Residues at the tip of the pore loop of NR3B-containing NMDA receptors determine Ca2+ permeability and Mg2+ block

NMDARs mediate peripheral and central sensitization contributing to chronic orofacial pain

Peripheral and central sensitizations of the trigeminal nervous system are the main mechanisms to promote the development and maintenance of chronic orofacial pain characterized by allodynia, hyperalgesia, and ectopic pain after trigeminal nerve injury or inflammation. Although the pathomechanisms of chronic orofacial pain are complex and not well known, sufficient clinical and preclinical evidence supports the contribution of the N-methyl-D-aspartate receptors (NMDARs, a subclass of ionotropic glutamate receptors) to the trigeminal nociceptive signal processing pathway under various pathological conditions. NMDARs not only have been implicated as a potential mediator of pain-related neuroplasticity in the peripheral nervous system (PNS) but also mediate excitatory synaptic transmission and synaptic plasticity in the central nervous system (CNS). In this review, we focus on the pivotal roles and mechanisms of NMDARs in the trigeminal nervous system under orofacial neuropathic and inflammatory pain. In particular, we summarize the types, components, and distribution of NMDARs in the trigeminal nervous system. Besides, we discuss the regulatory roles of neuron-nonneuronal cell/neuron-neuron communication mediated by NMDARs in the peripheral mechanisms of chronic orofacial pain following neuropathic injury and inflammation. Furthermore, we review the functional roles and mechanisms of NMDARs in the ascending and descending circuits under orofacial neuropathic and inflammatory pain conditions, which contribute to the central sensitization. These findings are not only relevant to understanding the underlying mechanisms, but also shed new light on the targeted therapy of chronic orofacial pain.

Presynaptic NMDA receptors control nociceptive transmission at the spinal cord level in neuropathic pain

Chronic neuropathic pain is a debilitating condition that remains challenging to treat. Glutamate N-methyl-D-aspartate receptor (NMDAR) antagonists have been used to treat neuropathic pain, but the exact sites of their actions have been unclear until recently. Although conventionally postsynaptic, NMDARs are also expressed presynaptically, particularly at the central terminals of primary sensory neurons, in the spinal dorsal horn. However, presynaptic NMDARs in the spinal cord are normally quiescent and are not actively involved in physiological nociceptive transmission. In this review, we describe the emerging role of presynaptic NMDARs at the spinal cord level in chronic neuropathic pain and the implications of molecular mechanisms for more effective treatment. Recent studies indicate that presynaptic NMDAR activity at the spinal cord level is increased in several neuropathic pain conditions but not in chronic inflammatory pain. Increased presynaptic NMDAR activity can potentiate glutamate release from primary afferent terminals to spinal dorsal horn neurons, which is crucial for the synaptic plasticity associated with neuropathic pain caused by traumatic nerve injury and chemotherapy-induced peripheral neuropathy. Furthermore, α2δ-1, previously considered a calcium channel subunit, can directly interact with NMDARs through its C-terminus to increase presynaptic NMDAR activity by facilitating synaptic trafficking of α2δ-1-NMDAR complexes in neuropathic pain caused by chemotherapeutic agents and peripheral nerve injury. Targeting α2δ-1-bound NMDARs with gabapentinoids or α2δ-1 C-terminus peptides can attenuate nociceptive drive form primary sensory nerves to dorsal horn neurons in neuropathic pain.

The Role of NMDA Receptors in Alzheimer's Disease

In Alzheimer’s disease (AD), early synaptic dysfunction is associated with the increased oligomeric amyloid-beta peptide, which causes NMDAR-dependent synaptic depression and spine elimination. Memantine, low-affinity NMDAR channel blocker, has been used in the treatment of moderate to severe AD. However, clear evidence is still deficient in demonstrating the underlying mechanisms and a relationship between NMDARs dysfunction and AD. This review focuses on not only changes in expression of different NMDAR subunits, but also some unconventional modes of NMDAR action.

The Enigma of the Dichotomic Pressure Response of GluN1-4a/b Splice Variants of NMDA Receptor: Experimental and Statistical Analyses

Professional deep-water divers, exposed to hyperbaric pressure (HP) above 1.1 MPa, develop High Pressure Neurological Syndrome (HPNS), which is associated with central nervous system (CNS) hyperexcitability. It was previously reported that HP augments N-methyl-D-aspartate receptor (NMDAR) synaptic response, increases neuronal excitability and potentially causes irreversible neuronal damage. Our laboratory has reported differential current responses under HP conditions in NMDAR subtypes that contain either GluN1-1a or GluN1-1b splice variants co-expressed in Xenopus laevis oocytes with all four GluN2 subunits. Recently, we reported that the increase in ionic currents measured under HP conditions is also dependent on which of the eight splice variants of GluN1 is co-expressed with the GluN2 subunit. We now report that the NMDAR subtype that contains GluN1-4a/b splice variants exhibited "dichotomic" (either increased or decreased) responses at HP. The distribution of the results is not normal thus analysis of variance (ANOVA) test and clustering analysis were employed for statistical verification of the grouping. Furthermore, the calculated constants of alpha function distribution analysis for the two groups were similar, suggesting that the mechanism underlying the switch between an increase or a decrease of the current at HP is a single process, the nature of which is still unknown. This dichotomic response of the GluN1-4a/b splice variant may be used as a model for studying reduced response in NMDAR at HP. Successful reversal of other NMDAR subtypes response (i.e., current reduction) may allow the elimination of the reversible malfunctioning short term effects (HPNS), or even deleterious long term effects induced by increased NMDAR function during HP exposure.

