Receptor-associated Glycine Site Research Articles

D-Cycloserine reverses scopolamine-induced object and place memory deficits in a spontaneous recognition paradigm in rats

d-Cycloserine (DCS) is a partial agonist of the glutamatergic N-methyl-d-aspartate (NMDA) receptor-associated glycine site, and it prevents the amnesic effects of the muscarinic receptor antagonist scopolamine in various memory tests in rodents. In the present study, we tested the hypothesis that DCS has anti-amnesic effects in scopolamine-induced deficits using spontaneous object recognition and place recognition tests. In both tests, scopolamine (0.5 mg/kg, i.p.) was systemically administered 60 min prior to testing, while DCS (7.5, 15, 30 mg/kg, i.p.) was administered 30 min before testing, which consisted of a sample phase (5 min), a delay interval (15 min) and a test phase (2 min). DCS treatment reversed scopolamine-induced deficits in discriminatory behavior during the test phase. However, DCS did not affect decreased object exploration itself or increased thigmotaxis in the open-field arena induced by scopolamine. These results support our hypothesis and suggest differential contributions of glutamatergic-cholinergic system interactions to recognition memory and non-mnemonic exploratory behaviors.

D-Cycloserine enhances spatial memory in spontaneous place recognition in rats

The effects of d-cycloserine (DCS), an exogenous partial agonist of N-methyl-d-aspartate (NMDA) receptor-associated glycine site, on spatial memory were investigated using spontaneous place recognition (SPR) test. In SPR test, rats were allowed to explore an arena where two identical objects were presented for 5min (sample phase), and after a delay of 24h they were placed again in the same arena but one of the two objects was moved to a novel place (test phase, 4min). Systemic administration of DCS before the sample phase, immediately after the sample phase, and before the test phase caused rats’ significant preference for the object in a novel place in the test phase, although in this condition rats without DCS treatment did not show any preference. DCS affected neither total object exploration time nor locomotor activity in the arena during testing. Results suggest the possibility that DCS can facilitate various processes of spatial memory including encoding, consolidation and retrieval, and that NMDA receptors play an important role in these memory processes.

D-Cycloserine in prelimbic cortex enhances relearning of an odor-reward associative task

We investigated the effects of bilateral infusions in the prelimbic cortex of d-cycloserine (DCS), a partial agonist of the NMDA receptor-associated glycine site. Wistar rats underwent a training session (acquisition, three trials) and a 24-h test (two retention and two relearning trials) of a rapidly learned olfactory discrimination task. Rats infused with DCS (10 μg/site) prior to training exhibited a significant enhancement of performance in such odor-reward task, especially in relearning.

3H](+)MK801 Radioligand Binding Assay at the N‐Methyl‐D‐Aspartate Receptor

The N-methyl-D-aspartate subtype of glutamate receptor is unusual in that it requires two endogenous agonists for activation. Thus, in addition to glutamate, the amino acid glycine (or possibly D-serine) is an essential co-agonist. This unit presents a radioligand binding protocol that detects ligand activity at the NMDA receptor-associated glycine site. This is a convenient approach that exploits the ability of NMDA receptor modulators to alter the kinetics of ligands that bind to the channel-blocking site of the NMDA receptor. This protocol takes advantage of one of the most potent and specific ligands that bind to this receptor, in this case [(3)H](+)MK801. Importantly, this assay can detect and differentiate agonists and antagonists that bind to the glycine site. A protocol for the measurement of glycine site activity with [(3)H](+)MK801 binding is provided, along with support protocols that provide information to aid in the design of assays of agonists and antagonists of the glycine site, and data analysis.

Effect of 5-fluoroindole-2-carboxylic acid (an antagonist of the NMDA receptor-associated glycine site) on the anticonvulsive activity of conventional antiepileptic drugs.

