tetrameric receptor subtypes with distinct functional properties and physiological roles. The majority NMDA receptors are composed of two GluN1 and two GluN2 subunits, which can assemble into four diheteromeric receptors subtypes composed of GluN1 and one type of GluN2 subunit (e.g., GluN1/2A), and presumably also six triheteromeric receptor subtypes composed of GluN1 and two different GluN2 subunits (e.g., GluN1/2A/2B). Furthermore, the GluN1 subunit exists as eight splice variants (e.g., GluN1-1a and GluN1-1b isoforms), and two different GluN1 isoforms can co-assemble to also form triheteromeric NMDA receptors (e.g., GluN1-1a/1b/2A). Here, we describe a method to faithfully express triheteromeric NMDA receptors in heterologous expression systems by controlling the identity of two of the four subunits. This method overcomes the problem that co-expression of three different NMDA receptor subunits generates two distinct diheteromeric receptor subtypes as well as one triheteromeric receptor subtype, thereby confounding studies that require a homogenous population of triheteromeric NMDA receptors. The method has been applied to selectively express recombinant triheteromeric GluN1/2A/2B, GluN1/2A/2C, GluN1/2B/2D, GluN1-1a/GluN1-1b/2A, GluN1-1a/GluN1-1b/2B receptors with negligible co-expression of the respective diheteromeric receptor subtypes. This method therefore enables quantitative evaluation of functional and pharmacological properties of triheteromeric NMDA receptors, some of which are abundant NMDA receptor subtypes in the adult brain.

Glycine: The Smallest Anti-Inflammatory Micronutrient

Glycine is a non-essential amino acid with many functions and effects. Glycine can bind to specific receptors and transporters that are expressed in many types of cells throughout an organism to exert its effects. There have been many studies focused on the anti-inflammatory effects of glycine, including its abilities to decrease pro-inflammatory cytokines and the concentration of free fatty acids, to improve the insulin response, and to mediate other changes. However, the mechanism through which glycine acts is not clear. In this review, we emphasize that glycine exerts its anti-inflammatory effects throughout the modulation of the expression of nuclear factor kappa B (NF-κB) in many cells. Although glycine is a non-essential amino acid, we highlight how dietary glycine supplementation is important in avoiding the development of chronic inflammation.

Leptin Contributes to Neuropathic Pain via Extrasynaptic NMDAR-nNOS Activation

Leptin is an adipocytokine that is primarily secreted by white adipose tissue, and it contributes to the pathogenesis of neuropathic pain in collaboration with N-methyl-D-aspartate receptors (NMDARs). Functional NMDARs are a heteromeric complex that primarily comprise two NR1 subunits and two NR2 subunits. NR2A is preferentially located at synaptic sites, and NR2B is enriched at extrasynaptic sites. The roles of synaptic and extrasynaptic NMDARs in the contribution of leptin to neuropathic pain are not clear. The present study examined whether the important role of leptin in neuropathic pain was related to synaptic or extrasynaptic NMDARs. We used a rat model of spared nerve injury (SNI) and demonstrated that the intrathecal administration of the NR2A-selective antagonist NVP-AAM077 and the NR2B-selective antagonist Ro25-6981 prevented and reversed mechanical allodynia following SNI. Administration of exogenous leptin mimicked SNI-induced behavioral allodynia, which was also prevented by NVP-AAM077 and Ro25-6981. Mechanistic studies showed that leptin enhanced NR2B- but not NR2A-mediated currents in spinal lamina II neurons of naïve rats. Leptin also upregulated the expression of NR2B, which was blocked by the NR2B-selective antagonist Ro25-6981, in cultured dorsal root ganglion (DRG) neurons. Leptin enhanced neuronal nitric oxide synthase (nNOS) expression, which was also blocked by Ro25-6981, in cultured DRG cells. However, leptin did not change NR2A expression, and the NR2A-selective antagonist NVP-AAM077 had no effect on leptin-enhanced nNOS expression. Our data suggest an important cellular link between the spinal effects of leptin and the extrasynaptic NMDAR-nNOS-mediated cellular mechanism of neuropathic pain.

Controlling the "Opioid Epidemic": A Novel Chemical Entity (NCE) to Reduce or Supplant Opiate Use for Chronic Pain

We report on the ongoing project “A Novel Therapeutic to Ameliorate Chronic Pain and Reduce Opiate Use.” Over 100 million adults in the U.S. suffer from intermittent or constant chronic pain, and chronic pain affects at least 10% of the world’s population. The primary pharmaceuticals for treatment of chronic pain have been natural or synthetic opioids and the use of opioids for pain treatment has resulted in what has been called an “epidemic” of opioid abuse, addiction and lethal overdoses. We have, through a process of rational drug design, generated a novel chemical entity (NCE) and have given it the name Kindolor. Kindolor is a non-opiate, non-addicting molecule that was developed specifically to simultaneously control the aberrant activity of three targets on the peripheral sensory system that are integral in the development and propagation of chronic pain. In our initial preclinical studies, we demonstrated the efficacy of Kindolor to reduce or eliminate chronic pain in five animal models. The overall goal of the project is to complete the investigational new drug (IND)-enabling preclinical studies of Kindolor, and once IND approval is gained, we will proceed to the clinical Phase Ia and 1b safety studies and a Phase 2a efficacy study. The work is in its second year, and the present report describes progress toward our overall goal of bringing our compound to a full Phase 2 ready stage.

