Slow deactivation kinetics of NMDA receptors containing NR1 and NR2D subunits in rat cerebellar Purkinje cells

Conformational basis of subtype-specific allosteric control of NMDA receptor gating

N-methyl-D-aspartate receptors are ionotropic glutamate receptors that are integral to synaptic transmission and plasticity. Variable GluN2 subunits in diheterotetrameric receptors with identical GluN1 subunits set very different functional properties, which support their individual physiological roles in the nervous system. To understand the conformational basis of this diversity, we assessed the conformation of the common GluN1 subunit in receptors with different GluN2 subunits using single-molecule fluorescence resonance energy transfer (smFRET). We established smFRET sensors in the ligand binding domain and modulatory amino-terminal domain to study an apo-like state and partially liganded activation intermediates, which have been elusive to structural analysis. Our results demonstrate a strong, subtype-specific influence of apo and glutamate-bound GluN2 subunits on GluN1 rearrangements, suggesting a conformational basis for the highly divergent levels of receptor activity, desensitization and agonist potency. Chimeric analysis reveals structural determinants that contribute to the subtype differences. Our study provides a framework for understanding GluN2-dependent functional properties and could open new avenues for subtype-specific modulation.

Therapeutic potential of N-methyl-D-aspartate receptor modulators in psychiatry

N-methyl-D-aspartate (NMDA) receptors mediate a slow component of excitatory synaptic transmission, are widely distributed throughout the central nervous system, and regulate synaptic plasticity. NMDA receptor modulators have long been considered as potential treatments for psychiatric disorders including depression and schizophrenia, neurodevelopmental disorders such as Rett Syndrome, and neurodegenerative conditions such as Alzheimer's disease. New interest in NMDA receptors as therapeutic targets has been spurred by the findings that certain inhibitors of NMDA receptors produce surprisingly rapid and robust antidepressant activity by a novel mechanism, the induction of changes in the brain that well outlast the presence of drug in the body. These findings are driving research into an entirely new paradigm for using NMDA receptor antagonists in a host of related conditions. At the same time positive allosteric modulators of NMDA receptors are being pursued for enhancing synaptic function in diseases that feature NMDA receptor hypofunction. While there is great promise, developing the therapeutic potential of NMDA receptor modulators must also navigate the potential significant risks posed by the use of such agents. We review here the emerging pharmacology of agents that target different NMDA receptor subtypes, offering new avenues for capturing the therapeutic potential of targeting this important receptor class.

GluD receptors are functional ion channels

In this article, we review contemporary evidence that GluD receptors are functional ion channels whose depolarizing currents contribute to their biological functions, akin to all other members of the ionotropic glutamate receptor (iGluR) family.

Potentiation of neuronal activity by tonic GluD1 current in brain slices

Ion channel function of native delta glutamate receptors (GluD_R ) is incompletely understood. Previously, we and others have shown that activation of Gαq protein-coupled receptors (GqPCR) produces a slow inward current carried by GluD1_R . GluD1_R also carries a tonic cation current of unknown cause. Here, using voltage-clamp electrophysiological recordings from adult mouse brain slices containing the dorsal raphe nucleus, we find no role of ongoing G-protein-coupled receptor activity in generating or sustaining tonic GluD1_R currents. Neither augmentation nor disruption of G protein activity affects tonic GluD1_R currents, suggesting that ongoing G-protein-coupled receptor activity does not give rise to tonic GluD1_R currents. Further, the tonic GluD1_R current is unaffected by the addition of external glycine or D-serine, which influences GluD2_R current at millimolar concentrations. Instead, GqPCR-stimulated and tonic GluD1_R currents are regulated by physiological levels of external calcium. In current-clamp recordings, block of GluD1_R channels hyperpolarizes the membrane by ~7 mV at subthreshold potentials, reducing excitability. Thus, GluD1_R carries a G-protein-independent tonic current that contributes to subthreshold neuronal excitation in the dorsal raphe nucleus.

Targeting NMDA Receptors at the Neurovascular Unit: Past and Future Treatments for Central Nervous System Diseases

The excitatory neurotransmission of the central nervous system (CNS) mainly involves glutamate and its receptors, especially N-methyl-D-Aspartate receptors (NMDARs). These receptors have been extensively described on neurons and, more recently, also on other cell types. Nowadays, the study of their differential expression and function is taking a growing place in preclinical and clinical research. The diversity of NMDAR subtypes and their signaling pathways give rise to pleiotropic functions such as brain development, neuronal plasticity, maturation along with excitotoxicity, blood-brain barrier integrity, and inflammation. NMDARs have thus emerged as key targets for the treatment of neurological disorders. By their large extracellular regions and complex intracellular structures, NMDARs are modulated by a variety of endogenous and pharmacological compounds. Here, we will present an overview of NMDAR functions on neurons and other important cell types involved in the pathophysiology of neurodegenerative, neurovascular, mental, autoimmune, and neurodevelopmental diseases. We will then discuss past and future development of NMDAR targeting drugs, including innovative and promising new approaches.

The regulation roles of Ca2+ in erythropoiesis: What have we learned?

Calcium (Ca²⁺) is an important second messenger molecule in the body, regulating cell cycle and fate. There is growing evidence that intracellular Ca²⁺ levels play functional roles in the total physiological process of erythroid differentiation, including the proliferation and differentiation of erythroid progenitor cells, terminal enucleation, and mature red blood cell aging and clearance. Moreover, recent research on the pathology of erythroid disorders has made great progress in the past decades, indicating that calcium ion hemostasis is closely related to ineffective erythropoiesis and increased sensitivity to stress factors. In this review, we summarized what is known about the functional roles of intracellular Ca²⁺ in erythropoiesis and erythrocyte-related diseases, with an emphasis on the regulation of the intracellular Ca²⁺ homeostasis during erythroid differentiation. An understanding of the regulation roles of Ca²⁺ homeostasis in erythroid differentiation will facilitate further studies and eventually molecular identification of the pathways involved in the pathological process of erythroid disorders, providing new therapeutic opportunities in erythrocyte-related disease.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

Slow excitatory synaptic currents generated by AMPA receptors

Decades of literature indicate that the AMPA-type glutamate receptor is among the fastest acting of all neurotransmitter receptors. These receptors are located at excitatory synapses, and conventional wisdom says that they activate in hundreds of microseconds, deactivate in milliseconds due to their low affinity for glutamate and also desensitize profoundly. These properties circumscribe AMPA receptor activation in both space and time. However, accumulating evidence shows that AMPA receptors can also activate with slow, indefatigable responses. They do so through interactions with auxiliary subunits that are able promote a switch to a high open probability, high-conductance 'superactive' mode. In this review, we show that any assumption that this phenomenon is limited to heterologous expression is false and rather that slow AMPA currents have been widely and repeatedly observed throughout the nervous system. Hallmarks of the superactive mode are a lack of desensitization, resistance to competitive antagonists and a current decay that outlives free glutamate by hundreds of milliseconds. Because the switch to the superactive mode is triggered by activation, AMPA receptors can generate accumulating 'pedestal' currents in response to repetitive stimulation, constituting a postsynaptic mechanism for short-term potentiation in the range 5-100 Hz. Further, slow AMPA currents span 'cognitive' time intervals in the 100 ms range (theta rhythms), of particular interest for hippocampal function, where slow AMPA currents are widely expressed in a synapse-specific manner. Here, we outline the implications that slow AMPA receptors have for excitatory synaptic transmission and computation in the nervous system.