5-Fluoroindole-2-carboxylic acid, an antagonist of the glycine site within the NMDA receptor complex, administered intraperitoneally in doses of 150 and 200 mg/kg, 120 min before electroconvulsions, significantly raised the convulsive threshold from 6.8 to 7.9 and 8.3 mA, respectively. At lower doses, it did not influence the threshold. However, lethality was observed 24h after administration of the threshold-elevating doses of this glycine site antagonist. 5-Fluoroindole-2-carboxylic acid (100 mg/kg), applied together with carbamazepine, valproate or phenobarbital, significantly reduced their ED50 values against maximal electroshock - from 13.9 to 7.5 mg/kg, from 291 to 242 mg/kg, and from 18.6 to 11.1 mg/kg, respectively. At the dose of 50 mg/kg, it also potentiated the protective activity of carbamazepine. However, 5-fluoroindole-2-carboxylic acid, up to 100 mg/kg, did not affect the anti-convulsive activity of diphenylhydantoin. When applied at doses equal to their ED50 values against maximal electroshock-induced convulsions, carbamazepine (13.9 mg/kg), phenobarbital (18.6 mg/kg) and valproate (291 mg/kg) did not affect the motor performance of mice in the chimney test. 5-Fluoroindole-2-carboxylic acid (100 mg/kg produced a significant motor impairment, at 50 mg/kg it did not affect the motor performance. The combined treatment of 5-fluoroindole-2-carboxylic acid (100 mg/kg) with carbamazepine, phenobarbital or valproate, providing a 50% protection against maximal electroshock, resulted in motor impairment. Only the combination of 5-fluoroindole-2-carboxylic acid (50 mg/kg) with carbamazepine (8.6 mg/kg) did not significantly influence this parameter. Almost all of the antiepileptic drugs studied, when administered at doses equal to their ED50 values against maximal electroshock, did not influence retention in the passive avoidance task, which is a measure of long-term memory. Only valproate (291 mg/kg) worsened long-term memory. The combined treatment of 5-fluoroindole-2-carboxylic acid (100 mg/kg) with carbamazepine or phenobarbital, providing a 50% protection against maximal electroshock, did not affect the retention. The combination of 5-fluoroindole-2-carboxylic acid (100 mg/kg) with valproate (242 mg/kg) caused a significant impairment of long-term memory and mortality of 50% of animals 24h following the administration. The results suggest that the blockade of the strychnine-insensitive glycine site may lead to an enhancement of the protective activity of some conventional antiepileptic drugs, which is associated with pronounced side-effects and lethality in some cases.

Tetrahydroaminoacridine, a cholinesterase inhibitor, and D-cycloserine, a partial NMDA receptor-associated glycine site agonist, enhances acquisition of spatial navigation.

The present study examines the efficacy of single and combined treatments with an antiocholinesterase, tetrahydroaminoacridine (THA, i.p.), and a glycine-B site partial agonist, D-cycloserine (DCS, i.p.) to alleviate water maze (WM) spatial navigation defect induced by medial septal (MS) lesion. THA 3 and DCS at 3 or 10 mg/kg improved acquisition of the WM test, but only DCS improved spatial bias. These drugs had no effect on consolidation. A combination of THA 3 and DCS 10 mg/kg enhanced WM acquisition more effectively than either of the treatments on their own. This suggests that combined modulation of acetylcholine and NMDA mechanisms may have greater therapeutic effect to stimulate cognitive dysfunctions.

Intracerebroventricular Infusion of the NMDA Receptor-Associated Glycine Site Antagonist 7-Chlorokynurenate Impairs Water Maze Performance but Fails to Block Hippocampal Long-Term Potentiation in Vivo

Most previous studies investigating the relationship betweenN-methyl-d-aspartate receptor-dependent synaptic plasticity and learning have employed drugs that either compete with glutamate for access to the primary agonist binding site (e.g.,d-2-amino-5-phosphopentanoic acid) or block the associated ion channel (e.g., dizocilpine). This study targeted the glycine receptor site located on the NMDA receptor complex. Chronic intracerebroventricular infusion of the glycine site antagonist 7-chlorokynurenate (7CK; 75 mM, 0.5 μl/h, icv, for up to 14 days) impaired performance of male Lister hooded rats during acquisition of a spatial reference memory task in the water maze. In addition, however, these animals showed sensorimotor deficits, including a prolonged righting reflex, ataxia, and difficulty in staying on the escape platform. On completion of behavioral testing, the rats were anesthetized with urethane and an attempt was made to induce LTP in the hippocampus ipsilateral to the infusion cannula. Both control and 7CK-infused animals displayed equivalent long-term potentiation (LTP) 60 min posttetanus. A novel analytical technique for assaying drug tissue levels involving high-performance liquid chromotography with fluorescence detection revealed that tissue levels of 7CK in hippocampus were extremely low and unlikely to be sufficient to affect LTP, as observed. These findings neither support nor compromise the LTP/learning hypothesis, but they illustrate some of the problems of using drugs to elucidate the neurobiological mechanisms of learning and memory and the importance of a within-subjects design incorporating behavioral, physiological, and biochemical measures.

