Effects of excitant amino acids on acoustic responses of inferior colliculus neurons

Iontophoretic application of the excitant amino acids (EAAs), glutamate, aspartate and N-methyl-D-aspartate (NMDA) resulted in increased acoustically evoked and spontaneous firing of most neurons in the central nucleus of inferior colliculus (ICC). The excitatory effects of these EAAs were blocked by simultaneous application of EAA antagonists which selectively block the NMDA receptor subtype, 2-amino-5-phosphonovalerate or D-alpha-aminoadipate and to a lesser extent with non-selective EAA antagonists, such as glutamic acid diethylester. Application of NMDA receptor-selective EAA antagonists alone greatly reduced the firing of most ICC neurons examined, but non-selective EAA antagonists either increased or produced little change in firing of most ICC neurons examined. In this and previous studies cholinergic agonists were found to increase the firing of ICC neurons, but the cholinergic agonists were less effective in exciting ICC neurons than EAA agonists. Cholinergic antagonists in a previous study were considerably less effective in inhibiting the discharge of ICC neurons than were the EAA antagonists in the present study. These results, in conjunction with previous neurochemical and anatomical localization studies, support a possible role of an EAA as a candidate for afferent excitatory transmitter in neurons of the inferior colliculus.

Selective vulnerability of the brain: new insights into the pathophysiology of stroke

Stroke is a major cause of morbidity and mortality in the United States with 250,000 cases per year. Cerebral ischemia is the largest category of stroke with cardiac arrest, profound hypotension, and vascular occlusion the principal causes. Traditional approaches to the treatment of ischemic stroke focus on maintaining cardiac output, blood pressure, cerebral blood flow, and on preventing thrombosis. Recently, attention has been focused on developing new therapies that are directed toward abnormal biochemical events at excitatory synapses. Ischemia causes impairment of brain energy metabolism and the release of excessive amounts of glutamate into the extracellular space. This process secondarily excites neurons and further depletes energy stores. The excitotoxic hypothesis of brain injury proposes that glutamate is a principal cause of damage in ischemia. Three components of this hypothesis have been tested and largely proved in experimental studies in tissue culture and in animal models of stroke. First, elevated concentrations of glutamate cause excessive excitation at a subset of glutamate receptors, the N-methyl-D-aspartate (NMDA) receptor. Second, excitation at this receptor leads to excessive influx of sodium chloride and water which causes acute neuronal damage, and calcium which causes delayed and more permanent damage. Third, pharmacologic blockade at the NMDA receptor-ion channel complex prevents ischemic neuronal damage. Studies using specific pharmacologic compounds that block glutamate's action hold particular promise for treating stroke in humans, including competitive antagonists at the NMDA glutamate binding site (for example, 2-amino-5-phosphonovalerate, AP5), noncompetitive antagonists at the calcium channel (for example, MK-801, dextromethorphan, ketamine), and agents that might be directed at the glycine, zinc, and magnesium sites.

A phencyclidine recognition site is associated with N-methyl-D-aspartate inhibition of carbachol-stimulated phosphoinositide hydrolysis in rat cortical slices

The effects of N-methyl-D-aspartate (NMDA) on muscarinic receptor-stimulated phosphoinositide (PI) hydrolysis in rat cortical slices were studied. NMDA inhibits carbachol-stimulated PI hydrolysis with an IC50 of 9.8 +/- 1.4 microM and a maximal inhibition of 70% at 100 microM. The inhibitory effect of NMDA is not due to increased metabolism of accumulated inositol phosphates. NMDA inhibition of carbachol-stimulated PI hydrolysis was significantly reduced in the absence of extracellular calcium. Although the inhibitory effect of NMDA is observed in the presence of 1.18 mM Mg2+, the concentration-response curve is slightly shifted to the left (5-fold) in the absence of extracellular Mg2+. Antagonists of NMDA-evoked excitations were effective inhibitors of the NMDA modulation of PI hydrolysis, including the competitive antagonist 2-amino-5-phosphonopentanoic acid and the noncompetitive antagonist MK-801. The rank order of potencies of the antagonists were MK-801 greater than phencyclidine = (-)-cyclazocine greater than ketamine = etoxadrol greater than N-allylnormetazocine greater than 2-amino-5-phosphonopentanoic acid. (+)-MK-801 and (-)-cyclazocine were more potent inhibitors, by 4-5-fold, of the NMDA response than their respective isomers, whereas N-allylnormetazocine isomers were approximately equipotent antagonists. The activity of dexoxadrol against NMDA inhibition of carbachol-stimulated PI hydrolysis could not be determined because of its antimuscarinic effects. The rank order of potencies of antagonists, the stereoselectivity of the isomers of MK-801, cyclazocine, and N-allylnormetazocine, and Mg2+ sensitivity of the NMDA inhibitory response suggest that a phencyclidine binding similar to the one located in the cation channel gated by NMDA receptors is associated with the NMDA receptor that modulates muscarinic-stimulated PI hydrolysis.

