Pharmacological characterization of heterodimeric NMDA receptors composed of NR 1a and 2B subunits: differences with receptors formed from NR 1a and 2A

Encephalitis with Autoantibodies against the Glutamate Kainate Receptors GluK2

OBJECTIVE

The objective of this study was to report the identification of antibodies against the glutamate kainate receptor subunit 2 (GluK2-abs) in patients with autoimmune encephalitis, and describe the clinical-immunological features and antibody effects.

METHODS

Two sera from 8 patients with similar rat brain immunostaining were used to precipitate the antigen from neuronal cultures. A cell-based assay (CBA) with GluK2-expressing HEK293 cells was used to assess 596 patients with different neurological disorders, and 23 healthy controls. GluK2-ab effects were determined by confocal microscopy in cultured neurons and electrophysiology in GluK2-expressing HEK293 cells.

RESULTS

Patients' antibodies precipitated GluK2. GluK2 antibody-specificity was confirmed by CBA, immunoprecipitation, GluK2-immunoabsorption, and GluK2 knockout brain immunohistochemistry. In 2 of 8 samples, antibodies reacted with additional GluK2 epitopes present in GluK1 or GluK3; in both, the reactivity was abrogated after GluK2 immuno-absorption. Six of 8 patients developed acute encephalitis and clinical or magnetic resonance imaging (MRI) features of predominant cerebellar involvement (4 presenting as cerebellitis, which in 2 patients caused obstructive hydrocephalus), and 2 patients had other syndromes (1 with cerebellar symptoms). One of the samples showed mild reactivity with non-kainate receptors (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors [AMPAR] and N-methyl-D-aspartate receptors [NMDAR]) leading to identify 6 additional cases with GluK2-abs among patients with anti-AMPAR (5/71) or anti-NMDAR encephalitis (1/73). GluK2-abs internalized GluK2 in HEK293 cells and neurons; these antibody-effects were reversible in neurons. A significant reduction of GluK2-mediated currents was observed in cells treated with patients' GluK2 serum following the time frame of antibody-mediated GluK2 internalization.

INTERPRETATION

GluK2-abs associate with an encephalitis with prominent clinicoradiological cerebellar involvement. The antibody effects are predominantly mediated by internalization of GluK2. ANN NEUROL 2021;90:107-123.

Ketamine and Ketamine Metabolite Pharmacology: Insights into Therapeutic Mechanisms

Ketamine, a racemic mixture consisting of (S)- and (R)-ketamine, has been in clinical use since 1970. Although best characterized for its dissociative anesthetic properties, ketamine also exerts analgesic, anti-inflammatory, and antidepressant actions. We provide a comprehensive review of these therapeutic uses, emphasizing drug dose, route of administration, and the time course of these effects. Dissociative, psychotomimetic, cognitive, and peripheral side effects associated with short-term or prolonged exposure, as well as recreational ketamine use, are also discussed. We further describe ketamine's pharmacokinetics, including its rapid and extensive metabolism to norketamine, dehydronorketamine, hydroxyketamine, and hydroxynorketamine (HNK) metabolites. Whereas the anesthetic and analgesic properties of ketamine are generally attributed to direct ketamine-induced inhibition of N-methyl-D-aspartate receptors, other putative lower-affinity pharmacological targets of ketamine include, but are not limited to, γ-amynobutyric acid (GABA), dopamine, serotonin, sigma, opioid, and cholinergic receptors, as well as voltage-gated sodium and hyperpolarization-activated cyclic nucleotide-gated channels. We examine the evidence supporting the relevance of these targets of ketamine and its metabolites to the clinical effects of the drug. Ketamine metabolites may have broader clinical relevance than was previously considered, given that HNK metabolites have antidepressant efficacy in preclinical studies. Overall, pharmacological target deconvolution of ketamine and its metabolites will provide insight critical to the development of new pharmacotherapies that possess the desirable clinical effects of ketamine, but limit undesirable side effects.

Coumarin-3-carboxylic acid derivatives as potentiators and inhibitors of recombinant and native N-methyl-D-aspartate receptors

N-Methyl-d-aspartate receptors (NMDARs) are known to be involved in a range of neurological and neurodegenerative disorders and consequently the development of compounds that modulate the function of these receptors has been the subject of intense interest. We have recently reported that 6-bromocoumarin-3-carboxylic acid (UBP608) is a negative allosteric modulator with weak selectivity for GluN2A-containing NMDARs. In the present study, a series of commercially available and newly synthesized coumarin derivatives have been evaluated in a structure-activity relationship (SAR) study as modulators of recombinant NMDAR activity. The main conclusions from this SAR study were that substituents as large as iodo were accommodated at the 6-position and that 6,8-dibromo or 6,8-diiodo substitution of the coumarin ring enhanced the inhibitory activity at NMDARs. These coumarin derivatives are therefore excellent starting points for the development of more potent and GluN2 subunit selective inhibitors, which may have application in the treatment of a range of neurological disorders such as neuropathic pain, epilepsy and depression. Surprisingly, 4-methyl substitution of UBP608 to give UBP714, led to conversion of the inhibitory activity of UBP608 into potentiating activity at recombinant GluN1/GluN2 receptors. UBP714 also enhanced NMDAR mediated field EPSPs in the CA1 region of the hippocampus. UBP714 is therefore a novel template for the development of potent and subunit selective NMDAR potentiators that may have therapeutic applicability in the treatment of patients with cognitive deficits or schizophrenia.

Structure-activity relationships for allosteric NMDA receptor inhibitors based on 2-naphthoic acid

Over-activation of N-methyl-d-aspartate (NMDA) receptors is critically involved in many neurological conditions, thus there has been considerable interest in developing NMDA receptor antagonists. We have recently identified a series of naphthoic and phenanthroic acid compounds that allosterically modulate NMDA receptors through a novel mechanism of action. In the present study, we have determined the structure-activity relationships of 18 naphthoic acid derivatives for the ability to inhibit the four GluN1/GluN2(A-D) NMDA receptor subtypes. 2-Naphthoic acid has low activity at GluN2A-containing receptors and yet lower activity at other NMDA receptors. 3-Amino addition, and especially 3-hydroxy addition, to 2-naphthoic acid increased inhibitory activity at GluN1/GluN2C and GluN1/GluN2D receptors. Further halogen and phenyl substitutions to 2-hydroxy-3-naphthoic acid leads to several relatively potent inhibitors, the most potent of which is UBP618 (1-bromo-2-hydroxy-6-phenylnaphthalene-3-carboxylic acid) with an IC(50) ∼ 2 μM at each of the NMDA receptor subtypes. While UBP618 is non-selective, elimination of the hydroxyl group in UBP618, as in UBP628 and UBP608, leads to an increase in GluN1/GluN2A selectivity. Of the compounds evaluated, specifically those with a 6-phenyl substitution were less able to fully inhibit GluN1/GluN2A, GluN1/GluN2B and GluN1/GluN2C responses (maximal % inhibition of 60-90%). Such antagonists may potentially have reduced adverse effects by not excessively blocking NMDA receptor signaling. Together, these studies reveal discrete structure-activity relationships for the allosteric antagonism of NMDA receptors that may facilitate the development of NMDA receptor modulator agents for a variety of neuropsychiatric and neurological conditions.

Anti-NMDA-receptor encephalitis: a severe, multistage, treatable disorder presenting with psychosis

Anti-NMDA-receptor encephalitis is a severe, treatable and potentially reversible disorder presenting with memory deficits, psychiatric symptoms and seizures. Initially described in young patients with ovarian teratoma, the disease is meanwhile increasingly recognized also in women without tumours, in men and in children. The presence of anti-glutamate receptor (type NMDA) autoantibodies in serum or cerebrospinal fluid is specific for this novel and widely underdiagnosed disorder. Early recognition is crucial since prognosis largely depends on adequate immunotherapy and, in paraneoplastic cases, complete tumour removal. Indirect immunofluorescence using NMDA-type glutamate receptors recombinantly expressed in human cells is a highly competent method for diagnosing anti-NMDA-receptor encephalitis.

