Uncompetitive Antagonists of the N‐Methyl‐D‐aspartate (NMDA) Receptors Alter the mRNA Expression of Proteins Associated with the NMDA Receptor Complex

Abstract: N‐methyl‐D‐aspartate (NMDA) receptor function appears to be under complex control during physiological and pharmacological states. We have investigated the effects of acute administration of uncompetitive NMDA receptor antagonists on mRNA levels of NMDA receptor subunits and on molecules known to cluster or phosphorylate the receptor utilizing in situ hybridization on rat brain sections. A high dose (5 mg/kg; 4 hr) of dizocilpine (MK‐801) decreased mRNA levels of NMDA receptor subunits NR2C and NR2B in the entorhinal and parietal cortices, respectively. MK‐801 increased mRNA levels of synapse‐associated protein‐90/postsynaptic density‐95 (SAP90/PSD‐95) and a γ‐isoform of protein kinase C (PKCγ) in cortical regions. Synapse‐associated protein‐97 (SAP97) mRNA levels were increased in the entorhinal cortex layer III after MK‐801 or after relatively high doses of other uncompetitive NMDA receptor antagonists: phencyclidine (15 mg/kg; 6 hr) and memantine (50 mg/kg; 6 hr). Memantine also increased SAP97 mRNA expression in other cortical regions, but this effect was not observed with MK‐801 or phencyclidine. NMDA receptor uncompetitive antagonists alter the expression of multiple receptor components and such events may ultimately play a role in adaptation or toxic responses.     

Increased expression of neuronal Src and tyrosine phosphorylation of NMDA receptors in rat brain after systemic treatment with MK-801

We have observed that systemic treatment with the uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 increases Src expression and NMDA receptor phosphorylation in rat brain. A partial cDNA encoding rat neuronal Src was isolated and its sequence was used to design specific oligonucleotide probes. Systemically administered MK-801 (5 mg/kg for 4 h) increased by 28+/-4% mRNA expression of neuronal Src in the superficial layers of the parietal cortex. This effect was observed at doses as low as 0.2 mg/kg. A similar, although more modest, induction was observed 6 h after phencyclidine (15 mg/kg) administration, but not after high doses of memantine and ketamine. The MK-801-induced effect was not blocked by pretreatment with clozapine. Consistent with the increase in mRNA levels, cortical Src protein was increased to 186 +/- 24% of control 24 h after MK-801 treatment. Total cellular Src activity was also increased in parietal cortex homogenates 4 h after MK-801 (5 mg/kg). Moreover, MK-801 treatment (0.5 mg/kg and 5 mg/kg for 4 h) increased tyrosine phosphorylation, but not protein levels, of the NMDA receptor subunit NR2A. These results provide evidence for a contribution of Src and tyrosine phosphorylation of NMDA receptors in the pharmacological actions of uncompetitive NMDA receptor antagonists.     

Atypical antipsychotics and a Src kinase inhibitor (PP1) prevent cortical injury produced by the psychomimetic, noncompetitive NMDA receptor antagonist MK-801.

Noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists such as phencyclidine, ketamine, and MK-801 produce schizophrenia-like psychosis in humans. The same NMDA antagonists injure retrosplenial cortical neurons in adult rats. We examined the effects of atypical antipsychotics and an inhibitor of nonreceptor tyrosine kinase pp60 (Src) on the cortical injury produced by MK-801. An atypical antipsychotic (either clozapine, ziprasidone, olanzapine, quetiapine, or risperidone) or vehicle was administered to adult female Sprague-Dawley rats. PP1 (Src inhibitor), PP3 (nonfunctional analog of PP1) or vehicle (DMSO) was administered to another group of animals. After pretreatment, animals were injected with MK-801, killed 24 h after the MK-801, and injury to retrosplenial cortex assessed by neuronal Hsp70 protein expression. All atypical antipsychotics examined significantly attenuated MK-801-induced cortical damage. PP1 protected compared to vehicle, whereas PP3 did not protect. The ED50s (decrease injury by 50%) were as follows: PP1 <0.1 mg/kg; olanzapine 0.8 mg/kg; risperdal 1 mg/kg; clozapine 3 mg/kg; ziprasidone 32 mg/kg; and quetiapine 45 mg/kg. The data show that the atypical antipsychotics tested as well as a Src kinase inhibitor prevent the injury produced by the psychomimetic MK-801, and the potency of the atypical antipsychotics for preventing cortical injury was roughly similar to the potency of these drugs for treating psychosis in patients.     