Disability-specific genes GRIN1, GRIN2 and CNR1 show injury-dependent protein expression in the lumbar spinal cord of CCI rats

The sensory changes triggered by peripheral nerve injury result from functional changes in both neurons and glia in the dorsal horn of the spinal cord. Whether the disrupted affective-motivational states often comorbid with injury-evoked changes in sensation are driven directly by these functional changes is a question only recently investigated. Using a combination of GeneChip microarrays and RT-PCR techniques we identified differences in mRNA expression unique to rats with sustained changes to their social behaviour following sciatic nerve chronic constriction injury (CCI). Amongst these changes were the mRNAs encoding several of the NMDA subunits and the CB1 receptor. However, as protein translation is not a necessary consequence of the upregulation or downregulation of genes we decided to evaluate the functional significance of our initial observations using immunohistochemical detection of their translated protein products to determine their location and abundance in the lumbar spinal cord. Spinal cord tissue from rats with ('Affected'), and without ('Unaffected') changes in social behaviour after CCI was compared with tissue from uninjured controls. The expression of NMDA-1 (NR1) subunit, NMDA-2D subunit, Cannabinoid Receptor 1 (CB1), Glucocorticoid Receptor (GR) and Glial Fibrillary Acidic Protein (GFAP) immunoreactivities was quantified for these rats and revealed that nerve injury increased the expression of NMDA-2D, CB1 and GFAP immunoreactivity compared to uninjured controls. However, these changes were not specific to rats whose social behaviours were 'Affected' or 'Unaffected' by the nerve injury. Our data thus suggest that the development and expression of changes in social behaviour seen in a proportion of rats following CCI are unlikely to be directly related to the spinal changes in NMDA-2D, CB1 and GFAP expression induced by the nerve injury.

Glycine transporter inhibitors: A new avenue for managing neuropathic pain

Interneurons operating with glycine neurotransmitter are involved in the regulation of pain transmission in the dorsal horn of the spinal cord. In addition to interneurons, glycine release also occurs from glial cells neighboring glutamatergic synapses in the spinal cord. Neuronal and glial release of glycine is controlled by glycine transporters (GlyTs). Inhibitors of the two isoforms of GlyTs, the astrocytic type-1 (GlyT-1) and the neuronal type-2 (GlyT-2), decrease pain sensation evoked by injuries of peripheral sensory neurons or inflammation. The function of dorsal horn glycinergic interneurons has been suggested to be reduced in neuropathic pain, which can be reversed by GlyT-2 inhibitors (Org-25543, ALX1393). Several lines of evidence also support that peripheral nerve damage or inflammation may shift glutamatergic neurochemical transmission from N-methyl-D aspartate (NMDA) NR1/NR2A receptor- to NR1/NR2B receptor-mediated events (subunit switch). This pathological overactivation of NR1/NR2B receptors can be reduced by GlyT-1 inhibitors (NFPS, Org-25935), which decrease excessive glycine release from astroglial cells or by selective antagonists of NR2B subunits (ifenprodil, Ro 25-6981). Although several experiments suggest that GlyT inhibitors may represent a novel strategy in the control of neuropathic pain, proving this concept in human beings is hampered by lack of clinically applicable GlyT inhibitors. We also suggest that drugs inhibiting both GlyT-1 and GlyT-2 non-selectively and reversibly, may favorably target neuropathic pain. In this paper we overview inhibitors of the two isoforms of GlyTs as well as the effects of these drugs in experimental models of neuropathic pain. In addition, the possible mechanisms of action of the GlyT inhibitors, i.e. how they affect the neurochemical and pain transmission in the spinal cord, are also discussed. The growing evidence for the possible therapeutic intervention of neuropathic pain by GlyT inhibitors further urges development of drugable compounds, which may beneficially restore impaired pain transmission in various neuropathic conditions.

Impaired social behaviors and minimized oxytocin signaling of the adult mice deficient in the N-methyl-d-aspartate receptor GluN3A subunit

The N-methyl-d-aspartate receptor (NMDAR) has been implicated in the pathophysiology of neurological diseases, such as schizophrenia, autism spectrum disorders (ASD), and Alzheimer's disease (AD), whose unique clinical hallmark is a constellation of impaired social and/or cognitive behaviors. GluN3A (NR3A) is a unique inhibitory subunit in the NMDAR complex. The role of GluN3A in social behavioral activities is obscure. In this study, we sought to evaluate altered social activities in adult GluN3A knockout (KO) mice. GluN3A KO mice spent less time in reciprocal social interaction in the social interaction test compared to wild-type (WT) mice. A social approach test using a three-chamber system confirmed that mice lacking GluN3A had lower sociability and did not exhibit a preference for social novelty. GluN3A KO mice displayed abnormal food preference in the social transmission of food preference task and low social interaction activity in the five-trial social memory test, but without social memory deficits. Using a home cage monitoring system, we observed reduced social grooming behavior in GluN3A KO mice. Signaling genes that might mediate the altered social behaviors were examined in the prefrontal cortex, hippocampus, and thalamus. Among nine genes examined, the expression of the oxytocin receptor was significantly lower in the prefrontal cortex of GluN3A KO mice than that in WT mice. Oxytocin treatment rescued social activity deficits in GluN3A KO mice. These findings support a novel idea that a chronic state of moderate increases in NMDAR activities may lead to downregulation of the oxytocin signaling and impaired behavioral activities that are seen in psychiatric/neurodegenerative disorders.

Properties of Triheteromeric N-Methyl-d-Aspartate Receptors Containing Two Distinct GluN1 Isoforms

N-Methyl-d-aspartate (NMDA)-type glutamate receptors mediate excitatory synaptic transmission in the central nervous system and play critical roles in many neuronal processes. The physiologic roles of NMDA receptors are shaped by their functional properties, which are highly dependent on subunit composition. Most NMDA receptors are assembled from two GluN1 and two GluN2 subunits, but diversity in subunit composition is made possible by eight GluN1 splice variants (i.e., isoforms) and four distinct GluN2 subunits (GluN2A-D). We demonstrate using Förster resonance energy transfer and fluorescence lifetime imaging that GluN1-1a and GluN1-1b isoforms, which include or lack residues encoded by exon 5, form triheteromeric GluN1-1a/GluN1-1b/GluN2A (1a/1b/2A) and GluN1-1a/GluN1-1b/GluN2B (1a/1b/2B) receptors. We describe the selective expression of NMDA receptors containing two different GluN1 isoforms, and show that triheteromeric 1a/1b/2A and 1a/1b/2B receptors exhibit intermediate deactivation kinetics and pharmacological properties compared with the respective diheteromeric GluN1-1a/GluN1-1a/GluN2 and GluN1-1b/GluN1-1b/GluN2 receptors. These results highlight the intriguing possibility that neurons can finely tune NMDA receptor signaling by shifting the ratio of expressed GluN1-1a and GluN1-1b isoforms. Furthermore, we evaluate the contribution of channel pore residues to magnesium block and calcium permeability. These data point to the asymmetric contribution of pore residues in GluN1 and GluN2 to magnesium block, and reveal that a single copy of pore residues from GluN3 subunits strongly attenuates magnesium block and calcium permeability of NMDA receptors. Thus, the selective expression of NMDA receptors containing two distinct GluN1 isoforms provides new opportunities to study functional properties relevant to neuronal receptors.