The role of N-methyl-D-aspartate receptor subunit NR2B in spinal cord in cancer pain

Cancer pain is one kind of the most common and severe kinds of chronic pain. No breakthrough regarding the mechanisms and therapeutics of cancer pains has yet been achieved. Based on the well established involvement of the NMDA (N-methyl-D-aspartate) receptor containing NR2B in inflammatory pain and neuropathic pain and the effective pain relief obtained with ketamine in cancer patients with intractable pain, we supposed that NR2B in the spinal cord was an important factor for cancer pain. In this study, we investigated the possible role of NR2B in the spinal cord using a murine model of bone cancer pain. C3H/HeJ mice were inoculated into the intramedullary space of the right femur with Osteosarcoma NCTC 2472 cells to induce ongoing bone cancer-related pain behaviors. At day 14 after operation, the expression of NR2B mRNA and NR2B protein in the spinal cord were higher in tumor-bearing mice compared to the sham mice. Intrathecal administration of 5 and 10 microg of NR2B subunit-specific NMDA receptor antagonist ifenprodil attenuated cancer-evoked spontaneous pain, thermal hyperalgesia and mechanical allodynia. These results suggest that NR2B in the spinal cord may participate in bone cancer pain in mice, and ifenprodil may be a useful alternative or adjunct therapy for bone cancer pain. The findings may lead to novel strategies for the treatment of bone cancer pain.

An endoplasmic reticulum retention signal located in the extracellular amino-terminal domain of the NR2A subunit of N-Methyl-D-aspartate receptors

N-Methyl-d-aspartate (NMDA) receptors play critical roles in complex brain functions as well as pathogenesis of neurodegenerative diseases. There are many NMDA isoforms and subunit types that, together with subtype-specific assembly, give rise to significant functional heterogeneity of NMDA receptors. Conventional NMDA receptors are obligatory heterotetramers composed of two glycine-binding NR1 subunits and two glutamate-binding NR2 subunits. When individually expressed in heterogeneous cells, most of the NR1 splice variants and the NR2 subunits remain in the endoplasmic reticulum (ER) and do not form homomeric channels. The mechanisms underlying NMDA receptor trafficking and functional expression remain uncertain. Using truncated and chimeric NMDA receptor subunits expressed in heterogeneous cells and hippocampal neurons, together with immunostaining, biochemical, and functional analyses, we found that the NR2A amino-terminal domain (ATD) contains an ER retention signal, which can be specifically masked by the NR1a ATD. Interestingly, no such signal was found in the ATD of the NR2B subunit. We further identified the A2 segment of the NR2A ATD to be the primary determinant of ER retention. These findings indicate that NR2A-containing NMDA receptors may undergo a different ER quality control process from NR2B-containing NMDA receptors.

Paraneoplastic anti-N-methyl-D-aspartate receptor encephalitis associated with ovarian teratoma

OBJECTIVE

To report the autoantigens of a new category of treatment-responsive paraneoplastic encephalitis.

METHODS

Analysis of clinical features, neuropathological findings, tumors, and serum/cerebrospinal fluid antibodies using rat tissue, neuronal cultures, and HEK293 cells expressing subunits of the N-methyl-D-aspartate receptor (NMDAR).

RESULTS

Twelve women (14-44 years) developed prominent psychiatric symptoms, amnesia, seizures, frequent dyskinesias, autonomic dysfunction, and decreased level of consciousness often requiring ventilatory support. All had serum/cerebrospinal fluid antibodies that predominantly immunolabeled the neuropil of hippocampus/forebrain, in particular the cell surface of hippocampal neurons, and reacted with NR2B (and to a lesser extent NR2A) subunits of the NMDAR. NR2B binds glutamate and forms heteromers (NR1/NR2B or NR1/NR2A/NR2B) that are preferentially expressed in the adult hippocampus/forebrain. Expression of functional heteromers (not single subunits) was required for antibody binding. Eleven patients had teratoma of the ovary (six mature) and one a mature teratoma in the mediastinum; five of five tumors examined contained nervous tissue that strongly expressed NR2 subunits and reacted with patients' antibodies. Tumor resection and immunotherapy resulted in improvement or full recovery of eight of nine patients (paralleled by decreased antibody titers); two of three patients without tumor resection died of neurological deterioration. Autopsies showed extensive microgliosis, rare T-cell infiltrates, and neuronal degeneration predominantly involving, but not restricted to, the hippocampus.

INTERPRETATION

Antibodies to NR2B- and NR2A-containing heteromers of the NMDAR associate with a severe but treatment-responsive encephalitis. Our findings provide a diagnostic test and suggest a model of autoimmune NMDAR-related encephalitis with broad implications for other immune-mediated disorders of memory, behavior, and cognition.

Increased vulnerability of hippocampal neurons with age in culture: temporal association with increases in NMDA receptor current, NR2A subunit expression and recruitment of L-type calcium channels

Excessive glutamate (Glu) stimulation of the NMDA-R is a widely recognized trigger for Ca(2+)-mediated excitotoxicity. Primary neurons typically show a large increase in vulnerability to excitotoxicity with increasing days in vitro (DIV). This enhanced vulnerability has been associated with increased expression of the NR2B subunit or increased NMDA-R current, but the detailed age-courses of these variables in primary hippocampal neurons have not been compared in the same study. Further, it is not clear whether the NMDA-R is the only source of excess Ca(2+). Here, we used primary hippocampal neurons to examine the age dependence of the increase in excitotoxic vulnerability with changes in NMDA-R current, and subunit expression. We also tested whether L-type voltage-gated Ca(2+) channels (L-VGCCs) contribute to the enhanced vulnerability. The EC(50) for Glu toxicity decreased by approximately 10-fold between 8-9 and 14-15 DIV, changing little thereafter. Parallel experiments found that during the same period both amplitude and duration of NMDA-R current increased dramatically; this was associated with an increase in protein expression of the NR1 and NR2A subunits, but not of the NR2B subunit. Compared to MK-801, ifenprodil, a selective NR2B antagonist, was less effective in protecting older than younger neurons from Glu insult. Conversely, nimodipine, an L-VGCC antagonist, protected older but not younger neurons. Our results indicate that enhanced excitotoxic vulnerability with age in culture was associated with a substantial increase in NMDA-R current, concomitant increases in NR2A and NR1 but not NR2B subunit expression, and with apparent recruitment of L-VGCCs into the excitotoxic process.

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.

Inducible expression and pharmacological characterization of recombinant rat NR1a/NR2A NMDA receptors

In this study, we have established a non-neuronal cell line stably and inducibly expressing recombinant NMDA receptors (NRs) composed of rat NR1a/NR2A subunits. EcR-293 cells were transfected with rat NR1a and NR2A cDNAs using the inducible mammalian expression vector pIND. Cell colonies resistant for the selecting agents were picked and tested for NR2A mRNA as well as protein expression using quantitative RT-PCR and flow cytometry based immunocytochemistry. Clonal cells expressing functional NMDA receptors were identified by measuring NMDA-evoked ion currents, and NMDA-induced increase in cytosolic free calcium concentration in whole-cell patch-clamp and fluorimetric calcium measurements, respectively. One clone named D5/H3, which exhibited the highest response to NMDA, was chosen to examine inducibility of the expression and for pharmacological profiling of recombinant NR1a/NR2A NMDA receptors. To check inducibility, NR2A subunit expression in D5/H3 cells treated with the inducing agent muristerone A (MuA) was compared with that in non-induced cells. Both NR2A mRNA and protein expression was several folds higher in cells treated with the inducing agent. As part of the pharmacological characterization, we examined the activation of the expressed NR1a/NR2A receptors as a function of increasing concentration of NMDA. NMDA-evoked concentration-dependent increases in cytosolic [Ca2+] with an EC50 value of 41 +/- 1 microM. In addition, whereas the NMDA response was concentration-dependently inhibited by the channel blocker MK-801 (IC50 = 58 +/- 6 nM), NR2B subunit selective NMDA receptor antagonists were ineffective. Thus, this cell line, which stably and inducibly expresses recombinant NR1a/NR2A NMDA receptors, can be a useful tool for testing NMDA receptor antagonists and studying their subunit selectivity.