Slow AMPA receptors in hippocampal principal cells

Glutamate receptor ion channels, including α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, mediate fast excitatory neurotransmission in the CNS. Previous work suggested that AMPA receptors produce a synaptic current with a millisecond duration. However, we find that about two-thirds of principal cells in the hippocampal CA1 region also express AMPA receptors with reduced desensitization that can stay active for half a second after repetitive stimuli. These slow AMPA receptors are expressed at about half of the synapses, with a flat spatial distribution. The increased charge transfer from slow AMPA receptors allows short-term potentiation from a postsynaptic locus and reliable triggering of action potentials. Biophysical and pharmacological observations imply slow AMPA receptors incorporate auxiliary proteins, and their activation lengthens miniature synaptic currents. These data indicate that AMPA receptors are a major source of synaptic diversity. Synapses harboring slow AMPA receptors could have unique roles in hippocampal function.

Contribution of NMDA Receptors to Synaptic Function in Rat Hippocampal Interneurons

The ability of neurons to produce behaviorally relevant activity in the absence of pathology relies on the fine balance of synaptic inhibition to excitation. In the hippocampal CA1 microcircuit, this balance is maintained by a diverse population of inhibitory interneurons that receive largely similar glutamatergic afferents as their target pyramidal cells, with EPSCs generated by both AMPA receptors (AMPARs) and NMDA receptors (NMDARs). In this study, we take advantage of a recently generated GluN2A-null rat model to assess the contribution of GluN2A subunits to glutamatergic synaptic currents in three subclasses of interneuron found in the CA1 region of the hippocampus. For both parvalbumin-positive and somatostatin-positive interneurons, the GluN2A subunit is expressed at glutamatergic synapses and contributes to the EPSC. In contrast, in cholecystokinin (CCK)-positive interneurons, the contribution of GluN2A to the EPSC is negligible. Furthermore, synaptic potentiation at glutamatergic synapses on CCK-positive interneurons does not require the activation of GluN2A-containing NMDARs but does rely on the activation of NMDARs containing GluN2B and GluN2D subunits.

Presynaptic release-regulating NMDA receptors in isolated nerve terminals: A narrative review

The existence of presynaptic, release‐regulating NMDA receptors in the CNS has been long matter of discussion. Most of the reviews dedicated to support this conclusion have preferentially focussed on the results from electrophysiological studies, paying little or no attention to the data obtained with purified synaptosomes, even though this experimental approach has been recognized as providing reliable information concerning the presence and the role of presynaptic release‐regulating receptors in the CNS. To fill the gap, this review is dedicated to summarising the results from studies with synaptosomes published during the last 40 years, which support the existence of auto and hetero NMDA receptors controlling the release of transmitters such as glutamate, GABA, dopamine, noradrenaline, 5‐HT, acetylcholine and peptides, in the CNS of mammals. The review also deals with the results from immunochemical studies in isolated nerve endings that confirm the functional observations.

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.

GRIN2D/GluN2D NMDA receptor: Unique features and its contribution to pediatric developmental and epileptic encephalopathy

N-methyl-d-aspartate receptors (NMDARs), a subset of ligand-gated ionotropic glutamate receptors, are critical for learning, memory, and neuronal development. However, when NMDAR subunits are mutated, a host of neuropathological conditions can occur, including epilepsy. Recently, genetic variation within the GRIN2D gene, which encodes the GluN2D subunit of the NMDAR, has been associated with a set of early-onset neurological diseases, notably developmental and epileptic encephalopathy (DEE). Importantly, patients with GRIN2D variants are largely refractory to conventional anti-epileptic drug (AED) treatment, highlighting the need to further understand the distinctive characteristics of GluN2D in neurological and pathological functions. In this review, we first summarize GluN2D's unique spatial and temporal expression patterns, electrophysiological profiles, and contributions to both pre- and postsynaptic signaling. Next, we review thirteen unique case studies from DEE patients harboring ten different causal GRIN2D variants. These patients are highly heterogenous, manifesting multiple seizure types, electroencephalographic recordings, and neurological and developmental outcomes. Lastly, this review concludes by highlighting the difficulty in treating patients with DEE-associated GRIN2D variants, and stresses the need for selective therapeutic agents delivered within a precise time window.

Developmental Changes of Synaptic and Extrasynaptic NMDA Receptor Expression in Rat Cerebellar Neurons In Vitro

Transient expression of different NMDA receptors (NMDARs) plays a role in development of the cerebellum. Whether similar processes undergo during neuronal differentiation in culture is not clearly understood. We studied NMDARs in cerebellar neurons in cultures of 7 and 21 days in vitro (DIV) using immunocytochemical and electrophysiological approaches. Whereas at 7 DIV, the vast majority of neurons were immunopositive for GluN2 subunits, further synaptoginesis was accompanied by the time-dependent loss of NMDARs. In contrast to GluN2B- and GluN2C-containing NMDARs, which at 7 DIV exhibited homogenous distribution in extrasynaptic regions, GluN2A-containing receptors were aggregated in spots both in cell bodies and dendrites. Double staining for GluN2A subunits and synaptophysin, a widely used marker for presynaptic terminals, revealed their co-localization in about 75% of dendrite GluN2A fluorescent spots, suggesting postsynaptic origin of GluN2A subunits. In agreement, diheteromeric GluN2A-containing NMDARs contributed to postsynaptic currents recorded in neurons throughout the timescale under study. Diheteromeric GluN2B-containing NMDARs escaped postsynaptic regions during differentiation. Finally, the developmental switch favored the expression of triheteromeric NMDARs assembled of 2 GluN1/1 GluN2B/1 GluN2C or GluN2D subunits in extrasynaptic regions. At 21 DIV, these receptors represented over 60% of the NMDAR population. Thus, cerebellar neurons in primary culture undergo transformations with respect to the expression of di- and triheteromeric NMDARs that should be taken into account when studying cellular aspects of their pharmacology and functions.

Orientation Tuning of Correlated Activity in the Developing Lateral Geniculate Nucleus

Neural circuits and the cells that comprise them undergo developmental changes in the spatial organization of their connections and in their temporal response properties. Within the lateral geniculate nucleus (LGN) of the dorsal thalamus, these changes have pronounced effects on the spatiotemporal receptive fields (STRFs) of neurons. An open and unresolved question is how STRF maturation affects stimulus-evoked correlated activity between pairs of LGN neurons during development. This is an important question to answer because stimulus-evoked correlated activity likely plays a role in establishing the specificity of thalamocortical connectivity and the receptive fields (RFs) of postsynaptic cortical neurons. Using multielectrode recording methods and white noise stimuli, we recorded neural activity from ensembles of LGN neurons in cats across early development. As expected, there was a progressive maturation of the spatial and temporal properties of visual responses. Using drifting bar stimuli and cross-correlation analysis, we also determined the orientation-tuning bandwidth of correlated activity between pairs of LGN neurons at different stages of development (Sillito and Jones, 2002; Andolina et al., 2007; Stanley et al., 2012; Kelly et al., 2014). Despite the larger RFs and slower responses of immature LGN neurons compared with mature neurons, our results show that correlated activity in the LGN was as tightly tuned for orientation early in development as it was in the adult. Closer examination revealed this age-invariant orientation tuning of correlated activity likely involves cellular mechanisms related to spike fatigue in young animals and a progressive decrease in response latency with development.SIGNIFICANCE STATEMENT Orientation tuning is a fundamental property of neurons in primary visual cortex. An important and unresolved question is how orientation tuning emerges during brain development. This study explores a potential mechanism for the establishment of orientation tuning based on correlated activity patterns among ensembles of maturing neurons in the lateral geniculate nucleus (LGN) of the thalamus. Results show that correlated activity between pairs of LGN neurons is more tightly tuned than predictions based simply on receptive field size, indicating that correlated activity has the properties needed to play an important role in the development of geniculocortical circuits and the emergence of cortical orientation tuning.

