N-Methyl-D-aspartate receptors: subunit assembly and trafficking to the synapse

Src-protein tyrosine kinases are required for cocaine-induced increase in the expression and function of the NMDA receptor in the ventral tegmental area

Cocaine-induced long-term potentiation of glutamatergic synapses in the ventral tegmental area (VTA) has been proposed as a key process that contributes to the development of addictive behaviors. In particular, the activation of ionotrophic glutamate NMDA receptor (NMDAR) in the VTA is critical for the initiation of cocaine sensitization. Here we show that application of cocaine both in slices and in vivo induced an increase in tyrosine phosphorylation of the NR2A, but not the NR2B subunit of the NMDAR in juvenile rats. Cocaine induced an increase in the activity of both Fyn and Src kinases, and the Src-protein tyrosine kinase (Src-PTKs) inhibitor, 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2), abolished both cocaine-induced increase in tyrosine phosphorylation of the NR2A subunit and the increase in the expression of NR1, NR2A, and NR2B in the VTA. Moreover, cocaine-induced enhancement in NMDAR-mediated excitatory post-synaptic currents was completely abolished by PP2. Taken together, these results suggest that acute cocaine induced an increase in the expression of NMDAR subunits and enhanced tyrosine phosphorylation of NR2A-containing NMDAR through members of the Src-PTKs. This in turn, increased NMDAR-mediated currents in VTA dopamine neurons. These results provide a potential cellular mechanism by which cocaine triggers NMDAR-dependent synaptic plasticity of VTA neurons that may underlie the development of behavioral sensitization.

TRPV1 mediates the uterine capsaicin-induced NMDA NR2B-dependent cross-organ reflex sensitization in anesthetized rats

Spinal cord-mediated cross-organ sensitization between the uterus and the lower urinary tract may underlie the high concurrence of obstetrical/gynecological inflammation and chronic pelvic pain syndrome characterized by urogenital pain. However, the neural pathway and the neurotransmitters involved are still unknown. We tested the hypothesis that the excitation of capsaicin-sensitive primary afferent fibers arising from the uterus through the stimulation of transient receptor potential vanilloid 1 (TRPV1) induces cross-organ sensitization on the pelvic-urethra reflex activity. Capsaicin (1–1,000 μM, 0.05 ml) was instilled into the uterus to induce cross-organ reflex sensitization. Activation of capsaicin-sensitive primary afferent fibers by capsaicin instillation into the uterine horn sensitized the pelvic-urethra reflex activity that was reversed by an intrauterine pretreatment with capsaizepine, a TRPV1-selective antagonist. Intrathecal injection of AP5, a glutamatergic N-methyl-d-aspartate (NMDA) antagonist, and Co-101244, an NMDA NR2B-selective antagonist, both abolished the cross-organ reflex sensitization caused by capsaicin instillation. These results demonstrated that TRPV1 plays a crucial role in contributing to the capsaicin-sensitive primary afferent fibers mediating the glutamatergic NMDA-dependent cross-organ sensitization between the uterus and the lower urinary tract when there is a tissue injury.

EphB receptor signaling in mouse spinal cord contributes to physical dependence on morphine

Cellular and molecular mechanisms underlying opioid tolerance and dependence remain elusive. We investigated roles of EphB receptor tyrosine kinases--which play important roles in synaptic connection and plasticity during development and in the matured nervous system--in development and maintenance of physical dependence on morphine in the mouse spinal cord (SC). Spinal administration of an EphB receptor blocking reagent EphB2-Fc prevents and/or suppresses behavioral responses to morphine withdrawal and associated induction of c-Fos and depletion of calcitonin gene-related peptide. Western blotting and immunohistochemical fluorescence staining demonstrates that EphB1 receptor protein is significantly up-regulated in the spinal dorsal horn following escalating morphine treatment. Chronic morphine exposure and withdrawal significantly increased phosphorylation of N-methyl-D-aspartate receptor subunit NR2B as well as the activated forms of extracellular signal-regulated kinase and the cAMP response element binding protein in SC. The increased levels of phosphorylation of these molecules, however, are significantly inhibited by the EphB receptor blocker. These findings indicate that EphB receptor signaling, probably by interacting with NR2B in SC, contributes to the development of opioid physical dependence and withdrawal effects. This novel role for EphB receptor signaling suggests that these molecules may be useful therapeutic targets for preventing, minimizing, or reversing the development of opiate dependence.

A NMDA receptor glycine site partial agonist, GLYX-13, simultaneously enhances LTP and reduces LTD at Schaffer collateral-CA1 synapses in hippocampus

N-methyl-D-aspartate glutamate receptors (NMDARs) are a key route for Ca2+ influx into neurons important to both activity-dependent synaptic plasticity and, when uncontrolled, triggering events that cause neuronal degeneration and death. Among regulatory binding sites on the NMDAR complex is a glycine binding site, distinct from the glutamate binding site, which must be co-activated for NMDAR channel opening. We developed a novel glycine site partial agonist, GLYX-13, which is both nootropic and neuroprotective in vivo. Here, we assessed the effects of GLYX-13 on long-term synaptic plasticity and NMDAR transmission at Schaffer collateral-CA1 synapses in hippocampal slices in vitro. GLYX-13 simultaneously enhanced the magnitude of long-term potentiation (LTP) of synaptic transmission, while reducing long-term depression (LTD). GLYX-13 reduced NMDA receptor-mediated synaptic currents in CA1 pyramidal neurons evoked by low frequency Schaffer collateral stimulation, but enhanced NMDAR currents during high frequency bursts of activity, and these actions were occluded by a saturating concentration of the glycine site agonist d-serine. Direct two-photon imaging of Schaffer collateral burst-evoked increases in [Ca2+] in individual dendritic spines revealed that GLYX-13 selectively enhanced burst-induced NMDAR-dependent spine Ca2+ influx. Examining the rate of MK-801 block of synaptic versus extrasynaptic NMDAR-gated channels revealed that GLYX-13 selectively enhanced activation of burst-driven extrasynaptic NMDARs, with an action that was blocked by the NR2B-selective NMDAR antagonist ifenprodil. Our data suggest that GLYX-13 may have unique therapeutic potential as a learning and memory enhancer because of its ability to simultaneously enhance LTP and suppress LTD.

