D-CPP-ene (SDZ EAA 494), a potent and competitive N-methyl-D-aspartate (NMDA) antagonist: effect on spontaneous activity and NMDA-induced depolarizations in the rat neocortical slice preparation, compared with other CPP derivatives and MK-801

Drug discrimination: A versatile tool for characterization of CNS safety pharmacology and potential for drug abuse

Drug discrimination studies for assessment of psychoactive properties of drugs in safety pharmacology and drug abuse and drug dependence potential evaluation have traditionally been focused on testing novel compounds against standard drugs for which drug abuse has been documented, e.g. opioids, CNS stimulants, cannabinoids etc. (e.g. Swedberg & Giarola, 2015), and results are interpreted such that the extent to which the test drug causes discriminative effects similar to those of the standard training drug, the test drug would be further characterized as a potential drug of abuse. Regulatory guidance for preclinical assessment of abuse liability by the European Medicines Agency (EMA, 2006), the U.S. Food and Drug Administration (FDA, 2010), the International Conference of Harmonization (ICH, 2009), and the Japanese Ministry of Health Education and Welfare (MHLW, 1994) detail that compounds with central nervous system (CNS) activity, whether by design or not, need abuse and dependence liability assessment. Therefore, drugs with peripheral targets and a potential to enter the CNS, as parent or metabolite, are also within scope (see Swedberg, 2013, for a recent review and strategy). Compounds with novel mechanisms of action present a special challenge due to unknown abuse potential, and should be carefully assessed against defined risk criteria. Apart from compounds sharing mechanisms of action with known drugs of abuse, compounds intended for indications currently treated with drugs with potential for abuse and or dependence are also within scope, regardless of mechanism of action. Examples of such compounds are analgesics, anxiolytics, cognition enhancers, appetite control drugs, sleep control drugs and drugs for psychiatric indications. Recent results (Swedberg et al., 2014; Swedberg & Raboisson, 2014; Swedberg, 2015) on the metabotropic glutamate receptor type 5 (mGluR5) antagonists demonstrate that compounds causing hallucinatory effects in humans did not exhibit clear discriminative effects when tested against classical drugs of abuse in drug discrimination studies, and were not self-administered by rats. However, these compounds did cause salient discriminative effects of their own in animals trained to discriminate them from no drug. Therefore, from a safety pharmacology perspective, novel compounds that do not cause discriminative effects similar to classical drugs of abuse, may still cause psychoactive effects in humans and carry the potential to maintain drug abuse, suggesting that proactive investigation of drug abuse potential is warranted (Swedberg, 2013). These and other findings will be discussed, and the application of drug discrimination procedures beyond the typical standard application of testing novel compounds against known and well characterized reference drugs will be addressed.

Presynaptic GluN2D receptors detect glutamate spillover and regulate cerebellar GABA release

Glutamate directly activates N-methyl-d-aspartate (NMDA) receptors on presynaptic inhibitory interneurons and enhances GABA release, altering the excitatory-inhibitory balance within a neuronal circuit. However, which class of NMDA receptors is involved in the detection of glutamate spillover is not known. GluN2D subunit-containing NMDA receptors are ideal candidates as they exhibit a high affinity for glutamate. We now show that cerebellar stellate cells express both GluN2B and GluN2D NMDA receptor subunits. Genetic deletion of GluN2D subunits prevented a physiologically relevant, stimulation-induced, lasting increase in GABA release from stellate cells [long-term potentiation of inhibitory transmission (I-LTP)]. NMDA receptors are tetramers composed of two GluN1 subunits associated to either two identical subunits (di-heteromeric receptors) or to two different subunits (tri-heteromeric receptors). To determine whether tri-heteromeric GluN2B/2D NMDA receptors mediate I-LTP, we tested the prediction that deletion of GluN2D converts tri-heteromeric GluN2B/2D to di-heteromeric GluN2B NMDA receptors. We find that prolonged stimulation rescued I-LTP in GluN2D knockout mice, and this was abolished by GluN2B receptor blockers that failed to prevent I-LTP in wild-type mice. Therefore, NMDA receptors that contain both GluN2D and GluN2B mediate the induction of I-LTP. Because these receptors are not present in the soma and dendrites, presynaptic tri-heteromeric GluN2B/2D NMDA receptors in inhibitory interneurons are likely to mediate the cross talk between excitatory and inhibitory transmission.