Glycine and D-serine decrease MK-801-induced hyperactivity in mice.

It is well known that the un-competitive N-methyl-D-aspartate (NMDA) receptor antagonist phencyclidine can induce a syndrome in humans that mimics both positive and negative symptoms of schizophrenia. In the light of this observation, it has been hypothesised that schizophrenia might be due to a hypofunction of central glutamate systems. A glycine agonist, by strengthening glutamatergic transmission, has been suggested to be useful as treatment. A crucial issue is the uncertainty regarding the degree of saturation of the glycine site associated with the NMDA receptor. The purpose of this study was to investigate if it is possible to strengthen NMDA receptor-mediated neurotransmission by modulating the associated glycine site. The effects of systemic and intraventricular administration of glycine. D-serine and L-serine on the hyperactivity induced in mice by the uncompetitive NMDA receptor antagonist MK-801 were tested. Systemically administered glycine and D-serine were found to decrease MK-801-induced hyperactivity. Intraventricularly administered D-serine in doses of 50 or 100 micrograms/side was found to decrease MK-801-induced hyperactivity during the second half hour of registration; L-serine given in the same doses did not affect the MK-801-induced hyperactivity during this period. These data may suggest that the NMDA receptor-associated glycine site is not saturated in vivo.

D-Serine as a Neuromodulator: Regional and Developmental Localizations in Rat Brain Glia Resemble NMDA Receptors

D-Serine is localized in mammalian brain to a discrete population of glial cells near NMDA receptors, suggesting that D-serine is an endogenous agonist of the receptor-associated glycine site. To explore this possibility, we have compared the immunohistochemical localizations of D-serine, glycine, and NMDA receptors in rat brain. In the telencephalon, D-serine is concentrated in protoplasmic astrocytes, which are abundant in neuropil in close vicinity to NMDA receptor 2A/B subunits. Ultrastructural examination of the CA1 region of hippocampus reveals D-serine in the cytosolic matrix of astrocytes that ensheath neurons and blood vessels, whereas NR2A/B is concentrated in dendritic spines. By contrast, glycine immunoreactivity in telencephalon is the lowest in brain. During postnatal week 2, D-serine levels in cerebellum are comparable to those in adult cerebral cortex but fall to undetectable levels by day 26. During week 2, we observe parallel ontogeny of D-serine in Bergmann glia and NR2A/B in Purkinje cells, suggesting a role for astrocytic D-serine in NMDA receptor-mediated synaptogenesis. D-Serine in the radial processes of Bergmann glia is also well positioned to regulate NMDA receptor-dependent granule cell migration. In the inner granule layer, D-serine is found transiently in protoplasmic astrocytes surrounding glomeruli, where it could regulate development of the mossy fiber/granule cell synapse. D-Serine seems to be the endogenous ligand of glycine sites in the telencephalon and developing cerebellum, whereas glycine predominates in the adult cerebellum, olfactory bulb, and hindbrain.

Reversion of β25–35-amyloid peptide-induced amnesia by NMDA receptor-associated glycine site agonists

The effects of d-cycloserine (DCS), a N-methyl-d-aspartate receptor-associated glycine site agonist, and milacemide (MIL), a glycine prodrug, were examined on learning impairments induced by administration of β25–35-amyloid peptide (3 nmol i.c.v.). Mice were examined for spontaneous alternation and step-down passive avoidance, 7 and 14 days after β25–35, respectively. The β25–35-induced deficits were reversed by DCS, 1–30 mg/kg i.p., or MIL, 3–100 mg/kg i.p., each drug being ineffective on control mice behaviours. These observations strengthen the therapeutic potential of glycine site agonists against the memory impairments induced by β-amyloid peptides.