LTD-like molecular pathways in developmental synaptic pruning

In long-term depression (LTD) at synapses in the adult brain, synaptic strength is reduced in an experience-dependent manner. LTD thus provides a cellular mechanism for information storage in some forms of learning. A similar activity-dependent reduction in synaptic strength also occurs in the developing brain and there provides an essential step in synaptic pruning and the postnatal development of neural circuits. Here we review evidence suggesting that LTD and synaptic pruning share components of their underlying molecular machinery and may thus represent two developmental stages of the same type of synaptic modulation that serve different, but related, functions in neural circuit plasticity. We also assess the relationship between LTD and synaptic pruning in the context of recent findings of LTD dysregulation in several mouse models of autism spectrum disorder (ASD) and discuss whether LTD deficits can indicate impaired pruning processes that are required for proper brain development.

Presynaptic GluN2D receptors detect glutamate spillover and regulate cerebellar GABA release

Glutamate directly activates N-methyl-d-aspartate (NMDA) receptors on presynaptic inhibitory interneurons and enhances GABA release, altering the excitatory-inhibitory balance within a neuronal circuit. However, which class of NMDA receptors is involved in the detection of glutamate spillover is not known. GluN2D subunit-containing NMDA receptors are ideal candidates as they exhibit a high affinity for glutamate. We now show that cerebellar stellate cells express both GluN2B and GluN2D NMDA receptor subunits. Genetic deletion of GluN2D subunits prevented a physiologically relevant, stimulation-induced, lasting increase in GABA release from stellate cells [long-term potentiation of inhibitory transmission (I-LTP)]. NMDA receptors are tetramers composed of two GluN1 subunits associated to either two identical subunits (di-heteromeric receptors) or to two different subunits (tri-heteromeric receptors). To determine whether tri-heteromeric GluN2B/2D NMDA receptors mediate I-LTP, we tested the prediction that deletion of GluN2D converts tri-heteromeric GluN2B/2D to di-heteromeric GluN2B NMDA receptors. We find that prolonged stimulation rescued I-LTP in GluN2D knockout mice, and this was abolished by GluN2B receptor blockers that failed to prevent I-LTP in wild-type mice. Therefore, NMDA receptors that contain both GluN2D and GluN2B mediate the induction of I-LTP. Because these receptors are not present in the soma and dendrites, presynaptic tri-heteromeric GluN2B/2D NMDA receptors in inhibitory interneurons are likely to mediate the cross talk between excitatory and inhibitory transmission.

GluN2D-containing NMDA receptors-mediate synaptic currents in hippocampal interneurons and pyramidal cells in juvenile mice

The differential regulation of the two major N-methyl-D-aspartate receptor (NMDAR) subunits GluN2A and GluN2B during development in forebrain pyramidal cells has been thoroughly investigated. In contrast, much less is known about the role of GluN2D, which is expressed at low levels and is downregulated following the second postnatal week. However, it appears that few cells, presumably interneurons, continue to express GluN2D also in juvenile mice. To investigate which hippocampal cell types express this subunit, we generated transgenic mice with EGFP-tagged GluN2D receptors. The expression of the transgene was confined to hippocampal interneurons, most of which were parvalbumin- and/or somatostatin-positive. Electrophysiological and morphological analyses showed that GluN2D was present mainly in fast spiking basket and axo-axonic cells. Based on pharmacological evidence and electrophysiological analysis of GluN2D knockout mice, we conclude that GluN2D-containing NMDARs mediate synaptic currents in hippocampal interneurons of young and juvenile mice and in CA1 pyramidal neurons of newborn mice.

Functional plasticity of the N-methyl-d-aspartate receptor in differentiating human erythroid precursor cells

Calcium signaling is essential to support erythroid proliferation and differentiation. Precise control of the intracellular Ca²⁺ levels in erythroid precursor cells (EPCs) is afforded by coordinated expression and function of several cation channels, including the recently identified N-methyl-d-aspartate receptor (NMDAR). Here, we characterized the changes in Ca²⁺ uptake and electric currents mediated by the NMDARs occurring during EPC differentiation using flow cytometry and patch clamp. During erythropoietic maturation, subunit composition and properties of the receptor changed; in proerythroblasts and basophilic erythroblasts, fast deactivating currents with high amplitudes were mediated by the GluN2A subunit-dominated receptors, while at the polychromatic and orthochromatic erythroblast stages, the GluN2C subunit was getting more abundant, overriding the expression of GluN2A. At these stages, the currents mediated by the NMDARs carried the features characteristic of the GluN2C-containing receptors, such as prolonged decay time and lower conductance. Kinetics of this switch in NMDAR properties and abundance varied markedly from donor to donor. Despite this variability, NMDARs were essential for survival of EPCs in any subject tested. Our findings indicate that NMDARs have a dual role during erythropoiesis, supporting survival of polychromatic erythroblasts and contributing to the Ca²⁺ homeostasis from the orthochromatic erythroblast stage to circulating red blood cells.

Opposing role of NMDA receptor GluN2B and GluN2D in somatosensory development and maturation

Development of correct topographical connections between peripheral receptors and central somatosensory stations requires activity-dependent synapse refinement, in which the NMDA type of glutamate receptors plays a key role. Here we compared functional roles of GluN2B (GluRε2 or NR2B) and GluN2D (GluRε4 or NR2D), two major regulatory subunits of neonatal NMDA receptors, in development of whisker-related patterning at trigeminal relay stations. Compared with control littermates, both the appearance of whisker-related patterning and the termination of the critical period, as assessed by unilateral infraorbital nerve transection, were delayed by nearly a day in the somatosensory cortex of GluN2B(+/-) mice but advanced by nearly a day in GluN2D(-/-) mice. Similar temporal shifts were found at subcortical relay stations in the thalamus and brainstem of GluN2B(+/-) and GluN2D(-/-) mice. In comparison, the magnitude of lesion-induced critical period plasticity in the somatosensory cortex, as assessed following row-C whisker removal, was normal in both mutants. Thus, GluN2B and GluN2D play counteractive roles in temporal development and maturation of somatosensory maps without affecting the magnitude of critical period plasticity. To understand the opposing action, we then examined neuronal and synaptic expressions of the two subunits along the trigeminal pathway. At each trigeminal station, GluN2B was predominant at asymmetrical synapses of non-GABAergic neurons, whereas GluN2D was selective to asymmetrical synapses of GABAergic neurons. Together, our findings suggest that GluN2B expressed at glutamatergic synapses on glutamatergic projection neurons facilitates refinement of ascending pathway synapses directly, whereas GluN2D expressed at glutamatergic synapses on GABAergic interneurons delays it indirectly.