Effects of competitive and non-competitive NMDA receptor antagonists on dopamine output in the shell and core subdivisions of the nucleus accumbens

The effects of acute intravenous administration of the non-competitive NMDA receptor antagonists, phencyclidine (PCP), dizocilpine (MK-801; (+)-5-methyl-10,11-dihydroxy-5H-dibenzo(a,b)cyclohepten-5,10-imine), and the competitive NMDA receptor antagonist CGP 39551 (DL-(E)-2-amino-4-methyl-5-phosphono-3-pentanoic acid) on extracellular dopamine concentrations were analyzed in the shell and core subdivisions of the nucleus accumbens (NAC), associated with limbic and motor functions, respectively. Extracellular dopamine concentrations were assessed utilizing differential normal pulse voltammetry in chloral hydrate anesthetized, pargyline pretreated rats. Intravenous administration of PCP (0.5 mg/kg) or MK-801 (0.1 mg/kg) both significantly elevated extracellular dopamine levels in the NAC shell but not in the core. However, administration of relatively low doses of the competitive NMDA receptor antagonist CGP 39551 (2.5 mg/kg) failed to affect dopamine output in either region. However, when a higher dose (10 mg/kg) was administered a significant elevation in dopamine output was obtained in the shell compared to the core. Our data demonstrate that non-competitive NMDA receptor antagonists evoke an accumbal dopamine output that is selective to limbic cortical related NAC regions. This profile is shared also by competitive NMDA receptor antagonists when given in high, but not low doses. Our results are compatible with the reported elicitation of PCP-like behavioral effects by competitive NMDA receptor antagonists when administered in relatively high doses. Moreover, these findings suggest that differences in the regional accumbal dopamine output between competitive and non-competitive NMDA receptor antagonists may be essentially attributable to the relative degree of NMDA receptor antagonism achieved by the drugs. This experimental model may afford a biochemical means to assess the psychotomimetic liability of NMDA receptor antagonists, a side effect that may reduce their usefulness as neuroprotective agents.     

Competitive and noncompetitive NMDA antagonist effects in rats trained to discriminate lever-press counts

The glutamate activated N-methyl-D-aspartate (NMDA) receptor may play a role in short-term memory processing. Among the evidence for this is that NMDA antagonists can impair accuracy in fixed consecutive number (FCN) tasks. This study was designed to further characterize this effect by examining NMDA antagonists differing in their cellular mechanisms of action. Rats were trained to respond under an FCN operant schedule, which required eight presses on one lever (counting lever) before one press at an alternate lever (reinforcement lever) would produce food reinforcement. The effects of three noncompetitive [MK-801 (0.01-0.56 mg/kg); phencyclidine (0.3-3.0 mg/kg); memantine (1-10 mg/kg)] and two competitive [SDZ EAA 494 (0.3-3.0 mg/kg) and NPC 17742 (2.0-16 mg/kg)] NMDA antagonists were analyzed. MK-801 and phencyclidine decreased accuracy at doses not reducing response rates. Memantine, and both of the competitive antagonists, also reduced accuracy, but did so only at doses that markedly reduced response rates. These results suggest that both the affinity and the site bound on the NMDA glutamate receptor by antagonists can determine their effects on FCN performance. Subsequent studies investigated whether SCH 23390, a dopamine D1 receptor antagonist, and NMDA could modulate the effects by phencyclidine and SDZ EAA 494, respectively, on FCN performance.     

Excitatory actions of NMDA receptor antagonists in rat entorhinal cortex and cultured entorhinal cortical neurons.