Trajectory events across hippocampal place cells require previous experience

Replay of hippocampal place cell sequences has been proposed as a fundamental mechanism of learning and memory. However, the standard interpretation of replay has been challenged by reports that similar activity is observed before experience ('preplay'). By the preplay account, pre-existing temporal sequences are mapped onto new experiences without learning sequential structure. Here we employed high density recording methods to monitor hundreds of place cells simultaneously while rats explored multiple novel environments. While we observed large numbers of synchronous spiking events before experience, they were not temporally correlated with subsequent experience. Multiple measures differentiated pre-experience and postexperience events that, taken together, defined the latter but not the former as trajectory-depicting. The formation of events with these properties was prevented by administration of an NMDA-receptor antagonist during experience. These results suggest that the sequential structure of behavioral episodes is encoded during experience and reexpressed as trajectory events.

Effects of modulators of N-methyl-D-aspartate receptor-mediated neurotransmission on diazepam discrimination in rats

N-methyl-D-aspartate (NMDA) antagonists share a number of pharmacological effects with GABA(A) agonists, including anxiolytic and anticonvulsant effects. This study evaluated the effects of site-selective NMDA antagonists in rats trained to discriminate the benzodiazepine diazepam from vehicle. As expected, diazepam produced robust discriminative stimulus effects and dose-dependently substituted for the training dose. Mixed results were obtained with competitive NMDA antagonists: whereas NPC 17742 partially substituted for diazepam, SDZ EAA 494 did not elicit responding on the diazepam-associated lever. Other site-selective NMDA antagonists, including the open channel blocker phencyclidine, the glycine-site antagonists ACEA 1021 and MDL 102,288, the polyamine-site antagonist arcaine, and the glutamate release inhibitor riluzole, failed to substitute for diazepam. Agonists at nonbenzodiazepine sites of the GABA(A) receptor complex were also tested for comparison purposes. The barbiturate pentobarbital and the neurosteroid Co 2-1068 partially substituted for diazepam. In contrast, the anticonvulsant carbamazepine failed to substitute even at a dose that substantially reduced response rates. These results suggest that substitution of NMDA antagonists for GABA(A) agonists is dependent upon the site at which the NMDA antagonist binds. Further, they suggest that similarities between the stimulus properties of GABA(A) agonists and NMDA antagonists are at least as strong as similarities among agonists acting at different sites on GABA(A) receptors.

Effects of Cyperus articulatus compared to effects of anticonvulsant compounds on the cortical wedge

Cyperus articulatus L. (Cyperaceae) is a plant commonly used in traditional medicine in Africa and Latin America to treat many diseases. The water extract from rhizomes of Cyperus articulatus concentration-dependently reduced spontaneous epileptiform discharges and NMDA-induced depolarisations in the rat cortical wedge preparation at concentrations at which AMPA-induced depolarisations are not affected. The two antiepileptic compounds, valproate and ethosuximide, possessed effect neither on epileptiform discharges nor on AMPA- and NMDA-induced depolarisations. Phenobarbital, pentobarbital and phenythoin inhibited both AMPA- and NMDA-induced depolarisations and spontaneous epileptiform discharges. The effects of Cyperus articulatus were very close to the effect of D-CPPene. D-CPPene also inhibited spontaneous epileptiform discharges and antagonised NMDA- but not AMPA-induced depolarisations. The extract of Cyperus articulatus could contain components acting as NMDA antagonists.

Inorganic iron complexes derived from the nitric oxide donor nitroprusside: competitive N-methyl-D-aspartate receptor antagonists with nanomolar affinity

Aquopentacyanoferrate(II), [Fe(II)H2O(CN)5]3-, is one of the photodegradation products of the vasodilator and nitric oxide donor nitroprusside. Earlier observations concerning the light dependence of N-methyl-D-aspartate (NMDA) receptor blockade by nitroprusside prompted us to examine the effects of this iron complex on the NMDA receptor. [Fe(II)H2O(CN)5]3- and two other related species, aminopentacyanoferrate(II) and aminopentacyanoferrate(III), were found to be highly potent, competitive, and selective NMDA receptor antagonists. In a binding assay for the transmitter recognition site on the NMDA receptor, these iron complexes displaced the radioligand [3H]CGP 39653 with nanomolar affinities. They did not displace radioligands labeling the channel ([3H]MK-801) or the glycine co-agonist ([3H]glycine) sites of the NMDA receptor, nor did they have any relevant affinities for a number of other neurotransmitter (alpha-adrenergic, 5-hydroxytryptamine, dopamine, opiate) receptors. The iron complexes blocked NMDA-induced depolarizations in rat cortical slices at submicromolar concentrations, whereas responses to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate were not affected. In another functional receptor assay (potentiation of [3H]MK-801 binding by glutamate under non-equilibrium conditions), Schild analysis demonstrated the competitive nature of the NMDA receptor antagonism. The pA2 values obtained from these experiments were similar to the pK(i) values derived from radioligand ([3H]CGP 39653) binding assays. To explain the high affinity and selectivity of these compounds for the NMDA receptor, a novel mechanism of antagonist-receptor interaction is proposed, involving a ligand exchange process in which a loosely bound species (here H2O or NH3) in the coordination sphere of the iron complex is replaced by a functional group of an amino acid side chain placed at the glutamate recognition site of the NMDA receptor, thereby hindering agonist binding.