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.

N-methyl-D-aspartate receptors in human erythroid precursor cells and in circulating red blood cells contribute to the intracellular calcium regulation

The presence of N-methyl-d-aspartate receptor (NMDAR) was previously shown in rat red blood cells (RBCs) and in a UT-7/Epo human myeloid cell line differentiating into erythroid lineage. Here we have characterized the subunit composition of the NMDAR and monitored its function during human erythropoiesis and in circulating RBCs. Expression of the NMDARs subunits was assessed in erythroid progenitors during ex vivo erythropoiesis and in circulating human RBCs using quantitative PCR and flow cytometry. Receptor activity was monitored using a radiolabeled antagonist binding assay, live imaging of Ca(2+) uptake, patch clamp, and monitoring of cell volume changes. The receptor tetramers in erythroid precursor cells are composed of the NR1, NR2A, 2C, 2D, NR3A, and 3B subunits of which the glycine-binding NR3A and 3B and glutamate-binding NR2C and 2D subunits prevailed. Functional receptor is required for survival of erythroid precursors. Circulating RBCs retain a low number of the receptor copies that is higher in young cells compared with mature and senescent RBC populations. In circulating RBCs the receptor activity is controlled by plasma glutamate and glycine. Modulation of the NMDAR activity in RBCs by agonists or antagonists is associated with the alterations in whole cell ion currents. Activation of the receptor results in the transient Ca(2+) accumulation, cell shrinkage, and alteration in the intracellular pH, which is associated with the change in hemoglobin oxygen affinity. Thus functional NMDARs are present in erythroid precursor cells and in circulating RBCs. These receptors contribute to intracellular Ca(2+) homeostasis and modulate oxygen delivery to peripheral tissues.

Nedd4 is a specific E3 ubiquitin ligase for the NMDA receptor subunit GluN2D

NMDA receptors are a family of glutamate-gated ion channels that regulate various CNS functions such as synaptic plasticity and learning. However hypo- or hyper-activation of NMDA receptors is critically involved in many neurological and psychiatric conditions such as pain, stroke, epilepsy, neurodegeneration, schizophrenia, and depression. Thus, it is important to identify mechanisms (such as by targeted ubiquitination) that regulate the levels of individual subtypes of NMDA receptors. In this study, we used a series of tagged, carboxy terminal constructs of GluN2D to identify associating proteins from rat brain. Of seven different GluN2D C-terminal fragments used as bait, only the construct containing amino acids 983-1097 associated with an E3 ubiquitin ligase, Nedd4. A direct interaction between GluN2D and Nedd4 was confirmed both in vivo and in vitro. This association is mediated by an interaction between GluN2D's C-terminal PPXY motif and the 2nd and 3rd WW domains of Nedd4. Of the four GluN2 subunits, Nedd4 directly interacted with GluN2D and also weakly with GluN2A. Nedd4 coexpression with GluN2D enhances GluN2D ubiquitination and reduces GluN1/GluN2D NMDA receptor responses. These results identify Nedd4 as a novel binding partner for GluN2D and suggest a mechanism for the regulation of NMDA receptors that contain GluN2D subunits through ubiquitination-dependent downregulation. This article is part of the Special Issue entitled 'Glutamate Receptor-Dependent Synaptic Plasticity'.

Selective Pharmacological Modulation of Pyramidal Neurons and Interneurons in the CA1 Region of the Rat Hippocampus

The hippocampus is a complex network tightly regulated by interactions between excitatory and inhibitory neurons. In neurodegenerative disorders where cognitive functions such as learning and memory are impaired this excitation-inhibition balance may be altered. Interestingly, the uncompetitive N-methyl-d-aspartate receptor (NMDAR) antagonist memantine, currently in clinical use for the treatment of Alzheimer’s disease, may alter the excitation-inhibition balance in the hippocampus. However, the specific mechanism by which memantine exerts this action is not clear. To better elucidate the effect of memantine on hippocampal circuitry, we studied its pharmacology on NMDAR currents in both pyramidal cells (PCs) and interneurons (Ints) in the CA1 region of the hippocampus. Applying whole-cell patch-clamp methodology to acute rat hippocampal slices, we report that memantine antagonism is more robust in PCs than in Ints. Using specific NMDAR subunit antagonists, we determined that this selective antagonism of memantine is attributable to specific differences in the molecular make-up of the NMDARs in excitatory and inhibitory neurons. These findings offer new insight into the mechanism of action and therapeutic potential of NMDA receptor pharmacology in modulating hippocampal excitability.

A fast model of voltage-dependent NMDA receptors

NMDA receptors are among the crucial elements of central nervous system models. Recent studies show that both conductance and kinetics of these receptors are changing voltage-dependently in some parts of the brain. Therefore, several models have been introduced to simulate their current. However, on the one hand, kinetic models-which are able to simulate these voltage-dependent phenomena-are computationally expensive for modeling of large neural networks. On the other hand, classic exponential models, which are computationally less expensive, are not able to simulate the voltage-dependency of these receptors, accurately. In this study, we have modified these classic models to endow them with the voltage-dependent conductance and time constants. Temperature sensitivity and desensitization of these receptors are also taken into account. We show that, it is possible to simulate the most important physiological aspects of NMDA receptor's behavior using only three to four differential equations, which is significantly smaller than the previous kinetic models. Consequently, it seems that our model is both fast and physiologically plausible and therefore is a suitable candidate for the modeling of large neural networks.

GluN1 splice variant control of GluN1/GluN2D NMDA receptors

NMDA receptors are ionotropic glutamate receptors that mediate a slow, Ca2+-permeable component of excitatory synaptic transmission in the central nervous system. Recombinant GluN1-1a/GluN2D receptors are characterized by low channel open probability and prolonged deactivation time course following the removal of agonist. Here, we show that the deactivation time course, agonist potency, and single channel properties of GluN2D-containing NMDA receptors are modulated by alternative RNA splicing of GluN1. Our results demonstrate that GluN1 exon 5, which encodes a 21-amino-acid insert in the amino-terminal domain, is a key determinant of GluN1/GluN2D receptor function. GluN1-1b/GluN2D receptors, which contain the residues encoded by exon 5, deactivate with a dual exponential time course described by a τFAST of 410 ms and a τSLOW of 1100 ms. This time course is 3-fold more rapid than that for exon 5-lacking GluN1-1a/GluN2D, which deactivates with a τFAST of 1100 ms and a τSLOW of 3400 ms. Exon 5-containing NMDA receptors also have a two-fold higher open probability (0.037) than exon 5-lacking receptors (0.017). Furthermore, inclusion of exon 5-encoded residues within the GluN1-1b subunit decreases the potency for the endogenous agonist l-glutamate. Evaluation of receptor kinetics for NMDA receptors containing mutated GluN1-1b subunits and wild-type GluN2D identified residue Lys211 in GluN1-1b as a key determinant of exon 5 control of the deactivation time course and glutamate potency. Evaluation of a kinetic model of GluN1/GluN2D gating suggests that residues encoded by exon 5 influence several rate-limiting steps. These data demonstrate that the GluN1 subunit is a key determinant of the kinetic and pharmacological properties of GluN2D-containing NMDA receptors.