We have characterized excitatory effects of non-competitive NMDA receptor antagonists MK-801, PCP, and ketamine in the rat entorhinal cortex and in cultured primary entorhinal cortical neurons using expression of immediate early gene c-fos as an indicator. NMDA receptor antagonists produced a strong and dose-dependent increase in c-fos mRNA and protein expression confined to neurons in the layer III of the caudal entorhinal cortex. Induction of c-fos mRNA is delayed and it is inhibited by antipsychotic drugs. Cultured entorhinal neurons are killed by high doses of MK-801 and PCP but c-fos expression is not induced in these neurons indicating that this in vitro model does not fully replicate the in vivo effects of PCP-like drugs in the entorhinal cortex. Excitatory effects of the NMDA receptor antagonists may be connected with the psychotropic side effects of these drugs and might become a useful model system to investigate neurobiology of psychosis.     

The mGlu2/3 receptor agonist LY379268 injected into cortex or thalamus decreases neuronal injury in retrosplenial cortex produced by NMDA receptor antagonist MK-801: possible implications for psychosis

The non-competitive NMDA receptor antagonists, including PCP (phencyclidine), ketamine, and MK-801 (dizocilpine) produce psychosis in humans and injure neurons in retrosplenial cortex in adult rodent brain. This study examined the effects of the metabotropic mGlu2/3 agonist LY379268 and antagonist LY341495 on cortical injury produced by systemic MK-801 (1 mg/kg i.p.) in adult female rats. Systemic injections of mGlu2/3 agonist LY379268, but not mGlu2/3 antagonist LY341495, decreased the injury in the retrosplenial cortex produced by systemic MK-801 as assessed by Hsp70 induction. Bilateral injections of LY379268, but not vehicle, into retrosplenial cortex or bilateral injections of LY379268 into anterior thalamus also decreased the injury in retrosplenial cortex produced by systemic MK-801. The data show that bilateral activation of mGlu2/3 glutamate receptors in cortex or anterior thalamus decreases the neuronal injury in retrosplenial cortex produced by systemic MK-801. Because antipsychotic medications decrease cortical injury produced by NMDA antagonists in rodents and decrease psychosis in humans, mGlu2/3 agonists that decrease cortical injury produced by NMDA antagonists in rodents might be evaluated for decreasing psychosis in people.     

Prepulse inhibition of the startle reflex in rats: effects of compounds acting at various sites on the NMDA receptor complex

Prepulse inhibition (PPI) of the startle reflex in rats is disrupted by N-methyl-D-aspartate (NMDA) receptor non-competitive antagonists (phencyclidine-like compounds). In order to explore more thoroughly the control exerted by NMDA receptors on PPI, we assessed the effects of i.p. administration, in Sprague-Dawley rats, of compounds acting as antagonists or agonists at the five binding sites of the NMDA receptor complex. The non-competitive NMDA receptor antagonists phencyclidine (1-6 mg/kg) and MK-801 (dizocilpine: 0.05-0.2 mg/kg) robustly and dose-dependently disrupted PPI. A similar effect was obtained with the competitive NMDA receptor antagonists CGS 19755 (1-20 mg/kg) and CPP (3-20 mg/kg), but not with the cation Mg2+ (100 and 200 mg/kg), the glycine/NMDA binding site antagonist L-701,324 (1-10 mg/kg), or the polyamine/NMDA binding site antagonist eliprodil (3-20 mg/kg). Potentiation of glutamatergic neurotransmission by NMDA (10-50 mg/kg), and the glycine/NMDA site partial agonist d-cycloserine (1-30 mg/kg) also failed to modify PPI, though d-cycloserine diminished PPI at higher doses (50-200 mg/kg). Co-administration of sub-threshold doses of CPP (3 mg/kg) and phencyclidine (2 mg/kg) resulted in an additive effect, disrupting PPI. In contrast, co-administration of L-701,324 (6 mg/kg) with phencyclidine (2 mg/kg), eliprodil (20 mg/kg), or CPP (3 mg/kg), did not disrupt PPI. These results demonstrate that PPI-disrupting effects can only be obtained with phencyclidine-like compounds and NMDA receptor competitive antagonists. Treatment with compounds that potentially augment glutamatergic tone were without effect. Finally, despite the permissive control of the glycine/NMDA binding site on glutamatergic neurotransmission, the glycine/NMDA binding site antagonist L-701,324 did not produce synergistic activity when combined with antagonists at the glutamate, polyamine/NMDA or phencyclidine-like compound binding sites.     