Assembly and forward trafficking of NMDA receptors (Review)

N-Methyl-D-aspartate (NMDA) receptors are a subclass of the excitatory, ionotropic L-glutamate neurotransmitter receptors. They are important for normal brain function being both primary candidates for the molecular basis of learning and memory and in the establishment of synaptic connections during the development of the central nervous system. NMDA receptors are also implicated in neurological and psychiatric disorders. Their dysfunction which is primarily due to either hypo- or hyper-activity is pivotal to these pathological conditions. There is thus a fine balance between NMDA receptor-mediated mechanisms in normal brain and those in diseased states where receptor homeostasis is perturbed. Receptor activity is due in part to the number of surface expressed receptors. Understanding the assembly and trafficking of this complex, heteromeric, neurotransmitter receptor family may therefore, be pivotal to understanding diseases in which their altered activity is evident. This article will review the current understanding of the mechanisms of NMDA receptor assembly, how this assembly is regulated and how assembled receptors are trafficked to their appropriate sites in post-synaptic membranes where they are integral components of a macromolecular signalling complex.

Repeated treatment with haloperidol, but not olanzapine, alters synaptic NMDA receptor composition in rat striatum

We here show that repeated administration of the first generation antipsychotic haloperidol, but not of the second generation olanzapine, significantly reduced the expression of NMDA subunit NR2A at striatal synapses, whereas both drugs decreased alphaCaMKII protein levels and autophosphorylation degree. Given that alterations in the localization of NMDA receptor regulatory subunits at synapses have been described in experimental parkinsonism, the haloperidol-induced effect on NMDA subunit localization might contribute to drug-induced parkinsonism induced by haloperidol.

Orexin-A modulates glutamatergic NMDA-dependent spinal reflex potentiation via inhibition of NR2B subunit

Glucose-sensitive neurons in the lateral hypothalamic area produce orexin-A (OxA) as well as orexin-B (OxB) and send their axons to the spinal dorsal horn, which predominantly expresses orexin receptor-1 (OX-1), showing a higher sensitivity to OxA. The purpose of the present study was to assess the effects of OxA on the induction of a novel form of activity-dependent reflex potentiation, spinal reflex potentiation (SRP), in the pelvic-urethral reflex activity. External urethra sphincter electromyogram in response to pelvic afferent nerve test stimulation (TS; 1/30 Hz) or repetitive stimulation (RS; 1 Hz) was recorded in anesthetized rats. TS evoked a baseline reflex activity, whereas RS produced SRP, which was abolished by intrathecal OxA (30 nM, 10 mul). Intrathecal SB-408124 (10 muM, 10 mul), an OX-1 antagonist, reversed the abolition on SRP caused by OxA. Although there is, so far, no NR2A- and NR2B-specific agonist available, N-methyl-d-aspartate (NMDA) reversed the abolition on the RS-induced SRP caused by the co-administration of OxA and Co-101244 (30 nM, 10 mul; an NMDA NR2B subunit antagonist), but it did not reverse the abolition by the co-administration of OxA and PPPA (300 nM, 10 mul; an NMDA NR2A subunit antagonist). In conclusion, the activation of descending orexinergic fibers may inhibit the repetitive afferent input-induced central sensitization of pelvic-urethral reflex activity and urethra hyperactivity, indicating that spinal orexinergic neural transmission may be a novel target for the treatment of patients with neuropathetic or postinflammatory pain of pelvic origin.

Developmental expression of NMDA and AMPA receptor subunits in vestibular nuclear neurons that encode gravity-related horizontal orientations

We examined the expression profile of subunits of ionotropic glutamate receptors [N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-proprionate (AMPA)] during postnatal development of connectivity in the rat vestibular nucleus. Vestibular nuclear neurons were functionally activated by constant velocity off-vertical axis rotation, a strategy to stimulate otolith organs in the inner ear. These neurons indicated Fos expression as a result. By immunodetection for Fos, otolith-related neurons that expressed NMDA/AMPA receptor subunits were identified as early as P7, and these neurons were found to increase progressively up to adulthood. Although there was developmental invariance in the percentage of Fos-immunoreactive neurons expressing the NR1, NR2A, GluR1, or GluR2/3 subunits, those expressing the NR2B subunit decreased from P14 onward, and those expressing the GluR4 subunit decreased in adults. These double-immunohistochemical data were corroborated by combined immuno-/hybridization histochemical data obtained from Fos-immunoreactive neurons expressing NR2B mRNA or GluR4 mRNA. The staining of both NR2B and GluR4 in the cytoplasm of these neurons decreased upon maturation. The percentage of Fos-immunoreactive neurons expressing the other ionotropic glutamate receptor subunits (viz. NR1, NR2A, GluR1, and GluR2/3) remained relatively constant throughout postnatal maturation. Triple immunofluorescence further demonstrated coexpression of NR1 and NR2 subunits in Fos-immunoreactive neurons. Coexpression of NR1 subunit with each of the GluR subunits was also observed among the Fos-immunoreactive neurons. Taken together, the different expression profiles of ionotropic glutamate receptor subunits constitute the histological basis for glutamatergic neurotransmission in the maturation of central vestibular connectivity for the coding of gravity-related horizontal head movements.

Effects of exercise on NMDA receptor subunit contributions to bidirectional synaptic plasticity in the mouse dentate gyrus

We examined synaptic plasticity in the dentate gyrus (DG) of the hippocampus in vitro in juvenile C57Bl6 mice (28-40 days of age), housed in control conditions with minimal enrichment (Controls) or with access to an exercise wheel (Runners). LTP expression was significantly greater in slices from Runners than in those from Controls, but could be blocked by APV in both groups. LTP was significantly reduced by NR2B subunit antagonists in both groups. NVP-AAM077, an antagonist with a higher preference for NR2A subunits over NR2B subunits, blocked LTP in slices from Runners and produced a slight depression in Control animals. LTD in the DG was also blocked by APV, but not by either of the NR2B specific antagonists. Strikingly, NVP-AAM077 prevented LTD in Runners, but not in Control animals, suggesting an increased involvement of NR2A subunits in LTD in animals that exercise. NVP-AAM077 did not block LTD in NR2A Knock Out (KO) animals that exercised, as expected. In an attempt to discern whether NMDA receptors located at extrasynaptic sites could play a role in the induction of LTD, DL-TBOA was used to block excitatory amino acid transport and increase extracellular glutamate levels. Under these conditions, LTD was not blocked by the co-application of a specific NR2B subunit antagonist in either group, but NVP-AAM077 again blocked LTD selectively in Runners. These results indicate that NR2A and NR2B subunits play a significant role in LTP in the DG, and that exercise can significantly alter the contribution of NMDA NR2A subunits to LTD.