Plasticity of NMDA receptor-mediated excitatory postsynaptic currents at perforant path inputs to dendrite-targeting interneurons

Synaptic plasticity of NMDA receptors (NMDARs) has been recently described in a number of brain regions and we have previously characterised LTP and LTD of glutamatergic NMDA receptor-mediated EPSCs (NMDAR-EPSCs) in granule cells of dentate gyrus. The functional significance of NMDAR plasticity at perforant path synapses on hippocampal network activity depends on whether this is a common feature of perforant path synapses on all postsynaptic target cells or if this plasticity occurs only at synapses on principal cells. We recorded NMDAR-EPSCs at medial perforant path synapses on interneurons in dentate gyrus which had significantly slower decay kinetics compared to those recorded in granule cells. NMDAR pharmacology in interneurons was consistent with expression of both GluN2B- and GluN2D-containing receptors. In contrast to previously described high frequency stimulation-induced bidirectional plasticity of NMDAR-EPSCs in granule cells, only LTD of NMDAR-EPSCs was induced in interneurons in our standard experimental conditions. In interneurons, LTD of NMDAR-EPSCs was associated with a loss of sensitivity to a GluN2D-selective antagonist and was inhibited by the actin stabilising agent, jasplakinolide. While LTP of NMDAR-EPSCs can be readily induced in granule cells, this form of plasticity was only observed in interneurons when extracellular calcium was increased above physiological concentrations during HFS or when PKC was directly activated by phorbol ester, suggesting that opposing forms of plasticity at inputs to interneurons and principal cells may act to regulate granule cell dendritic integration and processing.

Pre and post synaptic NMDA effects targeting Purkinje cells in the mouse cerebellar cortex

N-methyl-D-aspartate (NMDA) receptors are associated with many forms of synaptic plasticity. Their expression level and subunit composition undergo developmental changes in several brain regions. In the mouse cerebellum, beside a developmental switch between NR2B and NR2A/C subunits in granule cells, functional postsynaptic NMDA receptors are seen in Purkinje cells of neonate and adult but not juvenile rat and mice. A presynaptic effect of NMDA on GABA release by cerebellar interneurons was identified recently. Nevertheless whereas NMDA receptor subunits are detected on parallel fiber terminals, a presynaptic effect of NMDA on spontaneous release of glutamate has not been demonstrated. Using mouse cerebellar cultures and patch-clamp recordings we show that NMDA facilitates glutamate release onto Purkinje cells in young cultures via a presynaptic mechanism, whereas NMDA activates extrasynaptic receptors in Purkinje cells recorded in old cultures. The presynaptic effect of NMDA on glutamate release is also observed in Purkinje cells recorded in acute slices prepared from juvenile but not from adult mice and requires a specific protocol of NMDA application.

Development of NMDA NR2 subunits and their roles in critical period maturation of neocortical GABAergic interneurons

The goals of this research are to (1) determine the changes in the composition of NMDA receptor (NMDAR) subunits in GABAergic interneurons during critical period (CP); and (2) test the effect of chronic blockage of specific NR2 subunits on the maturation of specific GABAergic interneurons. Our data demonstrate that: (1) The amplitude of NMDAR mediated EPSCs (EPSCs(NMDAR) ) was significantly larger in the postCP group. (2) The coefficient of variation (CV), τ(decay) and half-width of EPSCs(NMDAR) were significantly larger in the preCP group. (3) A leftward shift in the half-activation voltages in the postCP vs. preCP group. (4) Using subunit-specific antagonists, we found a postnatal shift in NR2 composition towards more NR2A mediated EPSCs(NMDAR) . These changes occurred within a two-day narrow window of CP and were similar between fast-spiking (FS) and regular spiking (RSNP) interneurons. (5) Chronic blockage of NR2A, but not NR2B, decreased the expression of parvalbumin (PV), but not other calcium binding proteins in layer 2/3 and 4 of barrel cortex. (6) Chronic blockage of NR2A selectively affected the maturation of IPSCs mediated by FS cells. In summary, we have reported, for the first time, developmental changes in the molecular composition of NMDA NR2 subunits in interneurons during CP, and the effects of chronic blockage of NR2A but not NR2B on PV expression and inhibitory synaptic transmission from FS cells. These results support an important role of NR2A subunits in developmental plasticity of fast-spiking GABAergic circuits during CP.

MHC class I modulates NMDA receptor function and AMPA receptor trafficking

Proteins of the major histocompatibility complex class I (MHCI) are known for their role in immunity and have recently been implicated in long-term plasticity of excitatory synaptic transmission. However, the mechanisms by which MHCI influences synaptic plasticity remain unknown. Here we show that endogenous MHCI regulates synaptic responses mediated by NMDA-type glutamate receptors (NMDARs) in the mammalian central nervous system (CNS). The AMPA/NMDA ratio is decreased at MHCI-deficient hippocampal synapses, reflecting an increase in NMDAR-mediated currents. This enhanced NMDAR response is not associated with changes in the levels, subunit composition, or gross subcellular distribution of NMDARs. Increased NMDAR-mediated currents in MHCI-deficient neurons are associated with characteristic changes in AMPA receptor trafficking in response to NMDAR activation. Thus, endogenous MHCI tonically inhibits NMDAR function and controls downstream NMDAR-induced AMPA receptor trafficking during the expression of plasticity.

Control of assembly and function of glutamate receptors by the amino-terminal domain

The extracellular amino-terminal domains (ATDs) of the ionotropic glutamate receptor subunits form a semiautonomous component of all glutamate receptors that resides distal to the membrane and controls a surprisingly diverse set of receptor functions. These functions include subunit assembly, receptor trafficking, channel gating, agonist potency, and allosteric modulation. The many divergent features of the different ionotropic glutamate receptor classes and different subunits within a class may stem from differential regulation by the amino-terminal domains. The emerging knowledge of the structure and function of the amino-terminal domains reviewed here may enable targeting of this region for the therapeutic modulation of glutamatergic signaling. Toward this end, NMDA receptor antagonists that interact with the GluN2B ATD show promise in animal models of ischemia, neuropathic pain, and Parkinson's disease.