Effects of the Nitric Oxide Synthase Inhibitor -NAME on Recognition and Spatial Memory Deficits Produced by Different NMDA Receptor Antagonists in the Rat

There is consistent experimental evidence that noncompetitive antagonists of the N-methyl--aspartate (NMDA) receptor, such as ketamine, MK-801, and phencyclidine (PCP), impair cognition and produce psychotomimetic effects in rodents. Nitric oxide (NO) is considered as an intracellular messenger in the brain. The implication of NO in learning and memory is well documented. This study was designed to investigate the ability of the NO synthase inhibitor -NAME to antagonize recognition and spatial memory deficits produced by the NMDA receptor antagonists, MK-801 and ketamine, in the rat. -NAME (1–3 mg/kg) counteracted MK-801- (0.1 mg/kg) and ketamine (3 mg/kg)-induced performance impairments in the novel object recognition task. -NAME (10 mg/kg) attenuated ketamine (15 mg/kg)-induced spatial working memory and retention deficits in the radial water maze paradigm. -NAME, applied at 3 mg/kg, however, disrupted rodents'' performance in this spatial memory task. The present findings indicate (1) that -NAME is sensitive to glutamate hypofunction produced by other than PCP NMDA antagonists such as MK-801 and ketamine and (2) that -NAME alone differentially affects rodents'' spatial memory.     

Dopamine is not required for the hyperlocomotor response to NMDA receptor antagonists.

N-methyl-D-aspartate (NMDA) receptor antagonists can elicit symptoms in humans that resemble those seen in schizophrenic patients. Rodents manifest locomotor and stereotypic behaviors when treated with NMDA receptor antagonists such as phencyclidine (PCP) or dizocilpine maleate (MK-801); these behaviors are usually associated with an activated dopamine system. However, recent evidence suggests that increased glutamatergic transmission mediates the effects of these NMDA receptor antagonists. The role of dopamine in PCP- and MK-801-induced behavior (eg hyperlocomotion) remains unclear. We used dopamine-deficient (DD) mice in which tyrosine hydroxylase is selectively inactivated in dopaminergic neurons to determine whether dopamine is required for the locomotor and molecular effects of PCP and MK-801. DD mice showed a similar increase in locomotor activity and c-fos mRNA induction in the striatum in response to these NMDA receptor antagonists as control mice. Restoration of dopamine signaling in DD mice enhanced their locomotor response to PCP and MK-801. Administration of LY379268, a group II metabotropic glutamate receptor agonist that inhibits glutamate release, blocked PCP- and MK-801-induced hyperlocomotion in both DD and control mice. These results suggest that glutamate, rather than dopamine, is required for the locomotor and molecular effects of NMDA receptor antagonists, but that glutamate and dopamine can act cooperatively.     

Enhanced locomotor stimulation by NMDA receptor antagonists in alcohol-sensitive ANT rats

The ability of the antagonists for the N-methyl-D-aspartate (NMDA) type of glutamate receptor to modulate locomotor activity were compared in alcohol-sensitive (or alcohol-nontolerant, ANT) and alcohol-insensitive (or alcohol-tolerant, AT) rat lines. Both rat lines showed altered locomotor activity after acute injections of a competitive antagonist (LY235959), a glycine-site antagonist (L-701,324), or noncompetitive antagonists [MK-801, phencyclidine (PCP), and ketamine] of the NMDA receptor. MK-801 at 0.5 mg/kg caused a strong increase in horizontal activity in both rat lines, the effect being significantly greater in the ANT rats. There was a subpopulation among AT rats that was almost completely unresponsive to MK-801. This insensitivity to MK-801 correlated with the lack of c-fos induction in the retrosplenial and cingulate cortices. Fos immunoreactive cells in these brain regions after MK-801 treatment were more numerous in ANT than AT rats, although c-fos induction in the inferior olivary nucleus was similar in all animals after MK-801. The ANT rats showed greater locomotor stimulation also after ketamine and LY235959, while stimulation induced by PCP and depression induced by L-701,324 did not differ between the rat lines. The data suggest that altered NMDA receptor-mediated processes may correlate with differences in innate alcohol sensitivity in the ANT/AT rat model.     

