Patch clamp studies on the kinetics and selectivity of N-methyl-D-aspartate receptor antagonism by memantine (1-amino-3,5-dimethyladamantan)

Ethanol-induced inhibition of NMDA receptor channels

Ethanol is a potent inhibitor of the N-methyl-D-aspartate (NMDA)-receptor subtype of glutamate receptor in a number of brain areas. The mechanism of ethanol action has been investigated by means of patch-clamp recording of ionic currents and fura-2 measurement of intracellular Ca2+ concentration in cell culture systems; the subunit composition of NMDA receptors and their influence on the effect of ethanol was determined by molecular biology methods. Ethanol does not appear to interact with NMDA either at the glutamate recognition site of the receptor, or at any of the hitherto known multiple modulatory sites, such as the glycine or polyamine site. Moreover, ethanol does not cause an open channel block by itself and fails to interact with Mg2+ at the site where it causes open channel block. The ability of ethanol to inhibit responses to NMDA is dependent on the subunit combination of NMDA receptors. The NR1/NR2A and NR1/NR2B combinations are preferentially sensitive to ethanol inhibition. Chronic treatment with ethanol leads to an increase of the NMDA receptor number at the transcriptional and posttranscriptional level; the receptor function is also facilitated. This causes withdrawal-type seizures after termination of chronic treatment with ethanol. The inhibition of NMDA receptors by ethanol leads to the depression of excitatory synaptic potentials mediated by this type of excitatory amino acid receptor. Ethanol-induced disturbances in certain regions of the brain, i.e. hippocampus, nucleus accumbens or locus coeruleus may lead to cognitive disorders or drug dependence. Brain slices containing the locus coeruleus may be used as an in vitro test system to investigate the addictive properties of ethanol.

Memantine is a clinically well tolerated N-methyl-D-aspartate (NMDA) receptor antagonist--a review of preclinical data

N-methyl-D-aspartate (NMDA) receptor antagonists have therapeutic potential in numerous CNS disorders ranging from acute neurodegeneration (e.g. stroke and trauma), chronic neurodegeneration (e.g. Parkinson's disease, Alzheimer's disease, Huntington's disease, ALS) to symptomatic treatment (e.g. epilepsy, Parkinson's disease, drug dependence, depression, anxiety and chronic pain). However, many NMDA receptor antagonists also produce highly undesirable side effects at doses within their putative therapeutic range. This has unfortunately led to the conclusion that NMDA receptor antagonism is not a valid therapeutic approach. However, memantine is clearly an uncompetitive NMDA receptor antagonist at therapeutic concentrations achieved in the treatment of dementia and is essentially devoid of such side effects at doses within the therapeutic range. This has been attributed to memantine's moderate potency and associated rapid, strongly voltage-dependent blocking kinetics. The aim of this review is to summarise preclinical data on memantine supporting its mechanism of action and promising profile in animal models of chronic neurodegenerative diseases. The ultimate purpose is to provide evidence that it is indeed possible to develop clinically well tolerated NMDA receptor antagonists, a fact reflected in the recent interest of several pharmaceutical companies in developing compounds with similar properties to memantine.

