Enhancement of NMDA-induced current by the putative NR2B selective antagonist ifenprodil

Subunit-selective allosteric inhibition of glycine binding to NMDA receptors

NMDA receptors are ligand-gated ion channels that mediate excitatory neurotransmission in the brain and are involved in numerous neuropathological conditions. NMDA receptors are activated upon simultaneous binding of coagonists glycine and glutamate to the GluN1 and GluN2 subunits, respectively. Subunit-selective modulation of NMDA receptor function by ligand binding to modulatory sites distinct from the agonist binding sites could allow pharmacological intervention with therapeutically beneficial mechanisms. Here, we show the mechanism of action for 3-chloro-4-fluoro-N-[(4-[(2-(phenylcarbonyl)hydrazino)carbonyl]phenyl)methyl]-benzenesulfonamide (TCN-201), a new GluN1/GluN2A-selective NMDA receptor antagonist whose inhibition can be surmounted by glycine. Electrophysiological recordings from chimeric and mutant rat NMDA receptors suggest that TCN-201 binds to a novel allosteric site located at the dimer interface between the GluN1 and GluN2 agonist binding domains. Furthermore, we demonstrate that occupancy of this site by TCN-201 inhibits NMDA receptor function by reducing glycine potency. TCN-201 is therefore a negative allosteric modulator of glycine binding.

GluN2B subunit deletion reveals key role in acute and chronic ethanol sensitivity of glutamate synapses in bed nucleus of the stria terminalis

The bed nucleus of the stria terminalis (BNST) is a critical region for alcohol/drug-induced negative affect and stress-induced reinstatement. NMDA receptor (NMDAR)-dependent plasticity, such as long-term potentiation (LTP), has been postulated to play key roles in alcohol and drug addiction; yet, to date, little is understood regarding the mechanisms underlying LTP of the BNST, or its regulation by ethanol. Acute and chronic exposure to ethanol modulates glutamate transmission via actions on NMDARs. Despite intense investigation, tests of subunit specificity of ethanol actions on NMDARs using pharmacological approaches have produced mixed results. Thus, we use a conditional GluN2B KO mouse line to assess both basal and ethanol-dependent function of this subunit at glutamate synapses in the BNST. Deletion of GluN2B eliminated LTP, as well as actions of ethanol on NMDAR function. Further, we show that chronic ethanol exposure enhances LTP formation in the BNST. Using KO-validated pharmacological approaches with Ro25-6981 and memantine, we provide evidence suggesting that chronic ethanol exposure enhances LTP in the BNST via paradoxical extrasynaptic NMDAR involvement. These findings demonstrate that GluN2B is a key point of regulation for ethanol's actions and suggest a unique role of extrasynaptic GluN2B-containing receptors in facilitating LTP.

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.

The influences of juvenile diabetes on memory and hippocampal plasticity in rats: improving effects of glucagon-like peptide-1

Previous studies in children with diabetes found that hyperglycemia induces memory dysfunction. In this study, we investigated memory and synaptic plasticity in streptozotocine (STZ)-induced diabetic rats during the juvenile period. We further investigated the effects of glucagon-like peptide-1 (GLP-1) on the diabetes-induced profiles. STZ (85 mg/kg, i.p.) was administered to 17-day-old Wistar rats to induce type-1 juvenile diabetes mellitus (JDM). In the Y-maze test, JDM rats showed significant impairment of learning and memory, which were improved by GLP-1 (7-36) amide (1 microg/5 microl/rat, i.c.v.). Extracellular recording at Schaffer collateral synapses in the CA1 region of hippocampal slices showed that long-term potentiation and paired-pulse facilitation in JDM rats were similar to age-matched control rats. However, the input-output relation was strengthened, and long-term depression (LTD) and responses of N-methyl d-aspartic acid through NR2B subunits were weakened in the JDM rats. GLP-1 (7-36) amide (100 nM) increased the magnitude of LTD and the responses through NR2B in the JDM rats. These results indicate that the lack of LTD and NR2B responses may contribute to impairment of memory associated with JDM, suggesting the potential usefulness of GLP-1 in the treatment of memory dysfunction in JDM.