Influence of long-lasting administration of neuroleptics on cortical NMDA receptors and phencyclidine-induced deficit in the sensorimotor gating in rats.

The aim of this study was to examine the role of cortical NMDA receptors in the antipsychotic action of neuroleptics. Haloperidol (1 mg/kg/day) and clozapine (30 mg/kg/day) were administered to rats in drinking water. Autoradiographic and saturation binding analyses showed that a 3-month treatment with both haloperidol and clozapine increased the density of NMDA receptors labelled with [3H]CGP 39653 (a competitive antagonist) in the parietal and insular cortices. Haloperidol additionally increased the binding of that ligand in the frontal cortex. None of those neuroleptics influenced the binding of [3H]MK-801, an uncompetitive antagonist of NMDA receptors, in the frontal, parietal or insular cortices. A 6-week and a 3-month treatment with haloperidol antagonized the deficit of prepulse inhibition induced by phencyclidine (5 mg/kg s.c.). In contrast, short-term (4-day) administration of that neuroleptic was ineffective. The present study suggests that the increased density of cortical NMDA receptors, induced by long-term neuroleptic administration, may overcome the deficit of sensorimotor gating induced by phencyclidine. However, contribution of such an effect to the antipsychotic activity needs to be established.     

The behavioural effects of MK-801: a comparison with antagonists acting non-competitively and competitively at the NMDA receptor

The selective non-competitive NMDA receptor antagonist, MK-801, potently blocked convulsions induced in the mouse by N-methyl-DL-aspartic acid (NMDLA) with an i.v. ED50 dose of 0.2 mg/kg. Similar doses of MK-801 were also effective in blocking seizures induced by pentylenetetrazol (PTZ), electroshock and by sound in audiogenic seizure-prone animals. Other less selective non-competitive NMDA receptor antagonists including phencyclidine (PCP), thienylcyclohexylpiperidine (TCP), (+)-N-allylnormetazocine [+)-NANM, (+)-SKF 10,047) and ketamine also blocked NMDLA-induced seizures with a rank order of potency of MK-801 greater than PCP greater than TCP = (+)-NANM greater than ketamine. The competitive NMDA receptor antagonist, 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) blocked NMDLA-induced seizures with an ED50 of 4.5 mg/kg, 22- and 560-fold more potently than the competitive antagonists, 2-DL-amino-7-phosphonoheptanoic acid (2-APH) and 2-DL-amino-5-phosphonovaleric acid (2-APV), respectively. MK-801 was the most potent of the non-competitive antagonists to induce a motor syndrome including head weaving, body rolling, increased locomotion and ataxia, characteristic of the behavioural response to PCP in the mouse. The syndrome was also present following injection of the competitive NMDA receptor antagonists, although they were generally less potent (probably a reflection of poor brain penetration) and less efficacious than the non-competitive antagonists. For all compounds except CPP, the anticonvulsant ED50 dose was close to the minimum effective dose to induce motor stimulation: CPP was 5- to 10-fold more potent as an anticonvulsant.(ABSTRACT TRUNCATED AT 250 WORDS)     

Morphine-induced behavioral sensitization increased the mRNA expression of NMDA receptor subunits in the rat amygdala