Differential binding properties of [3H]dextrorphan and [3H]MK-801 in heterologously expressed NMDA receptors

The N-methyl-D-aspartate receptor (NMDAR) antagonists: MK-801, phencyclidine and ketamine are open-channel blockers with limited clinical value due to psychotomimetic effects. Similarly, the psychotomimetic effects of the dextrorotatory opioids, dextromethorphan and its metabolite dextrorphan, derive from their NMDAR antagonist actions. Differences in the use dependency of blockade, however, suggest that the binding sites for MK-801 and dextrorphan are distinct. In the absence of exogenous glutamate and glycine, the rate of association of [3H]MK-801 with wild-type NR1-1a/NR2A receptors was considerably slower than that for [3H]dextrorphan. Glutamate individually, and in the presence of the co-agonist glycine, had substantial effects on the specific binding of [3H]MK-801, while the binding of [3H]dextrorphan was not affected. Mutation of residues N616 and A627 in the NR1 subunit had a profound effect on [3H]MK-801 binding affinity, while that of [3H]dextrorphan was unaltered. In contrast, NR1 residues, W611 and N812, were critical for specific binding of [3H]dextrorphan to NR1-1a/NR2A complexes with no corresponding influence on that of [3H]MK-801. Thus, [3H]dextrorphan and [3H]MK-801 have distinct molecular determinants for high-affinity binding. The ability of [3H]dextrorphan to bind to a closed channel, moreover, indicates that its recognition site is shallower in the ion channel domain than that of MK-801 and may be associated with the extracellular vestibule of the NMDAR.

Altered Expression of Neurotransmitter-receptor Subunit and Uptake Site mRNAs in Hemimegalencephaly

PURPOSE

Hemimegalencephaly (HMEG) is characterized by unilateral hemispheric enlargement and severe cytoarchitectural abnormalities that are highly associated with intractable epilepsy. No studies have defined alterations in neurotransmitter-receptor subunit gene expression in HMEG. We hypothesize that a differential expression of excitatory amino acid and gamma-aminobutyric acid (GABA)A-receptor subunit messenger RNAs (mRNAs) exists in HMEG.

METHODS

The expression of mRNAs encoding 20 neurotransmitter-receptor subunits, synthetic enzymes, and uptake sites as well as select additional candidate genes was defined in HMEG samples (n=8) compared with homotopic control cortex specimens by using targeted complementary DNA (cDNA) arrays. Expression of GLT-1 (a glial glutamate transporter), EAAC-1 (neuronal glutamate transporter), and NMDA2B was corroborated by immunohistochemical, Western, and ligand-binding assays.

RESULTS

Differential expression of 11 neurotransmitter-related mRNAs was demonstrated in HMEG compared with control cortex. For example, expression of GLT-1 and GluR6 mRNAs was enhanced, whereas diminished expression of the neuronal glutamate transporter EAAC-1, GABAAalpha2, GABAAgamma2, GABAAgamma3, NMDA2B, GluR1, GluR2, GluR4, and GluR5 subunits occurred. Reduced NMDA2B subunit mRNA expression in HMEG was confirmed by receptor ligand-binding assays by using the NMDA2B-receptor antagonist ifenprodil, which revealed barely detectable levels of NMDA2B binding compared with that in control cortex.

CONCLUSIONS

Selective alterations occur in distinct neurotransmitter-receptor and -uptake sites in HMEG. Differential expression of neurotransmitter-receptor and -uptake sites in HMEG may contribute to epileptogenesis in HMEG.

Selective activation induced cleavage of the NR2B subunit by calpain

Although activation of calcium-activated neutral protease (calpain) by the NMDA receptor has been suggested to play critical roles in synaptic modulation and neurologic disease, the nature of its substrates has not been completely defined. In this study, we examined the ability of calpain to cleave the NMDA receptor in cultured hippocampal neurons. Activation of the NMDA receptor by agonist application led to rapid calpain-specific proteolysis of spectrin and decreased levels of NR2A/2B subunits. Cleavage of the NR2A/2B subunit created a 115 kDa product that retained the ability to bind 125I-MK-801 and is predicted to be active. Increases in levels of this product appeared within 5 min of NMDA receptor activation and were stable for periods of >30 min. Subtype-specific antibodies demonstrated that the NR2B subunit was cleaved in these primary cultures, but the NR2A subunit was not. An inhibitor of calpain blocked both the decrease of intact NR2B and the increase of the low molecular weight form, whereas neither caspase nor cathepsin inhibitors had an effect on these events. Cell surface biotinylation experiments demonstrated that the 115 kDa fragment remained on the cell surface. This NR2B fragment was also found in the rat hippocampus after transient forebrain ischemia, showing that this process also occurs in vivo. This suggests that calpain-mediated cleavage of the NR2B subunit occurs in neurons and gives rise to active NMDA receptor forms present on the cell surface after excitotoxic glutamatergic stimulation. Such forms could contribute to excitotoxicity and synaptic remodeling.

Expression of NMDA receptor NR1, NR2A and NR2B subunit mRNAs during development of the human hippocampal formation

The N-methyl-d-aspartate receptor plays a critical role in the formation and maintenance of synapses during brain development. In the rodent, changes in subunit expression and assembly of the heteromeric receptor complex accompany these maturational processes. However, little is known about N-methyl-d-aspartate receptor subunit expression during human brain development. We used in situ hybridization to examine the distribution and relative abundance of NR1, NR2A and NR2B subunit messenger ribonucleic acids in the hippocampal formation and adjacent cortex of 34 human subjects at five stages of life (neonate, infant, adolescent, young adult and adult). At all ages, the three messenger ribonucleic acids were expressed in all subfields, predominantly by pyramidal neurons, granule cells and polymorphic hilar cells. However, their abundance varied across ontogeny. Levels of NR1 messenger ribonucleic acid in CA4, CA3 and CA2 subfields were significantly lower in the neonate than all other age groups. In the dentate gyrus, subiculum and parahippocampal gyrus, NR2B messenger ribonucleic acid levels were higher in the neonate than in older age groups. NR2A messenger ribonucleic acid levels remained constant, leading to an age-related increase in NR2A/2B transcript ratio. We conclude that N-methyl-d-aspartate receptor subunit messenger ribonucleic acids are differentially expressed during postnatal development of the human hippocampus, with a pattern similar but not identical to that seen in the rodent. Changes in subunit composition may thus contribute to maturational differences in human hippocampal N-methyl-d-aspartate receptor function, and to their role in the pathophysiology of schizophrenia and other neurodevelopmental disorders.

Ethanol differentially inhibits homoquinolinic acid- and NMDA-induced neurotoxicity in primary cultures of cerebellar granule cells

The potency of ethanol to inhibit N-methyl-D-aspartate (NMDA) receptor functions may depend on the subunit composition of the NMDA receptors. We used a NR2A-B subunit-selective NMDA receptor agonist, homoquinolinic acid (HQ), and a subunit-unselective agonist, NMDA, to induce neurotoxicity in cerebellar granule cells and examined the neuroprotective actions of ethanol, as well as NR2A- and NR2B-subunit selective antagonists, respectively. HQ was a more potent neurotoxic agent than NMDA, as measured by the MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) assay. NR2A- and NR2B-selective NMDA receptor antagonists displayed quite similar neuroprotective potencies against the NMDA- and HQ-produced cell death, indicating that the higher potency of HQ to induce neurotoxicity cannot be simply explained by NR2A- or NR2B-subunit selectivity. As expected, ethanol (25 and 50 mM) attenuated the NMDA-induced neurotoxicity in a non-competitive manner by significantly reducing the maximum neurotoxicity produced by NMDA. By contrast, ethanol inhibited the HQ-induced neurotoxicity in a manner resembling a competitive-like interaction significantly increasing the EC50 value for HQ, without reducing the maximum neurotoxicity produced by HQ. These results suggest that HQ reveals either a novel site or a not previously observed mechanism of interaction between ethanol and NMDA receptors in rat cerebellar granule cell cultures.