Traumatic neuronal injury in vitro is attenuated by NMDA antagonists

Pure traumatic neuronal injury was modeled in dispersed neocortical cell cultures derived from fetal mice. A plastic stylet was used to tear the neuronal and glial cell layer; medium oxygen content, pH, and glucose remained unchanged. Adjacent to this local disruption, many neurons developed acute swelling and went on to degenerate over the next day, but glia were relatively spared. If the same mechanical insult was delivered in the presence of the N-methyl-D-aspartate (NMDA) antagonists dextrorphan or D-2-amino-5-phosphonovalerate, resultant neuronal degeneration was markedly reduced. The protective effect of these NMDA antagonists was concentration-dependent between 1 and 100 microM, with EC50 near 10 microM for both compounds. Present findings suggest that endogenous excitatory amino acids may participate significantly in the propagation of central neuronal cell loss in response to a purely mechanical insult.

Antitussive agents as N-methylaspartate antagonists: further studies

The relative potencies of ketamine and the morphinan derivatives dextrorphan, dextromethorphan, and levorphanol as antagonists of the excitatory actions of N-methylaspartate on rat spinal neurones in vivo were examined, both following their microelectrophoretic administration and, with the exception of levorphanol, after intravenous injection. Applied microelectrophoretically, dextrorphan was a more potent N-methylaspartate antagonist than ketamine, levorphanol, or dextromethorphan. After systemic administration, however, dextrorphan was rather less potent than ketamine in this respect, whereas dextromethorphan remained less potent than either ketamine or dextrorphan. Noscapine, an antitussive that lacks anticonvulsant activity, failed to reduce selectively responses to N-methylaspartate as did papaverine, an isoquinoline structurally related to noscapine, and triprolidine, an antihistamine commonly found in proprietary cough medicines. The results are discussed with particular reference to the potential of the compounds tested as anticonvulsant and neuroprotective agents in vivo.

The role of excitatory amino acids and NMDA receptors in traumatic brain injury

Brain injury induced by fluid percussion in rats caused a marked elevation in extracellular glutamate and aspartate adjacent to the trauma site. This increase in excitatory amino acids was related to the severity of the injury and was associated with a reduction in cellular bioenergetic state and intracellular free magnesium. Treatment with the noncompetitive N-methyl-D-aspartate (NMDA) antagonist dextrophan or the competitive antagonist 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid limited the resultant neurological dysfunction; dextrorphan treatment also improved the bioenergetic state after trauma and increased the intracellular free magnesium. Thus, excitatory amino acids contribute to delayed tissue damage after brain trauma; NMDA antagonists may be of benefit in treating acute head injury.

NMDA receptor antagonists can enhance or impair learning performance in animals

The effects of NMDA receptor antagonism on learning and memory were investigated using competitive (DL-2-amino-7-phosphonoheptanoate, AP7) and non-competitive (MK 801) blockers in three different learning tasks. Administration (i.p.) of drugs prior to training resulted in impaired learning performance in the place-navigation and dark-avoidance paradigms, and improved performance in the step-down passive avoidance task; however, using this treatment protocol, the possibility of drug-induced non-mnemonic effects modifying learning performance could not be excluded. Drug administration immediately post-trial had no effect in the place-navigation paradigm, and improved retention performance in the dark-avoidance and step-down avoidance tasks. The similar results obtained with both types of antagonist indicate that the observed effects are indeed due to NMDA receptor blockade, and hence that such blockade modifies learning in a task-dependent manner. Exclusion of non-mnemonic effects by using the post-trial treatment regime demonstrates that NMDA antagonists facilitate learning of passive avoidance tasks.