Single-channel properties of N-methyl-D-aspartate receptors containing chimaeric GluN2A/GluN2D subunits

Subtypes of NMDARs (N-methyl-D-aspartate receptors) display differences in their pharmacological and biophysical properties. The differences are, to a large extent, determined by the identities of the GluN2 (glutamate-binding) NMDAR subunits that are co-expressed with GluN1 (glycine-binding) subunits, which form the final tetrameric NMDAR assembly. Of the four GluN2 subunits that exist (termed A-D), NMDARs composed of GluN1/GluN2A and GluN1/GluN2D subunits display the greatest differences in their sensitivities to a variety of agonists, antagonists and channel blockers as well as showing marked differences in their single-channel conductances and deactivation kinetics. Here, we describe a series of experiments where we have generated and studied two chimaeric GluN2A/GluN2D subunits. The first chimaera, referred to as GluN2A(2D-M1M2M3), replaces the membrane-associated regions M1, M2 and M3 of the GluN2A subunit with the corresponding regions found in the GluN2D subunit. The second chimaera, GluN2A(2D-S1M1M2M3S2), replaces the same three membrane-associated regions of the GluN2A subunit plus the LBD (ligand-binding domain) with the corresponding regions of the GluN2D subunit. Our results show that the identity of the GluN2 LBD not only controls glutamate potency, but also influences the potency of the NMDAR co-agonist glycine, whereas the single-channel conductance and the duration of single activations of ion channels can be predicted by the identities of the M1-M3 regions and the LBD.

Extrasynaptic NR2D-containing NMDARs are recruited to the synapse during LTP of NMDAR-EPSCs

Long-term potentiation of NMDA-receptor-mediated synaptic transmission (NMDAR-LTP) is a little-understood form of plasticity. In the present study, we investigated whether NMDAR-LTP in the dentate gyrus involves recruitment of extrasynaptic NMDARs, because NMDARs are expressed both synaptically and extrasynaptically with evidence for subtype differences at different locations. We show that before induction of NMDAR-LTP, pharmacological inhibition of glutamate transporters resulted in glutamate spillover from the synapse and activation of extrasynaptic NMDARs. After the induction of NMDAR-LTP, such activation of extrasynaptic NMDARs was absent. Activation of extrasynaptic NMDARs after glutamate uptake inhibition also occurred when synaptic NMDARs were inhibited with MK801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate], and this extrasynaptically mediated NMDAR-EPSC was strongly reduced by prior induction of NMDAR-LTP. The extrasynaptic NMDARs were shown to be NR2D-containing, because the activation of extrasynaptic NMDARs by glutamate spillover was prevented by the NR2D-selective antagonists PPDA [(2R*,3S*)-1-(phenanthrenyl-2-carbonyl)piperazine-2,3-dicarboxylic acid] and UBP141. Further studies using selective antagonists for NR2A- and NR2B-containing NMDARs demonstrated that synaptic NMDARs are predominantly NR2A-containing and NR2B-containing receptors, whereas the extrasynaptic NMDARs are complex multimeric receptors with NR2A, NR2B, or NR2D subunits. Our results show that LTP of NMDAR-EPSCs involves movement of NMDARs from an extrasynaptic to a synaptic location and suggest a novel physiological role for extrasynaptic NMDARs.

Anatomical localization and expression pattern for the NMDA-2D receptor subunit in a rat model of neuropathic pain

The N-methyl-d-aspartate receptor (NMDAR) has been implicated in the etiology of chronic pain. In this regard, this study sought to characterize the localization and expression pattern for the NMDAR-2D subunit in a rat model of neuropathic pain. To this end, one group of rats, 3 weeks post-dorsal root rhizotomy (DRR) and a second group, 3 weeks post-spinal nerve ligation (SNL) and sham surgery, were generated. Dorsal root ganglia (DRG) and/or lumbar spinal cord were excised from DRR, naïve, SNL and sham rats. Both immunohistochemical and real-time PCR analysis confirmed discrete NMDAR-2D subunit expression within the DRG and dorsal horn. However, no overt differences in staining intensity or expression were noted between DRG and spinal cord sections obtained from the different surgical groups. Results also demonstrated that the NMDAR-2D subunit was present within Neu N+ cells in the spinal cord and DRG, but excluded from cells labeled with the astrocytic marker, GFAP, and the microglial maker, OX-42. Lastly, the NMDAR-2D subunit was not co-expressed within neurokinin-1 (NK-1)+ or neurofilament-52 (N-52)+ neurons, but the antibody did co-label a number of isolectin B4+ (IB4) DRG cells. Together, these findings seem to suggest that the NMDAR-2D receptor subunit is present within the cell body region of a population of small diameter sensory afferents and post-synaptically within second order dorsal horn neurons. Although these data suggest that the NMDAR-2D subunit is well poised anatomically to modulate pain neurotransmission, the expression pattern for this subunit is not altered in rats demonstrating the presence of neuropathic-like pain behavior.

Prion protein attenuates excitotoxicity by inhibiting NMDA receptors

It is well established that misfolded forms of cellular prion protein (PrP [PrP^C]) are crucial in the genesis and progression of transmissible spongiform encephalitis, whereas the function of native PrP^C remains incompletely understood. To determine the physiological role of PrP^C, we examine the neurophysiological properties of hippocampal neurons isolated from PrP-null mice. We show that PrP-null mouse neurons exhibit enhanced and drastically prolonged N-methyl-d-aspartate (NMDA)–evoked currents as a result of a functional upregulation of NMDA receptors (NMDARs) containing NR2D subunits. These effects are phenocopied by RNA interference and are rescued upon the overexpression of exogenous PrP^C. The enhanced NMDAR activity results in an increase in neuronal excitability as well as enhanced glutamate excitotoxicity both in vitro and in vivo. Thus, native PrP^C mediates an important neuroprotective role by virtue of its ability to inhibit NR2D subunits.

Emergence of brain-derived neurotrophic factor-induced postsynaptic potentiation of NMDA currents during the postnatal maturation of the Kolliker-Fuse nucleus of rat

The Kölliker-Fuse nucleus (KF) contributes essentially to respiratory pattern formation and adaptation of breathing to afferent information. Systems physiology suggests that these KF functions depend on NMDA receptors (NMDA-R). Recent investigations revealed postnatal changes in the modulation of glutamatergic neurotransmission by brain-derived neurotrophic factor (BDNF) in the KF. Therefore, we investigated postnatal changes in NMDA-R subunit composition and postsynaptic modulation of NMDA-R-mediated currents by BDNF in KF slice preparations derived from three age groups (neonatal: postnatal day (P) 1-5; intermediate: P6-13; juvenile: P14-21). Immunohistochemistry showed a developmental up-regulation of the NR2D subunit. This correlated with a developmental increase in decay time of NMDA currents and a decline of desensitization in response to repetitive exogenous NMDA applications. Thus, developmental up-regulation of the NR2D subunit, which reduces the Mg(2+) block of NMDA-R, causes these specific changes in NMDA current characteristics. This may determine the NMDA-R-dependent function of the mature KF in the control of respiratory phase transition. Subsequent experiments revealed that bath-application of BDNF progressively potentiated these repetitively evoked NMDA currents only in intermediate and juvenile age groups. Pharmacological inhibition of protein kinase C (PKC), as a downstream component of the BDNF-tyrosine kinase B receptor (trkB) signalling, prevented BDNF-induced potentiation of NMDA currents. BDNF-induced potentiation of NMDA currents in later developmental stages might be essential for synaptic plasticity during the adaptation of the breathing pattern in response to peripheral/central commands. The lack of plasticity in neonatal neurones strengthens the hypothesis that the respiratory network becomes permissive for activity-dependent plasticity with ongoing postnatal development.