Inducible expression and pharmacology of recombinant NMDA receptors, composed of rat NR1a/NR2B subunits

An ecdysone-inducible mammalian expression system was used to study expression of recombinant N-methyl-D-aspartate (NMDA) receptors. Human embryonic kidney (HEK) 293 cells expressing the regulatory vector pVgRXR (EcR 293 cells) were transfected with rat NR1a and NR2B cDNAs using the inducible vector pIND (Invitrogen). Inducible expression of the NR2B subunit in cell clone designated EcR/rNR1a2B was investigated using quantitative RT-PCR and flow cytometry based immunocytochemical methods. The mRNA level of the NR2B subunits in EcR/rNRa2B cells was dependent on the concentration of the ecdysone analogue inducing agent, muristerone A (MuA). Similarly, NR2B subunit protein expression was higher in cells pre-treated with the inducing agent. Functionally active NMDA receptors were also detected in EcR/rNR1a2B cells after MuA induction. In presence of the inducing factor, NMDA-evoked ion currents as well as increase in cytoplasmic calcium-concentrations were measured using whole-cell patch clamp and fluorometric calcium measuring techniques. The pharmacological profile of the expressed NMDA receptors was characterised by comparing the inhibitory activity of several NR2B subunit selective NMDA antagonists in EcR/rNR1a2B cells with that observed in primary cultures of rat cortical neurones. Whereas the efficacies of the NR2B subunit selective NMDA antagonists were similar in EcR/rNR1a2B cells and in neurones, their maximal inhibitory effects were significantly higher in cells expressing NR1a/NR2B recombinant receptors. This study demonstrates that recombinant NMDA receptors can be expressed in an inducible way in non-neuronal cell lines using the ecdysone-inducible mammalian expression system. Such cell lines can be suitable tools in high throughput functional screening for potential subtype selective modulators of the NMDA receptor.

Proteolysis of the N-methyl-d-aspartate receptor by calpain in situ

N-Methyl-D-aspartate (NMDA) receptors are calcium-permeable glutamate receptors that play putative roles in learning, memory, and excitotoxicity. NMDA receptor-mediated calcium entry can activate the calcium-dependent protease calpain, leading to substrate degradation. The major NMDA receptor 2 (NR2) subunits of the receptor are in vitro substrates for calpain at selected sites in the C-terminal region. In the present study, we assessed the ability of calpain-mediated proteolysis to modulate the NR1a/2A subtype in a heterologous expression system. Human embryonic kidney (HEK293t) cells, which endogenously express calpain, were cotransfected with NR1a/2A in addition to the calpain inhibitor calpastatin or empty vector as control. Receptor activation by glutamate and glycine as co-agonists led to calpain activation as measured by succinyl-L-leucyl-L-leucyl-L-valyl-L-tyrosyl-aminomethyl coumarin (Suc-LLVY-AMC). Calpain activation also resulted in the degradation of NR2A and decreased binding of (125)I-MK-801 ((125)I-dizocilpine) to NR1a/2A receptors. No stable N-terminal fragment of the NMDA receptor was formed after calpain activation, suggesting calpain regulation of NMDA receptor levels in ways distinct from that previously observed with in vitro cleavage. NR2 subunit constructs lacking the final 420 amino acids were not degraded by calpain. Agonist-stimulated NR1a/2A-transfected cells also had decreased calcium uptake and produced lower changes in agonist-stimulated intracellular calcium compared with cells cotransfected with calpastatin. Calpastatin had no effect on either calcium uptake or intracellular calcium levels when the NR2A subunit lacked the final 420 amino acids. These studies demonstrate that NR2A is a substrate for calpain in situ and that this proteolytic event can modulate NMDA receptor levels.

Specific proteolysis of the NR2 subunit at multiple sites by calpain

The NMDA subtype of glutamate receptor plays an important role in the molecular mechanisms of learning, memory and excitotoxicity. NMDA receptors are highly permeable to calcium, which can lead to the activation of the calcium-dependent protease, calpain. In the present study, the ability of calpain to modulate NMDA receptor function through direct proteolytic digestion of the individual NMDA receptor subunits was examined. HEK293t cells were cotransfected with the NR1a/2A, NR1a/2B or NR1a/2C receptor combinations. Cellular homogenates of these receptor combinations were prepared and digested by purified calpain I in vitro. All three NR2 subunits could be proteolyzed by calpain I while no actin or NR1a cleavage was observed. Based on immunoblot analysis, calpain cleavage of NR2A, NR2B and NR2C subunits was limited to their C-terminal region. In vitro calpain digestion of fusion protein constructs containing the C-terminal region of NR2A yielded two cleavage sites at amino acids 1279 and 1330. Although it has been suggested that calpain cleavage of the NMDA receptor may act as a negative feedback mechanism, the current findings demonstrated that calpain cleavage did not alter [(125)I]MK801 binding and that receptors truncated to the identified cleavage sites had peak intracellular calcium levels, (45)Ca uptake rates and basal electrophysiological properties similar to wild type.

The amino acid residue at sequence position 5 in the conantokin peptides partially governs subunit-selective antagonism of recombinant N-methyl-D-aspartate receptors

Whole cell voltage clamp recordings were performed to assess the ability of conantokin-G (con-G), conantokin-T (con-T), and a 17-residue truncated form of conantokin-R (con-R[1-17]) to inhibit N-methyl-d-aspartate (NMDA)-evoked currents in human embryonic kidney 293 cells transiently expressing various combinations of NR1a, NR1b, NR2A, and NR2B receptor subunits. Con-T and con-R[1-17] attenuated ion currents in cells expressing NR1a/NR2A or NR1a/NR2B. Con-G did not affect NMDA-evoked ionic currents in cells expressing NR1a/NR2A, but it showed inhibitory activity in cells expressing NR1a/NR2B receptors and the triheteromeric combination of NR1a/NR2A/NR2B. An Ala-rich con-G analog, con-G[Q6G/gamma7K/N8A/gamma10A/gamma14A/K15A/S16A/N17A] (Ala/con-G, where gamma is Gla), in which all nonessential amino acids were altered to Ala residues, manifested subunit specificity similar to that of con-G, suggesting that the replaced residues are not responsible for selectivity in the con-G framework. A sarcosine-containing con-T truncation analog, con-T[1-9/G1Src/Q6G], inhibited currents in NR1a/NR2A and NR1a/NR2B receptors, eliminating residues 10-21 as mediators of the broad subunit selectivity of con-T. In contrast to the null effects of con-G and Ala/con-G at a NR1a/NR2A-containing receptor, some inhibition ( approximately 40%) of NMDA-evoked currents was effected by these peptides in cells expressing NR1b/NR2A. This finding suggests that the presence of exon 5 in NR1b plays a role in the activity of the conantokins. Analysis of various conantokin analogs demonstrated that Leu(5) of con-G is an important determinant of conantokin selectivity. Taken as a whole, these results suggest that the important molecular determinants on conantokins responsible for NMDA receptor activity and specificity are discretely housed in specific residues of these peptides, thus allowing molecular manipulation of the NMDA receptor inhibitory properties of the conantokins.

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.