Conditioned taste aversions induced by phencyclidine and other antagonists of N-methyl-D-aspartate

Taste aversions can be conditioned in rats by a variety of psychoactive drugs, including those with reinforcing properties. Previous research, however, has not established clearly whether phencyclidine and related drugs are active in such procedures. The present study was carried out to investigate whether phencyclidine would induce a conditioned taste aversion and whether several other compounds (MK-801, the stereoisomers of NANM and ifenprodil) which, like phencyclidine, are known to antagonise the actions of N-methyl-D-aspartate (NMDA), would produce similar effects. When rats received injections of these compounds, after consuming a novel solution of saccharin, their subsequent consumption of the same solution decreased. The smallest doses of the different drugs which induced clear taste aversions were: phencyclidine 3 mg/kg, MK-801 0.3 mg/kg, (+)-NANM 10 mg/kg, (-)-NANM 3 mg/kg and ifenprodil 10 mg/kg. Thus, all the drugs were active. However, as neither the potencies nor the efficacies of the different compounds in inducing taste aversions correlated with their other behavioural effects or with their relative potencies in antagonising the effects of NMDA or in displacing phencyclidine from its binding sites, the mechanisms involved are unclear.

Pentobarbital-like discriminative stimulus effects of N-methyl-D-aspartate antagonists

The discriminative stimulus effects of competitive and noncompetitive N-methyl-D-aspartate (NMDA) antagonists were compared in rats trained to discriminate sodium pentobarbital (5.0 mg/kg i.p.) from saline under a two-lever fixed ratio 32 schedule of food reinforcement. The competitive NMDA antagonist 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) substituted for pentobarbital at doses that did not disrupt rates of responding. The proposed competitive NMDA antagonist NPC 12626 [2-amino-4,5-(1,2-cyclohexyl)-7-phosphonoheptanoic acid] also substituted for pentobarbital. The benzodiazepine antagonist Ro15-1788 did not antagonize the pentobarbital-like discriminative stimulus effects of CPP. The noncompetitive NMDA antagonists phencyclidine and MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine maleate] produced a maximum average of only 42 and 38%, respectively, pentobarbital-lever responding at doses that also substantially reduced response rates. These results suggest that the competitive NMDA antagonists CPP and NPC 12626 share discriminative stimulus properties with pentobarbital. However, the pentobarbital-like discriminative stimulus effects of CPP are probably not mediated through interaction with benzodiazepine receptors sensitive to Ro15-1788. In addition, because phencyclidine and MK-801 did not fully substitute for pentobarbital, these results provide further evidence for differences in the discriminative stimulus properties of competitive and noncompetitive NMDA antagonists.

Calpain I activation is specifically related to excitatory amino acid induction of hippocampal damage

Sustained stimulation of receptors for excitatory amino acids leads to both activation of the calcium-dependent cysteine protease calpain I and to the death of receptive neurons. Here, we have examined the relationship between the calpain I activation and neurodegeneration. Calpain I activation was manifested as increased levels of the major proteolytic fragments of the calpain substrate spectrin, detected and quantified by immunoblotting. Intraventricular administration of the excitatory amino acids kainate or N-methyl-D-aspartate (NMDA) produced calpain I-mediated spectrin degradation and hippocampal neuronal loss. The NMDA antagonist 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid selectively blocked NMDA- but not kainate-induced protease activation and hippocampal damage. Temporally, spectrin degradation preceded the onset of pyramidal cell degeneration monitored by silver-impregnation histochemistry. Only those doses of kainate (0.15-1 microgram) or NMDA (40-80 micrograms) sufficient to cause hippocampal damage markedly increased spectrin breakdown. Both the neuronal damage and calpain I activation induced by kainate occurred primarily in area CA3. Degeneration of hippocampal neurons evoked by colchicine was not accompanied by calpain activation, indicating that proteolysis is not stimulated simply as a secondary response to neuronal destruction. Thus, a close correspondence exists between excitatory amino acid induction of neuronal degeneration and of calpain I-mediated spectrin degradation. The results suggest that calpain I may be an intracellular mediator of excitatory amino acid action, and further, they support the hypothesis that calcium influx and calpain I activation are obligatory events in the initiation of excitatory amino acid neurotoxicity.