Excitatory amino acid antagonists alleviate convulsive and toxic properties of lindane in mice

Pesticides acting at GABAA receptors may induce convulsions in man and animals, but the mechanisms responsible for their convulsant activity are not fully explained. The following excitatory amino acid antagonists were studied for their protective action in mice intoxicated with chlorinated hydrocarbon insecticide lindane (gamma-hexachlorocyclohexane): the competitive NMDA antagonist: 3-(2-carboxypiperazine-4-yl)propenyl-1- phosphonic acid (D-CPPene, 20 mg/kg), the non-competitive NMDA antagonist: dizocilpine (MK-801, 0.4 mg/kg), the glycine site antagonist of NMDA receptor: 2-phenyl-1,3-propane-diol dicarbamate (felbamate, 400 mg/kg) and the competitive AMPA antagonist: 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline (NBQX, 100 mg/kg). Systemic administration of an antagonist prior to lindane resulted in a strong anticonvulsant effect. D-CPPene, MK-801 and NBQX produced a marked increase of CD50 values of lindane for clonic convulsions. All the antagonists protected animals against tonic convulsions. Toxicity of lindane was potently reduced, as assessed 2, 24 and 120 hr after administration of the pesticide. Our results demonstrate that excitatory amino acid antagonists reduce convulsant properties and toxicity of lindane, suggesting that excitatory amino acid neurotransmission may be involved in its central action.

Extracts from rhizomes of Cyperus articulatus (Cyperaceae) displace [3H]CGP39653 and [3H]glycine binding from cortical membranes and selectively inhibit NMDA receptor-mediated neurotransmission

The marshland plant Cyperus articulatus (Cyperaceae) is commonly used in traditional medicine in Africa and Latin America to treat a wide variety of human diseases ranging from headache to epilepsy. We tested the hypothesis that the purported anti-epileptic effect of this plant might be due to a functional inhibition of excitatory amino acid receptors. One or several component(s) contained in the extracts inhibited the binding of [3H]CGP39653 to the NMDA recognition site and of [3H]glycine to the strychnine-insensitive glycine site of the NMDA receptor complex from rat neocortex. Water extracts from rhizomes of Cyperus articulatus dose-dependently reduced spontaneous epileptiform discharges and NMDA-induced depolarizations in the rat cortical wedge preparation at concentrations at which AMPA-induced depolarizations were not affected. We conclude that the purported beneficial effects of Cyperus articulatus might at least partially be due to inhibition of NMDA-mediated neurotransmission.

Biphenyl-derivatives of 2-amino-7-phosphonoheptanoic acid, a novel class of potent competitive N-methyl-D-aspartate receptor antagonist--I. Pharmacological characterization in vitro

Omega-Phosphono-substituted alpha-amino acids have long been known to be antagonists at the N-methyl-D-aspartate (NMDA) receptor. D-2-Amino-5-phosphonopentanoic (D-AP5) and D-2-amino-7-phosphonoheptanoic (D-AP7) acids are the "prototype" compounds of this kind. Insertion of a biphenyl-moiety in the middle of the AP7 chain results in increased affinity and reverses the enantioselectivity from a D- to an L-form preference (Müller et al., (1992) Helv. Chim. Acta 75: 855-864). We describe here a series of substituted biphenyl-AP7-derivatives, the most potent of which have affinities (in a [3H]CGP-39653 binding assay using native and recombinant receptors) and potencies (antagonism of NMDA-induced depolarizations in a cortical wedge preparation; inhibition of glutamate-stimulated [3H]MK-801 binding under non-equilibrium conditions) in the low nanomolar range. Structure-activity relationships show that hydroxy-substitution at the C5-atom in the AP7-chain as well as substitution in the second phenyl ring with space filling (such as chloro-)groups in the para- and especially the ortho-position (extending the torsion angle of the two rings) increase affinity and potency of these compounds. They have no relevant affinities for the strychnine-insensitive glycine co-agonist site or the MK-801/PCP channel blocking site on the NMDA receptor complex. AMPA- and kainate-induced responses were not affected by biphenyl-analogues. These compounds also do not interact with a number of other neurotransmitter receptor sites, and they do not inhibit the uptake of [3H] glutamate in rat brain synaptosomes. However, they display affinities in the (sub)micromolar range for a non-NMDA, non-AMPA, non-kainate binding site for [3H]glutamate, measured in the presence of calcium chloride, the functional correlate of which has not yet been elucidated.