Increased expression of NR2A subunit does not alter NMDA-evoked responses in cultured fetal trisomy 16 mouse hippocampal neurons

The trisomy 16 (Ts16) mouse is an animal model for human trisomy 21 (Down's syndrome). The gene encoding the NR2A subunit of the NMDA receptor has been localized to mouse chromosome 16. In the present study, western blot analysis revealed a 2.5-fold increase of NR2A expression in cultured Ts16 embryonic hippocampal neurons. However, this increase did not affect the properties of NMDA-evoked currents in response to various modulators. The sensitivity of NMDA receptors to transient applications of NMDA, spermine, and Zn(2+) was investigated in murine Ts16 and control diploid cultured embryonic hippocampal neurons. Peak and steady-state currents evoked by NMDA were potentiated by spermine at concentrations < 1 mM, and inhibited by Zn(2+) in a dose-dependent and voltage-independent manner. No marked difference was observed between Ts16 and control diploid neurons for any of these modulators with regard to IC(50) and EC(50) values or voltage dependency. Additionally, inhibition by the NR2B selective inhibitor, ifenprodil, was similar. These results demonstrate that NMDA-evoked currents are not altered in cultured embryonic Ts16 neurons and suggest that Ts16 neurons contain similar functional properties of NMDA receptors as diploid control neurons despite an increased level of NR2A expression.

Selective alterations in glutamate and GABA receptor subunit mRNA expression in dysplastic neurons and giant cells of cortical tubers

The molecular pharmacologic basis of epileptogenesis in cortical tubers in the tuberous sclerosis complex is unknown. Altered transcription of genes encoding glutamatergic and gamma-aminobutyric acid (GABA)-ergic receptors and uptake sites may contribute to seizure initiation and may occur selectively in dysplastic neurons and giant cells. Arrays containing GABA A (GABAAR), GluR, NMDA receptor (NR) subunits, GAD65, the vesicular GABA transporter (VGAT), and the neuronal glutamate transporter (EAAC1) cDNAs were probed with amplified poly (A) mRNA from tubers or normal neocortex to identify changes in gene expression. Increased levels of EAAC1, and NR2B and 2D subunit mRNAs and diminished levels of GAD65, VGAT, GluR1, and GABAAR alpha1 and alpha2 were observed in tubers. Ligand-binding experiments in frozen tuber homogenates demonstrated an increase in functional NR2B-containing receptors. Arrays were then probed with poly (A) mRNA from single, microdissected dysplastic neurons, giant cells, or normal neurons (n = 30 each). Enhanced expression of GluR 3, 4, and 6 and NR2B and 2C subunit mRNAs was noted in the dysplastic neurons, whereas only the NR2D mRNA was upregulated in giant cells. GABAAR alpha1 and alpha2 mRNA levels were reduced in both dysplastic neurons and giant cells compared to control neurons. Differential expression of GluR, NR, and GABAAR mRNAs in tubers reflects cell-specific changes in gene transcription that argue for a distinct molecular phenotype of dysplastic neurons and giant cells and suggests that dysplastic neurons and giant cells make differential contributions to epileptogenesis in the tuberous sclerosis complex.

Chronic ethanol exposure attenuates the anti-apoptotic effect of NMDA in cerebellar granule neurons

Ethanol, added to primary cultures of cerebellar granule neurons simultaneously with NMDA, was previously shown to inhibit the anti-apoptotic effect of NMDA. The in vitro anti-apoptotic effect of NMDA is believed to mimic in vivo protection against apoptosis afforded by innervation of developing cerebellar granule neurons by glutamatergic mossy fibers. Therefore, the results suggested that the presence of ethanol in the brain at a critical period of development would promote apoptosis. In the present studies, we examined the effect of chronic ethanol exposure on the anti-apoptotic action of NMDA in cerebellar granule neurons. The neurons were treated with ethanol in vitro for 1-3 days in the absence of NMDA. Even after ethanol was removed from the culture medium, as ascertained by gas chromatography, the protective effect of added NMDA was significantly attenuated. The decreased anti-apoptotic effect of NMDA was associated with a change in the properties of the NMDA receptor, as indicated by a decrease in ligand binding, decreased expression of NMDA receptor subunit proteins, and decreased functional responses including stimulation of increases in intracellular Ca(2+) and induction of brain-derived neurotrophic factor expression. The latter effect may directly underlie the attenuated protective effect of NMDA in these neurons. The results suggest that ethanol exposure during development can have long-lasting effects on neuronal survival. The change in the NMDA receptor caused by chronic ethanol treatment may contribute to the loss of cerebellar granule neurons that is observed in animals and humans exposed to ethanol during gestation.

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

Ifenprodil has been widely used as an antagonist selective for NMDA receptors containing the NR2B subunit. Evidence suggests, however, that ifenprodil also increases NMDA receptor affinity. Using rat brain slices, we found that ifenprodil enhanced NMDA-induced current in both cortical and subcortical areas examined. To test whether the effect is due to an increase in NMDA receptor affinity, we compared the effect of ifenprodil on currents induced by different concentrations of NMDA. Consistent with the hypothesis, the enhancing effect (percent increase) was relatively constant at low NMDA concentrations. As NMDA concentration increased, however, the effect decreased. To test whether the effect is blocked when NMDA binding sites are saturated with NMDA, high concentrations of NMDA were applied. To partially block Ca(2+) influx and prevent cells from deteriorating, the experiments were performed in the presence of either MK801 or kynurenate, two noncompetitive antagonists. Under such conditions, ifenprodil not only failed to potentiate NMDA currents, but consistently suppressed the current. When the same concentration of NMDA was applied in the presence of the competitive antagonist CGP37849, ifenprodil regained its ability to potentiate NMDA currents. Furthermore, the higher the concentration of CGP37849 the more the NMDA current was potentiated by ifenprodil. These results, combined with previous studies, suggest that the enhancing effect is due to an increase in NMDA receptor affinity and is specific for responses induced by low NMDA concentrations. As NMDA concentration increases, the affinity-enhancing effect decreases. Consequently, the channel-suppressing effect becomes more prominent.

Declines in mRNA expression of different subunits may account for differential effects of aging on agonist and antagonist binding to the NMDA receptor

The purpose of the present study was to determine whether some of the age-related changes that occur in binding to the NMDA receptor complex can be accounted for by changes in subunit expression during the aging process. In situ hybridization for the NMDA subunits zeta1, epsilon1, and epsilon2, and receptor autoradiography, using the agonist glutamate and the competitive antagonist [(+/-)-2-carboxypiperazin-4-yl] propyl-1-phosphonic acid (CPP), were performed on sections from C57Bl/6 mice representing three different age groups (3, 10, and 30 months of age). There was a significant overall decrease between 3 and 30 month olds in the density of mRNA for the zeta1 subunit in the cortex and hippocampus, but only a few individual brain regions exhibited significant declines. The mRNA for the epsilon2 subunit was significantly decreased in a majority of cortical regions and in the dentate granule cells. Emulsion analysis indicated that the change in the density of epsilon2 subunit mRNA in the inner frontal cortex was primarily attributable to a decrease in the amount of messages per cell. Age-related changes in mRNA density of the epsilon2 subunit correlated with changes in NMDA-displaceable [(3)H]glutamate binding, and mRNA density changes in the zeta1 subunit showed a significant relationship with changes in [(3)H]CPP binding in the 30-month-old mice. These results suggest that changes during aging in the expression of different subunits of the NMDA receptor may account for the differential effects of aging on agonist versus antagonist binding to the NMDA binding site.

Subunit- and site-specific pharmacology of the NMDA receptor channel

N-Methyl-D-aspartate (NMDA) receptor channels play important roles in various physiological functions such as synaptic plasticity and synapse formation underlying memory, learning and formation of neural networks during development. They are also important for a variety of pathological states including acute and chronic neurological disorders, psychiatric disorders, and neuropathic pain syndromes. cDNA cloning has revealed the molecular diversity of NMDA receptor channels. The identification of multiple subunits with distinct distributions, properties and regulation, implies that NMDA receptor channels are heterogeneous in their pharmacological properties, depending on the brain region and the developmental stage. Furthermore, mutation studies have revealed a critical role for specific amino acid residues in certain subunits in determining the pharmacological properties of NMDA receptor channels. The molecular heterogeneity of NMDA receptor channels as well as their dual role in physiological and pathological functions makes it necessary to develop subunit- and site-specific drugs for precise and selective therapeutic intervention. This review summarizes from a molecular perspective the recent advances in our understanding of the pharmacological properties of NMDA receptor channels with specific references to agonists binding sites, channel pore regions, allosteric modulation sites for protons, polyamines, redox agents, Zn2+ and protein kinases, phosphatases.