The magnitude of hippocampal long term depression depends on the synaptic location of activated NR2-containing N-methyl-D-aspartate receptors

Activation of N-methyl-D-aspartate receptors (NMDARs) is the first step in the induction of certain forms of synaptic plasticity in the hippocampus. In the adult rat hippocampus, NMDARs are composed almost exclusively of NR1 and NR2 subunits with NR1 subunits being mainly associated with either NR2A and/or NR2B subunits. The role played by the different subunits in synaptic plasticity is still controversial. In the present study, we used two different long term depression (LTD) -inducing protocols (electrical and chemical stimulation) to show that activation of NR2A-containing NMDAR subunits leads to the induction of LTD. We also demonstrated that extrasynaptic NR2B-containing NMDARs regulate the magnitude of LTD by exerting a control over the function of synaptic NR2A-containing NMDARs while having no effect on plasticity in the absence of synaptic receptor activation. Taken as a whole, these experiments demonstrate that NMDAR subunits play different roles according to their nature (NR2A or NR2B) and location (synaptic versus extrasynaptic). This sheds new light on the functional role of extrasynaptic NR2B containing-NMDARs. These results are particularly important for a better understanding of certain pathological disorders associated with glutamatergic overactivity.

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.

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

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

Virally mediated knock-down of NR2 subunits ipsilateral to the deprived eye blocks ocular dominance plasticity

NMDA receptors (NMDARs) are important in developmental plasticity in the visual cortex. The NR2A and NR2B subunits of this receptor develop with different time courses, suggesting that they play different roles in plasticity. To understand the role of the NR2B subunit, we knocked-down NR2B gene expression in visual cortex by injecting a recombinant adenovirus containing an antisense NR2B oligonucleotide. To assess knock-down, we injected the recombinant adenovirus into the right visual cortex of rats (p22) or mice (p30). Eight days later we perfused the animals and processed the visual cortex for NMDAR subunit immunoreactivity (IR). NR2B-IR was depleted dramatically in the neuropil near the injection. Depletion was more modest in the neuronal somata. Surprisingly, NR2A-IR was also reduced, but NR1-IR was not reduced. To assess the functional effects of depletion, we measured ocular dominance plasticity with monocular deprivation (MD). We compared mice receiving the NR2B antisense virus with mice receiving virus containing only the GFP sequence and mice receiving no injection. All injections were between p26 and p29 in the right cortex and bilateral recordings were performed 6-8 days later. Animals receiving the antisense virus lost plasticity if the right eye was deprived. If the left eye was deprived, the cortex was normally plastic bilaterally. Injection of control virus had no effect on plasticity. The data indicate that ocular dominance plasticity requires normal NMDARs in the hemisphere ipsilateral to the deprived eye but not in the hemisphere contralateral to the deprived eye.

Blockade and enhancement of glutamate receptor responses in Xenopus oocytes by methylated arsenicals

Pentavalent and trivalent organoarsenic compounds belong to the major metabolites of inorganic arsenicals detected in humans. Recently, the question was raised whether the organic arsenicals represent metabolites of a detoxification process or methylated species with deleterious biological effects. In this study, the effects of trivalent arsenite (AsO(3) (3-); iA(III)), the pentavalent organoarsenic compounds monomethylarsonic acid (CH(3)AsO(OH)(2); MMA(V)) and dimethylarsinic acid ((CH(3))(2)AsO(OH); DMA(V)) and the trivalent compounds monomethylarsonous acid (CH(3)As(OH)(2), MMA(III)) and dimethylarsinous acid ((CH(3))(2)As(OH); DMA(III)) were tested on glutamate receptors and on voltage-operated potassium and sodium channels heterologously expressed in Xenopus oocytes. Membrane currents of ion channels were measured by conventional two-electrode voltage-clamp techniques. The effects of arsenite were tested in concentrations of 1-1,000 micromol/l and the organic arsenical compounds were tested in concentrations of 0.1-100 micromol/l. We found no significant effects on voltage-operated ion channels; however, the arsenicals exert different effects on glutamate receptors. While MMA(V) and MMA(III) significantly enhanced ion currents through N-methyl-D: -aspartate (NMDA) receptor ion channels with threshold concentrations <10 micromol/l, DMA(V) and DMA(III) significantly reduced NMDA-receptor mediated responses with threshold concentrations <0.1 micromol/l; iA(III) had no effects on glutamate receptors of the NMDA type. MMA(III) and DMA(V) significantly reduced ion currents through alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-receptor ion channels with threshold concentrations <10 micromol/l (MMA(III)) and <1 micromol/l (DMA(V)). MMA(V) and iA(III) had no significant effects on glutamate receptors of the AMPA type. The effects of MMA(V), MMA(III), DMA(V) and DMA(III )on glutamate receptors point to a neurotoxic potential of these substances.