Serum differentially modifies the transcription and translation of NMDAR subunits in retinal neurons

The N-methyl-D-aspartate type of glutamate receptor (NMDAR) plays a major role in the vertebrate retina. Expression of NR1 splice-variants and NR2 subunits in the retina differs from that in the brain, suggesting a tissue-specific heteromeric assembly of NMDARs. We previously demonstrated that serum alters retinal glutamate receptor properties. In order to relate this effect to NMDAR subunit composition, we here studied the effect of serum on the expression of NMDAR subunits and splice-variants in chick retinal neurons in primary culture. Our results show that mRNA and protein expression of NR1 alternative splice-variants and NR2 subunits are differentially modified by glutamate contained in serum. Such alteration suggests that NMDAR structure is reversed to embryonic heteromeric composition, through the control of subunit availability. The present findings could be relevant for the understanding of the lack of effect in the retina, of drugs which have been shown to protect cortical neurons from glutamate-induced excitotoxicity in those pathological or clinical conditions in which the retina is exposed to serum.

CREB modulates the functional output of nucleus accumbens neurons: a critical role of N-methyl-D-aspartate glutamate receptor (NMDAR) receptors

Nucleus accumbens (NAc) medium spiny neurons cycle between two states, a functionally inactive downstate and a functionally active upstate. Here, we show that activation of the transcription factor cAMP-response element-binding protein (CREB), a common molecular response to several drugs of abuse, increases both duration of the upstate and action potential firing during the upstate. This effect of CREB is mediated by enhanced N-methyl-d-aspartate glutamate receptor (NMDAR) function: increased CREB activity increases both NMDAR-mediated synaptic currents and surface level of NMDARs, while inhibition of NMDARs abolishes the effect of CREB on upstate duration. Furthermore, mimicking the effect of CREB by pharmacological enhancement of NMDAR function in the NAc in vivo suppressed novelty- and cocaine-elicited locomotor activity. These findings suggest that by enhancing NMDAR-mediated synaptic transmission, CREB activation promotes the proportion of time NAc neurons spend in the upstate. This effect, along with the CREB enhancement of NAc membrane excitability (Dong, Y., Green, T., Saal, D., Marie, H., Neve, R., Nestler, E. J., and Malenka, R. C. (2006) Nat. Neurosci. 9, 475-477), may counteract drug-induced maladaptations in the NAc and thus ameliorate the addictive state.

Amygdala infusions of an NR2B-selective or an NR2A-preferring NMDA receptor antagonist differentially influence fear conditioning and expression in the fear-potentiated startle test

Within the amygdala, most N-methyl-D-aspartic acid (NMDA) receptors consist of NR1 subunits in combination with either NR2A or NR2B subunits. Because the particular subunit composition greatly influences the receptors' properties, we investigated the contribution of both subtypes to fear conditioning and expression. To do so, we infused the NR1/NR2B receptor antagonist CP101,606 (0.5, 1.5, or 4.5 microg/amygdala) or the NR1/NR2A-preferring antagonist NVP-AAM077 (0.075, 0.25, 0.75, or 2.5 microg/amygdala) into the amygdala prior to either fear conditioning (i.e., light-shock pairings) or fear-potentiated startle testing. CP101,606 nonmonotonically disrupted fear conditioning but did not disrupt fear expression. NVP-AAM077 dose-dependently disrupted fear conditioning as well as fear expression. The results suggest that amygdala NR1/NR2B receptors play a special role in fear memory formation, whereas NR1/NR2A receptors participate more generally in synaptic transmission.

Interactions between the NR2B receptor and CaMKII modulate synaptic plasticity and spatial learning

The NR2B subunit of the NMDA receptor interacts with several prominent proteins in the postsynaptic density, including calcium/calmodulin-dependent protein kinase II (CaMKII). To determine the function of these interactions, we derived transgenic mice expressing a ligand-activated carboxy-terminal NR2B fragment (cNR2B) by fusing this fragment to a tamoxifen (TAM)-dependent mutant of the estrogen receptor ligand-binding domain LBD(G521R). Here, we show that induction by TAM allows the transgenic cNR2B fragment to bind to endogenous CaMKII in neurons. Activation of the LBD(G521R)-cNR2B transgenic protein in mice leads to the disruption of CaMKII/NR2B interactions at synapses. The disruption decreases Thr286 phosphorylation of alphaCaMKII, lowers phosphorylation of a key CaMKII substrate in the postsynaptic membrane (AMPA receptor subunit glutamate receptor 1), and produces deficits in hippocampal long-term potentiation and spatial learning. Together our results demonstrate the importance of interactions between CaMKII and NR2B for CaMKII activity, synaptic plasticity, and learning.

NMDA receptor NR2B subunit over-expression increases cerebellar granule cell migratory activity

Glutamate acting on NMDA receptors (NMDARs) is known to influence cerebellar granule cell migration. Subunit composition of NMDARs in granule cells changes characteristically during development: NR2B subunit containing receptors are abundant during migration towards the internal granule cell layer but are gradually replaced by NR2A and/or NR2C subunits once the final position is reached. Cerebellar granule cell migration was investigated using mutant mouse lines either with a deletion of the NR2C gene (NR2C(-/-) mice) or expressing NR2B instead of the NR2C subunit (NR2C-2B mice). BrdU-labeling revealed that over-expression of NR2B increased granule cell translocation in vivo, while the lack of NR2C subunit did not have any detectable effects on cell migration. Cellular composition of wild-type and mutant dissociated cerebellar granule cell cultures isolated from 10-day-old cerebella were similar, but NR2C-2B cultures had elevated level of NR2B subunits and intracellular Ca2+ imaging revealed higher sensitivity towards the addition of NR2B-selective antagonist in vitro. Time-lapse videomicroscopic observations revealed that average migratory velocity and the proportion of translocating cell bodies were significantly higher in NR2C-2B than in wild-type cultures. Our results provide evidence that NR2B-containing NMDARs can have specialized roles during granule cell migration and can increase migratory speed.