Repeated seizure-associated long-lasting changes of N-methyl-D-aspartate receptor properties in the developing rat brain

Glutamate NMDA receptor has been implicated in brain developmental processes as well as in excitotoxicity and seizure mediation. A previous study has shown that an acute episode of seizures for 30 min in rats altered NMDA receptor characteristics, mainly in the very immature animal. In order to assess whether receptor modifications may also account for long-lasting cerebral disabilities, medium- and long-term consequences of repeated seizures in developing rats on brain NMDA receptor properties were investigated. Seizures were induced once a day for 3 consecutive days, either from post-natal day 5 (P5) to P7 or from P15 to P17. NMDA receptors were then analysed at P15, P25 and P60 (adulthood) by measuring specific binding of [3H]MK-801 on brain membrane preparations. In addition, allosteric modulation of NMDA receptors by exogenous glutamate and glycine was investigated. Seizures from P5 to P7 led to a 22% increase in the density of [3H]MK-801 binding sites measured at P15, but did not affect NMDA receptor density or affinity at P25 or P60. P15-P17 seizures led to a 21% decrease in the density of binding sites and to a 33% decrease in receptor dissociation constant at P25, while they were without effect at P60. Moreover, P5-P7 and P15-P17 seizures were both associated with a suppression of the glutamate/glycine-induced receptor activation at P60. These modifications might account for long-term alterations in cerebral excitability or plasticity after early convulsive disorders, with regards to altered cognitive capacities, epileptogenesis and brain susceptibility to recurrent seizures.

Ethanol sensitivity of NMDA receptor function in developing cerebellar granule neurons

The mechanism by which ethanol inhibits the function of the NMDA subtype of glutamate receptor has not been elucidated. One possibility that has been suggested is that NMDA receptor subunit composition influences the sensitivity of the receptor to ethanol. We have taken advantage of developmental changes in subunit composition of the NMDA receptor in cultured neurons to examine possible changes in the effect of ethanol. We found an increase in expression of the NR2A subunit, and a decrease in expression of the NR2B subunit of the NMDA receptor in primary cultures of cerebellar granule neurons over time in culture, with no significant change in NR1 expression. This change in NR2 subunit expression was associated with the expected changes in functional properties of the NMDA receptor (measured as the NMDA-induced increase in intracellular Ca2+), i.e., ifenprodil sensitivity and glycine potency were higher when there was a relatively greater proportion of NR2B in the cultured neurons. However, the potency of ethanol to inhibit NMDA receptor function was lower when there was a greater proportion of NR2B subunits. Previous studies showed that ethanol inhibition of NMDA receptor function in cerebellar granule neurons resulted from an ethanol-induced decrease in potency of the co-agonist, glycine, and that this effect of ethanol was blocked by inhibitors of protein kinase C. Our current results suggest that the lower potency of ethanol to inhibit the response of NMDA receptors when cerebellar granule neurons are expressing a greater proportion of NR2B subunits is a result of the higher affinity of the NMDA receptors for endogenous levels of glycine at this point in time.

Development of an inducible NMDA receptor stable cell line with an intracellular Ca2+ reporter

Cytotoxicity associated with NMDA receptor activation has impeded the establishment of cell lines expressing recombinant subtypes of this ligand-gated ion channel class. To circumvent this toxicity, we describe in this report the use of a potent inducible promoter in the construction of a cell line stably expressing the NR1a/NR2A subtype of the NMDA receptor. Western blot analysis using subunit selective antibodies revealed that NR2A subunits were constitutively expressed in this cell line, whereas expression of NR1a subunits was tightly regulated by tetracycline. Upon tetracycline removal, electrophysiological recordings using the patch clamp technique indicated the expression of functional receptors with biophysical and pharmacological properties corresponding to those expected of the NR1a/NR2A subtype. In addition, we utilized this cell line with the recombinant membrane targeted Ca2+ reporter, aequorin, in a functional assay of NMDA receptor activation. An evaluation of the coupling efficiency of NMDA receptor activation and aequorin response, as well as the pharmacological profile of this assay, illustrates the suitability of this cell line and the Ca2+ reporter assay to functionally identify novel NMDA receptor antagonists.

Actions of butyrophenones and other antipsychotic agents at NMDA receptors: relationship with clinical effects and structural considerations

Haloperidol inhibits NMDA receptors with higher affinity for NMDA receptors composed of NR1/2B compared with NR1/2A. To assess whether the clinical effects of haloperidol and other antipsychotic agents are mediated through this site on NMDA receptors and to examine structure activity relationships at this site, we examined the ability of a variety of drugs with neuroleptic actions to inhibit NMDA receptor function. Many antipsychotic agents inhibit 125I-MK 801 binding to the NMDA receptor with IC50 values in the micromolar range. The rank order of potency for inhibition of binding to adult rat forebrain was trifluperidol (TFP) > clozapine = fluphenazine = reduced haloperidol = spiperone = trifluoperazine = butaclamol >> pimozide = risperidone = sulpiride. These findings match the molecular biological specificity of the agents, with trifluperidol having a marked preference for NR1/2B (epsilon2) receptors. Mutations at epsilon2E201, which alter the effects of haloperidol, also decrease the affinity of TFP but not other modulators, showing that the effect of TFP but not other modulators is mediated by this residue of the NMDA receptor. The present results demonstrate that while TFP acts on NMDA receptors in a manner similar to haloperidol, other antipsychotic agents do not share the specific pharmacological properties of this action, suggesting that their clinical mechanism is not mediated by this receptor.

Binding sites for permeant ions in the channel of NMDA receptors and their effects on channel block

We report the presence of binding sites for permeant monovalent cations at the internal and external entrances to the channel of NMDA receptors. We measured the effects of changing internal cesium (Cs+) and external sodium (Na+) concentrations on the channel-blocking kinetics of the adamantane derivatives IEM-1754 and IEM-1857. Binding of Na+, or of Cs+ after it permeates the channel, to sites at the external channel entrance prevents blockers from entering the channel. Binding of Na+ to a blocked channel prevents blocker unbinding. Cs+ binding to a site at the internal channel entrance prevents IEM-1754 from occupying the deeper of its two sites of block. The results show the critical effects of permeant ions on the kinetics, affinity and voltage-dependence of channel blockers.

Characterization of desensitization in recombinant N-methyl-D-aspartate receptors: comparison with native receptors in cultured hippocampal neurons

In the present study we have characterized the effect of Ca2+, glycine, and agonist concentration on inactivation and desensitization in native and recombinant N-methyl-d-aspartate (NMDA) receptors. In agreement with earlier studies on neurons, we found that in the presence of saturating glycine concentrations, lowering [Ca2+]o, will decrease inactivation of NMDA receptors in cultured hippocampal neurons. However, unlike native NMDA receptors under the same recording conditions, recombinant receptors did not exhibit Ca2+-dependent inactivation. We also show that the glycine-insensitive desensitization observed in the recombinant receptors is subunit dependent, as NR1a2A and NR1a2B receptors significantly desensitized while the NR1a2C combination did not. Furthermore, we show this form of desensitization in NR1a2A receptors is due to classic agonist-induced desensitization. In addition, we demonstrate the presence of glycine-dependent desensitization in recombinant receptors. The ability of glycine to inhibit desensitization correlates to the rank order of glycine's affinity for potentiating the peak response for each subtype. Finally, using ifenprodil in the presence of high and low glycine concentrations, we present evidence that both 2A-like and 2B-like subtypes of receptors can independently coexist in single neurons.