This study was designed to evaluate the effect of repeated morphine treatment on rat behavioral responses. In the genetic section, the mRNA expression of NMDA receptor subunits (NR1 and NR2A) was measured in certain areas of the male rat brain (striatum, prefrontal cortex, hippocampus, hypothalamus and amygdala). In the behavioral section, the effect of repeated morphine treatment on animal models such as locomotion, oral stereotypy, and state-dependent memory in a passive avoidance test was evaluated in the presence or absence of MK801 (NMDA receptor antagonist). Our results showed that chronic morphine treatment, followed by a 7-day (but not 24-hour) washout period, potentiated the effect of test doses of morphine, which is referred to as behavioral sensitization. Meanwhile, pretreatment of animals with MK801 (0.1 and 0.25 mg/kg), 30 min before a test dose of morphine (5 mg/kg), failed to attenuate the locomotion and oral stereotypy in the behavioral sensitization state. Interestingly, a higher dose of MK801 (0.25 mg/kg) decreased memory retrieval induced by morphine (2.5 mg/kg) in state-dependent memory. This effect may be due to the intrinsic motor enhancer property of higher doses of MK801, rather than the blockade of NMDA receptors. It can be concluded that MK801 does not affect morphine-induced behavioral sensitization in the expression phase. In the genetic section of the study, results of quantitative real-time RT-PCR clearly indicated that morphine sensitization increased the expression of NMDA receptor subunits mRNA in the amygdala (NR1 by 104% and NR2A by 85%), while the other areas of the brain were unaffected. Maenwhile, no change in the mRNA levels was observed in non-sensitized animals (chronic morphine treatment followed by a 24-hour washout period). In summary, the present study indicates that repeated morphine treatment followed by long-term (7-day washout) induces behavioral sensitization and causes a delayed increase in mRNA levels of NMDA receptor subunits in the rat amygdala. Meanwhile, it has previously been reported that the amygdala is involved in behavioral sensitization. Thus, it can be concluded that the increase in NMDA receptor expression is associated with morphine-induced behavioral sensitization.     

The dopamine D3 receptor mediates locomotor hyperactivity induced by NMDA receptor blockade

N-methyl-D-aspartate (NMDA)/glutamate receptor antagonists, like phencyclidine, generate schizophrenic-like symptoms in humans and behavioural abnormalities in animals, such as hyperactivity. We investigated the role of the dopamine D(3) receptor in locomotor hyperactivity produced in mice by dizocilpine (MK-801), another NMDA receptor antagonist, at a low dose (0.12 mg/kg). BP 897, a highly D(3) receptor-selective partial agonist, or nafadotride, a preferential D(3) receptor antagonist, both at low doses (1 mg/kg and lower), had no effects on spontaneous activity and completely inhibited MK-801-induced hyperactivity. Clozapine, an atypical antipsychotic, produced the same effect as BP 897 and nafadotride. Haloperidol, a typical antipsychotic, reduced both spontaneous activity and MK-801-induced hyperactivity. In D(3) receptor knockout mice, MK-801-induced hyperactivity was weaker than that observed in wild-type mice while BP 897 and nafadotride were inactive. On the contrary, the effects of clozapine and haloperidol, which target multiple receptors in addition to the D(3) receptor, were almost completely preserved in D(3) receptor knockout mice. Our results show that hyperactivity produced by a low dose of MK-801 is dependent upon D(3) receptor stimulation and constitutes the first simple response to assess the in vivo activity of D(3) receptor-selective drugs. In addition, since D(3) receptor antagonists and antipsychotics produced very similar effects, our results add to the growing evidence suggesting that D(3) receptor blockade might produce antipsychotic effects.     

Effects of the NMDA antagonists CPP and MK-801 on radial arm maze performance in rats

The dose- and time-dependent effects of N-methyl-D-aspartate receptor/channel antagonists on radial 8-arm maze performance were examined in rats. Both CPP (1.0-30 mg/kg), a competitive NMDA antagonist, and MK-801 (0.1-1.0 mg/kg), a noncompetitive NMDA antagonist, produced dose-dependent increases in the number of errors made to sample all 8 baited arms. The effective doses of both drugs produced maximal performance impairments 2 hr after IP injection, and no effects after 24 hr. In a second radial arm maze task where only 4 arms were baited, CPP (10 mg/kg) had a somewhat greater effect on the number of working memory errors than on reference memory errors. MK-801 (0.1, 0.33 mg/kg) had no effects on either this task or on a task involving a 1-hr delay between correct choices 4 and 5 on the 8 choice task. CPP (10 mg/kg), however, impaired performance on this latter task. These results indicate that doses of NMDA antagonists, sufficient to block hippocampal long-term potentiation, also disrupt radial arm maze performance.     