ERK activation causes epilepsy by stimulating NMDA receptor activity

The ERK MAPK signalling pathway is a highly conserved kinase cascade linking transmembrane receptors to downstream effector mechanisms. To investigate the function of ERK in neurons, a constitutively active form of MEK1 (caMEK1) was conditionally expressed in the murine brain, which resulted in ERK activation and caused spontaneous epileptic seizures. ERK activation stimulated phosphorylation of eukaryotic translation initiation factor 4E (eIF4E) and augmented NMDA receptor 2B (NR2B) protein levels. Pharmacological inhibition of NR2B function impaired synaptic facilitation in area cornus ammonicus region 3 (CA3) in acute hippocampal slices derived from caMEK1-expressing mice and abrogated epilepsy in vivo. In addition, expression of caMEK1 caused phosphorylation of the transcription factor, cAMP response element-binding protein (CREB) and increased transcription of ephrinB2. EphrinB2 overexpression resulted in increased NR2B tyrosine phosphorylation, which was essential for caMEK1-induced epilepsy in vivo, since conditional inactivation of ephrinB2 greatly reduced seizure frequency in caMEK1 transgenic mice. Therefore, our study identifies a mechanism of epileptogenesis that links MAP kinase to Eph/Ephrin and NMDA receptor signalling.

Cortico-striatal synaptic defects and OCD-like behaviours in Sapap3-mutant mice

Obsessive-compulsive disorder (OCD) is an anxiety-spectrum disorder characterized by persistent intrusive thoughts (obsessions) and repetitive actions (compulsions). Dysfunction of cortico-striato-thalamo-cortical circuitry is implicated in OCD, although the underlying pathogenic mechanisms are unknown. SAP90/PSD95-associated protein 3 (SAPAP3; also known as DLGAP3) is a postsynaptic scaffolding protein at excitatory synapses that is highly expressed in the striatum. Here we show that mice with genetic deletion of Sapap3 exhibit increased anxiety and compulsive grooming behaviour leading to facial hair loss and skin lesions; both behaviours are alleviated by a selective serotonin reuptake inhibitor. Electrophysiological, structural and biochemical studies of Sapap3-mutant mice reveal defects in cortico-striatal synapses. Furthermore, lentiviral-mediated selective expression of Sapap3 in the striatum rescues the synaptic and behavioural defects of Sapap3-mutant mice. These findings demonstrate a critical role for SAPAP3 at cortico-striatal synapses and emphasize the importance of cortico-striatal circuitry in OCD-like behaviours.

Rapid NMDA receptor phosphorylation and oxidative stress precede striatal neurodegeneration after hypoxic ischemia in newborn piglets and are attenuated with hypothermia

The basal ganglia of newborns are extremely vulnerable to hypoxic ischemia (HI). Striatal neurons undergo prominent necrosis after HI. The mechanisms for this degeneration are not well understood. Postasphyxic hypothermia ameliorates the striatal necrosis, but the mechanisms of hypothermia-induced neuroprotection are not known. We used a newborn piglet model of hypoxic-asphyxic cardiac arrest to test the hypotheses that N-methyl-d-aspartate receptor activation and free radical damage coexist, prior to neurodegeneration, early after resuscitation, and that these changes are attenuated with hypothermia. Piglets were subjected to 30min of hypoxia followed by 7min of airway occlusion, causing asphyxic cardiac arrest, and then were resuscitated and survived normothermically for 5min, 3h, or 6h, or hypothermically for 3h. By 6h of normothermic recovery, 50% of neurons in putamen showed ischemic cytopathology. Striatal tissue was fractionated into membrane or soluble proteins and was assayed by immunoblotting for carbonyl modification, phosphorylation of the N-methyl-d-aspartate receptor subunit NR1, and neuronal nitric oxide synthase. Significant accumulation of soluble protein carbonyls was present at 3h (196% of control) and 6h (142% of control). Phosphorylation of serine-897 of NR1 was increased significantly at 5min (161% of control) and 3h (226% of control) after HI. Phosphorylation of serine-890 of NR1 was also increased after HI. Membrane-associated neuronal nitric oxide synthase was increased by 35% at 5min. Hypothermia attenuated the oxidative damage and the NR1 phosphorylation in striatum. We conclude that neuronal death signaling in newborn striatum after HI is engaged rapidly through N-methyl-d-aspartate receptor activation, neuronal nitric oxide synthase recruitment, and oxidative stress. Postasphyxic, mild whole body hypothermia provides neuroprotection by suppressing N-methyl-d-aspartate receptor phosphorylation and protein oxidation.

Cytokines and neuronal ion channels in health and disease

The biophysical properties and the spatial distribution of ion channels define the signaling characteristics of individual neurons. Function, number localization, and ratio of receptor and ion channels are dynamically modulated in response to diverse stimuli and undergo dynamic changes in both physiological and pathological conditions. Increasing evidence indicates that cytokines may specifically interact with receptor and ion channels regulating neuronal excitability, synaptic plasticity, and injury. Interleukin (IL)-1beta and tumor necrosis factor (TNF)-alpha, two proinflammatory cytokines implicated in various pathophysiological conditions of the CNS, have been particularly studied. Literature data indicate that these cytokines (1) directly and promptly modulate ion channel activity, (2) exert different (and often opposite) effects on the same channels, and (3) act on ion channels both at physiological and pathological concentrations. Consequently, cytokines are now regarded as novel neuromodulators, opening important perspectives in the current view of brain behavior.

Altered NMDA receptor trafficking in a yeast artificial chromosome transgenic mouse model of Huntington's disease

Overactivation of NMDA receptors (NMDARs) is believed to play a role in degeneration of striatal medium-sized spiny neurons (MSNs) in Huntington's disease (HD). This hereditary disorder is caused by an expansion >35 in the polyglutamine (polyQ) region of the protein huntingtin (htt). Previous work has shown that NMDAR current, calcium signaling, and/or toxicity are enhanced in striatal MSNs in a variety of transgenic mice and cellular models of HD, but whether the enhancement is specific for MSNs or correlated with mutant htt (mhtt) polyQ length is not known. Furthermore, the mechanism underlying the increase in NMDAR activity has not been elucidated. Here we report polyQ length-dependent enhancement of peak NMDAR current density by mhtt in cultured MSNs, but not cortical neurons, from the yeast artificial chromosome (YAC) transgenic HD mouse model. We also observed a shift of NMDAR subunits NR1 and NR2B from internal pools to the plasma membrane and a significantly faster rate of NMDAR insertion to the surface in YAC72 MSNs. In comparing YAC72 with wild-type striatal tissue, subcellular fractionation revealed a relative enrichment of NR1 C2'-containing NMDARs in the vesicle/microsome-enriched fraction, and coimmunoprecipitation experiments demonstrated an increased proportion of NR1 C2' isoforms associated with NR2 subunits, which may contribute to faster forward trafficking of these receptors. Our results suggest that altered NMDAR trafficking may underlie potentiation of NMDAR-mediated current and toxicity in the YAC72 HD mouse model. This polyQ length-dependent, neuronal-specific change in NMDAR activity induced by mhtt may contribute to selective neuronal degeneration in HD.

