The pharmacology of SDZ EAA 494, a competitive NMDA antagonist

An examination of NMDA receptor contribution to conditioned responding evoked by the conditional stimulus effects of nicotine

RATIONALE

Research using a drug discriminated goal-tracking (DGT) task showed that the N-methyl-D: -aspartate (NMDA) channel blocker MK-801 (dizocilpine) reduced the nicotine-evoked conditioned response (CR).

OBJECTIVES

Given the unknown mechanism of the effect, Experiment 1 replicated the MK-801 results and included tests with NMDA receptor ligands. Experiments 2a and 2b tested whether MK-801 pretreatment blocked DGT via a state-dependency effect.

METHODS

In Experiment 1, adult male Sprague-Dawley rats received intermittent access to liquid sucrose following nicotine (0.4 mg base/kg); no sucrose was delivered on intermixed saline sessions. Conditioning was indicated by increased anticipatory dipper entries (goal-tracking) on nicotine compared to saline sessions. Antagonism and/or substitution tests were conducted with MK-801, phencyclidine, CGP 39551, d-CPPene (SDZ EAA 494), Ro 25,6981, L-701,324, ACPC, and NMDA. In Experiment 2a, rats received nicotine and sucrose on every session-no intermixed saline sessions without sucrose. Tests combined MK-801 or the non-competitive nicotinic acetylcholine receptor antagonist, mecamylamine with either nicotine or saline. Experiment 2b had sucrose delivered on saline sessions and no sucrose on intermixed nicotine sessions followed by MK-801 antagonism tests of the saline CS.

RESULTS

MK-801 and phencyclidine dose-dependently attenuated the CR in Experiment 1. Ro-25,6981 enhanced the CR, but did not substitute for nicotine. Other ligands showed inconsistent effects. In Experiment 2a, MK-801 pretreatment reduced goal-tracking when given before nicotine and saline test sessions; mecamylamine pretreatment had no effect. In Experiment 2b, MK-801 dose-dependently attenuated the saline-evoked CR.

CONCLUSIONS

Combined, the results suggest that MK-801 blocks discriminated goal-tracking by virtue of state-changing properties.

Comparative pharmacology of the human NMDA-receptor subtypes R1-2A, R1-2B, R1-2C and R1-2D using an inducible expression system

Pharmacological characterization of N-methyl-D-aspartate (NMDA) receptors has been hampered by the difficulty to outwit cytotoxicity after functional expression in recombinant systems. In this study a muristerone-inducible expression system for the NNMDA-R1 subunit was used. This was combined with constitutive expression of NMDA-R2A, 2B, 2C and 2D in different cell clones. After establishment of the cell lines, quantitative RT-PCR demonstrated the inducibility of the NNMDA-R1 subunit, and verified the expression of the NMDA-R2 subunits in the different cell clones. Functional responses were characterized using calcium influx through the ion channel as a robust assay system. Stimulation of the NMDA-receptor subtypes in the different cell lines led to calcium transients which were rising gradually, peaked after 30-160 s and declined thereafter very slowly. The expression of the four different NMDA-receptor subtypes in the same cellular background allowed a direct pharmacological comparison of the different receptors. Glutamate showed the highest potency at the NMDA-R1-2D. NMDA displayed at all subtypes a lower potency compared to glutamate and was a partial agonist except at the NMDA-R1-2D. 20 antagonists were tested in this study and the pharmacological characterization of the inhibition of glutamate-evoked elevation of intracellular free Ca(2+) revealed a distinct rank order of antagonist potency for each receptor subtype. These data illustrate that assessment of calcium transients upon receptor stimulation in the same cellular background is a powerful tool to compare the functional effects of compounds acting at the different NMDA-R2 receptors.

N-Methyl-D-aspartate receptors as a target for improved antipsychotic agents: novel insights and clinical perspectives

RATIONALE

Activation of "co-agonist" N-methyl-D-aspartate (NMDA) and Glycine(B) sites is mandatory for the operation of NMDA receptors, which play an important role in the control of mood, cognition and motor function.

OBJECTIVES

This article outlines the complex regulation of activity at Glycine(B)/NMDA receptors by multiple classes of endogenous ligand. It also summarizes the evidence that a hypoactivity of Glycine(B)/NMDA receptors contributes to the pathogenesis of psychotic states, and that drugs which enhance activity at these sites may possess antipsychotic properties.