NMDA receptor contribution to the climbing fiber response in the adult mouse Purkinje cell

Among integrative neurons displaying long-term synaptic plasticity, adult Purkinje cells seemed to be an exception by lacking functional NMDA receptors (NMDA-Rs). Although numerous anatomical studies have shown both NR1 and NR2 NMDA-R subunits in adult Purkinje cells, patch-clamp studies failed to detect any NMDA currents. Using more recent pharmacological and immunodetection tools, we demonstrate here that Purkinje cells from adult mice respond to exogenous NMDA application and that postsynaptic NMDA-Rs carry part of the climbing fiber-mediated EPSC (CF-EPSC), with undetectable contribution from presynaptic or polysynaptic NMDA currents. We also detect NR2-A/B subunits in adult Purkinje cells by immunohistochemistry. The NMDA-mediated CF-EPSC is barely detectable before 3 weeks postnatal. From the end of the third week, the number of cells displaying the NMDA-mediated CF-EPSC rapidly increases. Soon, this EPSC becomes detectable in all the Purkinje cells but is still very small. Its amplitude continues to increase until 12 weeks after birth. In mature Purkinje cells, we show that the NMDA-Rs contribute to the depolarizing plateau of complex spikes and increase their number of spikelets. Together, these observations demonstrate that mature Purkinje cells express functional NMDA receptors that become detectable in CF-EPSCs at approximately 21 d after birth and control the complex spike waveform.

Different roles for AMPA and NMDA receptors in transmission at the immature retinogeniculate synapse

The relay of information at the retinogeniculate synapse, the connection between retina and visual thalamus, begins days before eye opening and is thought to play an important role in the maturation of neural circuits in the thalamus and visual cortex. Remarkably, during this period of development, the retinogeniculate synapse is immature, with single retinal ganglion cell inputs evoking an average peak excitatory postsynaptic current (EPSC) of only about 40 pA compared with 800 pA in mature synapses. Yet, at the mature synapse, EPSCs >400 pA are needed to drive relay neuron firing. This raises the question of how small-amplitude EPSCs can drive transmission at the immature retinogeniculate synapse. Here we find that several features of the immature synapse, compared with the mature synapse, contribute to synaptic transmission. First, although the peak amplitude of EPSC is small, the decay time course of both alpha-amino-3-hydroxy-5-methyl-4isoxazolepropionic acid receptor (AMPAR) and N-methyl-d-aspartate receptor (NMDAR) currents is significantly slower. The prolonged time course of NMDAR currents is a result of the presence of both NR2B and NR2C/D subunits. In addition, the extended presence of neurotransmitter released prolongs the synaptic current time course. Second, reduced sensitivity to magnesium block results in significantly greater synaptic charge transfer through NMDAR. Third, AMPAR currents contribute to the spike latency, but not to temporal precision, at the immature synapse. Furthermore, intrinsic excitability is greater. These properties enable immature synapses with predominantly NMDARs and little or no AMPARs to contribute to the relay of information from retina to visual cortex.

Long-Lasting NMDA Receptor-Mediated EPSCs in Mouse Striatal Medium Spiny Neurons

Excitatory postsynaptic currents (EPSCs) from dorsolateral medium spiny neurons (MSNs) were recorded in cortico-striatal slice preparations from postnatal day 6–8 (P6-8) and >P12 wild-type mice and mice that were lacking either the NR2A or the NR2C subunit of the N-methyl-d-aspartate (NMDA) receptor. EPSCs were elicited by stimulation of the excitatory afferents and the NMDA and non-NMDA receptor-mediated components were pharmacologically isolated. The ratio of these components decreased with development and was significantly reduced only between age-matched +/+ and NR2A −/− neurons. In many MSNs, the NMDA-EPSC decay was characterized by the presence of a slow exponential component with a time constant lasting >1 s regardless of genotype or age. In the NR2A −/−, no developmental increase in the decay time (Tw) of the NMDA-EPSCs was observed although it was almost twofold longer than in +/+ MSNs. NR1/NR2B antagonists were ineffective in reducing the slow NMDA-EPSCs at all ages. Input-output studies revealed differences in stimulation threshold sensitivity of MSNs based on stimulus location. High-threshold responders were preferentially identified with stimulation from intracortical locations that produced considerably faster NMDA-EPSCs, whereas low-threshold responders were mainly elicited with stimulation more proximal to the striatum and exhibited slower NMDA-EPSCs. A low-affinity competitive antagonist of NMDA receptors failed to alter the decay of NMDA-EPSCs elicited from either location, suggesting that glutamate spillover is not responsible for the long-lasting NMDA-EPSCs. Our data are consistent with the expression of a unique NMDA receptor complex in MSNs with very slow deactivation kinetics.

Mg2+ and memantine block of rat recombinant NMDA receptors containing chimeric NR2A/2D subunits expressed in Xenopus laevis oocytes

N-methyl-d-aspartate receptors (NMDARs) display differences in their sensitivity to the channel blockers Mg(2+) and memantine that are dependent on the identity of the NR2 subunit present in the receptor-channel complex. This study used two-electrode voltage-clamp recordings from Xenopus laevis oocytes expressing recombinant NMDARs to investigate the actions of Mg(2+) and memantine at the two NMDARs displaying the largest differences in sensitivity to these blockers, namely NR1/NR2A and NR1/NR2D NMDARs. In addition, NR2A/2D chimeric subunits have been employed to examine the effects of pore-forming elements and ligand-binding domains (LBD) on the potency of the block produced by each of these inhibitors. Our results show that, as previously documented, NR2D-containing NMDARs are less sensitive to voltage-dependent Mg(2+) block than their NR2A-containing counterparts. The reduced sensitivity is determined by the M1M2M3 membrane-associated regions, as replacing these regions in NR2A subunits with those found in NR2D subunits results in a approximately 10-fold reduction in Mg(2+) potency. Intriguingly, replacing the NR2A LBD with that from NR2D subunits results in a approximately 2-fold increase in Mg(2+) potency. Moreover, when responses mediated by NR1/NR2A NMDARs are evoked by the partial agonist homoquinolinate, rather than glutamate, Mg(2+) also displays an increased potency. Memantine block of glutamate-evoked currents is most potent at NR1/NR2D NMDARs, but no differences are observed in its ability to inhibit NR2A-containing or NR2A/2D chimeric NMDARs. We suggest that the potency of block of NMDARs by Mg(2+) is influenced not only by pore-forming regions but also the LBD and the resulting conformational changes that occur following agonist binding.

Novel regional and developmental NMDA receptor expression patterns uncovered in NR2C subunit-beta-galactosidase knock-in mice

NMDA receptor "knock-in" mice were generated by inserting the nuclear beta-galactosidase reporter at the NR2C subunit translation initiation site. Novel cell types and dynamic patterns of NR2C expression were identified using these mice, which were unnoticed before because reagents that specifically recognize NR2C-containing receptors are non-existent. We identified a transition zone from NR2C-expressing neurons to astrocytes in an area connecting the retrosplenial cortex and hippocampus. We demonstrate that NR2C is expressed in a subset of S100beta-positive/GFAP-negative glial cells in the striatum, olfactory bulb and cerebral cortex. We also demonstrate novel areas of neuronal expression such as retrosplenial cortex, thalamus, pontine and vestibular nuclei. In addition, we show that during cerebellar development NR2C is expressed in transient caudal-rostral gradients and parasagittal bands in subsets of granule cells residing in the internal granular layer, further demonstrating heterogeneity of granule neurons. These results point to novel functions of NR2C-containing NMDA receptors.