Functional NR2B- and NR2D-containing NMDA receptor channels in rat substantia nigra dopaminergic neurones

NMDA receptors regulate burst firing of dopaminergic neurones in the substantia nigra pars compacta (SNc) and may contribute to excitotoxic cell death in Parkinson's disease (PD). In order to investigate the subunit composition of functional NMDA receptors in identified rat SNc dopaminergic neurones, we have analysed the properties of individual NMDA receptor channels in outside-out patches. NMDA (100 nm) activated channels corresponding to four chord conductances of 18, 30, 41 and 54 pS. Direct transitions were observed between all conductance levels. Between 18 pS and 41 pS conductance levels, direct transitions were asymmetric, consistent with the presence of NR2D-containing NMDA receptors. Channel activity in response to 100 nm or 200 microm NMDA was not affected by zinc or TPEN (N,N,N',N'-tetrakis-[2-pyridylmethyl]-ethylenediamine), indicating that SNc dopaminergic neurones do not contain functional NR2A subunits. The effect of the NR2B antagonist ifenprodil was complex: 1 microm ifenprodil reduced open probability, while 10 microm reduced channel open time but had no effect on open probability of channels activated by 100 nm NMDA. When the concentration of NMDA was increased to 200 microm, ifenprodil (10 microm) produced the expected reduction in open probability. These results indicate that NR2B subunits are present in SNc dopaminergic neurones. Taken together, these findings indicate that NR2D and NR2B subunits form functional NMDA receptor channels in SNc dopaminergic neurones, and suggest that they may form a triheteromeric NMDA receptor composed of NR1/NR2B/NR2D subunits.

Synaptic distribution of the NR1, NR2A and NR2B subunits of the N-methyl-d-aspartate receptor in the rat lumbar spinal cord revealed with an antigen-unmasking technique

Glutamate is the main excitatory neurotransmitter in the spinal cord and acts on several types of receptor, including N-methyl-d-aspartate (NMDA) receptors, which play an important role in synaptic plasticity and chronic pain. Three families of NMDA receptor subunit have been identified: NR1, NR2 (A-D) and NR3 (A and B). NMDA receptors are heteromeric channels that contain NR1 with at least one NR2 subunit. There is extensive evidence that NMDA receptors are present in spinal cord but little is known about their synaptic distribution. We have used an antigen-unmasking method involving pepsin treatment to reveal NR1, NR2A and NR2B subunits and have compared their distribution with that of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor GluR2 subunit, which is thought to be present at most glutamatergic synapses throughout the spinal cord. After pepsin treatment, punctate labelling was seen with antibodies against each of these subunits. Although NR1 puncta were present throughout the grey matter, NR2A was concentrated in laminae III-IV and NR2B in laminae I-II. The majority of puncta labelled with each NMDA receptor antibody were GluR2-immunoreactive, which suggests that they were present at synapses, and this was confirmed with electron microscopy for the NR1 and NR2A antibodies. However, many GluR2-immunoreactive puncta did not show NMDA receptor immunoreactivity. In laminae I-II, most NR2B puncta were also NR1-immunoreactive and a similar arrangement was found for NR2A/NR1 in laminae III-IV. These results suggest that many, but not all, glutamatergic synapses in the spinal cord possess NMDA receptors and that subunit composition varies in different regions.

The NR2B-Selective N-Methyl-D-aspartate Receptor Antagonist Ro 25-6981 [(±)-(R*,S*)-α-(4-Hydroxyphenyl)-β-methyl-4-(phenylmethyl)-1-piperidine Propanol] Potentiates the Effect of Nicotine on Locomotor Activity and Dopamine Release in the Nucleus Accumbens