Infusion of NMDA antagonists into the nucleus reticularis pontis oralis inhibits the maximal electroshock seizure response

The nucleus reticularis pontis oralis (RPO) is necessary for the expression of tonic hindlimb extension (THE) in maximal electroshock (MES) seizures of rats. Previous work in this laboratory has demonstrated that both systemic administration and focal RPO microinfusion of D-cycloserine inhibits THE. The purpose of the present study was to characterize specific components of the NMDA receptor/ionophore complex that regulate the anticonvulsant activity mediated by the RPO. Bilateral RPO microinfusion of the competitive NMDA antagonists (-)AP7 and D-CPP as well as the uncompetitive antagonist dizocilpine ((+)MK-801) inhibited THE in a dose-related fashion. Bilateral RPO microinfusion of NMDA did not affect the THE response to MES but did induce convulsions resembling audiogenic seizures in genetically epilepsy prone rats. Bilateral RPO microinfusion of the strychnine-insensitive glycine site partial agonist D-cycloserine and the antagonist 5,7-dichlorokynurenic acid inhibited THE. The strychnine-insensitive glycine partial agonists (+)HA-966 and ACPC, as well as the agonists glycine and D-serine, did not significantly affect the THE response. Strychnine microinfusions in the RPO had no effect on THE. The results support a hypothesis that the RPO is a site of anticonvulsant drug action in MES and indicate that either competitive or uncompetitive NMDA antagonist action regulates the anticonvulsant activity mediated by the RPO. The role of the strychnine-insensitive glycine site in the regulation of the anticonvulsant activity medicated by the RPO is uncertain.

Antagonist of N-methyl-D-aspartate receptors partially prevent the development of cocaine sensitization

Behavioral sensitization to cocaine was tested for in rats pretreated with MK-801, a noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, or D-3-(2-carboxypiperazin-4-yl)-1-propenyl-1-phosphonic acid (D-CPPene), a competitive NMDA antagonist. A 5-day regimen of once-daily cocaine (15 mg/kg) injections yielded sensitization to cocaine (15 mg/kg)-induced behavioral activation. Cocaine sensitization was partially prevented by MK-801 (0.25 mg/kg) or D-CPPene (20 mg/kg) pretreatment. These results differ from previous reports that NMDA receptor antagonists completely prevented the development of stimulant sensitization. While raising questions about methodological differences among laboratories studying this issue, our findings suggest that sensitization may involve mechanisms dependent on NMDA-receptor function as well as NMDA receptor-independent mechanisms.

Influence of combined treatment with NMDA and non-NMDA receptor antagonists on electroconvulsions in mice

alpha-Amino-3-hydroxy-5-methyl-isoxazole-4-propionate/kainate (AMPA/kainate) receptor antagonists (at subthreshold doses against electroconvulsions), 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466 at maximally 5 mg/kg) and 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(F)quinoxaline (NBQX at maximally 20 mg/kg) enhanced the protective effects of NMDA receptor antagonists, MK-801 (dizocilpine) or 2-(2-carboxypiperazine-4-yl)-1-propenyl-1-phosphonic acid (D-CPP-ene), against electroconvulsions. Similarly, MK-801 or D-CPP-ene reduced the ED50 values of both NBQX and GYKI 52466 against maximal electroshock. The adverse effects of D-CPP-ene, evaluated in the chimney and rotorod tests, were potentiated by both GYKI 52466 (2.5 mg/kg) and NBQX (10 mg/kg). Also, D-CPP-ene (0.1 mg/kg) worsened the motor performance of mice pretreated with GYKI 52466 in the rotorod test. Neither MK-801 (0.025 mg/kg) nor D-CPP-ene (0.1 mg/kg) affected the NBQX-induced impairment of motor coordination. Similarly, GYKI 52466 (2.5 mg/kg) or NBQX (10 mg/kg) did not influence the performance of mice treated with MK-801 (0.2 mg/kg). It may be concluded that the blockade of more than one subtype of glutamate receptors leads to a more pronounced anticonvulsive effect when compared with the effect of blockade of an individual receptor subtype. In some cases more efficient seizure protection was not associated with increased adverse effects.