Maternal separation stress leads to resilience against neuropathic pain in adulthood

Early life stress (ELS) leads to a permanent reprogramming of biochemical stress response cascades that may also be relevant for the processing of chronic pain states such as neuropathy. Despite clinical evidence, little is known about ELS-related vulnerability for neuropathic pain and the possibly underlying etiology.

In the framework of experimental studies aimed at investigating the respective relationships we used the established ELS model of maternal separation (MS). Rat dams and neonates were separated for 3 h/day from post-natal day 2–12. At adulthood, noxious mechanical and thermal thresholds were assessed before and during induction of neuropathic pain by chronic constriction injury (CCI). The potential involvement of spinal glutamatergic transmission, glial cells, pro-inflammatory cytokines and growth factors was studied by using qPCR.

MS per se did not modify pain thresholds. But, when exposed to neuropathic pain, MS rats exhibited a marked reduction of thermal sensitivity and a delayed development of mechanical allodynia/hyperalgesia when compared to control animals. Also, MS did not alter glucocorticoid receptor mRNA levels, but prevented the CCI-induced down-regulation of NR1 and NR2 sub-units of the NMDA receptor and of the glutamate transporter EAAT3 as observed at 21 days post-surgery. Additionally, CCI-provoked up-regulation of glial cell markers was either prevented (GFAP for astrocytes) or dampened (Iba1 for microglia) by MS. Pro-inflammatory cytokine mRNA expression was either not affected (IL-6) or reduced (IL-1β) by MS shortly after CCI. The growth factors GDNF and NGF were only slightly downregulated 4 days after CCI in the MS-treated animals. The changes in glutamatergic signaling, astroglial and cytokine activation as well as neurotrophin expression could, to some extent, explain these changes in pain behavior. Taken together, the results obtained in the described experimental conditions support the mismatch theory of chronic stress where an early life stress, rather than predisposing individuals to certain pathologies, renders them resilient.

A novel substituted aminoquinoline selectively targets voltage-sensitive sodium channel isoforms and NMDA receptor subtypes and alleviates chronic inflammatory and neuropathic pain

Recent understanding of the systems that mediate complex disease states, has generated a search for molecules that simultaneously modulate more than one component of a pathologic pathway. Chronic pain syndromes are etiologically connected to functional changes (sensitization) in both peripheral sensory neurons and in the central nervous system (CNS). These functional changes involve modifications of a significant number of components of signal generating, signal transducing and signal propagating pathways. Our analysis of disease-related changes which take place in sensory neurons during sensitization led to the design of a molecule that would simultaneously inhibit peripheral NMDA receptors and voltage sensitive sodium channels. In the current report, we detail the selectivity of N,N-(diphenyl)-4-ureido-5,7-dichloro-2-carboxy-quinoline (DCUKA) for action at NMDA receptors composed of different subunit combinations and voltage sensitive sodium channels having different α subunits. We show that DCUKA is restricted to the periphery after oral administration, and that circulating blood levels are compatible with its necessary concentrations for effects at the peripheral cognate receptors/channels that were assayed in vitro. Our results demonstrate that DCUKA, at concentrations circulating in the blood after oral administration, can modulate systems which are upregulated during peripheral sensitization, and are important for generating and conducting pain information to the CNS. Furthermore, we demonstrate that DCUKA ameliorates the hyperalgesia of chronic pain without affecting normal pain responses in neuropathic and inflammation-induced chronic pain models.

Antagonism of the Prokineticin System Prevents and Reverses Allodynia and Inflammation in a Mouse Model of Diabetes

Neuropathic pain is a severe diabetes complication and its treatment is not satisfactory. It is associated with neuroinflammation-related events that participate in pain generation and chronicization. Prokineticins are a new family of chemokines that has emerged as critical players in immune system, inflammation and pain. We investigated the role of prokineticins and their receptors as modulators of neuropathic pain and inflammatory responses in experimental diabetes. In streptozotocin-induced-diabetes in mice, the time course expression of prokineticin and its receptors was evaluated in spinal cord and sciatic nerves, and correlated with mechanical allodynia. Spinal cord and sciatic nerve pro- and anti-inflammatory cytokines were measured as protein and mRNA, and spinal cord GluR subunits expression studied. The effect of preventive and therapeutic treatment with the prokineticin receptor antagonist PC1 on behavioural and biochemical parameters was evaluated. Peripheral immune activation was assessed measuring macrophage and T-helper cytokine production. An up-regulation of the Prokineticin system was present in spinal cord and nerves of diabetic mice, and correlated with allodynia. Therapeutic PC1 reversed allodynia while preventive treatment blocked its development. PC1 normalized prokineticin levels and prevented the up-regulation of GluN2B subunits in the spinal cord. The antagonist restored the pro-/anti-inflammatory cytokine balance altered in spinal cord and nerves and also reduced peripheral immune system activation in diabetic mice, decreasing macrophage proinflammatory cytokines and the T-helper 1 phenotype. The prokineticin system contributes to altered sensitivity in diabetic neuropathy and its inhibition blocked both allodynia and inflammatory events underlying disease.

Calcium-permeable ion channels in pain signaling

The detection and processing of painful stimuli in afferent sensory neurons is critically dependent on a wide range of different types of voltage- and ligand-gated ion channels, including sodium, calcium, and TRP channels, to name a few. The functions of these channels include the detection of mechanical and chemical insults, the generation of action potentials and regulation of neuronal firing patterns, the initiation of neurotransmitter release at dorsal horn synapses, and the ensuing activation of spinal cord neurons that project to pain centers in the brain. Long-term changes in ion channel expression and function are thought to contribute to chronic pain states. Many of the channels involved in the afferent pain pathway are permeable to calcium ions, suggesting a role in cell signaling beyond the mere generation of electrical activity. In this article, we provide a broad overview of different calcium-permeable ion channels in the afferent pain pathway and their role in pain pathophysiology.