Solubilization of quisqualate-sensitive [3H]glutamate binding activity from rat retina

Binding activity of a putative central neurotransmitter, L-glutamic acid, was examined in the supernatant preparations solubilized from rat retinal membranes by Nonidet P-40. [3H]Glutamate binding activity increased linearly with increasing concentrations of the solubilized proteins up to 15 micrograms. The binding activity reached an equilibrium within 10 min at 2 degrees C, while increasing with incubation time up to 60 min at 30 degrees C. Addition of an excess of nonradioactive glutamate rapidly decreased the activity at 30 degrees C. Scatchard analysis revealed that the solubilized retinal binding activity consisted of a single component with a KD of 0.25 microM and a Bmax of 57.4 pmol/mg protein. The solubilized binding activity exhibited a stereospecificity and a structure selectivity to L-glutamate, and was abolished by quisqualate, L-glutamate diethyl ester, and DL-2-amino-3-phosphonopropionate. None of the other agonists and antagonists for the central excitatory amino acid receptors affected the binding activity. Reduction of incubation temperature from 30 degrees C to 2 degrees C resulted in a drastic attenuation of the binding activity due to decrement of the number of the apparent binding sites. Cation-exchange column chromatography revealed that unidentified radioactive material was in fact formed during the incubation of [3H]glutamate with the retinal preparations at 30 degrees C. These results suggest that retinal [3H]glutamate binding activity may be derived at least in part from the quisqualate-sensitive membranous enzyme with a stereospecific and structure-selective high affinity for the central neurotransmitter.

Glycine reverses the effect of HA-966 on NMDA responses in cultured rat cortical neurons and in chick retina

The effects of glycine on NMDA antagonism by a series of excitatory amino acid antagonists were tested in two functional in vitro models: NMDA induced [3H]GABA release from cultured mouse cortical neurons and NMDA evoked spreading depression in chick retina. In both models glycine reversed the NMDA antagonism by HA-966. Also NMDA block by kynurenic acid and by DNQX were partly reversed by glycine. However, CNQX, D-APV, ketamine and MK 801 showed the same NMDA antagonism in the absence and presence of glycine.

N-methyl-D-aspartate antagonists: ready for clinical trial in brain ischemia?

Antagonists of the N-methyl-D-aspartate (NMDA) subclass of glutamate receptors may offer a new approach for the treatment of ischemic brain injury. This strategy is supported by a well-developed scientific foundation and encouraging results in a variety of in vivo and in vitro experimental models. Several specific antagonists, including MK-801, dextrorphan, dextromethorphan, and ketamine, have already been used at low doses in humans for other indications and are potential candidates for Phase I clinical trials.

Synthesis and pharmacology of a series of 3- and 4-(phosphonoalkyl)pyridine- and -piperidine-2-carboxylic acids. Potent N-methyl-D-aspartate receptor antagonists

We recently prepared a series of 3- and 4-(phosphonoalkyl)pyridine- and -piperidine-2-carboxylic acids as antagonists of neurotransmission at N-methyl-D-aspartate (NMDA) preferring receptors. NMDA antagonists may prove to be useful therapeutic agents, for instance, as anticonvulsants, in the treatment of neurodegenerative disorders such as Alzheimer's disease and in the prevention of neuronal damage that occurs during cerebral ischemia. The compounds prepared were evaluated for their ability to displace [3H]CPP binding (an assay shown to be selective for compounds that bind at the NMDA receptor) and for their ability to block NMDA-induced lethality in mice (an assay that is also specific for competitive and noncompetitive NMDA antagonists). Two of the compounds, cis-4-(phosphonomethyl)piperidine-2-carboxylic acid (11a) and cis-4-(3-phosphonoprop-1-yl)piperidine-2-carboxylic acid (11c) proved to be potent NMDA antagonists. 11a and 11c displaced [3H]CPP binding with IC50's of 95 and 120 nM, respectively, and both protected mice from NMDA-induced lethality, with MEDs (minimum effective dose, the dose at which three of the five animals tested survived) of 10 and 40 mg/kg ip, respectively. The rest of the compounds prepared were weakly active or inactive in these assays. The pattern of activity observed for this series parallels that observed for the acyclic series of omega-phosphono-alpha-amino acids, where AP5 and AP7 possessed NMDA antagonist activity while AP6 and AP8 were inactive. Reduction of conformational mobility by incorporation of the piperidine ring led to enhanced potency relative to the acyclic analogues.