Neuroprotection in relation to retinal ischemia and relevance to glaucoma

Management of glaucoma is directed at the control of intraocular pressure (IOP), yet it is recognized now that increased IOP isjust an important risk factor in glaucoma. Therapy that prevents the death of ganglion cells is the main goal of treatment, but an understanding of the causes of ganglion cell death and precisely how it occurs remains speculative. Present information supports the working hypothesis that ganglion cell death may result from a particular form of ischemia. Support for this view comes from the fact that not all types of retinal ischemia lead to the pathologic findings seen in glaucomatous retinas or to cupping in the optic disk area. Moreover, in animal experiments in which ischemia is caused by elevated IOP, a retinal abnormality similar to that seen in true glaucoma is produced, whereas after occlusion of the carotid arteries a different pattern of damage is found. In ischemia, glutamate is released, and this initiates the death of neurons that contain ionotropic glutamate (NMDA) receptors. Elevated glutamate levels exist in the vitreous humor of patients with glaucoma, and NMDA receptors exist on ganglion cells and a subset of amacrine cells. Experimental studies have shown that a variety of agents can be used to prevent the death of retinal neurons (particularly ganglion cells) induced by ischemia. These agents are generally those that block NMDA receptors to prevent the action of the released glutamate or substances that interfere with the subsequent cycle of events that lead to cell death. The major causes of cell death after activation of NMDA receptors are the influx of calcium into cells and the generation of free radicals. Substances that prevent this cascade of events are, therefore, often found to act as neuroprotective agents. For a substance to have a role as a neuroprotective agent in glaucoma, it would ideally be delivered topically to the eye and used repeatedly. It is, therefore, of interest that betaxolol, a beta-blocker presently used to reduce IOP in humans, also has calcium channel-blocking functions. Moreover, experimental studies show that betaxolol is an efficient neuro protective agent against retinal ischemia in animals, when injected directly into the eye or intraperitoneally.

Brain penetration and in vivo recovery of NMDA receptor antagonists amantadine and memantine: a quantitative microdialysis study

PURPOSE

To determine free brain concentrations of the clinically used uncompetitive NMDA antagonists memantine and amantadine using microdialysis corrected for in vivo recovery in relations to serum, CSF and brain tissue levels and their in vitro potency at NMDA receptors.

METHODS

Microdialysis corrected for in vivo recovery was used to determine brain ECF concentrations after steady-state administration of either memantine or amantadine. Additionally CSF, serum, and brain tissue were analyzed.

RESULTS

Following 7 days of infusion of memantine or amantadine (20 and 100 mg/kg/day respectively) whole brain concentrations were 44-and 16-fold higher than free concentrations in serum respectively. The free brain ECF concentration of memantine (0.83 +/- 0.05 microM) was comparable to free serum and CSF concentrations. In case of amantadine, it was lower. A higher in vivo than in vitro recovery was found for memantine.

CONCLUSIONS

At clinically relevant doses memantine reaches a brain ECF concentration in range of its affinity for the NMDA receptor and close to its free serum concentration. This is not the case for amantadine and different mechanisms of action may be operational.

Open channel block of NMDA receptors by conformationally restricted analogs of milnacipran and their protective effect against NMDA-induced neurotoxicity

We investigated the blocking effect of the conformationally restricted analogs of milnacipran on NMDA receptors by recording the whole-cell currents of Xenopus oocytes injected with rat brain mRNA and the single channel currents of cultured hippocampal neurons under voltage-clamp conditions. Their protective effect against excitotoxicity was also investigated on cultured cortex neurons. All conformationally restricted analogs examined blocked activated NMDA receptors, though their structures were quite different from known NMDA receptor blockers. The analogs with a (1S, 2R, 1'S)-configuration such as PPDC ((1S, 2R)-1-phenyl-2[(S)-1-aminopropyl]-N,N-diethylcyclopropanecarboxamide+ ++) had lower IC50 values than those with other configurations. The empirical Hill coefficients for each compound were close to unity, indicating a 1:1 stoichiometry for the block. PPDC decreased the maximum responses to both N-methyl D-aspartate (NMDA) and glycine without altering their dissociation constants. The blocking effect was enhanced on hyperpolarization. PPDC had no effects on other glutamate receptor subtypes (AMPA, kainate, and metabotropic glutamate receptors) or other neurotransmitter receptors (GABA(A), 5HT2C, and AChM1 receptors) produced by the oocytes. PPDC decreased the mean open time of NMDA receptors without decreasing their elementary conductance. The microscopic blocking rate constant was 2.8x10(7) M(-1)s(-1). The macroscopic unblocking rate constant of PPDC was much faster than that of MK-801. Only the analogs with the (1S, 2R, 1'S)-configuration protected the cultures against NMDA-induced neurotoxicity, though they failed to protect against kainate-induced neurotoxicity. These results show that conformationally restricted analogs, at least PPDC, selectively blocked open channels of NMDA receptors.