Differentiation of glycine antagonist sites of N-methyl-D-aspartate receptor subtypes. Preferential interaction of CGP 61594 with NR1/2B receptors

The binding site for the co-agonist glycine on N-methyl-D-aspartate (NMDA) receptors has been mapped to the NR1 subunit whereas binding of the principal agonist glutamate is mediated by the NR2 subunits. Using the novel glycine site antagonist and photoaffinity label CGP 61594, distinct contributions of the NR2 subunit variants to the glycine antagonist binding domains of NMDA receptor subtypes are demonstrated. High affinity sites for CGP 61594 were exclusively displayed by NR1/2B receptors, as shown by their co-distribution with the NR2B subunit, by subunit-selective immunoprecipitation and by functional analysis of NR1/2B receptors expressed in Xenopus oocytes (inhibitory potency, IC50 = 45 +/- 11 nM). Other NMDA receptor subtypes are clearly distinguished by reduced inhibitory potencies for CGP 61594, being low for NR1/2A and NR1/2D receptors (IC50 = 430 +/- 105 nM and 340 +/- 61 nM, respectively) and intermediate for NR1/2C receptors (IC50 = 164 +/- 27 nM). Glycine antagonist sites with low and intermediate affinity for [3H]CGP 61594 were detected also in situ by radioligand binding in brain areas predominantly expressing the NR2A and NR2C subunits, respectively. Thus, [3H]CGP 61594 is the first antagonist radioligand that reliably distinguishes the glycine site of NMDA receptor subtypes. [3H]CGP 61594 is a promising tool to identify the NR2 subunit domains that contribute to differential glycine antagonist sites of NMDA receptor subtypes.

Investigation of the actions and antagonist activity of some polyamine analogues in vivo

1. The ability of three putative polyamine antagonists to antagonize behavioural changes induced by spermine was assessed. 2. Injection of an excitotoxic dose of spermine (100 microg, i.c.v.) in mice results in the development of a characteristic behavioural profile, which has two temporally distinct phases. The early events include clonic convulsions, and the later, more general excitation, includes tremor and culminates in the development of a fatal tonic convulsion. 3. Co-administration of arcaine (25 microg, i.c.v.) potentiated the early phase effects after spermine injection, but antagonized the development of spermine-induced tonic convulsions. A larger dose of arcaine (50 microg, i.c.v.) given alone resulted in the development of spermine-like body tremor and convulsions. It therefore appears that arcaine is not a pure polyamine antagonist in vivo, but may be a partial agonist. 4. Similarly, 1,10-diaminodecane appeared to act as a partial agonist in vivo, although it was less potent than arcaine. 5. In contrast, diethylenetriamine (DET) effectively inhibited the development of the early effects of spermine, but was ineffective against the spermine-induced CNS excitation and tonic convulsions. 6. It is concluded that none of the putative polyamine antagonists tested behaved as effective polyamine antagonists in vivo, although each produced some antagonism.

The NR2B-specific interactions of polyamines and protons with the N-methyl-D-aspartate receptor

Many compounds exhibit NR2B-specific modulation of the N-methyl-D-aspartate receptor, although their mechanism(s) of action are largely unknown. Using chimeric NR2A/NR2B subunits, we have located a region of NR2B (amino acids 138-238) which regulated glycine-independent polyamine stimulation. Mutation of glutamate 201 in this region affected stimulation by polyamines in the order E201D < E201A < E201N < E201R. The relief of proton inhibition of the N-methyl-D-aspartate-induced currents mediated by these mutant receptors correlated with the reduction in glycine-independent polyamine stimulation. Electrophysiological evidence with a triple mutant of NR2A further supports the hypothesis that polyamine stimulation may be linked to the relief of tonic inhibition by protons and demonstrates the crucial role of amino acids 200 and 201 in polyamine stimulation. Polyamines and protons, therefore, share common NR2B determinants.

Decreased expression of N-methyl-D-aspartate receptor 1 messenger RNA in select regions of Alzheimer brain

An antisense oligonucleotide probe was used to examine the expression of gene encoding the obligatory NMDAR1 subunit of the N-methyl-D-aspartate receptor in the hippocampus and adjacent cortical areas (entorhinal and perirhinal cortices) of seven Alzheimer patients and in the same brain regions of seven control individuals. Both groups were matched according to age, sex, cause of death, post mortem delay, and tissue storage time. Densitometric analysis of in situ hybridization autoradiograms revealed a 34% (P<0.05) decrease in NMDAR1 messenger RNA levels in layer III of the entorhinal cortex in Alzheimer brains. Similar deficits. although statistically not significant, were observed in layers II and IV-VI of the entorhinal cortex, and in granule cells of the dentate gyrus. Reduced levels of NMDAR1 messenger RNA were also found in layers II-VI of the perirhinal cortex (41 53% decrease, P<0.02). There were no changes in NMDAR1 messenger RNA expression in the CA1, hilus, or subiculum. Both Alzheimer and control group show substantial intersubject variation in levels of NMDAR1 messenger RNA. The analysis of emulsion-dipped tissue revealed a trend toward a decrease in the number of silver grains overlying individual neurons in the CA1, entorhinal cortex, and granule cell layer of some Alzheimer patients. No significant relationship was detected between the levels of NMDAR1 messenger RNA and post mortem delay, tissue storage, age of the subjects, or mini mental state exam score either in control or Alzheimer individuals. In contrast, a strong inverse correlation between NMDAR1 expression and disease duration was found. These data suggest that reduction in expression of the NMDAR1 gene observed in certain regions of Alzheimer hippocampus and adjacent cortical regions is specific for the disease itself. We postulate that reduced transcript levels may reflect either regional cell loss or anomalies in glutamatergic input to the hippocampus and entorhinal cortex in Alzheimer's disease. When followed by changes at the receptor subunit protein level, altered expression of the NMDAR1 gene in Alzheimer brain may contribute, through the formation of N-methyl-D-aspartate receptors with different properties, to the previously reported modified N-methyl-D-aspartate receptor ligand binding, abnormal vulnerability of select neuronal populations to excitotoxic insult, and may also be involved in learning and memory deficits.

Chronic corticosterone treatment induces parallel changes in N-methyl-D-aspartate receptor subunit messenger RNA levels and antagonist binding sites in the hippocampus

Some of the effects of glucocorticoids on the function and neuronal plasticity of the hippocampus are mediated by N-methyl-D-aspartate receptor activation. We tested the hypothesis that chronic corticosterone administration increases N-methyl-D-aspartate receptor expression in the hippocampus of the rat. We used in situ hybridization histochemistry to measure the messenger RNA levels for the NR1, NR2A and NR2B subunits of the N-methyl-D-aspartate receptor and [3H]dizocilpine maleate (a non-competitive antagonist) binding to measure N-methyl-D-aspartate receptor density. Since corticosterone depresses circulating testosterone levels, we also examined whether the effects of corticosterone are mediated by or interact with the effects of testosterone. In the intact animal, corticosterone increased messenger RNA levels for NR2A and NR2B but not NR1 subunits of the N-methyl-D-aspartate receptor in all regions of the hippocampus. Testosterone had no significant effect on messenger RNA levels of any of the subunits. The subunit composition determines the functional and pharmacological properties of the N-methyl-D-aspartate receptor. We used ifenprodil inhibition of [3H]dizocilpine maleate binding, which has been used to distinguish NR2A- from NR2B-containing receptors, to determine whether corticosterone altered the proportion of high- and low-affinity sites for ifenprodil in parallel with the changes in subunit messenger RNA levels. Corticosterone increased the density of [3H]dizocilpine maleate binding sites without changing the dissociation constant for [3H]dizocilpine maleate or the proportion of high- and low-affinity sites for ifenprodil. These data suggest that the effects of corticosterone on hippocampal function are mediated, in part, by parallel increases in NR2A and NR2B subunit levels and the number of receptor channel binding sites.