D-serine: a new word in the glutamatergic neuro-glial language

Gliotransmission is a process in which astrocytes are dynamic elements that influence synaptic transmission and synaptogenesis. The best-known gliotransmitters are glutamate and ATP. However, in the past decade, it has been demonstrated that D-serine, a D-amino acid, acts as a gliotransmitter in glutamatergic synapses. The physiological relevance of D-serine is sustained by the way in which it modulates the action of glutamatergic neurotransmission, neuronal migration and long-term potentiation (LTP). In addition, the synthesis and degradation mechanisms of D-serine have been proposed as potential therapeutic targets for the treatment of Alzheimer's disease, schizophrenia and related disorders. In the present review, detailed information is provided about the physiological and physiopathological relevance of D-serine, including metabolic and regulation aspects.

Structure and trafficking of NMDA and GABAA receptors

The fidelity of synaptic function is dependent on the expression of the appropriate neurotransmitter receptor subtype, the targeting and trafficking of receptors to synapses as well as the regulation of the actual number of receptors at synapses. GABAA (γ-aminobutyric acid type A) receptors and NMDA (N-methyl-D-aspartate) receptors are both examples of ligand-gated, heteromeric neurotransmitter receptors whose cell-surface expression is dynamic and tightly regulated. NMDA receptors are localized at excitatory synapses. These synapses are highly structured but dynamic, with the interplay between NMDA receptors and NMDA receptor-associated scaffolding proteins regulating the expression of functional cell-surface synaptic and extrasynaptic receptors. Based on current information, inhibitory synapses seem to be less ordered, and a GABAA receptor equivalent of PSD-95 (postsynaptic density-95), the scaffolding molecule pivotal to the organization of NMDA receptor complexes at synapses, is yet to be validated. In the present paper, processes regulating the trafficking, assembly and molecular organization of both NMDA receptors and GABAA receptors will be discussed.

NMDA receptor function: subunit composition versus spatial distribution

NMDA receptors (NMDARs) play a pivotal role in the regulation of neuronal communication and synaptic function in the central nervous system. The subunit composition and compartmental localization of NMDARs in neurons affect channel activity and downstream signaling. This review discusses the distinct NMDAR subtypes and their function at synaptic, perisynaptic, and extrasynaptic sites of excitatory and inhibitory neurons. Many neurons express more than one of the modulatory NR2 subunits that participate in the formation of di- and/or triheteromeric channel assemblies (e.g., NR1/NR2A, NR1/NR2B, and/or NR1/NR2A/NR2B). Depending on the subunit composition and presence or absence of intracellular binding partners along the postsynaptic membrane, these NMDAR subtypes are allocated to distinct synaptic inputs converging onto a neuron or are distributed differentially among synaptic or extrasynaptic sites. These sites can carry NR2A and NR2B subunits, supporting the hypothesis that the spatial distribution of scaffolding and signaling complexes critically determines the full spectrum of NMDAR signaling.

From blue states to up states: a regional view of NMDA-ethanol interactions

This article summarizes the proceedings of a symposium at the 2005 Research Society on Alcoholism Meeting in Santa Barbara, California, organized and cochaired by John J. Woodward and Dorit Ron. The purpose of the symposium was to discuss recent findings that extend our understanding of the importance of the N-methyl-D-aspartate (NMDA) receptor as a target for ethanol action in the brain. These receptors are ligand-gated ion channels that are activated by the neurotransmitter glutamate and are critically involved in many forms of synaptic plasticity including those associated with learning and memory. In the first presentation, Dorit Ron presented data showing how activation of Fyn or Src tyrosine kinases differentially regulated the cell surface expression and activity of NR2A and NR2B containing NMDA receptors. Danny Winder discussed the effects of ethanol on NMDA receptor-dependent long-term potentiation in the bed nucleus of the stria terminalis (BNST), a brain region associated with the interaction between stress and drug/alcohol use. In the third presentation, Marisa Roberto described adaptations in the expression and function of NMDA receptors in the central nucleus of the amygdala following chronic exposure to ethanol. Finally, John Woodward described the effects of ethanol on the activity of neurons in deep layers of the prefrontal cortex using a novel slice coculture preparation.

D-serine signalling in the brain: friend and foe

Neurons and glia talk to each other at synapses. Glia sense the level of synaptic activity and consequently regulate its efficacy via the release of neuromodulators. One such glia-derived modulator is D-serine, an amino acid that serves as an endogenous ligand for the strychnine-insensitive glycine-binding site of NMDA glutamate receptors. Here, we provide an overview of recent findings on the mechanisms of its synthesis, release and clearance at synapses, with an emphasis on the dichotomy of behaviour of this novel messenger in the brain. The discovery of the good and ugly faces of this gliotransmitter is an important issue of modern neuroscience that has repercussions for the treatment of brain disorders.

Intrinsic and extrinsic determinants of ion channel localization in neurons

Neurons are an extremely diverse group of excitable cells with a wide variety of morphologies including complex dendritic trees and very long axons. The electrical properties of neurons depend not only on the types of ion channels and receptors expressed, but also on where these channels are located in the cell. Two extreme examples that illustrate the subcellular polarized nature of neurons and the tight regulation of ion channel localization can be seen at the axon initial segment and the node of Ranvier. The axon initial segment is important for initiation of action potentials in the axon, whereas the node of Ranvier is required for the rapid, faithful and efficient propagation of action potentials along the axon. Given the similarity of their functions it is not surprising that nearly every protein component of the axon initial segment is also found at the node. However, there is one very important difference between these two sites: nodes require extrinsic, glial-derived factors in order to form, whereas the axon initial segment is intrinsically determined by the neuron. This mini-review discusses recent results that have begun to clarify the intrinsic and extrinsic mechanisms underlying formation of nodes and axon initial segments, and poses several important unanswered questions regarding their unique mechanisms of formation.