Reversion of β 25–35-amyloid peptide-induced amnesia by NMDA receptor-associated glycine site agonists

The effects of d-cycloserine (DCS), a N-methyl- d-aspartate receptor-associated glycine site agonist, and milacemide (MIL), a glycine prodrug, were examined on learning impairments induced by administration of β 25–35-amyloid peptide (3 nmol i.c.v.). Mice were examined for spontaneous alternation and step-down passive avoidance, 7 and 14 days after β 25–35, respectively. The β 25–35-induced deficits were reversed by DCS, 1–30 mg/kg i.p., or MIL, 3–100 mg/kg i.p., each drug being ineffective on control mice behaviours. These observations strengthen the therapeutic potential of glycine site agonists against the memory impairments induced by β-amyloid peptides.

Effects of NMDA receptor modulation on hippocampal type 2 theta activity in rats

1. 1. The present study was designed to investigate whether pharmacological modulation of N-methyl- d-aspartate (NMDA) receptor function could modify hippocampal type 2 theta activity in the dentate gyrus of rats. 2. 2. The effects of pre-recording administration of d-cycloserine (DCS: 1.0, 3.0 and 9.0 mg/kg, i.p.), a partial agonist at the NMDA receptor associated glycine site, and MK-801 (0.1 mg/kg, i.p.), a noncompetitive NMDA receptor antagonist, were examined in freely moving rats. 3. 3. Using adult Wistar rats, which had recording electrodes implanted unilaterally into the hilus of dentate gyrus, we recorded five 4 sec epochs of awake-immobility-related hippocampal EEG activity bands (1–20 Hz) 40 min after d-cycloserine and 2 hr after administration of MK-801. 4. 4. In the off-line analysis, the spectral power and the frequency at the maximal theta power were calculated. 5. 5. d-cycloserine (1.0–9.0 mg/kg) did not affect the frequency at the maximal theta power. However, the dose of 3.0 mg/kg, though not the 1.0 or 9.0 mg/kg doses, significantly increased the spectral power of the hippocampal immobility-related EEG activity. 6. 6. In line with the previous findings, 0.1 mg/kg MK-801 decreased both the frequency at the maximal theta power as well as the spectral power of hippocampal type 2 EEG activity. 7. 7. The present data show a clear relationship between NMDA receptors and hippocampal type 2 theta activity and suggest that the pharmacological modulation of the receptor function, using appropriate doses of glycine binding site agonist, d-cycloserine, may be a possible means to positively modulate the immobility-related hippocampal EEG activity.

Reversal of parkinsonian symptoms by intrastriatal and systemic manipulations of excitatory amino acid and dopamine transmission in the bilateral 6-OHDA lesioned marmoset

We have investigated the potential of alleviating parkinsonian symptoms by manipulating excitatory amino acid (EAA) transmission, by several different pharmacological means, in a novel primate model of parkinsonism. The model is based on a two-stage bilateral 6-hydroxydopamine lesion procedure in marmosets, which produces a stable but marked parkinsonian condition. Parkinsonian symptoms were reversed in a dose-dependent manner by systemic administration of levodopa and intrastriatal injections of apomorphine administered into either the caudate nucleus or the putamen. (R)-HA-966, a partial agonist for the NMDA receptor associated glycine site, also alleviated parkinsonian symptoms when injected intrastriatally but not when injected systemically. Systemic injection of enadoline, a kappa opiate which blocks release of EAAs, reduced parkinsonian symptoms when injected systemically, though it did not restore completely normal motor behaviour. In contrast, ifenprodil, an antagonist for the NMDA receptor-associated polyamine modulatory site, when injected systemically at an optimal dose, resulted in apparently normal motor behaviour. These data suggest that attenuation of EAA transmission could be used to treat parkinsonism.

Neurochemical and behavioural investigations of the NMDA receptor-associated glycine site in the rat striatum: functional implications for treatment of parkinsonian symptoms.