Effects of continual intravenous posttreatment with D-CPP-ene, a potent competitive N-methyl-D-aspartate receptor antagonist, on rat brain edema induced by injection of triethyltin into the cerebral hemisphere

Brain edema was produced by injecting triethyltin (TET) into the right cerebral hemisphere via the internal carotid artery in rats. TET induced a dose-related increase in mortality rate and brain water content. Immediately after TET-injection (2 mg/head), saline, glycerol (125 mg/ml) or the N-methyl-D-aspartate receptor antagonist (R)-4-(3-Phosphono-2-propenyl)-2-piperazine carboxylic acid (D-CPP-ene; 0.083 and 0.25 mg/ml) was continually infused via the right internal jugular vein at 20 microliters/min for 6 h. The mortality rate and brain water content were significantly decreased after infusion of 0.25 mg/ml D-CPP-ene, but only somewhat reduced after glycerol infusion when compared with the saline group. The results suggest that continual intravenous posttreatment with D-CPP-ene is useful for treatment of brain edema.

The pharmacology of SDZ EAA 494, a competitive NMDA antagonist

SDZ EAA 494 (D-CPPene) was characterized as a competitive NMDA antagonist, having a pA2 value against NMDA depolarizations in frog spinal cord and rat neocortex of 6.7-6.8 and a pKi of 7.5 in a [3H]CGP39653 binding assay, with no action on other receptors or amine reuptake. The compound was orally active in rodent maximal electroshock models with an ED50 of around 16 mg/kg, was protective in rats even 24 hours after oral application and had an oral therapeutic index of around 8. Muscle relaxation, ataxia, flattened body posture and reduced acquisition of a passive avoidance task, suggesting potential effects on memory formation, occurred at supra-anticonvulsant doses in rodents, with PCP-like stimulatory effects produced only by high i.p. doses or constant i.v. infusions. This favourable profile is discussed in relation to the negative outcome of a recent trial of the compound in patients with intractable epilepsy. The conclusion is drawn that standard models for screening new anticonvulsants are inappropriate to seeking drugs active in patients with a protracted convulsive history. The anti-ischaemic action of SDZ EAA 494 encourages further testing in brain trauma, in which the anticonvulsant action of the compound may be an added benefit.

Pretreatment with a competitive NMDA antagonist D-CPPene attenuates focal cerebral infarction and brain swelling in awake rats

The purpose of the study was to assess effects of the competitive N-methyl-D-aspartate (NMDA) receptor antagonist D-(E)-4-(3-phosphonoprop-2-enyl)piperazine-2-carboxylic acid (D-CPPene) upon focal cerebral infarction and brain oedema in the rat. Focal cerebral ischaemia was produced by permanent occlusion of the middle cerebral artery under halothane anaesthesia. The anaesthetic gas was discontinued immediately after the occlusion and the rats were killed 24 hours later. Cerebral infarction and brain swelling were each assessed on the frozen brain sections at 8 predetermined coronal planes. Pretreatment with D-CPPene (4.5 mg/kg i.v. followed by continuous infusion at 3 mg/kg/h until sacrifice) 15 minutes prior to MCA occlusion, significantly reduced the volume of infarction in the cerebral hemisphere by 29% (p < 0.05). Brain swelling, obtained by subtracting the nonischaemic hemispheric volume from the ischaemic hemispheric volume, was significantly reduced with D-CPPene treatment and the mean reduction in swelling (34% less than the controls: p < 0.001) proportionately similar to the decrease in infarct volume in the same animals. These data indicate that systemic administration of the competitive NMDA receptor antagonist D-CPPene has neuroprotective effects against ischaemic brain damage, and the reduction in brain swelling occurs in parallel with the reduction in ischaemic damage.