A critical interaction between NR2B and MAGUK in L-DOPA induced dyskinesia

Abnormal function of NMDA receptor has been suggested to be correlated with the pathogenesis of Parkinson's disease (PD) as well as with the development of l-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesia. Here we show that NMDA receptor NR2 subunits display specific alterations of their subcellular distribution in striata from unilateral 6-hydroxydopamine-lesioned, L-DOPA-treated dyskinetic, and L-DOPA-treated nondyskinetic rats. Dyskinetic animals have significantly higher levels of NR2A subunit in the postsynaptic compartment than all other experimental groups, whereas NR2B subunit shows a significant reduction in both dopamine-denervated and dyskinetic rats. These events are paralleled by profound modifications of NMDA receptor NR2B subunit association with interacting elements, i.e., members of the membrane-associated guanylate kinase (MAGUK) protein family postsynaptic density-95, synapse-associated protein-97 and synapse-associated protein-102. Treatment of nondyskinetic animals with a synthetic peptide (TAT2B) able to affect NR2B binding to MAGUK proteins as well as synaptic localization of this subunit in nondyskinetic rats was sufficient to induce a shift of treated rats toward a dyskinetic motor behavior. These data indicate abnormal NR2B redistribution between synaptic and extrasynaptic membranes as an important molecular disturbance of the glutamatergic synapse involved in L-DOPA-induced dyskinesia.

The nonkinase phorbol ester receptor alpha 1-chimerin binds the NMDA receptor NR2A subunit and regulates dendritic spine density

Abnormalities in dendritic spines have long been associated with cognitive dysfunction and neurodevelopmental delay, whereas rapid changes in spine shape underlie synaptic plasticity. The key regulators of cytoskeletal reorganization in dendrites and spines are the Rho GTPases, which modify actin polymerization in response to synaptic signaling. Rho GTPase activity is modulated by multiple regulatory proteins, some of which have been found to associate with proteins localized to spines. Here, we show that the nonkinase phorbol ester receptor alpha1-chimerin is present in dendrites and spines, where it binds to the NMDA receptor NR2A subunit in a phorbol ester-dependent manner. Alpha1-chimerin contains a GTPase activating (GAP) domain, with activity toward the Rho family member Rac1. Overexpression of alpha1-chimerin in cultured hippocampal neurons inhibits formation of new spines and removes existing spines. This reduction in spine density is mediated by Rac1 inhibition, because it depends critically on the presence of a functional GAP domain. Conversely, depletion of alpha1-chimerin leads to an increase in spine density, indicating that a basal inhibition of Rac1 maintains the number of spines at a submaximal level. The ability of alpha1-chimerin to modulate spine number requires an interaction with the NMDA receptor, because an alpha1-chimerin mutant that binds weakly to NR2A fails to decrease spine density. Together, these results suggest that alpha1-chimerin is able to modulate dendritic spine morphology by binding to synaptic NMDA receptors and locally inactivating Rac1.

A specific N-terminal residue in Kv1.5 is required for upregulation of the channel by SAP97

We have previously reported that SAP97 enhancement of hKv1.5 currents requires an intact Kv1.5 N-terminus and is independent of the PDZ-binding motif at the C-terminus of the channel [J. Eldstrom, W.S. Choi, D.F. Steele, D. Fedida, SAP97 increases Kv1.5 currents through an indirect N-terminal mechanism, FEBS Lett. 547 (2003) 205-211]. Here, we report that an interaction between the two proteins can be detected under certain conditions but their interaction is irrelevant to the enhancement of channel expression. Instead, a threonine residue at position 15 in the hKv1.5 N-terminus is critically important. Mutation of this residue, which lies within a consensus site for phosphorylation by protein kinase C, to an alanine, completely abrogated the effect of SAP97 on channel expression. Although we were unable to detect phosphorylation of this residue, specific inhibition of kinase C by Calphostin C eliminated the increase in wild-type hKv1.5 currents associated with SAP97 overexpression suggesting a role for this kinase in the response.

Contribution of NR2A and NR2B NMDA subunits to bidirectional synaptic plasticity in the hippocampus in vivo

It has recently been proposed that activation of the NR2A subunit results in Long-term potentiation (LTP) induction, whereas activation of the NR2B subunit results in long-term depression (LTD) induction. The present study undertakes to replicate these findings in vivo to determine if a role for specific subunits in synaptic plasticity can be shown in the intact brain. Field recordings were made from the CA1 subfield of the hippocampus using Schaffer collateral stimulation in anesthetized male Sprague-Dawley rats. Antagonists of the N-methyl-D-aspartate receptors NR2A and NR2B subunits were administered by either intraperitoneal (i.p.) or intrahippocampal (i.h.) injections to assess their involvement in LTP (100 Hz stimuli) and LTD (200 Paired-burst stimuli). i.h. injection of Ro25-6981 (100 microM) significantly attenuated hippocampal LTP expression and completely blocked LTD expression. When administered i.p., Ro25-6981 (6 mg/kg) again blocked LTD, but did not significantly diminish the expression of LTP. When NVP-AAM077 was administered i.h. (80 microM) both LTP and LTD were completely abolished. The administration of this compound i.p. (1.2 mg/kg) also significantly attenuated LTP, but did not affect LTD. These data suggest that both NR2A and NR2B subunits can play roles in LTP and LTD in the hippocampus in vivo.

The synaptic localization of NR2B-containing NMDA receptors is controlled by interactions with PDZ proteins and AP-2

The NMDA receptor (NMDAR) is a component of excitatory synapses and a key participant in synaptic plasticity. We investigated the role of two domains in the C terminus of the NR2B subunit--the PDZ binding domain and the clathrin adaptor protein (AP-2) binding motif--in the synaptic localization of NMDA receptors. NR2B subunits lacking functional PDZ binding are excluded from the synapse. Mutations in the AP-2 binding motif, YEKL, significantly increase the number of synaptic receptors and allow the synaptic localization of NR2B subunits lacking PDZ binding. Peptides corresponding to YEKL increase the synaptic response within minutes. In contrast, the NR2A subunit localizes to the synapse in the absence of PDZ binding and is not altered by mutations in its motif corresponding to YEKL of NR2B. This study identifies a dynamic regulation of synaptic NR2B-containing NMDARs through PDZ protein-mediated stabilization and AP-2-mediated internalization that is modulated by phosphorylation by Fyn kinase.

Exercise affects glutamate receptors in postsynaptic densities from cortical mice brain

Physical activity has been proposed as a behavior intervention that promotes mental health and some of the benefits induced by exercise have been related to the glutamatergic system. Indeed, glutamate is the most abundant excitatory neurotransmitter in brain. Thus, we evaluated if voluntary exercise in mice could modulate glutamatergic synapses at level of postsynaptic density (PSD). Through Western blot, we found that exercise during 1 month increased glutamatergic-related protein content in PSD from cortex of mice. Exercise increased the immunocontent of GluR1 (129%), SAP-97 (179%), GRIP-1 (129%), and in less extent, GluR2/3 (118%) and PSD-95 (112%) proteins. The overall content of NMDA subunits R1, R2A and R2B were not altered in mice that had exercised, however, the phosphorylated NMDA subunits, phospho-NMDAR1 (150%), and phospho-NMDAR2B (183%) showed a strong increase. Because exercise increased the content of phosphorylated forms of NMDA receptors, we evaluated the binding of MK-801, a specific ligand that binds to open NMDA channel. Exercise increased the binding of MK-801 in cortical cellular membranes in 51%. Altogether, our results point to a modulation of glutamatergic synapses by exercise with likely implications in the exercise-induced mental health.