NMDA antagonists block restraint-induced increase in extracellular DOPAC in rat nucleus accumbens

The effects of the N-methyl-D-aspartate (NMDA) receptor antagonists CPP, TCP, PK 26124 and ifenprodil, and of the minor tranquillizer diazepam on stress-induced changes of dopamine metabolism in the nucleus accumbens were investigated in the rat. Dopamine metabolism was assessed by measuring the extracellular levels of 3,4-dihydroxyphenylacetic acid (DOPAC) by means of in vivo differential pulse voltammetry with electrochemically pretreated carbon fiber electrodes. Physical immobilization of the rats for 4 min caused a marked and long-lasting increase in extracellular DOPAC levels in the nucleus accumbens. A similar, though shorter-lasting, augmentation of extracellular DOPAC was observed in the nucleus accumbens after systemic administration of the anxiogenic agent methyl-beta-carboline-3-carboxylate (beta-CCM) (10 mg/kg s.c.). Pretreatment with CPP (1 mg/kg i.p.), TCP (3 mg/kg i.p.), PK 26124 (3 mg/kg i.p.), ifenprodil (3 mg/kg i.p.) or diazepam (2 mg/kg i.p.) totally antagonized the immobilization-induced increase in extracellular DOPAC in the nucleus accumbens. Diazepam and the benzodiazepine (omega 1-2) receptor antagonist flumazenil (30 mg/kg i.p.), but not ifenprodil, also antagonized the beta-CCM-induced activation of dopamine metabolism in the nucleus accumbens. Finally, systemic administration of haloperidol (25 micrograms/kg i.p.) increased the extracellular concentrations of DOPAC in the nucleus accumbens, but pretreatment with ifenprodil (3 mg/kg i.p.) did not modify this response. These data indicate that NMDA receptor antagonists prevent the activation of dopamine metabolism in the nucleus accumbens caused by immobilization stress but not by beta-CCM-induced anxiogenic stimulation. These results suggest that NMDA receptor antagonists may possess an anxiolytic-like action in the rodent, which is exerted via neuroanatomical circuits distinct from those acted upon by diazepam.

NMDA receptor antagonists that bind to the strychnine-insensitive glycine site and inhibit NMDA-induced Ca2+ fluxes and [3H]GABA release

We have examined the actions of putative antagonists of the strychnine-insensitive glycine-mediated modulation of the N-methyl-D-aspartate (NMDA) receptor using [3H]MK801 binding, Ca2+ influx and [3H]GABA release assays. Kynurenic acid and HA-966 inhibited [3H]MK801 binding, NMDA and glycine induced Ca2+ influx measured using fura-2 and NMDA and glycine simulated [3H]GABA release. The effects of kynurenic acid could be partially overcome by the addition of excess glutamate and glycine, indicating limited selectivity for the glycine binding site. In addition, a component of the action of kynurenic acid was insensitive to agonist concentration, indicating a third action of kynurenic acid at high concentrations. In contrast, HA-966 was 100-fold selective for the glycine compared to the NMDA site. HA-966 only partially inhibited [3H]MK801 binding (IC50 19.7 microM), NMDA-induced Ca2+ influx and neurotransmitter release. The failure of HA-966 to completely block NMDA responses, even at high concentrations, suggests that glycine may not be an absolute requirement for the activation of NMDA receptors under these experimental conditions.