RESULTS

Polymorphisms in several genes known to interact with NMDA receptors are related to an altered risk for schizophrenia, and psychotic patients display changes in levels of mRNA encoding NMDA receptors, including the NR1 subunit on which Glycine(B) sites are located. Schizophrenia is also associated with an overall decrease in activity of endogenous agonists at Glycine(B)/NMDA sites, whereas levels of endogenous antagonists are elevated. NMDA receptor "open channel blockers," such as phencyclidine, are psychotomimetic in man and in rodents, and antipsychotic agents attenuate certain of their effects. Moreover, mice with genetically invalidated Glycine(B)/NMDA receptors reveal similar changes in behaviour. Finally, in initial clinical studies, Glycine(B) agonists and inhibitors of glycine reuptake have been found to potentiate the ability of "conventional" antipsychotics to improve negative and, albeit modestly, cognitive and positive symptoms. In contrast, therapeutic effects of clozapine are not reinforced, likely since clozapine itself enhances activity at NMDA receptors.

CONCLUSIONS

Reduced activity at NMDA receptors is implicated in the aetiology of schizophrenia. Correspondingly, drugs that (directly or indirectly) increase activity at Glycine(B) sites may be of use as adjuncts to other classes of antipsychotic agent. However, there is an urgent need for broader clinical evaluation of this possibility, and, to date, there is no evidence that stimulation of Glycine(B) sites alone improves psychotic states.

Antiepileptic drugs and agents that inhibit voltage-gated sodium channels prevent NMDA antagonist neurotoxicity

N-methyl-D-aspartate (NMDA) glutamate receptor antagonists are used in clinical anesthesia and are being developed as therapeutic agents for preventing neurodegeneration in stroke, epilepsy, and brain trauma. However, the ability of these agents to produce neurotoxicity in adult rats and psychosis in adult humans compromises their clinical usefulness. In addition, an NMDA receptor hypofunction (NRHypo) state might play a role in neurodegenerative and psychotic disorders, like Alzheimer's disease, bipolar disorder and schizophrenia. Thus, developing pharmacological means of preventing these NRHypo-induced effects could have significant clinically relevant benefits. NRHypo neurotoxicity appears to be mediated by a complex disinhibition mechanism that results in the excessive stimulation of certain vulnerable neurons. Here we report our findings that five agents (phenytoin, carbamazepine, valproic acid, lamotrigine, and riluzole), thought to possess anticonvulsant activity because they inhibit voltage-gated sodium channels, prevent NRHypo neurotoxicity. The ability of tetrodotoxin, a highly selective inhibitor of voltage-gated sodium channels, to prevent the same neurotoxicity suggests that inhibition of this ion channel is the likely mechanism of action of these five agents. We also found that three other anticonvulsants (felbamate, gabapentin and ethosuximide), whose mechanism is less clear, also prevent NRHypo neurotoxicity, suggesting that inhibition of voltage-gated sodium channels is not the only mechanism via which anticonvulsants can act to prevent NRHypo neurotoxicity. Several of these agents have been found to be of clinical use in bipolar disorder. It would be of interest to determine whether these agents might have therapeutic benefits for conditions in which a NRHypo state may exist.

Severe impairment of NMDA receptor function in mice carrying targeted point mutations in the glycine binding site results in drug-resistant nonhabituating hyperactivity

NMDA receptor hypofunction has been implicated in the pathophysiology of schizophrenia, and pharmacological and genetic approaches have been used to model such dysfunction. We previously have described two mouse lines carrying point mutations in the NMDA receptor glycine binding site, Grin1(D481N) and Grin1(K483Q), which exhibit 5- and 86-fold reductions in receptor glycine affinity, respectively. Grin1(D481N) animals exhibit a relatively mild phenotype compatible with a moderate reduction in NMDA receptor function, whereas Grin1(K483Q) animals die shortly after birth. In this study we have characterized compound heterozygote Grin1(D481N/K483Q) mice, which are viable and exhibited biphasic NMDA receptor glycine affinities compatible with the presence of each of the two mutated alleles. Grin1(D481N/K483Q) mice exhibited a marked NMDA receptor hypofunction revealed by deficits in hippocampal long-term potentiation, which were rescued by the glycine site agonist d-serine, which also facilitated NMDA synaptic currents in mutant, but not in wild-type, mice. Analysis of striatal monoamine levels revealed an apparent dopaminergic and serotonergic hyperfunction. Behaviorally, Grin1(D481N/K483Q) mice were insensitive to acute dizocilpine pretreatment and exhibited increased startle response but normal prepulse inhibition. Most strikingly, mutant mice exhibited a sustained, nonhabituating hyperactivity and increased stereotyped behavior that were resistant to suppression by antipsychotics and the benzodiazepine site agonist Zolpidem. They also displayed a disruption of nest building behavior and were unable to perform a cued learning paradigm in the Morris water maze. We speculate that the severity of NMDA receptor hypofunction in these mice may account for their profound behavioral phenotype and insensitivity to antipsychotics.