Protein phosphatase modulates the phosphorylation of spinal cord NMDA receptors in rats following intradermal injection of capsaicin

The present study investigates the role of serine/threonine protein phosphatase 2A (PP2A) in the modulation of the phosphorylation of the NR1 and NR2B subunits of NMDA receptors in the spinal cord of rats following intradermal injection of capsaicin. The effects of a specific inhibitor of PP2A, fostriecin, on the expression of NR1, phospho-NR1, NR2B, and phospho-NR2B subunits of the NMDA receptor in the spinal cord of rats following noxious stimulation were examined. After continually perfusing with ACSF or fostriecin (3 microM) through a previously implanted microdialysis fiber for 30 min, central sensitization was initiated by injection of capsaicin into the plantar surface of the left paw of rats. The spinal cord was removed at different time points (30, 60, 90, 120, 180 min) after intradermal injection of capsaicin. Western blots were performed to examine the expression of NMDA subunits in spinal cord tissue by using specific antibodies. We found that the upregulated phosphorylation of both NR1 and NR2B subunits induced by capsaicin injection was significantly potentiated by the PP2A inhibitor without affecting the NR1 and NR2B protein itself. These results suggest that PP2A may have a regulatory effect on central sensitization induced by noxious stimuli in the periphery by regulating the phosphorylation state of NMDA receptors.

Ethanol Alters Trafficking and Functional N-Methyl-D-aspartate Receptor NR2 Subunit Ratio via H-Ras

The N-methyl-D-aspartate receptor (NMDAR) plays a critical role in synaptic plasticity and is one of the main targets for alcohol (ethanol) in the brain. Trafficking of the NMDAR is emerging as a key regulatory mechanism that underlies channel activity and synaptic plasticity. Here we show that exposure of hippocampal neurons to ethanol increases the internalization of the NR2A but not NR2B subunit of the NMDAR via the endocytic pathway. We further observed that ethanol exposure results in NR2A endocytosis through the activation of H-Ras and the inhibition of the tyrosine kinase Src. Importantly, ethanol treatment alters functional subunit composition from NR2A/NR2B- to mainly NR2B-containing NMDARs. Our results suggest that addictive drugs such as ethanol alter NMDAR trafficking and subunit composition. This may be an important mechanism by which ethanol exerts its effects on NMDARs to produce alcohol-induced aberrant plasticity.

Postsynaptic IP3 receptor-mediated Ca2+ release modulates synaptic transmission in hippocampal neurons

Ca(2+)-dependent mechanisms are important in regulating synaptic transmission. The results herein indicate that whole-cell perfusion of inositol 1,4,5-trisphosphate receptor (IP(3)R) agonists greatly enhanced excitatory postsynaptic current (EPSC) amplitudes in postsynaptic hippocampal CA1 neurons. IP(3)R agonist-mediated increases in synaptic transmission changed during development and paralleled age-dependent increases in hippocampal type-1 IP(3)Rs. IP(3)R agonist-mediated increases in EPSC amplitudes were inhibited by postsynaptic perfusion of inhibitors of Ca(2+)/calmodulin, PKC and Ca(2+)/calmodulin-dependent protein kinase II. Postsynaptic perfusion of inhibitors of smooth endoplasmic reticulum (SER) Ca(2+)-ATPases, which deplete intracellular Ca(2+) stores, also enhanced EPSC amplitudes. Postsynaptic perfusion of the IP(3)R agonist adenophostin (AdA) during subthreshold stimulation appeared to convert silent to active synapses; synaptic transmission at these active synapses was completely blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Postsynaptic IP(3)R-mediated Ca(2+) release also produced a significant increase in spontaneous EPSC frequency. These results indicate that Ca(2+) release from intracellular stores play a key role in regulating the function of postsynaptic AMPARs.

Subunit Assembly of N-Methyl-d-aspartate Receptors Analyzed by Fluorescence Resonance Energy Transfer

N-methyl-d-aspartate (NMDA) receptors play major roles in synaptic transmission and plasticity, as well as excitotoxicity. NMDA receptors are thought to be tetrameric complexes mainly composed of NMDA receptor (NR)1 and NR2 subunits. The NR1 subunits are required for the formation of functional NMDA receptor channels, whereas the NR2 subunits modify channel properties. Biochemical and functional studies indicate that subunits making up NMDA receptors are organized into a dimer of dimers, and the N termini of the subunits are major determinants for receptor assembling. Here we used a biophysical approach, fluorescence resonance energy transfer, to analyze the assembly of intact, functional NMDA receptors in living cells. The results showed that NR1, NR2A, and NR2B subunits could form homodimers when they were expressed alone in HEK293 cells. Subunit homodimers were also found existing in heteromeric NMDA receptors formed between NR1 and NR2 subunits. These findings are consistent with functional NMDA receptors being arranged as a dimer of dimers. In addition, our data indicated that the conformation of NR1 subunit homodimers was affected by the partner NR2 subunits during the formation of heteromeric receptor complexes, which might underlie the mechanism by which NR2 subunits modify NMDA receptor function.

Glutamate receptor trafficking: endoplasmic reticulum quality control involves ligand binding and receptor function

The glutamate receptor (GluR) agonist-binding site consists of amino acid residues in the extracellular S1 and S2 domains in the N-terminal and M3-M4 loop regions, respectively. In the present study, we sought to confirm that the conserved ligand-binding residues identified in the AMPA receptor S1S2 domains also participate in ligand binding of GluR6 kainate receptors. Amino acid substitutions were made in the GluR6 parent at R523, T690, and E738 to alter their potential interactions with ligand. Mutant receptors were expressed in human embryonic kidney 293 cells, confirmed by Western blot analysis, and tested by [3H]kainate binding and patch-clamp recording. Each of the binding site mutations was sufficient to reduce [3H]kainate binding to undetectable levels and eliminate functional responses to glutamate or kainate. As with our studies of other nonfunctional mutants (Fleck et al., 2003), immunocytochemical staining and cell-surface biotinylation studies showed that the mutant receptors were retained intracellularly and did not traffic to the cell surface. Endoglycosidase-H digests and colocalization with endoplasmic reticulum (ER) markers demonstrated that the mutant receptors are immaturely glycosylated and retained in the ER. Immunoprecipitation, native PAGE, and functional studies confirmed that the GluR6-binding site mutants are capable of multimeric assembly, indicating their retention in the ER does not result from a gross protein folding error. Together, these results confirm the role of R523, T690, and E738 directly in ligand binding to GluR6 and further support our previous report that nonfunctional GluRs are retained intracellularly by a functional checkpoint in ER quality control.