Effects of phencyclidine and other N-methyl-D-aspartate antagonists on the schedule-controlled behavior of rats

The behavioral effects of phencyclidine (PCP) were compared with those of several compounds known to antagonize the actions of N-methyl-D-aspartate using two patterns of schedule-controlled responding in rats. Rates of variable interval responding suppressed by punishment were increased greatly by the benzodiazepine chlorodiazepoxide and showed small increases after MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine] . However, no consistent increases in response rates were produced by PCP, by the stereoisomers of N-allylnormetazocine (NANM: SKF 10,047) or by the anti-ischemic drug, ifenprodil. Small doses of PCP did increase rates of unpublished variable interval responding, as did a low dose of MK-801. Timing behavior maintained by a differential reinforcement of low rate schedule was disrupted by all the compounds studied. Response rates were increased by at least one dose of PCP, MK-801, (+)-NANM and 3-(2-carboxypiperazine-4-yl)propyl-1-phosphonic acid. The effect of MK-801, however, was considerably greater than that of the other compounds. Ifenprodil and (-)-NANM did not increase rates of responding but, at high doses, produced decreases in reinforcement frequency indicating that efficient timing behavior had been disrupted. These results show that although PCP, MK-801 and (+)-NANM produce generally similar behavioral effects, there may also be some differences between the compounds, notably a more consistent effect of MK-801 on punished responding. These behavioral effects may be related to antagonism of N-methyl-D-aspartate but ifenprodil, which is also an N-methyl-D-aspartate antagonist, does not show a similar behavioral profile.

N-methyl-D-aspartate/glycine and quisqualate/kainate receptors expressed in Xenopus oocytes: antagonist pharmacology

Quantitative pharmacological studies were done to determine the properties of excitatory amino acid receptors expressed in Xenopus oocytes injected with rat brain mRNA. Smooth currents with properties indicative of N-methyl-D-aspartate (NMDA) and quisqualate/kainate receptors were observed in mRNA-injected oocytes. Schild analysis of currents evoked by NMDA indicated that the EAA receptor antagonist D-2-amino-5-phosphonovalerate (D-APV) exerted a competitive block of the oocyte NMDA receptor, because the Schild regression was linear with a slope not significantly different from unity (1.03 +/- 0.025) up to 100 microM D-APV. The pA2 estimated for D-APV antagonism of NMDA currents (5.87 +/- 0.043) was nearly identical to that for D-APV as an L-aspartate antagonist (pA2 = 5.86 +/- 0.073, slope = 0.97 +/- 0.036), suggesting that these two agonists are selective for NMDA receptors in oocytes up to concentrations well above 1 mM. 6-Nitro-7-cyano-quinoxaline-2,3-dione (CNQX) reduced the maximum NMDA response significantly (70% reduction by 15 microM CNQX) but had no effect on the NMDA EC50. CNQX exerted a mixed competitive-noncompetitive block of the glycine site on NMDA receptors; 15 microM CNQX increased the glycine EC50 by 5-fold and reduced the maximum glycine response by 35%. In addition, CNQX exerted a potent and competitive antagonism of currents evoked by kainate. The Schild regression was linear up to 30 microM CNQX with a slope of 1.02 +/- 0.014 and a pA2 of 6.53 +/- 0.029. The block of kainate or NMDA currents by 2 microM CNQX was not voltage dependent. D-APV exerted a weak antagonism of kainate-evoked currents, with a pA2 of 3.39 +/- 0.044, but the slope of the Schild regression was slightly less than 1 (0.90 +/- 0.03). These data demonstrate a clear pharmacological distinction between receptors that mediate the kainate- and NMDA-induced currents and quantify the potency of CNQX and D-APV acting at NMDA/glycine and quisqualate/kainate receptors. The implications of these data for the identification of EAA receptors in oocytes and the classification of neuronal EAA receptors are discussed.