The glutamatergic cortico-striatal and subthalamo-entopeduncular pathways are both overactive in parkinsonism. Previous behavioural investigations have shown that intra-entopeduncular injection of either NMDA-site or glycine-site antagonists results in alleviation of parkinsonian symptoms, although injection of the former is associated with the appearance of anaesthetic-like side effects. These behavioural differences may be mediated by action on different NMDA receptor subtypes. Recent neurochemical and molecular pharmacological studies have indicated the existence of NMDA receptor subtypes which display differential modulation by glycine. In the present study, three potential modes of NMDA antagonism were differentiated in vitro by effects on [3H]-glycine binding to striatal sections. Specific [3H]-glycine binding was totally displaced by the glycine partial agonist (R)-HA-966; the NMDA-site antagonist D-CPP had no effect; and the NMDA-site antagonist D-AP5 displaced [3H]-glycine binding in a subpopulation of glycine sites. The anti-parkinsonian effects of (R)-HA-966, D-CPP and D-AP5 were assessed by intra-striatal injection in reserpine-treated rats and 6-OHDA-lesioned rats. Injection of (R)-HA-966 and D-CPP resulted in alleviation of parkinsonian akinesia, although the latter elicited anaesthetic-like side effects; D-AP5 was ineffective as an anti-parkinsonian agent. (R)-HA-966 was also effective as an anti-parkinsonian agent when administered systemically in the reserpine-treated rat. These data suggest that different classes of NMDA antagonist mediate different behavioural responses within the parkinsonian striatum. The behavioural response produced may depend on the exact nature of the conformational change induced by the antagonist and the location of the subtype most sensitive to that class of compound. Selection of a specific mode of NMDA receptor antagonism or targeting of striatal NMDA receptor subtypes may form the basis of a novel therapeutic approach to Parkinson's disease.

The NMDA receptor complex modulates clonidine-induced increases in growth hormone levels in rats

Intraperitoneal administration of clonidine (50 μg/kg) produced increases in growth hormone levels in male Wistar rats. Pretreatment with NMDA receptor antagonists including (±)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP/NMDA site), ifenprodril (polyamine site), and dizocilpine maleate (MK-801) or phencyclidine (PCP) (channel blockers) did not have any significant effect on clonidine-induced increases in growth hormone levels. In contrast, pretreatment with 5,7-dichlorokynurenic acid and 6,7-dinitroquinoxaline-2,3-dione (DNQX) (NMDA receptor-associated glycine site antagonists) significantly attenuated clonidine-induced increases in growth hormone levels. Attenuation of clonidine's effect on growth hormone levels by NMDA receptor-associated glycine site antagonists appears most likely due to an interaction between their effects on the NMDA receptor complex with growth hormone releasing factor.

Anticonvulsant activity of antagonists and partial agonists for the NMDA receptor-associated glycine site in the kindling model of epilepsy

Several previous studies have shown that competitive and uncompetitive N-methyl- d-aspartate (NMDA) receptor antagonists are ineffective as anticonvulsants in fully amygdala kindled rats, i.e. a widely-used model of complex-partial seizures (limbic epilepsy). In the present experiments, different categories of ligands for the NMDA receptor-associated glycine modulatory site were evaluated in the kindling model in rats. For this purpose, the ‘silent’ glycine antagonist 7-chlorokynurenic acid (7-CKA), the low efficacy glycine partial agonist (+)-HA-966( R(+)-3-amino-1-hydroxypyrrolid-2-one), and the high efficacy glycine partial agonist d-cycloserine (D-CS) were used. In view of the poor brain penetration of some of these compounds after systemic administration, all drugs were injected bilaterally into the lateral ventricles of fully kindled rat. Anticonvulsant activity was evaluated by determination of the threshold for induction of focal afterdischarges in the amygdala (ADT) and by monitoring seizure parameters (seizure severity, seizure duration, afterdischarge duration) at ADT current. In addition to anticonvulsant activity, the behavioural adverse effects of test drugs were determined. All three drugs increased the focal seizure threshold, although at different potencies. In terms of injected dosages and percentage ADT increase, the most potent compound was 7-CKA, followed by (+)-HA-966 and D-CS. None of the drugs induced behavioural adverse effects or motor impairment (measured in the rotarod test) at anticonvulsant doses. However, D-CS produced proconvulsant effects at higher doses. The data demonstrate that pharmacological manipulation of the glycine modulatory site of the NMDA receptor is an effective means of increasing seizure threshold in amygdala-kindled rats. In this respect, glycine antagonists and partial agonists are clearly superior to competitive or uncompetitive NMDA receptor antagonists. Systemically active glycine antagonists or partial agonists may thus provide a novel strategy for treatment of limbic seizures.