Modulation of the micturition reflex pathway by intravesical electrical stimulation: an experimental study in the rat

Intravesical electrical stimulation (IVES) is used clinically to improve bladder evacuation in patients with inadequate micturition contractions. The procedure involves field stimulation of Adelta bladder mechanoreceptor afferents resulting in a prolonged enhancement of the micturition reflex. The aim of the present experimental study in the rat was to identify the site for this neuromodulation, whether it was due to sensitization of bladder mechanoreceptors, to enhancement of transmission in the central micturition reflex pathway, or to improved effectiveness of the peripheral motor system of the bladder. The experiments were performed on female rats, anesthetized by alpha-chloralose. Multi-unit afferent or efferent activity was recorded from bladder pelvic nerve branches during repeated cystometries before and after IVES. The specific antagonist CPPene was used to block central glutaminergic receptors of NMDA type. Micturition threshold volume decreased significantly after IVES. The afferent threshold volume, peak response, and pressure sensitivity were unchanged as were the peak efferent activity and bladder contractility. There was no efferent activity until just before the micturition contraction. The IVES-induced decrease in micturition threshold was blocked by prior administration of the NMDA (N-methyl-D-aspartic acid) antagonist CPPene (3-(2-carboxypiperazin-4-yl)-1-propenyl-1-phosphonic acid). The findings indicate that the IVES-induced modulation of the micturition reflex is due to an enhanced excitatory synaptic transmission in the central micturition reflex pathway. The observed modulation may account for the clinical beneficial effect of IVES treatment.

Decrement in operant performance produced by NMDA receptor antagonists in the rat: tolerance and cross-tolerance

Current perspectives on the clinical use of NMDA receptor antagonists infer repeated administration schedules for the management of different pathological states. The development of tolerance and cross-tolerance between different NMDA receptor antagonists may be an important factor contributing to the clinical efficacy of these drugs. The present study aimed to characterize the development of tolerance and cross-tolerance to the ability of various site-selective NMDA receptor antagonists to produce a decrement of operant responding (multiple extinction 9 s fixed-interval 1-s schedule of water reinforcement). Acute administration of D-CPPen (SDZ EAA 494; 1-5.6 mg/kg), dizocilpine (MK-801; 0.03-0.3 mg/kg), memantine (0.3-17 mg/kg), ACEA-1021 (10-56 mg/kg), and eliprodil (1-30 mg/kg) differentially affected operant responding. Both increases and decreases in response rates and accuracy of responding were observed. Repeated preexposure to D-CPPen (5.6 mg/kg, once a day for 7 days) attenuated a behavioral disruption produced by an acute challenge with D-CPPen or ACEA-1021, but potentiated the effects of dizocilpine, memantine, and eliprodil. Based on the present results, one can suggest that the repeated administration of a competitive NMDA receptor antagonist differentially affects the functional activity of various sites on NMDA receptor complex.

The NMDA antagonist EAA 494 does not impair working memory in an operant DNMTP task in rats

There is contrasting evidence for an impairment of spatial working memory in operant delayed matching/or nonmatching to position (DMTP/DNMTP) tasks, as both delay-dependent and -independent disruption of choice accuracy has been found following N-methyl-D-aspartate (NMDA) receptor blockade. Using a within-subjects experimental design, the effect of the competitive NMDA receptor antagonist, EAA 494 (D-CPP-ene) (1, 1.5, 2 mg/kg IP 30 min prior), on working memory was investigated in male Lister Hooded rats pretrained to the DNMTP task (0-16-s delay in intervals). Metal barriers were inserted between the food magazine and levers to inhibit the use of mediating strategies, such as orientation towards the correct lever during the delay interval, because this behavior may contribute to the delay-dependent disruption noted in previous studies. It was found that EAA 494 did not modify working memory either in the presence or absence of barriers. However, a dose-dependent impairment of task performance was recorded, notably in the presence of barriers. These results indicate that competitive blockade of NMDA receptors with EAA 494 does not result in impaired working memory in rats and parallel the lack of effect of the compound upon working memory in humans. Activation of NMDA receptors does not appear to be essential for the performance of spatial tasks requiring working memory.