Selective depression of N-methyl-D-aspartate-mediated responses by dextrorphan in the hippocampal slice in rat

The effects of dextrorphan (DX) and dextromethorphan (DM) on responses to excitatory amino acids in the CA1 region of the hippocampus of the rat were studied using extracellular and intracellular recording in in vitro slices of brain. Dextrorphan selectively and non-competitively blocked depolarizations evoked by focally-applied N-methyl-D,L-aspartate (NMA), recorded by both extracellular and intracellular techniques. Quisqualate (QUIS) responses and evoked field potentials were not affected by DX. Epileptiform activity elicited in Mg2+-free solution was suppressed by DX. Dextrorphan had no effect on resting membrane potential or input resistance. The antagonism of NMA by DX was dose-dependent with an EC50 of 0.65 microM; DM was also effective but considerably less potent. In the paradigm used in the present study, DX did not produce the clear use-dependent block observed in the presence of MK-801. These data suggest that DX, the metabolite of the widely used antitussive DM, is a potent NMDA antagonist with a potential role as an anticonvulsant and neuroprotective agent.

Tricyclic antidepressants block N-methyl-D-aspartic acid-induced lethality in mice

It has been suggested on the basis of in vitro studies that tricyclic antidepressants interact with the N-methyl-D-aspartic acid (NMDA)-receptor complex to block the action of NMDA. The present study showed that tricyclic antidepressants prevented lethality produced by a large dose of NMDA. The potency of the drugs in preventing NMDA-induced lethality correlated with the inhibition of [3H]-MK-801 binding at the NMDA receptor complex, and not with effects on amine uptake. These in vivo data support the in vitro data of Reynolds & Miller (1988a,b).

The N-methyl-D-aspartate antagonists aminophosphonovaleric acid and MK-801 reduce anoxic damage to dentate granule and CA1 pyramidal cells in the rat hippocampal slice

The effect of the N-methyl-D-aspartate antagonists, aminophosphonovaleric acid and MK-801, on irreversible transmission loss subsequent to anoxia was examined using the hippocampal slice preparation. A population spike was recorded from either the dentate granule cells or the CA1 pyramidal cells and the amplitude of this spike was compared before and 60 min following anoxia. After 10 min of anoxia the dentate granule cells recovered to 16 +/- 7% (mean +/- SE) of their preanoxic level when untreated and to 54 +/- 15% when treated with aminophosphonovaleric acid (APV). In slices treated with MK-801 the population spikes recorded from dentate granule cells recovered to 85 +/- 4% of their preanoxic level after 10 min of anoxia. Untreated CA1 pyramidal cells recovered to 8 +/- 3% of their preanoxic amplitude after 5 min of anoxia; they recovered to 59 +/- 6% when treated with MK-801 and 31 +/- 13% when treated with APV. The recovery of slices treated with the drugs was significantly different from that of untreated slices. ATP levels were measured in both the dentate and the CA1 region of slices. ATP in both regions fell less during anoxia when the slices were pretreated with either APV or MK-801. These differences between drug-treated and untreated tissue were significant with APV and MK-801. These differences between drug-treated and untreated tissue were significant with APV and MK-801 in dentate tissue after 10 min of anoxia and with MK-801 in CA1 tissue after 5 min of anoxia. This reduced fall in ATP during anoxia was accompanied by better physiological recovery after anoxia. We conclude that these NMDA antagonists provide protection against anoxic damage to dentate granule and CA1 pyramidal cells in this in vitro hippocampal preparation.(ABSTRACT TRUNCATED AT 250 WORDS)

A local circuit neocortical synapse that operates via both NMDA and non-NMDA receptors

1. In slices of rat neocortex, spike triggered averaging was employed to record in one neurone the excitatory postsynaptic potential (e.p.s.p.) generated by a spike in another, neighbouring neurone. When recorded at different membrane potentials, some of these e.p.s.ps exhibited a voltage relation typical of neuronal responses to N-methyl-D-aspartate (NMDA). 2. Selective NMDA antagonists reduced the amplitude of these e.p.s.ps, but had little effect on their early rising phase. In contrast, a less selective excitatory amino acid antagonist reduced all phases of the e.p.s.p. 3. By analyzing single axon e.p.s.ps we have been able to establish that the synaptic input to one cortical cell, delivered by a single presynaptic cortical cell, operates simultaneously via NMDA and non-NMDA amino acid receptors.