The NMDA receptor antagonist MK-801 increases morphine catalepsy and lethality.

Interactions between excitatory amino acids and opioids were examined by studying the ability of the noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 to affect morphine catalepsy and lethality. MK-801 (0.3 mg/kg) reduced the ED50 for morphine-induced catalepsy from approximately 30 mg/kg to less than 10 mg/kg, and reduced the LD50 for morphine from approximately 100 mg/kg to approximately 10 mg/kg. Lower doses of MK-801 did not affect morphine catalepsy or lethality. MK-801, in the absence of morphine, produced no catalepsy or lethality at doses up to 3.0 mg/kg; at 0.3 mg/kg MK-801 caused weaving, body rolling and ataxis, as previously described, while at 3.0 mg/kg animals appeared to lose muscle tone, becoming limp. These results demonstrate that blockade of NMDA receptors can dramatically potentiate morphine catalepsy and lethality, and suggest a potential dangerous interaction with opioids in the clinical use of NMDA receptor antagonists.     

NMDA receptor 2C subunit is selectively decreased by MK-801 in the entorhinal cortex

Administration of the non-competitive NMDA receptor antagonist MK-801 (5.methyl-10,11-dihydro-5H-dibenzo[1, d]cyclohepten-5-10-imine) produces paradoxicl neurotoxicity in limbic cortical regions which incluide the entorhinal cortex. The expression of NMDAR-2C but not -2A, -2B, or -2D subunits was significantly decreased in rat entorhinal cortex layer III following MK-801 administration. These results suggest an important role for the NMDAR-2C subunit in the response to MK-801-induced neurotoxicity in brain regions highly vulnerable to injury.     

The effects of non-competitive NMDA receptor antagonists on rats exposed to hyperbaric pressure

The high pressure neurological syndrome (HPNS) occurs when man or animals are exposed to hyperbaric pressure. Four non-competitive N-methyl-D-aspartate (NMDA) antagonists - MK-801, phencyclidine (PCP), SKF 10,047 and ketamine were tested in rats for effects on the HPNS. All drugs were injected i.p. prior to compression; ketamine was also infused i.v. Control rats received saline. Rats were exposed individually to increasing helium pressure (PO2 0.5 atmospheres absolute ATA). Three endpoints were used to assess HPNS: onset pressures for tremor, myoclonus and convulsions. Neither MK-801 (0.03 and 0.3 mg/kg) nor SKF 10,047 (50 mg/kg) had any effect on the onset pressures for tremor, myoclonus or convulsions, although the type of seizure was modified from the clonic/tonic seizure seen in controls to purely clonic. PCP (5 mg/kg) had no effect on the endpoints, but pressure enhanced the excitation and stereotypy seen at 1 ATA. Ketamine (100 mg/kg i.p.) did not affect tremor or myoclonus; ketamine infused i.v. at pressure only prevented tremor and myoclonus at 'anaesthetizing' concentrations. Our results show that these non-competitive NMDA antagonists had little effect on HPNS, in contrast to competitive NMDA antagonists, such as AP7, which are highly effective. Possible explanations for this lack of effect include (1) interactions with NMDA receptor channels are pressure dependent; (2) other actions of these antagonists override their effects on the NMDA receptor channel.     

In vivo labelling of the NMDA receptor channel complex by [3H]MK-801

An in vivo radioligand binding assay for the N-methyl-D-aspartate (NMDA) receptor channel complex in the mouse brain has been developed using the non-competitive NMDA receptor antagonist [3H]MK-801. In vivo binding of [3H]MK-801 was displaced by MK-801 (ED50 = 0.17 mg/kg i.p.), (-)-MK-801 (1.0 mg/kg), thienylcyclohexylpiperidine (1.8 mg/kg), etoxadrol (5.1 mg/kg) and (+)-SKF 10,047 (34.5 mg/kg). The potency of these drugs in this in vivo binding assay was highly correlated (r = 0.97) with their functional effects as antagonists of N-methyl-DL-aspartate-induced tonic convulsions.