Activation of GRs-Akt-nNOs-NR2B signaling pathway by second dose GR agonist contributes to exacerbated hyperalgesia in a rat model of radicular pain

Central Akt, neuronal nitric oxide synthase (nNOS) and N-methyl-D-aspartate receptor subunit 2B (NR2B) play key roles in the development of neuropathic pain. Here we investigate the effects of glucocorticoid receptors (GRs) on the expression and activation of spinal Akt, nNOS and NR2B after chronic compression of dorsal root ganglia (CCD). Thermal hyperalgesia test and mechanical allodynia test were used to measure rats after intrathecal injection of GR antagonist mifepristone or GR agonist dexamethasone for 21 days postoperatively. Expression of spinal Akt, nNOS, NR2B and their phosphorylation state after CCD was examined by western blot. The effects of intrathecal treatment with dexamethasone or mifepristone on nociceptive behaviors and the corresponding expression of Akt, nNOS and NR2B in spinal cord were also investigated. Intrathecal injection of mifepristone or dexamethasone inhibited PWMT and PWTL in CCD rats. However, hyperalgesia was induced by intrathecal injection of dexamethasone on days 12 to 14 after surgery. Treatment of dexamethasone increased the expression and phosphorylation levels of spinal Akt, nNOS, GR and NR2B time dependently, whereas administration of mifepristone downregulated the expression of these proteins significantly. GRs activated spinal Akt-nNOS/NR2B pathway play important roles in the development of neuropathic pain in a time-dependent manner.

GluN2B and GluN2D NMDARs dominate synaptic responses in the adult spinal cord

The composition of the postsynaptic ionotropic receptors that receive presynaptically released transmitter is critical not only for transducing and integrating electrical signals but also for coordinating downstream biochemical signaling pathways. At glutamatergic synapses in the adult CNS an overwhelming body of evidence indicates that the NMDA receptor (NMDAR) component of synaptic responses is dominated by NMDARs containing the GluN2A subunit, while NMDARs containing GluN2B, GluN2C, or GluN2D play minor roles in synaptic transmission. Here, we discovered NMDAR-mediated synaptic responses with characteristics not described elsewhere in the adult CNS. We found that GluN2A-containing receptors contribute little to synaptic NMDAR responses while GluN2B dominates at synapses of lamina I neurons in the adult spinal cord. In addition, we provide evidence for a GluN2D-mediated synaptic NMDAR component in adult lamina I neurons. Strikingly, the charge transfer mediated by GluN2D far exceeds that of GluN2A and is comparable to that of GluN2B. Lamina I forms a distinct output pathway from the spinal pain processing network to the pain networks in the brain. The GluN2D-mediated synaptic responses we have discovered in lamina I neurons provide the molecular underpinning for slow, prolonged and feedforward amplification that is a fundamental characteristic of pain.

From molecular phylogeny towards differentiating pharmacology for NMDA receptor subtypes

In order to decode the roles that N-methyl-D-aspartate (NMDA) receptors play in excitatory neurotransmission, synaptic plasticity, and neuropathologies, there is need for ligands that differ in their subtype selectivity. The conantokin family of Conus peptides is the only group of peptidic natural products known to target NMDA receptors. Using a search that was guided by phylogeny, we identified new conantokins from the marine snail Conus bocki that complement the current repertoire of NMDA receptor pharmacology. Channel currents measured in Xenopus oocytes demonstrate conantokins conBk-A, conBk-B, and conBk-C have highest potencies for NR2D containing receptors, in contrast to previously characterized conantokins that preferentially block NR2B containing NMDA receptors. Conantokins are rich in γ-carboxyglutamate, typically 17-34 residues, and adopt helical structure in a calcium-dependent manner. As judged by CD spectroscopy, conBk-C adopts significant helical structure in a calcium ion-dependent manner, while calcium, on its own, appears insufficient to stabilize helical conformations of conBk-A or conBk-B. Molecular dynamics simulations help explain the differences in calcium-stabilized structures. Two-dimensional NMR spectroscopy shows that the 9-residue conBk-B is relatively unstructured but forms a helix in the presence of TFE and calcium ions that is similar to other conantokin structures. These newly discovered conantokins hold promise that further exploration of small peptidic antagonists will lead to a set of pharmacological tools that can be used to characterize the role of NMDA receptors in nervous system function and disease.

N-methyl-D-aspartate receptor subunit composition in the rat trigeminal principal nucleus remains constant during postnatal development and following neonatal denervation

N-methyl-D-aspartate receptors (NMDARs) play a major role in various forms of developmental and adult synaptic plasticity (Lopez de Armentia M, Sah P (2003) J Neurosci 23:6876-6883). Activity-dependent shifts in NR2 subunits of the NMDARs have been proposed to be the molecular basis of critical period plasticity. Several supporting examples have been reported; however it is not clear whether the relationship between NMDAR subunit changes and neural plasticity are correlative or causal, nor whether such a relationship is universal across all sensory pathways with developmental plasticity. In the present study, we used voltage-clamp recording techniques to investigate whether subunit composition of NMDARs changes during development and after neonatal denervation in the principal sensory nucleus (PrV) of the trigeminal nerve. Relative AMPA receptor contribution to synaptic transmission increased linearly by the second postnatal week in the normal PrV. Denervation by peripheral nerve damage did not alter this process. We took the weighted decay time constant (τw) of NMDAR-mediated EPSCs as an index for NMDAR subunit composition. The τw measurement and Western blot analysis revealed that NMDARs contained both NR2A and NR2B subunits. The NR2A/NR2B ratio did not change during postnatal development or after neonatal denervation. Thus, critical period plasticity-related pattern formation in the PrV does not depend on changes in subunit composition of NMDARs. The mechanism underlying developmental synaptic plasticity in the PrV differs from those in higher trigeminal centers and other brain structures.