D-cycloserine, a novel cognitive enhancer, improves spatial memory in aged rats

D-cycloserine, a partial agonist of the NMDA receptor-associated glycine site, can enhance cognition. The present experiment examines the behavioral effects of D-cycloserine on cognitive deficits in male Fischer-344 rats, 24 months old. Rats 24 months old ( n = 42) received either vehicle or one of 3 doses of D-cycloserine prior to testing. Young rats, 4 months old ( n = 13), received vehicle prior to testing. Place discrimination and repeated acquisition were tested in the water maze and a variety of sensorimotor tasks were given. Aging impaired performance in all tasks. D-cycloserine improved performance in place discrimination and repeated acquisition. No doses affected sensorimotor function. These results support the hypothesis that D-cycloserine has cognition enhancing properties and that it may be useful in treating disorders involving cognitive impairment.

Anxiolytic-like effects of [formula omitted] glycine receptor ligands in the rat potentiated startle test

Competitive and non-competitive NMDA receptor complex antagonists have been shown to be active in various models of anxiolytic activity. This study examined the effects of ligands at the NMDA receptor-associated glycine site and two competitive NMDA receptor antagonists on the fear-potentiated startle response model for anxiolytic activity. The results show that the NMDA receptors antagonist, 2-amino-7-phosphonoheptanoate (30 mg/kg) and 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate (3 mg/kg), the glycine receptor antagonist 7-chlorokynurenate (100 mg/kg), and the glycine receptor partial agonists 3-amino-1-hydroxy-2-pyrrolidinone ((+)-HA-966) (30 mg/kg), 1-aminocyclopropane carboxylate (200–500 mg/kg) and D-cycloserine (30–300 mg/kg) blocked the potentiated startle effect. These results extend the findings of earlier studies showing anxiolytic-like effects of NMDA antagonists and glycine receptor ligands by demonstrating their effectiveness in the rat potentiated startle paradigm. These results also demonstrate the anxiolytic potential of D-cycloserine.

Pharmacological characteristics of cyclic homologues of glycine at the [formula omitted] receptor-associated glycine site

In Xenopus oocytes, injected with mRNA from the brain of the rat, the characteristics of the cyclic homologues of glycine, ACPC, ACBC and cycloleucine have been examined. 1-Aminocyclopropane-1-carboxylate was a potent agonist at the NMDA-associated glycine site (EC 50 = 0.09 ±0.02 μM) and exhibited characteristics consistent with a partial agonist. 1-Aminocyclobutane-1-carboxylate, in addition to its previously described antagonist properties, was found to possess agonist properties of low efficacy. Furthermore, ACBC did not completely block NMDA/glycine-induced currents, which is also consistent with partial agonist characteristics. In addition, small concentrations of glycine (<3 μM) did not alter the potency of ACBC, possibly suggesting that it is not simply a competitive glycine antagonist. Cycloleucine was a very weak glycine antagonist. These results suggest that as the size of the ring of cyclic homologues of glycine increases, there is a resulting transition from agonist to mixed agonist/antagonist to antagonist properties.

The glycine site modulates NMDA-mediated changes of intracellular free calcium in cultures of hippocampal neurons

Recent evidence indicates that the N- methyl- d-aspartate (NMDA) receptor-channel complex contains a glycine subunit whose activation may be necessary for channel operation. It has been previously shown that stimulation of the NMDA receptor leads to an increase in intracellular ionic Ca 2+ ([Ca 2+] i); therefore, we examined the role of the NMDA receptor-associated glycine site in modulating [Ca 2+] i using the fluorescent dye Fura II in hippocampal neuron cultures. A 3-s pulse of 200 μM NMDA resulted in a mean [Ca 2+] i increase of 363 nM above the average resting concentration of 122 nM. Perfusion of the glycine site antagonist 7-chlorokynurenate (Cl-Kyn) essentially eliminated the NMDA-induced alteration in [Ca 2+] i. Either 40 μM glycine or 50 μM d-serine completely reversed the effect of Cl-Kyn, indicating that the drug was acting at the glycine site. The NMDA receptor antagonists 2-amino-5-phosphonovalerate (AP5) and ketamine, which bind to the glutamate recognition site and the ion channel, respectively, also blocked the NMDA-mediated [Ca 2+] i response; however, glycine or d-serine did not reverse this effect. These data show that the glycine binding site coupled to the NMDA receptor modulates the NMDA-mediated increase in [Ca 2+] i. Antagonists of the glycine site provide a new tool to investigate and possibly control neuroplasticity and neurotoxicity related to the NMDA receptor complex.