Memantine in severe dementia: results of the 9M-Best Study (Benefit and efficacy in severely demented patients during treatment with memantine)

OBJECTIVES

To assess clinical efficacy and safety of memantine--an uncompetitive N-methyl-D-aspartate (NMDA) antagonist--in moderately severe to severe primary dementia.

MATERIALS AND METHODS

Dementia was defined by DSM-III-R criteria and severity was assessed by the Global Deterioration Scale (stages 5-7) and the Mini-Mental State Examination (< 10 points). Primary endpoints were the Clinical Global Impression of Change (CGI-C) rated by the physician, and the Behavioural Rating Scale for Geriatric Patients (BGP), subscore 'care dependence', rated by the nursing staff. Secondary endpoints included the modified D-Scale (Arnold/Ferm).

RESULTS

The ITT sample comprised 166 patients and 151 patients were treated per protocol. At 12-week ITT endpoint analysis, 82 received memantine 10 mg per day, 84 placebo. Dementia was in 49% of the Alzheimer type and in 51% of the vascular type (CT, Hachinski score). A positive response in the CGI-C was seen in 73% versus 45% in favour of memantine (stratified Wilcoxon p < 0.001), independent of the etiology of dementia. The results in the BGP subscore 'care dependence' were 3.1 points improvement under memantine and 1.1 points under placebo (p = 0.016). A coincident response of the two independent target variables was observed in 61.3% (memantine) versus 31.6% (placebo). Secondary endpoint analysis of the D-Scale assessing basic ADL functions support the primary results. Regarding the safety profile, no significant differences between treatment groups were observed.

CONCLUSIONS

The results of this trial support the hypothesis that memantine treatment leads to functional improvement and reduces care dependence in severely demented patients.

Effects of NMDA receptor antagonists on nociceptive responses in vivo: comparison of antagonists acting at the glycine site with uncompetitive antagonists

We have shown that members of a new series of tricyclic pyridophthalazine diones, defined as glycineB site NMDA antagonists in vitro, are selective and systemically active NMDA antagonists in vivo. In electrophysiological tests in alpha-chloralose anaesthetised rats, these compounds reduced nociceptive reflex responses. In conscious rats they displayed analgesic properties. These glycineB antagonists were compared electrophysiologically with several uncompetitive NMDA channel blockers. The degree of voltage dependence previously reported in vitro related to the effectiveness of the agents against different amplitude nociceptive responses of spinal cord neurones in vivo.

Amino-alkyl-cyclohexanes are novel uncompetitive NMDA receptor antagonists with strong voltage-dependency and fast blocking kinetics: in vitro and in vivo characterization