Ion channel blockers for the treatment of neuropathic pain

Neuropathic pain, a severe chronic pain condition characterized by a complex pathophysiology, is a largely unmet medical need. Ion channels, which underlie cell excitability, are heavily implicated in the biological mechanisms that generate and sustain neuropathic pain. This review highlights the biological evidence supporting the involvement of voltage-, proton- and ligand-gated ion channels in the neuropathic pain setting. Ion channel modulators at different research or development stages are reviewed and referenced. Ion channel modulation is one of the main avenues to achieve novel, improved neuropathic pain treatments. Voltage-gated sodium and calcium channel and glutamate receptor modulators are likely to produce new, improved agents in the future. Rationally targeting subtypes of known ion channels, tackling recently discovered ion channel targets or combining drugs with different mechanism of action will be primary sources of new drugs in the longer term.

Ionotropic glutamate receptors in spinal nociceptive processing

Glutamate is the predominant excitatory transmitter used by primary afferent synapses and intrinsic neurons in the spinal cord dorsal horn. Accordingly, ionotropic glutamate receptors mediate basal spinal transmission of sensory, including nociceptive, information that is relayed to supraspinal centers. However, it has become gradually more evident that these receptors are also crucially involved in short- and long-term plasticity of spinal nociceptive transmission, and that such plasticity have an important role in the pain hypersensitivity that may result from tissue or nerve injury. This review will cover recent findings on pre- and postsynaptic regulation of synaptic function by ionotropic glutamate receptors in the dorsal horn and how such mechanisms contribute to acute and chronic pain.

Targeting the NMDA receptor subunit NR2B for the treatment of neuropathic pain

Neuropathic pain is generally defined as a chronic pain state resulting from peripheral or central nerve injury, or both. An effective treatment for neuropathic pain is still lacking. The NMDA receptor, one type of the ionotropic glutamate receptors, is known to be important for triggering long-lasting changes in synapses. NMDA receptor-dependent synaptic plasticity plays roles not only in physiological functions such as learning and memory, but also in unwanted pathological conditions such as chronic pain. This review addresses recent progress on NMDA receptors in neuropathic pain, with particular emphasis on the NR2B-subunit-containing receptors. The expression and function of NMDA receptors in synaptic plasticity in the pain transmission pathway from dorsal root ganglia to the anterior cingulate cortex is reviewed, and preclinical and clinical investigations of selective NMDA receptor in neuropathic pain are discussed. The NMDA receptors, in particular NR2B-containing NMDA receptors, serve as promising targets for treatment of neuropathic pain.

Potentiation of spinal NMDA-mediated nociception by cocaine- and amphetamine-regulated transcript peptide via PKA and PKC signaling pathways in rats

Our previous study showed that cocaine- and amphetamine-regulated transcript peptide fragment 55-102 (CARTp) specifically potentiated spinal N-methyl-D-aspartate (NMDA)-mediated nociceptive transmission in vivo and in vitro. The cellular mechanisms underlying CARTp potentiation of NMDA receptor function remains unclear. The present study was carried out to test the hypothesis that CARTp changes the phosphorylated state of NMDA receptors by activating intracellular signals and subsequently increasing the function of NMDA receptors. We found that the potentiating effect of CARTp on spinal NMDA-induced hyperalgesia in rats was reduced by intrathecal pretreatment with KT5720 (a selective PKA inhibitor) or GF109206X (a selective PKC inhibitor), but was increased by pretreatment with calyculin A (a protein phosphatase inhibitor). In the in vitro electrophysiological study, CARTp potentiation of NMDA-induced depolarizations was blocked by superfusion of PKA or PKC inhibitor applied 10 min before the application of CARTp. The levels of phosphoserine 897 on the NR1 subunit (pNR1-ser897) and phosphoserine 896 on the NR1 subunit (pNR1-ser896) in the dorsal horn of spinal lumbar segments significantly increased following CARTp superfusion in vitro or intrathecal injection in vivo. The increases in pNR1-ser897 and pNR1-ser896 in the in vivo and in vitro studies were inhibited by pretreatment with KT5720 and GF109206X, respectively. The results provide the first evidence that CARTp increases the phosphorylation of NMDA receptor NR1 subunit via activation of PKA and PKC signals, which may play a crucial role in CARTp regulation of spinal NMDA receptor-mediated nociceptive responses.

Ifenprodil induced antinociception and decreased the expression of NR2B subunits in the dorsal horn after chronic dorsal root ganglia compression in rats

BACKGROUND

Spinal N-methyl D-aspartate receptors play an important role in the pathogenesis of neuropathic pain, and administration of N-methyl D-aspartate receptor antagonists can attenuate this hyperpathia. Ifenprodil is an antagonist selective for N-methyl D-aspartate receptor 2B (NR2B) subunits. Several researches have reported effective analgesia of ifenprodil in animal models of neuropathic pain. We extended this work to include chronic compression of the dorsal root ganglia (CCD).

METHODS

The paw withdrawal mechanical threshold and paw withdrawal thermal latency tests were used to assess mechanical allodynia and thermal hyperalgesia after a CCD operation and intrathecal injection of ifenprodil. We used immunohistochemistry and immunoblotting to investigate the effect of ifenprodil on NR2B subunits expression in CCD rats.

RESULTS

The data revealed increased expression of NR2B subunits in the superficial dorsal horn in CCD rats. We found that, in addition to a marked suppression of thermal hyperalgesia and mechanical allodynia, intrathecal injection ifenprodil treatment causes a decreased expression of NR2B in the spinal cord.

CONCLUSIONS

These data suggest that ifenprodil induced antinociception in CCD rats and provided further evidence for the important role of NR2B subunits in the development of neuropathic pain.