The mechanism of action and pharmacological specificity of the anticonvulsant NMDA antagonist MK-801: a voltage clamp study on neuronal cells in culture

1. Some possible molecular mechanisms of action of the anxiolytic, anticonvulsant and neuroprotective agent MK-801 have been examined in 'whole-cell' voltage clamp recordings performed on rat hippocampal and cortical neurones, bovine adrenomedullary chromaffin cells and N1E-115 neuroblastoma cells maintained in cell culture. 2. Transmembrane currents recorded from rat hippocampal and cortical neurones in response to locally applied N-methyl-D-aspartate (NMDA) were antagonized by MK-801 (0.1-3.0 microM). Blockade was use-dependent, and little influenced by transmembrane potential. MK-801 (3 microM) had no effect on currents evoked by kainate (100 microM). 3. The antagonism of NMDA-induced currents by MK-801 was only slowly and incompletely reversed when the cell membrane potential was clamped at -60 mV during washout. Prolonged applications of NMDA at +40, but not -60 mV during washout, markedly accelerated recovery from block. 4. In contrast to MK-801, ketamine (10 microM) blocked NMDA-induced currents in a voltage-dependent manner. Blockade increased with membrane hyperpolarization and was completely reversible upon washout. 5. MK-801 (1-10 microM) produced a voltage- and concentration-dependent block of membrane currents elicited by ionophoretically applied acetylcholine (ACh) recorded from bovine chromaffin cells. The block was readily reversible upon washout. 6. gamma-Aminobutyric acidA (GABAA) receptor-mediated chloride currents of chromaffin cells were unaffected by MK-801 (1-100 microM). In contrast, such currents were potentiated by diazepam (1 microM). MK-801 (100 microM) had no effect on currents evoked by GABA on hippocampal neurones. 7. MK-801 (10 microM) had little effect on membrane currents recorded from N1E-115 neuroblastoma cells in response to ionophoretically applied 5-hydroxytryptamine (5-HT). Such currents were antagonized by the 5-HT3 receptor antagonist GR 38032F (1 nM) and also by MK-801 at high concentration (100 microM). 8. Voltage-activated, tetrodotoxin-sensitive, sodium currents of chromaffin cells were unaffected by 10 microM MK-801. However, at a relatively high concentration (100 microM), MK-801 reduced the amplitude of such currents to approximately 77% of control. 9. The relevance of the present results to the central actions of MK-801 is discussed.

The competitive N-methyl-D-aspartate (NMDA) antagonist CGS 19755 attenuates the rate-decreasing effects of NMDA in rhesus monkeys without producing ketamine-like discriminative stimulus effects

The purported competitive excitatory amino acid antagonist CGS 19755 was compared to the non-competitive antagonists ketamine and MK-801 in three rhesus monkeys discriminating between 1.78 mg/kg of ketamine and saline while responding under a fixed-ratio 100 schedule of food presentation. MK-801 substituted completely for the ketamine discriminative stimulus and was 32 times more potent than ketamine as a discriminative stimulus. CGS 19755 was studied using single and cumulative dosing procedures up to a dose of 10.0 mg/kg; for all conditions, CGS 19755 produced responding exclusively on the saline lever and had only modest rate-decreasing effects. N-Methyl-D-aspartate administered alone also did not produce ketamine-appropriate responding but did decrease response rates in a dose-related manner. N-Methyl-D-aspartate eliminated responding in all monkeys at doses of 5.6-10.0 mg/kg. MK-801 and ketamine antagonized the rate-decreasing effects of N-methyl-D-aspartate, however, ketamine was most effective as an antagonist at doses that decreased response rates when administered alone. CGS 19755 also attenuated the rate-decreasing effects of N-methyl-D-aspartate and shifted the N-methyl-D-aspartate dose-effect curve more than 5-fold to the right. The magnitude of antagonism of N-methyl-D-aspartate appeared to be somewhat greater with CGS 19755 than with MK-801 or ketamine. Thus, a competitive (CGS 19755) and some non-competitive (MK-801 and ketamine) excitatory amino acid antagonists can attenuate the rate-decreasing effects of N-methyl-D-aspartate.(ABSTRACT TRUNCATED AT 250 WORDS)