Effects of N-methyl-D-aspartate receptor antagonists on reinstatement of cocaine-seeking behavior by priming injections of cocaine or exposures to cocaine-associated cues in rats

The reinstatement of extinguished cocaine self-administration behavior was studied in rats pretreated with N-methyl-D-aspartate receptor antagonists. Rats were trained to self-administer intravenous cocaine (0.32 mg/kg/infusion) during five consecutive daily sessions that were followed by five consecutive daily extinction sessions, during which cocaine was unavailable and cocaine-associated cues (sound and light) were absent. Neither the competitive N-methyl-D-aspartate receptor antagonist D-CPPene (0.3-3 mg/kg) nor the low-affinity N-methyl-D-aspartate receptor channel blocker memantine (1-10 mg/kg) reinstated extinguished responding. Priming injections of intravenous cocaine (Experiment 1), and exposures to cocaine-associated stimuli (buzzer and light; Experiment 2) engendered responding on the reinforced lever in excess of that on the non-reinforced lever. In Experiment 1, administration of D-CPPene or memantine prior to the priming injection of cocaine eliminated the difference between reinforced-lever and non-reinforced-lever response rates. For both D-CPPene and memantine, however, this effect was largely due to increased responding upon the non-reinforced lever rather than to decreased reinforced-lever responding. In Experiment 2, D-CPPene, but not memantine, abolished in a dose-dependent manner the selective increase in reinforced-lever over non-reinforced-lever responding that was induced by exposures to cocaine-related stimuli. This effect of D-CPPene was not due to increased non-reinforced-lever responding. These data help define the boundaries within which N-methyl-D-aspartate receptor antagonists can prevent reinstatement of cocaine-seeking behavior (e.g. type of antagonist used and reinstatement procedure).

Modulation of the mesolimbic dopamine system by glutamate: role of NMDA receptors

Glutamate has been shown to modulate motor behavior, probably via N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors that are involved in the control of the mesolimbic dopamine (DA) system, that is, the ventral tegmental area (VTA)-nucleus accumbens (NAC). In the present study, we investigated the effects of uncompetitive (MK-801) and competitive [DL-2-amino-5-phosphonopentanoic acid (AP-5), CGP 40116] NMDA receptor antagonists and NMDA and AMPA on DA release in the mesolimbic system and on motor behavior. Systemic injection and intrategmental infusion of MK-801 increased DA levels in the VTA, but the systemic administration enhanced DA exclusively in the NAC and increased motor behavior. In contrast, intrategmental infusion of AP-5, but not the systemic administration of its lipophilic analogue CGP 40116, decreased the DA release in the two regions without affecting motor behavior. NMDA and AMPA infusion into the VTA increased DA levels in both areas. This increase was accompanied by a strong motor behavioral stimulation after NMDA but only a moderate increase after AMPA infusion. The present results indicate that mesolimbic DA neurons are controlled by the glutamatergic system and that the effects of uncompetitive and competitive NMDA receptor antagonists on DA release are mediated by an interaction with different brain areas. These findings may account for the different effects of NMDA receptor ligands on motor behavior.

Prolongation of morphine analgesia by competitive NMDA receptor antagonist D-CPPene (SDZ EAA 494) in rats

A possible future clinical application of NMDA receptor antagonists is the control of the development of opiate analgesic tolerance. Therefore, the ability of NMDA receptor antagonists to modify the acute analgesic effects of opiates becomes increasingly important. The present study sought to evaluate the analgesic potency of combined administration of morphine (5-20 mg/kg) and a competitive NMDA receptor antagonist D-CPPene (SDZ EAA 494; 3-(2-carboxypiperazin-4-yl)-1-propenyl-1-phosphonic acid; 0.3-5.6 mg/kg) in the tail-flick and tail-pinch tests with rats. It was found that D-CPPene significantly increased the duration of morphine analgesia, but there was hardly any evidence for potentiation of morphine analgesia shortly after morphine administration. This effect could only in part be attributed to the D-CPPene-induced disruption of the development of 'learned hyperresponsiveness' (i.e., acquisition of decreased latencies to escape from repeated exposures to noxious stimulation). In addition, the plasma concentration of morphine was not affected by concurrent treatment with D-CPPene.