Glutamate receptor pores

Glutamate receptors are ligand-gated ion channels that mediate fast excitatory synaptic transmission throughout the central nervous system. Functional receptors are homo- or heteromeric tetramers with each subunit contributing a re-entrant pore loop that dips into the membrane from the cytoplasmic side. The pore loops form a narrow constriction near their apex with a wide vestibule toward the cytoplasm and an aqueous central cavity facing the extracellular solution. This article focuses on the pore region, reviewing how structural differences among glutamate receptor subtypes determine their distinct functional properties.

Structure-based discovery of antagonists for GluN3-containing N-methyl-D-aspartate receptors

NMDA receptors are ligand-gated ion channels that assemble into tetrameric receptor complexes composed of glycine-binding GluN1 and GluN3 subunits (GluN3A-B) and glutamate-binding GluN2 subunits (GluN2A-D). NMDA receptors can assemble as GluN1/N2 receptors and as GluN3-containing NMDA receptors, which are either glutamate/glycine-activated triheteromeric GluN1/N2/N3 receptors or glycine-activated diheteromeric GluN1/N3 receptors. The glycine-binding GluN1 and GluN3 subunits display strikingly different pharmacological selectivity profiles. However, the pharmacological characterization of GluN3-containing receptors has been hampered by the lack of methods and pharmacological tools to study GluN3 subunit pharmacology in isolation. Here, we have developed a method to study the pharmacology of GluN3 subunits in recombinant diheteromeric GluN1/N3 receptors by mutating the orthosteric ligand-binding pocket in GluN1. This method is suitable for performing compound screening and characterization of structure-activity relationship studies on GluN3 ligands. We have performed a virtual screen of the orthosteric binding site of GluN3A in the search for antagonists with selectivity for GluN3 subunits. In the subsequent pharmacological evaluation of 99 selected compounds, we identified 6-hydroxy-[1,2,5]oxadiazolo[3,4-b]pyrazin-5(4H)-one (TK80) a novel competitive antagonist with preference for the GluN3B subunit. Serendipitously, we also identified [2-hydroxy-5-((4-(pyridin-3-yl)thiazol-2-yl)amino]benzoic acid (TK13) and 4-(2,4-dichlorobenzoyl)-1H-pyrrole-2-carboxylic acid (TK30), two novel non-competitive GluN3 antagonists. These findings demonstrate that structural differences between the orthosteric binding site of GluN3 and GluN1 can be exploited to generate selective ligands.

Open-channel blockade is less effective on GluN3B than GluN3A subunit-containing NMDA receptors

The GluN3 subunits of the N-methyl-d-aspartate (NMDA) receptor are known to reduce its Ca²⁺ permeability and Mg²⁺ sensitivity, however, little is known about their effects on other channel blockers. cRNAs for rat NMDA receptor subunits were injected into Xenopus oocytes and responses to NMDA and glycine were recorded using two electrode voltage clamp. Channel block of receptors containing GluN1-1a/2A, GluN1-1a/2A/3A or GluN1-1a/2A/3B subunits was characterised using Mg²⁺, memantine, MK-801, philanthotoxin-343 and methoctramine. IC₅₀ values for Mg²⁺ and memantine increased when receptors contained GluN3A subunits and were further increased when they contained GluN3B, e.g. IC₅₀s at − 75 mV for block of GluN1-1a/2A, GluN1-1a/2A/3A and GluN1-1a/2A/3B receptors respectively were 4.2, 22.4 and 40.1 μM for Mg²⁺, and 2.5, 7.5 and 17.5 μM for memantine. Blocking activity was found to be fully or partially restored when G or R (at the N and N + 1 sites respectively) were mutated to N in GluN3A. Thus, the changes cannot be attributed to the loss of the N or N + 1 sites alone, but rather involve both sites or residues elsewhere. Block by MK-801 and philanthotoxin-343 was also reduced by GluN3A, most strongly at − 100 mV but not at − 50 mV, and by GluN3B at all V_h. Methoctramine was the least sensitive to introduction of GluN3 subunits suggesting a minimal interaction with the N and N + 1 sites. We conclude that GluN3B-containing receptors provide increased resistance to channel block compared to GluN3A-containing receptors and this must be due to differences outside the deep pore region (N site and deeper).

Negative modulation of NMDA receptor channel function by DREAM/calsenilin/KChIP3 provides neuroprotection?

N-methyl-D-aspartate receptors (NMDARs) are glutamate-gated ion channels highly permeable to calcium and essential to excitatory neurotransmission. The NMDARs have attracted much attention because of their role in synaptic plasticity and excitotoxicity. Evidence has recently accumulated that NMDARs are negatively regulated by intracellular calcium binding proteins. The calcium-dependent suppression of NMDAR function serves as a feedback mechanism capable of regulating subsequent Ca²⁺ entry into the postsynaptic cell, and may offer an alternative approach to treating NMDAR-mediated excitotoxic injury. This short review summarizes the recent progress made in understanding the negative modulation of NMDAR function by DREAM/calsenilin/KChIP3, a neuronal calcium sensor (NCS) protein.

NMDA receptor signaling: death or survival?

Glutamate-induced neuronal damage is mainly caused by overactivation of N-methyl-D-aspartate (NMDA) receptors. Conversely, normal physiological brain function and neuronal survival require adequate activation of NMDA receptors. Studies have revealed that NMDA receptor-induced neuronal death or survival is mediated through distinct subset of NMDA receptors triggering different intracellular signaling pathways. Here we discuss recent advances in the characterization of NMDA receptors in neuronal protection, emphasizing subunit-specific role, which contributes to temporal-spatial distribution, subcellular localization and diverse channel properties of NMDA receptors.