Alterations in N-methyl-D-aspartate receptor subunits in primary sensory neurons following acid-induced esophagitis in cats

The excitatory amino acid glutamate plays an important role in the development of neuronal sensitization and the ionotropic N-methyl-d-aspartate receptor (NMDAR) is one of the major receptors involved. The objective of this study was to use a cat model of gastroesophageal reflux disease (GERD) to investigate the expression of the NR1 and NR2A subunits of NMDAR in the vagal and spinal afferent fibers innervating the esophagus. Two groups of cats (Acid-7D and PBS-7D) received 0.1 N HCl (pH 1.2) or 0.1 M PBS (pH 7.4) infusion in the esophagus (1 ml/min for 30 min/day for 7 days), respectively. NR1 splice variants (both NH(2) and COOH terminals) and NR2A in the thoracic dorsal root ganglia (DRGs), nodose ganglia (NGs), and esophagus were evaluated by RT-PCR, Western blot, and immunohistochemistry. Acid produced marked inflammation and a significant increase in eosinophil peroxidase and myeloperoxidase contents compared with PBS-infused esophagus. The NR1-4 splice variant gene exhibited a significant upregulation in DRGs and esophagus after acid infusion. In DRGs, NGs, and esophagus, acid infusion resulted in significant upregulation of NR1 and downregulation of NR2A subunit gene expression. A significant increase in NR1 polypeptide expression was observed in DRGs and NGs from Acid-7D compared with control. In conclusion, long-term acid infusion in the cat esophagus resulted in ulcerative esophagitis and differential expressions of NR1 and NR2A subunits. It is possible that these changes may in part contribute to esophageal hypersensitivity observed in reflux esophagitis.

Functional identification of NR2 subunits contributing to NMDA receptors on substance P receptor-expressing dorsal horn neurons

NMDA receptors are important elements in pain signaling in the spinal cord dorsal horn. They are heterotetramers typically composed of two NR1 and two of four NR2 subunits: NR2A-2D. Mice lacking specific NR2 subunits show deficits in pain transmission yet subunit location in the spinal cord remains unclear. We have combined electrophysiological and pharmacological approaches to investigate the composition of functional NMDA receptors expressed by lamina I, substance P receptor-expressing (NK1R+) neurons, as well as NK1R- neurons. Under low Mg²⁺ conditions (100 μM), the conductance of NMDA receptors at -90 mV (g(-90 mV)) with NR2A or NR2B subunits (NR2A/B) is low compared to conductance measured at the membrane potential where the inward current is maximal or maximal inward current (MIC) (ratio of ~0.07 calculated from Kuner and Schoepfer, 1996). For NR2C or NR2D subunits (NR2C/D), the ratio is higher (ratio ~0.4). NK1R+ and NK1R- neurons express NMDA receptors that give ratios ~0.28 and 0.16, respectively, suggesting both types of subunits are present in both populations of neurons, with NK1R+ neurons expressing a higher percentage of NR2C/D type NMDA receptors. This was confirmed using EAB318, an NR2A/B preferring antagonist, and UBP141, a mildly selective NR2C/D antagonist to increase and decrease the g(-90 mV)/g(MIC) ratios in both subpopulations of neurons.

NMDA receptor 2B subunit-mediated synaptic transmission in the superficial dorsal horn of peripheral nerve-injured neuropathic mice

Previous research has shown that peripheral inflammation and peripheral nerve injury alter the properties of NMDA receptors in the spinal dorsal horn. However, there is no direct evidence that demonstrates the influence of peripheral nerve injury on NMDA receptor-mediated synaptic transmission in the spinal dorsal horn. Using whole cell tight-seal methods, NMDA receptor-mediated excitatory postsynaptic currents (NMDA EPSCs) were recorded from superficial dorsal horn neurons in adult mouse spinal cord slices. Peripheral nerve injury-induced changes in the pharmacological and electrophysiological properties of synaptic NMDA receptors were studied. The ratio of the amplitude of NMDA EPSCs to that of non-NMDA EPSCs was larger in nerve-ligated neuropathic mice than in sham-operated control mice. The decay phase of the NMDA EPSCs was slower in nerve-ligated neuropathic mice. The NR2B subunit-specific NMDA receptor antagonist ifenprodil (10 microM) reduced the amplitude of the NMDA EPSCs and shortened their decay phase. The sensitivity of NMDA EPSCs to ifenprodil was significantly larger in nerve-ligated neuropathic mice than in sham-operated control mice. Single-cell RT-PCR analysis performed on superficial dorsal horn neurons showed that the incidence of NR2A mRNA-expressing neurons was reduced in nerve-ligated neuropathic mice. This result, together with the electrophysiological findings, suggests that the subunit composition of the subsynaptic NMDA receptors in the superficial dorsal horn was altered by peripheral nerve injury. Pharmacological and electrophysiological changes observed in the present experiments might be the underlying causes of the hyperalgesia and allodynia induced by peripheral nerve injury and inflammation.

N-methyl-D-aspartate (NMDA) and the regulation of mitogen-activated protein kinase (MAPK) signaling pathways: a revolving neurochemical axis for therapeutic intervention?

Excitatory synaptic transmission in the central nervous system (CNS) is mediated by the release of glutamate from presynaptic terminals onto postsynaptic channels gated by N-methyl-D-aspartate (NMDA) and non-NMDA (AMPA and KA) receptors. Extracellular signals control diverse neuronal functions and are responsible for mediating activity-dependent changes in synaptic strength and neuronal survival. Influx of extracellular calcium ([Ca(2+)](e)) through the NMDA receptor (NMDAR) is required for neuronal activity to change the strength of many synapses. At the molecular level, the NMDAR interacts with signaling modules, which, like the mitogen-activated protein kinase (MAPK) superfamily, transduce excitatory signals across neurons. Recent burgeoning evidence points to the fact that MAPKs play a crucial role in regulating the neurochemistry of NMDARs, their physiologic and biochemical/biophysical properties, and their potential role in pathophysiology. It is the purpose of this review to discuss: (i) the MAPKs and their role in a plethora of cellular functions; (ii) the role of MAPKs in regulating the biochemistry and physiology of NMDA receptors; (iii) the kinetics of MAPK-NMDA interactions and their biologic and neurochemical properties; (iv) how cellular signaling pathways, related cofactors and intracellular conditions affect NMDA-MAPK interactions and (v) the role of NMDA-MAPK pathways in pathophysiology and the evolution of disease conditions. Given the versatility of the NMDA-MAPK interactions, the NMDA-MAPK axis will likely form a neurochemical target for therapeutic interventions.