Early expression of AMPA receptors and lack of NMDA receptors in developing rat climbing fibre synapses

Whether nascent glutamatergic synapses acquire their AMPA receptors constitutively or via a regulated pathway triggered by pre-existing NMDA receptor activation is still an open issue. Here, we provide evidence that some glutamatergic synapses develop without expressing NMDA receptors. Using immunocytochemistry, we showed that synapses between developing rat climbing fibres and Purkinje cells expressed GluR2-containing AMPA receptors as soon as they were formed (i.e. on embryonic day 19) but never carried detectable NMDA receptors. This was confirmed by electrophysiological recordings. Excitatory synaptic currents were recorded in Purkinje cells as early as P0. However, no NMDA receptor-mediated component was found in either spontaneous or evoked synaptic responses. In addition, we ruled out a possible role of extrasynaptic NMDA receptors by showing that AMPA receptor clustering at nascent climbing fibre synapses was not modified by chronic in utero NMDA receptor blockade.

Gbetagamma-dependent and Gbetagamma-independent basal activity of G protein-activated K+ channels

Cardiac and neuronal G protein-activated K+ channels (GIRK; Kir3) open following the binding of Gbetagamma subunits, released from Gi/o proteins activated by neurotransmitters. GIRKs also possess basal activity contributing to the resting potential in neurons. It appears to depend largely on free Gbetagamma, but a Gbetagamma-independent component has also been envisaged. We investigated Gbetagamma dependence of the basal GIRK activity (A(GIRK,basal)) quantitatively, by titrated expression of Gbetagamma scavengers, in Xenopus oocytes expressing GIRK1/2 channels and muscarinic m2 receptors. The widely used Gbetagamma scavenger, myristoylated C terminus of beta-adrenergic kinase (m-cbetaARK), reduced A(GIRK,basal) by 70-80% and eliminated the acetylcholine-evoked current (I(ACh)). However, we found that m-cbetaARK directly binds to GIRK, complicating the interpretation of physiological data. Among several newly constructed Gbetagamma scavengers, phosducin with an added myristoylation signal (m-phosducin) was most efficient in reducing GIRK currents. m-phosducin relocated to the membrane fraction and did not bind GIRK. Titrated expression of m-phosducin caused a reduction of A(GIRK,basal) by up to 90%. Expression of GIRK was accompanied by an increase in the level of Gbetagamma and Galpha in the plasma membrane, supporting the existence of preformed complexes of GIRK with G protein subunits. Increased expression of Gbetagamma and its constitutive association with GIRK may underlie the excessively high A(GIRK,basal) observed at high expression levels of GIRK. Only 10-15% of A(GIRK,basal) persisted upon expression of both m-phosducin and cbetaARK. These results demonstrate that a major part of Ibasal is Gbetagamma-dependent at all levels of channel expression, and only a small fraction (<10%) may be Gbetagamma-independent.

Ligand Binding Is a Critical Requirement for Plasma Membrane Expression of Heteromeric Kainate Receptors

Intracellular trafficking of ionotropic glutamate receptors is controlled by multiple discrete determinants in receptor subunits. Most such determinants have been localized to the cytoplasmic carboxyl-terminal domain, but other domains in the subunit proteins can play roles in modulating receptor surface expression. Here we demonstrate that formation of an intact glutamate binding site also acts as an additional quality-control check for surface expression of homomeric and heteromeric kainate receptors. A key ligand-binding residue in the KA2 subunit, threonine 675, was mutated to either alanine or glutamate, which eliminated affinity for the receptor ligands kainate and glutamate. We found that plasma membrane expression of heteromeric GluR6/KA2(T675A) or GluR6/KA2(T675E) kainate receptors was markedly reduced compared with wild-type GluR6/KA2 receptors in transfected HEK 293 and COS-7 cells and in cultured neurons. Surface expression of homomeric KA2 receptors lacking a retention/retrieval determinant (KA2-R/A) was also reduced upon mutation of Thr-675 and elimination of the ligand binding site. KA2 Thr-675 mutant subunits were able to co-assemble with GluR5 and GluR6 subunits and were degraded at the same rate as wild-type KA2 subunit protein. These results suggest that glutamate binding and associated conformational changes are prerequisites for forward trafficking of intracellular kainate receptors following multimeric assembly.

NMDA receptors are movin' in

Dynamic modulation of the number of postsynaptic glutamate receptors is considered one of the main mechanisms for altering the strength of excitatory synapses in the central nervous system (CNS). However, until recently N-methyl-d-aspartate (NMDA) receptors were considered relatively stable once in the plasma membrane, especially in comparison with alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors that are internalized at a high rate. A series of recent studies has changed this viewpoint by revealing that NMDA receptors are subject to constitutive as well as agonist-induced internalization through clathrin-mediated endocytosis. Surprisingly, agonist-induced internalization is not dependent on current flow through the NMDA channel, and the receptors are primed for this type of internalization by selective stimulation of the glycine site but not of the glutamate site. Endocytosis of NMDA receptors provides a fundamental mechanism for dynamic regulation of the number of NMDA receptors at synapses, which might be important for physiological and pathological functioning of the CNS.

Why do many NMDA antagonists fail, while others are safe and effective at blocking excitotoxicity associated with dementia and acute injury?

Similar to drug development programs for stroke and traumatic brain injury, programs developed for Alzheimer's disease (AD) have not been very effective in treating dementia. Recently, researchers have explored modulating excitatory synaptic neurotransmission via the N-methyl-D-aspartate receptor (NMDAR) to treat AD. However, many investigators doubt that NMDA antagonists are safe and effective for treating persons with AD because they have failed in stroke and trauma programs. This article explores the role of NMDA-mediated excitotoxicity in AD, reviews how the NMDAR functions, highlights the side effects and alternate signaling pathways that are initiated from NMDAR activation, provides examples of NMDA antagonists that do not exhibit the typical side effects, and discusses why some NMDA antagonist compounds are effective and safe in limiting cascades of excitotoxicity in dementia or acute brain injury.