It has been proposed that nicotine-stimulated locomotor activity (LMA) and nicotine-induced dopamine (DA) release in the mesocorticolimbic DA system is partly regulated by glutamate receptors, particularly N-methyl-D-aspartate (NMDA) receptors. The functional characteristics of NMDA receptors depend on their subunit composition (NR1 in combination with NR2A-D). In the present study, we investigated the effect of the NR2B-selective NMDA receptor antagonist Ro 25-6981 [(+/-)-(R*,S*)-alpha-(4-hydroxyphenyl)-beta-methyl-4-(phenylmethyl)-1-piperidine propanol] on nicotine-stimulated LMA and nicotine-induced DA release in the nucleus accumbens (NAcc) in rats. Ro 25-6981 (3 and 10 mg/kg i.p.) given 10 min prior to a high dose (0.6 mg/kg s.c.) or a subthreshold dose (0.1 mg/kg s.c.) of nicotine potentiated nicotine-stimulated LMA with no effect when administered alone. Similarly, administration of a low dose (0.05 mg/kg i.p.) of the noncompetitive NMDA receptor antagonist MK-801 (dizocilpine maleate) had no effect on LMA by itself but potentiated nicotine-induced (0.1 mg/kg) LMA. However, pretreatment with the competitive NMDA receptor antagonist CGP39551 [(E)-(+/-)-2-amino-4-methyl-5-phosphono-3-pentenoic acid ethyl ester] (10 mg/kg i.p.) did not potentiate the LMA effect of 0.1 mg/kg nicotine as seen with Ro 25-6981. In vivo microdialysis revealed a significant increase of DA release in the NAcc in response to nicotine (0.1 mg/kg s.c.). In analogy to our LMA data, Ro 25-6981 (10 mg/kg i.p.) significantly potentiated the nicotine-induced DA release, although it had no effect on DA release when given alone. The data suggest that, compared with other subunits of the NMDA receptor, the NR2B subunit might play a different role in the reinforcing effects of nicotine.

NMDA currents and receptor protein are downregulated in the amygdala during maintenance of fear memory

The amygdala plays a critical role in fear conditioning, a model of emotional learning and cue-induced anxiety. In the lateral amygdala, fear conditioning is associated with an enduring increase in synaptic strength mediated through AMPA receptors and with a reduction in paired-pulse facilitation, reflecting an increased probability of neurotransmitter release. Here we show that NMDA-mediated transmission in the thalamic-to-lateral amygdala pathway is not facilitated after fear conditioning, although probability of transmitter release is enhanced. Rather, the EC50 for NMDA receptor (NR)-mediated current is shifted threefold to fourfold to the right in fear-conditioned animals, suggesting a postsynaptic alteration in NMDA receptors in the maintenance phase of fear memory. Furthermore, the ability of nonselective and subunit-selective antagonists of NMDA receptors to block NMDA receptor-mediated EPSCs is reduced in lateral amygdala neurons from fear-conditioned animals, suggesting a reduction in NMDA receptors at thalamolateral amygdala synapses. In addition, Western blots show a reduction in phosphorylated-NR1, NR2A, and NR2B subunit protein expression in amygdalas from fear-conditioned animals. These data indicate that postsynaptic mechanisms are involved in synaptic plasticity in the thalamoamygdala pathway in fear conditioning and raise the possibility that: (1) downregulation of the NMDA receptor may protect against excitotoxicity of unchecked NMDA receptor recruitment during induction and consolidation of fear memories, (2) reduced NMDA current and protein may allow persistence of the "capacity to reactivate" amygdala pathways in NMDA receptor-dependent fear memories, or (3) a persistent long-term depression of NMDA transmission may occur after fear learning.

The N-methyl-D-aspartate receptor subunit NR2B: localization, functional properties, regulation, and clinical implications

The N-methyl-D-aspartate (NMDA) receptor is an example of a heteromeric ligand-gated ion channel that interacts with multiple intracellular proteins by way of different subunits. NMDA receptors are composed of seven known subunits (NR1, NR2A-D, NR3A-B). The present review focuses on the NR2B subunit of the receptor. Over the last several years, an increasing number of reports have demonstrated the importance of the NR2B subunit in a variety of synaptic signaling events and protein-protein interactions. The NR2B subunit has been implicated in modulating functions such as learning, memory processing, pain perception, and feeding behaviors, as well as being involved in a number of human disorders. The following review provides a summary of recent findings regarding the structural features, localization, functional properties, and regulation of the NR2B subunit. The review concludes with a section discussing the role of NR2B in human diseases.