The competitive NMDA antagonist, D-CPP-ene, potentiates the anticonvulsant activity of conventional antiepileptics against maximal electroshock-induced seizures in mice

D-CPP-ene[3-(2-carboxy-piperazine-4-yl)-1-propenyl-1-phosphonic acid; a competitive antagonist of N-methyl-D-aspartic acid] in a dose of 2 mg/kg (i.p.) significantly increased the threshold for electroconvulsions. When given in a dose half that affecting the electroconvulsive threshold, D-CPP-ene potentiated the anticonvulsant activity of carbamazepine, diazepam, diphenylhydantoin, phenobarbital and valproate against maximal electroshock (50 mA)-induced seizures in mice. However, this NMDA antagonist did not influence the plasma levels of the antiepileptic drugs studied, so a pharmacokinetic interaction, in terms of total plasma levels at least, is not probable. The chimney test and retention test in mice revealed that the combined treatment of D-CPP-ene at 1.0 mg/kg (i.p.) with either diazepam, diphenylhydantoin, phenobarbital or valproate (providing a 50% protection against maximal electroshock convulsions) resulted in motor impairment and caused impairment of long-term memory. On the other hand, a combination of D-CPP-ene and carbamazepine was devoid of adverse effects. It can be concluded that the potential utility of D-CPP-ene in combination with conventional antiepileptic drugs does not seem promising, except for carbamazepine.

The effect of the non-NMDA receptor antagonist GYKI 52466 and NBQX and the competitive NMDA receptor antagonist D-CPPene on the development of amygdala kindling and on amygdala-kindled seizures

A competitive (NBQX) and a non-competitive (GYKI 52466) AMPA antagonist, and a competitive NMDA antagonist (D-CPPene) were tested against the development of kindling and against fully kindled seizures in amygdala-kindled rats. GYKI 52466, 10 mg/kg given i.p. 5 min prior to electrical stimulation in fully kindled animals, reduces both the cortical after-discharge duration and the behavioural seizure score. GYKI 52466, 20 mg/kg, reduces seizure score and after-discharge duration significantly (after 5-30 min) but the animals show severe motor side effects and an irregular cortical and hippocampal EEG. Administration of GYKI 52466, 10 mg/kg, prior to kindling stimulation on days 3-8, does not slow the development of kindling. NBQX, 20 mg/kg or 40 mg/kg i.p., 30 min prior to stimulation, significantly reduces the seizure score in fully kindled animals. NBQX 20 mg/kg i.p. has no effect on the development of kindling. D-CPPene, 8 mg/kg or 12 mg/kg, 120 min prior to stimulation reduces the behavioural seizure score in fully kindled animals. D-CPPene, 8 mg/kg on days 3-8, delays the development of kindling. NMDA receptors play a key role in the kindling process. Expression of kindled seizures involves non-NMDA and NMDA receptors.

Excitatory amino acid receptors: a gallery of new targets for pharmacological intervention

The excitatory amino acids (EAAs) L-glutamate and L-aspartate are the most abundant amino acids in brain and play a number of roles in maintaining neuronal function. Among these are their use as protein constituents, as key intermediates in ammonia metabolism, and as precursors for other neurotransmitters. Given the widespread distribution of EAA-containing neurons, these transmitters are likely to be involved in virtually all central nervous system functions, with abnormalities in neurotransmission contributing to the symptoms of a host of neurological and psychiatric disorders. Because of the importance of EAAs in maintaining the functional integrity of the central nervous system, efforts are underway to design agents capable of regulating the activity of these transmitters for therapeutic gain. Inasmuch as potential side effects preclude a generalized modification of this system, strategies must be found to alter EAA neurotransmission in selected brain regions. In this regard, pharmacological data suggest several functionally distinct EAA receptors, a finding confirmed by cloning studies which hint at an even larger family of sites. Moreover, it appears that some excitatory amino acid receptor complexes are composed of interacting sites which orchestrate receptor function, and there is evidence that EAA receptors may influence the activity of one another. Thus, there appear to be numerous sites that can be targeted to selectively modify excitatory amino acid neurotransmission in brain. Besides the agonist recognition site for each receptor subtype, other targets include regulatory subunits, ion channels and components of receptor-coupled second messenger systems.

Do N-methyl-D-aspartate antagonists have disproportionately greater effects on brain swelling than on ischemic damage in focal cerebral infarction?

Using a frozen section technique, we have assessed the effects of N-methyl-D-aspartate (NMDA) antagonist upon brain swelling caused by ischemic brain edema in a rat model of focal cerebral infarction. Although pretreatment with the competitive NMDA antagonist, D-CPPene or the noncompetitive NMDA antagonist, CNS 1102 reduced both the volumes of infarction and ischemic edema in the cerebral hemisphere, mean reduction in brain edema was proportionately similar to decrease in infarct volume in the same animals (correlation coefficient, r = 0.82, p < 0.001). There was, therefore, no evidence of disproportionately greater effects with NMDA antagonist upon brain edema.