CP-101,606, a potent neuroprotectant selective for forebrain neurons

The neuroprotective activity of (1S,2S)-1-(4-hydroxyphenyl)-2-(4-hydroxy-4-phenylpiperidino)-1-propanol (CP-101,606), an N-methyl-D-aspartate (NMDA) receptor antagonist structurally similar to ((+/-)-(R*,S*)-alpha-(4-hydroxyphenyl)-beta-methyl-4-(phenylmethyl)-1-++ +piperidineethanol (ifenprodil), was investigated in neurons in primary culture. CP-101,606 potently and efficaciously protected hippocampal neurons from glutamate toxicity but was > 900-fold less effective for cerebellar granule neurons. The neuroprotective activity in the hippocampal neurons is mediated through a high affinity binding site distinct from the agonist and thienylcyclohexylpiperidine (TCP) binding sites of the NMDA receptor. Autoradiography indicates the CP-101,606 binding site is localized in forebrain, most notably in hippocampus and the outer layers of cortex. The functional selectivity for hippocampal neurons, forebrain localization of binding sites, and structural relation to ifenprodil suggest that CP-101,606 is an NMDA antagonist highly selective for NR2B subunit containing receptors.

The competitive NMDA receptor antagonist SDZ 220-581 reverses haloperidol-induced catalepsy in rats

The present studies investigated whether SDZ 220-581 ((S)-alpha-amino 2'chloro-5-(phosphonomethyl)[1,1'-biphenyl]-3-propanoic acid), a potent, competitive antagonist at the NMDA glutamate receptor subtype, reversed haloperidol-induced catalepsy in rats, a widely used model of Parkinson's disease. SDZ 220-581 (0.32-3.2 mg/kg i.p.) dose- and time-dependently reduced the time spent in an abnormal position induced by haloperidol (1.0 mg/kg s.c.). Compared to other NMDA receptor antagonists the rank order of potency was MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine) > SDZ 220-581 > SDZ EAA 494 (D-CPPene: (S)-(E)-4-(3-phosphonoprop-2-enyl)-piperazine-2-carboxylic acid) > SDZ EAB 515 ((S)-alpha-amino-5-(phosphonomethyl)[1,1'-biphenyl]-3-propanoic acid). Since it has been demonstrate that SDZ 220-581 counters the effects of L-dihydroxyphenylalanine (L-DOPA) on the motor disturbances of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-pre-treated primates, the results suggest that the reversal of haloperidol-induced catalepsy by competitive NMDA receptor antagonists may not be predictive of efficacy in other models of Parkinson's disease.

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.

Biphenyl-derivatives of 2-amino-7-phosphono-heptanoic acid, a novel class of potent competitive N-methyl-D-aspartate receptor antagonists--II. Pharmacological characterization in vivo

A selection of biphenyl-analogues of 2-amino-7-phosphonoheptanoic acid (AP7), N-methyl-D-aspartate (NMDA) receptor antagonists with high affinity in vivo efficacy. The lead compound SDZ EAB 515 was found to inhibit L-phenylalanine uptake by the large neutral amino acid carrier in vitro and in vivo; active transport may thus confer a good bioavailability to this class of compounds. CNS effects were demonstrated by significant changes in 2-deoxyglucose-uptake in various brain regions at doses from 1 to 10 mg/kg i.p. With the most active agent, SDZ 220-581, full protection against maximal electroshock seizures (MES) was obtained at oral doses of 10 mg/kg in rats and in mice. The compound had a fast onset (< or = 1 hr) and a long duration (> or = 24 hr) of action. Motor-debilitating effects (impairment of rotarod performance) occurred at doses about 10 times higher than those required for protection against MES. Neuroprotective activity was demonstrated by the ability of the compounds to reduce the extent of quinolinic acid-induced striatal lesions in rats, in the dose range of 3-15 mg/kg (i.p.) or 10-50 mg/kg (p.o.). In the middle cerebral artery occlusion (MCAO) model of focal cerebral ischemia in rats, the test compounds reduced the infarct size by 40-50% when given i.v. before or by 20-30% when given i.v. 1 hr after MCAO. SDZ 220-581 provided 20-30% protection at > or = 2 x 10 mg/kg p.o. This compound also showed analgesic activity at low oral doses in a model of neuropathic pain, although higher doses were required in model of mechanical inflammatory hyperalgesia. Unexpectedly, SDZ 220-581 at low s.c. doses counteracted the antiparkinsonian effects of L-DOPA in MPTP-treated marmosets. (Sub)chronic administration of SDZ 220-581 did not reduce its ability to protect against quinolinic acid neurotoxicity, and no upregulation of NMDA receptors was detected using a [3H]CGP-39653 binding assay. In conclusion, from a series of biphenyl-AP7-derivatives, SDZ 220-581 is clearly the most active compound in vivo. Its pharmacological profile with a good, long-lasting oral activity might open up novel therapeutic applications for competitive NMDA receptor antagonists.