NMDA receptor antagonist treatment at the time of nerve injury prevents injury-induced changes in spinal NR1 and NR2B subunit expression and increases the sensitivity of residual pain behaviours to subsequently administered NMDA receptor antagonists

Spinal NMDA receptors (NMDA R) are important in neuropathic sensitisation and acute administration of antagonists can provide temporary attenuation of sensitisation. If establishment of the chronic pain state could be prevented by brief administration of such agents at or around the time of nerve injury (pre-emptive analgesia) it might be possible to avoid many of the unacceptable side effects associated with repeated administration of these or other antagonists. Several reports describe aspects of effective pre-emptive analgesia from NMDA R antagonists in animal models of neuropathic pain. The first aim of the present study was to make a direct comparison of changes in mechanical allodynia, cold allodynia and thermal hyperalgesia following nerve injury, demonstrating their increasing degree of susceptibility to pre-emptive NMDA R antagonist treatment. Secondly, we used immunoblotting and immunohistochemistry to investigate the effects of nerve injury on NMDA receptor subunit expression, revealing increased expression of NR2B, but not NR2A and reduced NR1 in the superficial dorsal horn. These changes were attenuated following NMDA receptor antagonist pre-treatment. Thirdly, we investigated the pharmacological properties of residual mechanical allodynia and cold allodynia that remained after pre-emptive treatment and revealed a greater sensitivity to NMDA R antagonists. These findings indicate that in addition to a marked suppression of thermal hyperalgesia and cold allodynia, pre-emptive treatment with NMDA R antagonist causes a lasting change in spinal NMDA R complexes such that remaining mechanical allodynia should be more effectively targeted by NMDA R antagonists.

Differential expression of NMDA receptor subunits between neurons containing and not containing enkephalin in the mouse embryo spinal cord

We transfected cultures of mouse spinal cord slices with the enhanced green fluorescent protein (GFP) gene driven by the promoter for preproenkephalin, using the particle-mediated gene transfer system adapted for small neurons in the superficial dorsal horn, and observations were made after 4-6 days in vitro. A considerable number of cells in the superficial dorsal horn were observed to express GFP fluorescence, reminiscent of the previously reported distribution of enkephalinergic neurons in the spinal cord. The number of GFP-expressing neurons increased in response to forskolin application. Reverse transcription-polymerase chain reaction (RT-PCR) analysis of single neurons revealed that the N-methyl-d-aspartate (NMDA) receptor NR2B subunit is expressed more frequently in enkephalinergic neurons, and the NR2A subunit more frequently in non-enkephalinergic neurons. These observations suggest that expression of NMDA receptor subunits is controlled differentially in distinct populations of neurochemically identified neurons in the spinal cord. Biolistic particle-mediated gene transfection seems useful for identifying neuronal phenotypes in organotypic cultures of the spinal cord.

Overexpression of NGF or GDNF alters transcriptional plasticity evoked by inflammation

Transcriptional changes evoked in nociceptive sensory neurons by inflammatory injury play a substantial role in the generation of and recovery from painful hypersensitivity. Transgenic mice overexpressing nerve growth factor (NGF) or glial cell line-derived neurotrophic factor (GDNF) in the skin possess a greatly increased number of nociceptors. Surprisingly, NGF-overexpressers display reduced hypersensitivity and recovered more rapidly in response to inflammation, suggesting a compensatory suppression of nociceptive transmission in these mice. To determine whether these transgenic mice show changes in inflammation-evoked transcriptional plasticity, we examined the expression of a panel of genes implicated in nociceptive signaling in response to injection of complete Freund's adjuvant into the hindpaw. Relative mRNA levels were quantified 1, 4 and 15 days after injection using real-time PCR. In wild type mice CFA injection elicited a reproducible pattern of altered gene expression that returned to baseline over a 2-week period. In mice overexpressing NGF or GDNF the expression patterns for several genes were substantially altered; these changes in injury-evoked patterns of gene expression suggest the existence of endogenous regulatory mechanisms that can compensate for increased nociceptive input by modulating the expression of a limited subset of genes.

Effects of the N-methyl-D-aspartate receptor antagonist perzinfotel [EAA-090; [2-(8,9-dioxo-2,6-diazabicyclo[5.2.0]non-1(7)-en-2-yl)-ethyl]phosphonic acid] on chemically induced thermal hypersensitivity

Perzinfotel [EAA-090; [2-(8,9-dioxo-2,6-diazabicyclo[5.2.0]non-1(7)-en-2-yl)-ethyl]phosphonic acid] is a selective, competitive N-methyl-D-aspartate (NMDA) receptor antagonist with high affinity for the glutamate site. The current study evaluated whether perzinfotel would have antinociceptive effects or block thermal hypersensitivity associated with the administration of chemical irritants in rats. Perzinfotel lacked antinociceptive effects but dose- and time-dependently blocked prostaglandin E(2) (PGE(2))- and capsaicin-induced thermal hypersensitivity in a warm-water tail-withdrawal assay in rats. Doses of 10 mg/kg intraperitoneal or 100 mg/kg oral blocked PGE(2)-induced hypersensitivity by 60 to 80%. The magnitude of reversal was greater than other negative modulators of the NMDA receptor studied, such as uncompetitive channel blockers (e.g., memantine, dizocilpine, and ketamine), a NR2B selective antagonist (e.g., ifenprodil), and other glutamate antagonists [e.g., selfotel, 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP), D,L-(E)-2-amino-4-propyl-5-phosphono-3-pentenoic acid (CGP-39653)], up to doses that suppressed operant rates of responding. In contrast to other negative modulators of the NMDA receptor studied, which typically decreased operant rates of responding at doses that lacked antinociceptive effects, perzinfotel did not modify response rates at doses that blocked irritant-induced thermal hypersensitivity. Collectively, these studies demonstrate that perzinfotel has therapeutic ratios for effectiveness versus adverse effects superior to those seen with other competitive and uncompetitive NMDA receptor antagonists studied.