Competitive as well as uncompetitive N-methyl-D-aspartate receptor antagonists affect cortical neuronal morphology and cerebral glucose metabolism

The studies examined the effects of three antagonists (CPP, CGS 19755, and CGP 37849) that act competitively at the glutamate recognition site of the NMDA receptor complex on cortical neuronal morphology and cerebral limbic glucose metabolism. Responses were compared to the effects of dizocilpine, an uncompetitive NMDA receptor ion channel antagonist as a positive control. CGS 19755 and CGP 37849 (100 mg kg-1 i.p.) caused vacuolation in cortical pyramidal neurons in the posterior cingulate cortex four hours after dosing and this dose of CGP 37849 caused a pattern of limbic glucose metabolism activation similar to that seen after dizocilpine. CPP was without effect at 100 mg/kg i.p. probably due to poor brain penetration. The data indicates that the functional consequences (structural and metabolic) of NMDA receptor blockade with NMDA antagonists acting competitively at the glutamate recognition site and uncompetitively in the receptor ion channel are ultimately the same. Comparisons of the potential therapeutic window for CGS 19755 and CGP 37849 with dizocilpine (neuroprotection versus vacuolation) suggests that the window for the competitive antagonists is greater. This indicates that the potential therapeutic window for the different classes of NMDA antagonists may vary with the site in the receptor complex at which they interact.

Characterization of the cerebroprotective efficacy of the competitive NMDA receptor antagonist CGP40116 in a rat model of focal cerebral ischemia: an in vivo magnetic resonance imaging study

The cerebroprotective properties of the competitive NMDA antagonist D-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid (CGP 40116) were evaluated in a rat model of focal cerebral ischemia. CGP 40116 (5-40 mg/kg i.v.) was injected immediately following permanent occlusion of the left middle cerebral artery (MCA). MK 801 (1 or 3 mg/kg i.v.), D-CPPene (20 mg/kg i.v.), and CGS 19755 (40 mg/kg i.v.) were used for comparison. Lesion volume was assessed using in vivo magnetic resonance imaging, which in initial experiments with parallel histological determinations proved to be an accurate method for the measurement of brain infarction and the determination of a cerebroprotective drug effect. CGP 40116 dose-dependently reduced the volume of cortical infarction, with an ED50 of 11 mg/kg i.v. and a maximal effect equivalent to a 62% reduction in cortical edema volume. Its cerebroprotective efficacy was thus comparable to that of MK 801. The rank order of potency for the NMDA antagonists was MK 801 > CGP 40116 approximately D-CPPene > CGS 19755. Neuroprotection by CGP 40116 was still apparent when treatment was started 30 min after MCA occlusion. It is concluded that CGP 40116 is an effective cerebroprotectant with potential clinical utility for amelioration of focal cerebral ischemic damage.

Antagonism of non-NMDA receptors inhibits handling-induced, strychnine-potentiated convulsions

The effects of blockade of the alpha-amino-3-hydroxy-5-methyl-isoxazole-4-proprionate (AMPA) and kainate subtypes of NBQX (2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline) and compared to the effects of N-methyl-D-aspartate (NMDA) receptor blockade with D-CPPene (D-(E)-4-(3-phosphonoprop-2-enyl)-piperazine-2-carboxylic acid), a competitive NMDA antagonist. Both compounds exerted peak blocking activity 30 min after intraperitoneal administration. Using this pretreatment interval, a dose-response relationship for blocking handling-induced, strychnine-potentiated convulsions was generated for each compound. D-CPPene blocked seizures with an ED50 of 0.72 (0.59-0.87) mg/kg and NBQX blocked seizures with an ED50 of 68.0 (36.72-125.94) mg/kg. These results indicate that both NMDA and non-NMDA subtypes of excitatory amino acid receptors are activated in handling-induced, strychnine-potentiated convulsions.

Efficacy of D-CPPene, a competitive N-methyl-D-aspartate antagonist in focal cerebral ischemia in the rat

The effect of a novel and potent competitive N-methyl-D-aspartate (NMDA) antagonist D-(E)-4-(3-phosphonoprop-2-enyl)piperazine-2-carboxylic acid (D-CPPene) upon ischemic brain damage has been examined in a rat model of focal cerebral ischemia. Focal cerebral ischemia was produced by permanent occlusion of the left middle cerebral artery (MCA). The animals were sacrificed 24 h after MCA occlusion and the amount of ischemic brain damage was assessed at 8 predetermined coronal planes. Pretreatment with D-CPPene (1.5, 4.5 or 15 mg/kg, i.v.), initiated 15 min prior to MCA occlusion (followed by constant infusion at 1, 3 or 10 mg/kg/h), produced dose-dependent reductions in the volumes of infarction; the dose of 4.5 mg/kg being the most effective (reduced by 37%; P < 0.01). These results indicate that systemic administration of the competitive NMDA antagonist D-CPPene has neuroprotective effects in a model of focal cerebral ischemia and define the dose dependency of its neuroprotective effects.