NMDA receptors in layer 4 spiny stellate cells of the mouse barrel cortex contain the NR2C subunit

In layer 4 of the somatosensory cortex, the glutamatergic synapses that interconnect spiny stellate (SpS) neurons, which are the major targets of thalamocortical input, differ from most other neocortical excitatory synapses in that they have an extremely large NMDA receptor (NMDAR)-mediated component that is relatively insensitive to voltage-dependent Mg2+ blockade. We now report that this unique feature of the NMDA response reflects the distinctive subunit composition of the underlying receptors. We studied NMDAR-mediated miniature EPSCs (mEPSCs) and NMDA channel currents in tangential brain slices of mouse barrel cortex, which exclusively contain layer 4. NMDAR-mediated mEPSCs in SpS neurons were prominent at negative membrane potentials, and NMDA channels in outside-out patches excised from the somata of the same neurons had relatively low conductance and reduced susceptibility to Mg2+ block. These are characteristic features of heteromeric NMDAR assemblies that contain the NR2C subunit. Some patches also contained NMDA channels with higher conductance and a greater sensitivity to Mg2+. In the neocortex of transgenic mice in which a beta-galactosidase (lacZ) indicator gene was controlled by the NR2C promoter, the lacZ indicator was densely expressed in layer 4. In current-clamp recordings, blockade of NMDARs caused hyperpolarization and an increase in apparent input resistance. Our data demonstrate that the SpS neurons of layer 4 functionally express NR2C subunits; this is the likely explanation for their ability to generate large NMDAR-mediated EPSPs that are effective at resting potential, without previous depolarization.

Cell type-specific changes in spontaneous and minimally evoked excitatory synaptic activity in hippocampal CA1 interneurons of kainate-treated rats

The epileptiform activity in the kainic acid (KA) model of epilepsy arises from complex changes in excitation and inhibition. To assess the involvement of excitatory drive onto inhibitory interneurons in this epileptiform activity, we examined changes in spontaneous and minimally evoked excitatory post-synaptic currents (sEPSCs and eEPSCs) in CA1 interneurons in stratum oriens/alveus (O/A) and stratum radiatum (RAD) in rat hippocampal slices after KA treatment. The frequency and amplitude of sEPSCs and the amplitude of eEPSCs were unchanged in O/A interneurons, but the EPSC kinetics were significantly slower. These changes appear to be due to altered kinetics and voltage-dependent properties of the NMDA component of EPSCs in O/A interneurons. In contrast, sEPSCs and eEPSCs in RAD interneurons did not change after KA treatment. The distinct changes in excitatory synaptic activity in interneurons differentially involved in feedback (O/A) versus feedforward (RAD) inhibition suggest a cell type-specific reorganization of excitatory synapses after KA treatment. These modifications in excitatory input to interneurons could contribute to the maintenance of inhibition of CA1 pyramidal cells after KA treatment, or may also create network conditions favourable to epileptiform activity.

Target-cell-specific left-right asymmetry of NMDA receptor content in schaffer collateral synapses in epsilon1/NR2A knock-out mice

Input-dependent left-right asymmetry of NMDA receptor epsilon2 (NR2B) subunit allocation was discovered in hippocampal Schaffer collateral (Sch) and commissural fiber pyramidal cell synapses (Kawakami et al., 2003). To investigate whether this asymmetrical epsilon2 allocation is also related to the types of the postsynaptic cells, we compared postembedding immunogold labeling for epsilon2 in left and right Sch synapses on pyramidal cells and interneurons. To facilitate the detection of epsilon2 density difference, we used epsilon1 (NR2A) knock-out (KO) mice, which have a simplified NMDA receptor subunit composition. The labeling density for epsilon2 but not zeta1 (NR1) and subtype 2/3 glutamate receptor (GluR2/3) in Sch-CA1 pyramidal cell synapses was significantly different between the left and right hippocampus with opposite directions in strata oriens and radiatum; the left to right ratio of epsilon2 labeling density was 1:1.50 in stratum oriens and 1.44:1 in stratum radiatum. No significant difference, however, was detected in CA1 stratum radiatum between the left and right Sch-GluR4-positive (mostly parvalbumin-positive) and Sch-GluR4-negative interneuron synapses. Consistent with the anatomical asymmetry, the amplitude ratio of NMDA EPSCs to non-NMDA EPSCs in pyramidal cells was approximately two times larger in right than left stratum radiatum and vice versa in stratum oriens of epsilon1 KO mice. Moreover, the amplitude of long-term potentiation in the Sch-CA1 synapses of left stratum radiatum was significantly larger than that in the right corresponding synapses. These results indicate that the asymmetry of epsilon2 distribution is target cell specific, resulting in the left-right difference in NMDA receptor content and plasticity in Sch-CA1 pyramidal cell synapses in epsilon1 KO mice.

Cultivated keratinocytes express N-methyl-d-aspartate receptors of the NMDAR2D type

N-methyl-D-aspartate receptors (NMDAR) can regulate the intracellular calcium concentration of keratinocytes (KC) and seem to be important for their growth and differentiation. The objective of this study was to identify the subtype(s) of this receptor expressed by KC in vitro. The mRNA was isolated from primary cultures of KC as well as from a KC cell line (HaCaT) and expression of the NMDAR subtypes determined by using RT-PCR. At the mRNA level, we found expression of only the constant NMDAR1 as well as the subtype NMDAR2D. In contrast to the other subtypes of NMDAR, NMDAR2D is characterized by low influence of magnesium to the receptor function. This characteristic is consistent with previously published functional investigations in KC. The identification of the NMDAR2D subtype in KC may be of value for the development of new therapeutic approaches.

Gene expression evidence for remodeling of lateral hypothalamic circuitry in cocaine addiction

By using high-density oligonucleotide arrays, we profiled gene expression in reward-related brain regions of rats that developed escalated cocaine intake after extended access to cocaine (6 h per day). Rats allowed restricted daily access to cocaine (only 1 h) that displayed a stable level of cocaine intake and cocaine naive rats were used for controls. Four analysis methods were compared: Affymetrix microarray suite 4 and microarray suite 5, which use perfect-match-minus-mismatch models, and dchip and rma, which use perfect-match-only models to generate expression values. Results were validated by RT-PCR in individual animals from an independent replication of the experiment. A small number of genes was associated with escalated cocaine intake (ESC genes). Unexpectedly, of the brain regions examined [prefrontal cortex, nucleus accumbens, septum, lateral hypothalamus (LH), amygdala, and ventral tegmental area], the LH was the most transcriptionally responsive in escalation of cocaine intake. Most of the ESC genes identified are also expressed during synaptogenesis and synaptic plasticity and include genes that code for several presynaptic and postsynaptic proteins involved in neurotransmission. These results suggest that LH intrinsic circuitry undergoes a structural reorganization during escalation of cocaine use. This remodeling of LH circuitry could contribute to the chronic deficit in reward function that has been hypothesized to drive the transition to drug addiction. Results also support the value of using multiple analysis strategies to identify the most robust changes in gene expression and to compensate for the biases that affect each strategy.