The ultrastructural localisation of the N-methyl-D-aspartate NR2B receptor subunit in rat lumbar spinal cord

Glutamate together with its N-methyl-d-aspartate (NMDA) receptors has an important role in the transmission of stimuli in the spinal cord. Whilst the expression of the various NMDA receptor subunits within the spinal cord has been investigated the subcellular location of the NMDA NR2B subunit has yet to be definitively established. Both mRNA and light microscopical studies have failed to unequivocally demonstrate the proposed pre-synaptic location of this subunit. This has been proposed from pharmacological data and is thought to underlie the apparent analgesic properties of selective NR2B antagonists. Using pre-embedding immunohistochemistry combined with electron microscopy our findings provide the first definitive morphological evidence for both a pre- and post-synaptic localisation of NR2B/containing NMDA receptors, and suggest expression by astrocytes, in the rat lumbar spinal cord.

Glutamate and tachykinin receptors in central sensitization of withdrawal reflexes in the decerebrated rabbit

This study assessed the involvement of NMDA and group I metabotropic glutamate receptors, and tachykinin NK1 and NK3 receptors, in central sensitization of withdrawal reflexes in the decerebrated rabbit. Reflexes evoked in the ankle flexor tibialis anterior and the knee flexor semitendinosus by electrical stimulation at the base of the toes were enhanced for 29-63 min after application of 20% mustard oil to the tips of the toes. Selective antagonists of mGlu1, mGlu5, NMDA and NR2B-subunit-containing NMDA glutamate receptors, as well as NK1, and NK3 receptors, and a non-selective blocker of all tachykinin receptors, were assessed for their effects on the magnitude and duration of the increase in reflexes induced by mustard oil. Dizocilpine, an antagonist of all NMDA receptors (1 mg intrathecal) abolished facilitation of tibialis anterior reflexes and significantly reduced the magnitude and duration of increase of the semitendinosus response. The NR2B-subtype selective antagonist CP-101,606 decreased the magnitude of facilitation of both reflexes but had no effect on duration of enhancement. Selective antagonists for the mGlu1 (CPCCOEt, 1-3 mg intrathecal), mGlu5 (MPEP, 0.2-1 mg intrathecal), NK1 (L-733,060, 0.3 mg intrathecal) or NK3 (SR 142,801, 1 mg kg(-1) i.v.) receptors had no effect on the amplitude or duration of sensitization. However, the non-selective tachykinin receptor blocker ZD-6021 (0.3 mg intrathecal) reduced the amplitude but not the duration of sensitization in the flexor reflexes. Combination of ZD-6021 with CP-101,606 (doses as above) decreased both aspects of the sensitization response. Dizocilpine reduced reflexes evoked from the heel per se, and dizocilpine, CP-101,606 and ZD-6021 reduced arterial blood pressure. Otherwise the drugs used had no effects on baseline variables. The present data confirm the importance of NMDA receptors as a critical part of the process of central sensitization, provide no evidence for a role of metabotropic glutamate receptors, and show that simultaneous blockade of all tachykinin receptors is required to reveal their role in hyperalgesia. The data further indicate that a combined pharmacological approach offers a potential way forward for the development of new antihyperalgesic agents.

An antisense oligonucleotide to the N-methyl-D-aspartate (NMDA) subunit NMDAR1 attenuates NMDA-induced nociception, hyperalgesia, and morphine tolerance

We determined whether the i.t. administration of an 18-mer phosphodiester antisense oligodeoxynucleotide (ODN) that reduces the expression of the rat NMDAR1 subunit of the N-methyl-d-aspartate (NMDA) receptor would affect nociceptive behaviors and prevent the development of morphine tolerance. Rats received 5 microl of i.t. saline, 30 nM antisense, or mismatch ODN twice a day for 5 days (NMDA-induced nociception, NMDA-induced thermal hyperalgesia, NR1 mRNA, and ligand binding studies) or for 3 days (formalin study). For the tolerance study, 5 days of ODNs or saline were followed by 3 days of concurrent administration of ODNs or saline (twice a day) and i.t. morphine (three times a day). Antisense, but not mismatch, results in the reduction of formalin phase 2 flinching by 50%, the spinal cord dorsal horn levels of NMDAR1 mRNA by 30%, and ligand binding by 50%. The i.t. ED(50) for NMDA-induced nociceptive behaviors is doubled, and thermal hyperalgesia is blocked by antisense treatment. The effects of antisense on NMDA-induced nociception and thermal hyperalgesia are completely reversed by discontinuing antisense. The coadministration of antisense with increasing doses of i.t. morphine for 3 days attenuates the development of morphine tolerance. These results demonstrate that an in vivo antisense targeting of the NMDAR1 subunit results in antihyperalgesic effects and a partial blockade of spinal morphine tolerance. They provide additional support for the critical role of the NMDA receptor in these forms of spinal nociception and in the development of morphine tolerance and suggest the potential therapeutic utility of this approach.

The role of N-methyl-D-aspartate (NMDA) receptors in pain: a review

There is accumulating evidence to implicate the importance of N-methyl-D-aspartate (NMDA) receptors to the induction and maintenance of central sensitization during pain states. However, NMDA receptors may also mediate peripheral sensitization and visceral pain. NMDA receptors are composed of NR1, NR2 (A, B, C, and D), and NR3 (A and B) subunits, which determine the functional properties of native NMDA receptors. Among NMDA receptor subtypes, the NR2B subunit-containing receptors appear particularly important for nociception, thus leading to the possibility that NR2B-selective antagonists may be useful in the treatment of chronic pain.

A conditional deletion of the NR1 subunit of the NMDA receptor in adult spinal cord dorsal horn reduces NMDA currents and injury-induced pain

To determine the importance of the NMDA receptor (NMDAR) in pain hypersensitivity after injury, the NMDAR1 (NR1) subunit was selectively deleted in the lumbar spinal cord of adult mice by the localized injection of an adenoassociated virus expressing Cre recombinase into floxed NR1 mice. NR1 subunit mRNA and dendritic protein are reduced by 80% in the area of the virus injection, and NMDA currents, but not AMPA currents, are reduced 86-88% in lamina II neurons. The spatial NR1 knock-out does not alter heat or cold paw-withdrawal latencies, mechanical threshold, or motor function. However, injury-induced pain produced by intraplantar formalin is reduced by 70%. Our results demonstrate conclusively that the postsynaptic NR1 receptor subunit in the lumbar dorsal horn of the spinal cord is required for central sensitization, the central facilitation of pain transmission produced by peripheral injury.