Intracerebroventricular application of competitive and non-competitive NMDA antagonists induce similar effects upon rat hippocampal electroencephalogram and local cerebral glucose utilization

In this study we have used electrophysiological and metabolic markers to investigate the effects of competitive and non-competitive NMDA antagonists in rats after central or peripheral administration. The non-competitive antagonist, MK-801, induced dose-dependent suppression of rat hippocampal EEG energy both after intraperitoneal (i.p.) and intracerebroventricular (i.c.v.) application. Similar effects were observed after i.p. and i.c.v. application of the competitive antagonist, DL-CPP-ene. Whereas the MK-801 was more potent after i.p. application, DL-CPP-ene was more potent after i.c.v. administration. Intracerebroventricular administration of MK-801 and DL-CPP-ene resulted in similar changes in the pattern of local cerebral glucose utilization in the olfactory tubercle and regions of the limbic system such as the anteroventral thalamus, hippocampus and entorhinal cortex. Intravenous (i.v.) administration of MK-801 induced increases in glucose metabolism similar to those observed after i.c.v. application. In contrast, i.v. administration of DL-CPP-ene induced only small decreases of glucose utilization in several regions of the central sensory system. Thus the blockade of glutamatergic (NMDA) transmission results in decreased hippocampal EEG activity which is paralleled by increased metabolic activity in this area. We conclude from EEG recordings and [14C]2-deoxyglucose uptake experiments that both non-competitive and competitive NMDA antagonists produce the same pattern of alterations after i.c.v. administration. Apparent differences in efficacy after peripheral administration may be largely due to differences in bioavailability.

N-methyl-D-aspartate receptor antagonists and channel blockers have different effects upon a spinal seizure model in mice

Spinal seizures in mice induced by handling following pretreatment with a subconvulsive dose of strychnine could be blocked by competitive N-methyl-D-aspartate (NMDA) receptor antagonists (D-, L-, DL-CPPene (CPPene = (E)-4-(3-phophonoprop-2-enyl)-piperazine-2-carboxylic acid), D-AP5 (D-2-amino-5-phophonovalerate)) and compounds acting at receptor-coupled modulatory sites (R-HA 966, ifenprodil). NMDA cation channel antagonists (MK-801, phencyclidine) however, resulted in ataxia, tremor and loss of righting. There are differences between NMDA antagonists acting via the receptor and the cation channel in this model of spinal seizure.

Evaluation of a competitive NMDA antagonist (D-CPPene) in feline focal cerebral ischemia

The effects of a competitive, N-methyl-D-aspartate (NMDA) receptor antagonist, D(-)E-4-(3-phosphonoprop-2-enyl)-piperazine-2-carboxylic acid (D-CPPene), on the volume of ischemic brain damage was assessed by quantitative histological study in 35 chloralose-anesthetized cats. Focal cerebral ischemia was produced by permanent occlusion of one middle cerebral artery and the animals were killed by transcardiac perfusion fixation 6 hours later. Pretreatment with D-CPPene (1.5, 4.5, or 15 mg/kg, administered intravenously 15 minutes prior to occlusion, with subsequent drug infusions to maintain a plateau in the plasma drug concentrations) effected dose-dependent reductions in the volume of ischemic brain damage. At the highest dose studied (15 mg/kg, plus an infusion of 170 micrograms/kg/min), D-CPPene reduced the volume of ischemic damage in the cerebral cortex by more than 75% compared to vehicle-treated control animals. The plasma concentration of D-CPPene, which is associated with a half maximal reduction in the volume of ischemic damage, was estimated to be 24 micrograms/ml during the initial 120 minutes after the middle cerebral artery occlusion. Treatment with D-CPPene (15 mg/kg, plus an infusion of 170 micrograms/kg/min) initiated 1 hour after occlusion reduced the volume of ischemic brain damage in the cerebral cortex by 30%, but this response did not achieve statistical significance. Precise definition of dose dependency for the anti-ischemic effects of NMDA antagonists and the therapeutic time window are influenced greatly by brain pharmacokinetics of the agents.(ABSTRACT TRUNCATED AT 250 WORDS)