N-methyl-D-aspartate receptors expressed in a nonneuronal cell line mediate subunit-specific increases in free intracellular calcium

N-methyl-D-aspartate (NMDA) receptors can mediate cell death in neurons and in non-neuronal cells that express recombinant NMDA receptors. In neurons, increases in intracellular calcium correlate with NMDA receptor-mediated death, supporting a key role for loss of cellular calcium homeostasis in excitotoxic cell death. In the present study, free intracellular calcium concentrations were examined in response to activation of recombinant NMDA receptors expressed in human embryonic kidney 293 cells. Intracellular calcium was measured in transfected cell populations by cotransfection with the calcium-sensitive, bioluminescent protein aequorin and by single cell imaging with the fluorescent calcium indicator fluo-3. Agonist application to NR1/2A or NR1/2B-transfected cells elicited robust rises in intracellular calcium. NR1/2A responses were inhibited by the noncompetitive antagonists MK-801 and dextromethorphan and were dependent on extracellular calcium but not on intracellular calcium stores. In contrast, no detectable intracellular calcium responses were observed in NR1/2C-transfected cells. These findings indicate that NMDA receptors in the absence of other neuron-specific factors can mediate increases in intracellular calcium with subunit specificity and extracellular calcium dependence.

Anticonvulsant effect of reduced NMDA receptor expression in audiogenic DBA/2 mice

Pretreatment of DBA/2 mice (n = 14-15 per group) with an 18-mer antisense probe to the NMDA-receptor submit NR1 (2 x 1 micrograms, or 2 x 83 pmol, NR1 antisense probe intracerebroventricularly, -29 and -7 h before testing for seizure response) resulted in almost complete suppression of sound-induced clonic seizures. A saline-treated group gave a 100% seizures response, while the group treated with NR1 antisense probe gave a 7% seizure response to the sound stimulus. The group treated with NR1 nonsense-probe showed no anticonvulsant protection (93% seizure response). The anticonvulsant protection observed following NR1 antisense administration was of relatively short duration, with seizure response gradually returning to control levels 12 to 24 h following the termination of antisense administration. When NR1 receptor levels were assessed by receptor autoradiography ([3H]-MK 801 and -CGP 39653 binding) in the same groups of mice, significant (20%) reductions in NR1 levels were observed in the retrosplenial cortex and the overall cortex. The seizure-induced expression of c-fos and NGFI-A in thalamus, hypothalamus, inferior colliculus and medical geniculate seen in vehicle- and NR1 nonsense-treated mice was completely blocked by NR1 antisense pretreatment.

Regional and subunit specific changes in NMDA receptor mRNA and immunoreactivity in mouse brain following chronic ethanol ingestion

Chronic ethanol treatment of mice has been shown to result in increased binding of dizocilpine and glutamate to hippocampal NMDA receptors. These changes were suggested to reflect an increase in NMDA receptor number that may underlie certain signs of the ethanol withdrawal syndrome. However, there was no change in binding of a competitive NMDA receptor antagonist, or of ligand binding to the glycine co-agonist site on the receptor after chronic ethanol treatment. Differential changes in the binding of particular ligands at the NMDA receptor suggested the possibility that chronic ethanol ingestion might selectively affect the expression of particular NMDA receptor subunits. Our current work demonstrates that chronic ethanol ingestion by mice, which results in the generation of physical dependence, also produces increases in the NMDA receptor NR1 subunit protein in the hippocampus and cerebellum (approximately 50% and 95%, respectively), and produces increases in the NR2A subunit protein in the hippocampus and cortex (approximately 25% and 40%, respectively). However, the mRNA levels for these subunits were not increased in the respective brain areas by the same ethanol treatment. The changes in NMDA receptor subunit expression in discrete areas of the brain may contribute to the previously observed changes in ligand binding and, possibly, signs of ethanol withdrawal.

Interactions between ifenprodil and the NR2B subunit of the N-methyl-D-aspartate receptor

Ifenprodil is an atypical noncompetitive modulator of the N-methyl-D-aspartate (NMDA) receptor (NR) which demonstrates a 140-fold preference for NR2B over NR2A subunits, although the molecular basis for this subunit specificity is unknown. We have made chimeric receptors by fusing the murine forms of NR2A (epsilon 1) and NR2B (epsilon 2) to localize the high affinity determinants of ifenprodil inhibition on the 2B subunit. Binding experiments with 125I-MK-801 implicated the region between amino acids 198 and 356 of NR2B for high affinity ifenprodil interaction. Site-directed mutants at Arg-337 showed that this residue is absolutely required for high affinity ifenprodil inhibition. Polyamines also modulate the NMDA receptor with a preference for NR2B subunits, and the pharmacology of these agents overlaps with ifenprodil. Although the determinants of the polyamine enhancement of iodo-MK-801 binding also localize to the NH2 terminus of NR2B, the point mutants at Arg-337 form receptors that are polyamine-stimulated at wild type levels. In addition, polyamine stimulation depends on the expression of NR1 splice variants, whereas high affinity ifenprodil inhibition is independent of NR1 isoform expression. These studies provide evidence that ifenprodil and polyamines interact at discrete sites on the NR2B subunit.

Investigation of the involvement of the N-methyl-D-aspartate receptor macrocomplex in the development of spermine-induced CNS excitation in vivo

1. The involvement of the N-methyl-D-aspartate (NMDA) receptor macrocomplex in the development of spermine-induced CNS excitation in vivo was investigated. 2. Injection of 100 micrograms of spermine into the left lateral cerebral ventricle of female Laca mice (20-25 g) resulted in the development of two distinct phases of CNS excitatory effects which were quantified by a scoring system. 3. The first phase effects occurred within minutes of injection and generally lasted for about 1 h. Most mice showed scratching of the upper body, frequent face washing and some mice developed clonic convulsions. By about 2 h after injection, the second phase of effects began to develop in the form of body tremor which worsened with time and culminated in fatal tonic convulsions, generally within 8 h of injection. 4. Pretreatment of the mice with dizocilpine (0.3 mg kg-1, i.p.) resulted in antagonism of the first phase of spermine-induced effects, but a higher dose (0.3 mg kg-1, (x2), i.p.) was necessary to inhibit the second phase effects. 5. Whereas the glutamate antagonist, 3-((R)-2-carboxypiperazin-4-yl) propyl-1-phosphonic acid (D-CPP) (10, 20 mg kg-1, i.p.), the glycine antagonist 7-chlorokynurenate (10, 30, 50 nmol, i.c.v.), or the polyamine antagonist ifenprodil (30, 60 mg kg-1, i.p.) antagonized the first phase of effects produced by spermine, these agents given as monotherapy, were ineffective against the development of the second phase of effects. 6. Co-administration of ifenprodil with either D-CPP or 7-chlorokynurenate resulted in a dose-dependent antagonism of the development of the second phase of spermine-induced effects. 7. It is concluded that the development of the two temporally distinct phases of spermine-induced effects may be mediated by pharmacologically distinct mechanisms, although the results suggest that the NMDA receptor macrocomplex may be involved in both phases of effects. Furthermore, a moderate dose of D-CPP or 7-chlorokynurenate appears to enhance the inhibitory potential of ifenprodil in vivo.

Characterisation of a functional polyamine site on rat mast cells: association with a NMDA receptor macrocomplex

Polyamines can modulate activation of N-methyl-D-aspartate (NMDA) receptors by binding to a specific polyamine site associated with a NMDA receptor macrocomplex. Polyamines induce histamine release from mast cells, although the mechanism had not been defined. We have examined whether spermine, a natural polyamine, and compound 48/80, regarded as a synthetic polyamine, activate mast cells by a polyamine site associated with a NMDA receptor macrocomplex. Spermine induced secretion of histamine from rat peritoneal mast cells and rat brain mast cells in a concentration-dependent manner. Rat peritoneal mast cells were used as a model system to explore the effects of NMDA antagonists on polyamine-induced histamine release. Ifenprodil, MK801 and arcaine inhibited histamine secretion from mast cells exposed to polyamines; the percentage inhibition was greater against spermine than compound 48/80. These data support the proposal that spermine (and possibly compound 48/80) induce histamine release from mast cells by interacting with a specific polyamine site on a NMDA receptor complex.