NMDA receptors and L-type voltage-gated calcium channels contribute to long-term potentiation and different components of fear memory formation in the lateral amygdala

Long-term potentiation (LTP) at sensory input synapses to the lateral amygdala (LA) is a candidate mechanism for memory storage during fear conditioning. We evaluated the effect of L-type voltage-gated calcium channel (VGCC) and NMDA receptor (NMDAR) blockade in LA on LTP at thalamic input synapses induced by two different protocols in vitro and on fear memory in vivo. When induced in vitro by pairing weak presynaptic stimulation with strong (spike eliciting) postsynaptic depolarization, LTP was dependent on VGCCs and not on NMDARs, but, when induced by a form of tetanic stimulation that produced prolonged postsynaptic depolarization (but not spikes), LTP was dependent on NMDARs and not on VGCCs. In behavioral studies, bilateral infusions of NMDAR antagonists into the LA impaired both short-term and long-term memory of fear conditioning, whereas VGCC blockade selectively impaired long-term memory formation. Collectively, the results suggest that two pharmacologically distinct forms of LTP can be isolated in the LA in vitro and that a combination of both contribute to the formation of fear memories in vivo at the cellular level.

NMDA receptor antagonists as analgesics: focus on the NR2B subtype

Ifenprodil and a group of related compounds are selective antagonists of NR2B-containing NMDA receptors. These compounds are antinociceptive in a variety of preclinical pain models and have a much lower side-effect profile compared with other NMDA receptor antagonists. It remains unclear whether the improved safety of these compounds is due to their subtype selectivity or to a unique mode of inhibition of the receptor. Human trials have so far confirmed the good tolerability of these subtype-selective NMDA receptor antagonists; however, whether they are as effective as other NMDA receptor antagonists in pain patients remains to be demonstrated.

Pharmacological characterization of interactions of RO 25-6981 with the NR2B (epsilon2) subunit

We used ligand binding to ascertain whether the pharmacological actions of RO 25-6981 [(R:(*), S:(*))-alpha-(4-hydroxyphenyl)-beta-methyl-4-(phenylmethyl)-1-piperidinepropanol] match those of other NR2B (epsilon2) subunit specific agents. RO 25-6981 inhibited binding of 125I-MK801 [iodo-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohept-5,10-imine maleate] to receptors made from NR1a/epsilon2 but not NR1a/epsilon1. Increasing the concentration of spermidine did not change the efficacy of RO 25-6981 and minimally changed the IC(50) value. Chimeric epsilon1/epsilon2 receptors demonstrated that the structural determinants for high affinity actions of RO 25-6981 were contained completely within the first 464 amino acids, but no receptor retained wildtype features when the size of the epsilon2 component was decreased further. Epsilon1Q336R receptors were more inhibited by ifenprodil and RO 25-9681 than wildtype epsilon1 receptors in ligand binding assays but not in functional assays. Selected mutations of epsilon2E200 and epsilon2E201 also decreased the sensitivity of receptors to ifenprodil and RO 25-6981. These results suggest that RO 25-6981 shares structural determinants with ifenprodil and other modulators in the NR2B subunit.

Dynamic modulation of NMDA-induced responses by ifenprodil in rat prefrontal cortex

Ifenprodil is known to inhibit channel opening of NMDA receptors containing the NR2B subunit. However, it has also been shown to increase NMDA receptor affinity for glutamate-site agonists, including NMDA. The coexistence of the two opposing effects may explain why ifenprodil can either enhance or suppress an NMDA response depending on the level of NMDA binding and thus the NMDA concentration. Using whole cell recordings in rat prefrontal cortical slices, we report here that the effect of ifenprodil also depends on the speed and the direction of change of NMDA concentration. As shown previously, ifenprodil increased the inward current induced by low concentrations of NMDA applied through a local Y-tube perfusion system. However, the rising phase of the current was less enhanced compared to the falling phase. Increasing the speed of rising of NMDA concentration further reduced the enhancing effect of ifenprodil. When pressure ejection was used to produce even faster NMDA responses, the entire rising phase including the peak of the response was suppressed by ifenprodil, while the falling phase remained enhanced. These results are consistent with the suggestion that ifenprodil decreases both the association and dissociation rates of NMDA from NMDA receptors, and suggest that ifenprodil affects slow and fast NMDA responses in different manners. In particular, this study suggests that ifenprodil inhibits the rising phase of a fast NMDA response by suppressing both channel opening and the association of NMDA with NMDA receptors and that this inhibition can occur even when the level of NMDA binding is low.