The present study characterized the in vitro NMDA receptor antagonistic properties of novel amino-alkyl-cyclohexane derivatives and compared these effects with their ability to block excitotoxicity in vitro and MES-induced convulsions in vivo. The 36 amino-alkyl-cyclohexanes tested displaced [3H]-(+)-MK-801 binding to rat cortical membranes with K(i)s between 1.5 and 143 microM. Current responses of cultured hippocampal neurones to NMDA were antagonized by the same compounds with a wide range of potencies (IC50s of 1.3-245 microM, at -70 mV) in a use- and strongly voltage-dependent manner (delta 0.55-0.87). The offset kinetics of NMDA receptor blockade was correlated with equilibrium affinity (Corr Coeff. 0.87 P < 0.0001). As an example, MRZ 2/579 (1-amino-1,3,3,5,5-pentamethyl-cyclohexane HCl) had similar blocking kinetics to those previously reported for memantine (K(on) 10.67 +/- 0.09 x 10(4) M(-1) s(-1), K(off) 0.199 +/- 0.02 s(-1), K(d) = K(off)/K(on) = 1.87 microM c.f. IC50 of 1.29 microM). Most amino-alkyl-cyclohexanes were protective against glutamate toxicity in cultured cortical neurones (e.g. MRZ 2/579 IC50 2.16 +/- 0.03 microM). Potencies in the three in vitro assays showed a relatively strong cross correlation (all corr. coeffs. > 0.72, P < 0.0001). MRZ 2/579 was also effective in protecting hippocampal slices against 7 min. hypoxia/hypoglycaemia-induced reduction of fEPSP amplitude in CA1 with an EC50 of 7.01 +/- 0.24 microM. MRZ 2/579 showed no selectivity between NMDA receptor subtypes expressed in Xenopus oocytes but was somewhat more potent than in patch clamp experiments-IC50s of 0.49 +/- 0.11, 0.56 +/- 0.01 microM, 0.42 +/- 0.04 and 0.49 +/- 0.06 microM on NR1a/2A /2B, /2C and 2/D, respectively. In contrast, memantine and amantadine were both 3-fold more potent at NR1a/2C and NR1a/2D than NR1a/2A receptors. All Merz amino-alkyl-cyclohexane derivatives inhibited MES-induced convulsions in mice with ED50s ranging from 3.6 to 130 mg/kg i.p. The in vivo and in vitro potencies correlated indicating similar access of most compounds to the CNS. MRZ 2/579 administered at 10 mg/kg resulted in peak plasma concentrations of 5.3 and 1.4 microM following i.v. and p.o. administration respectively, which then declined with a half life of around 170-210 min. Analysis of A.U.C. concentrations indicates a p.o./i.v. bioavailability ratio for MRZ 2/579 of 60%. MRZ 2/579 injected i.p. at a dose of 5 mg/kg resulted in peak brain extracellular fluid (ECF) concentrations of 0.78 microM (brain microdialysates). Of the compounds tested MRZ 2/579, 2/615, 2/632, 2/633, 2/639 and 2/640 had affinities, kinetics and voltage-dependency most similar to those of memantine and had good therapeutic indices against MES-induced convulsions. We predict that these amino-alkyl-cyclohexanes, which all had methyl substitutions at R1, R2, and R5, at least one methyl or ethyl at R3 or R4 and a charged amino-containing substitution at R6, could be useful therapeutics in a wide range of CNS disorders proposed to involve disturbances of glutamatergic transmission.

NMDA receptor characterization and subunit expression in rat cultured mesencephalic neurones

1. NMDA-induced changes in free intracellular Ca2+ concentration ([Ca2+]i) were determined in individual cultured rat mesencephalic neurones by the fura-2 method. mRNA expression encoding NMDA receptor subunits (NR1, NR2A-D) was examined by RT-PCR. 2. NMDA (1-100 microM, plus 10 microM glycine) induced a concentration-dependent increase in [Ca2+]i (EC50 = 5.7 microM). The effect of NMDA was virtually insensitive to tetrodotoxin (0.3 microM) and nitrendipine (1 microM), but dependent on extracellular Ca2+. 5,7-Dichlorokynurenic acid (10 microM), a specific antagonist at the glycine binding site on the NMDA receptor, abolished the NMDA response. 3. Memantine, an open-channel blocker, and ifenprodil, a preferential non-competitive NR1/NR2B receptor antagonist diminished the NMDA effect with an IC50 value of 0.17 and 1 microM, respectively. Ethanol at 50 and 100 mM caused about 25 and 45%-inhibition, respectively. 4. Agarose gel analysis of the PCR products followed by ethidium bromide fluorescence or CSPD chemiluminescence detection revealed an almost exclusive expression of the NR1 splice variants lacking exon (E) 5 and E22. The 3' splice form without both E21 and E22 exceeded that containing E21 by approximately 4 fold. The relative amounts of NR2A, NR2B, NR2C corresponded to approximately 1:2:1. NR2D mRNA was also detectable. 5. In conclusion, mesencephalic neurones bear ethanol-sensitive NMDA receptors which might be involved in the development of ethanol dependence and withdrawal. The high affinity of NMDA to this receptor, its sensitivity to ifenprodil and memantine may suggest that the mesencephalic NMDA receptor comprises the NR1 splice variant lacking E5, NR2B, and NR2C, respectively.

Open channel block and alteration of N-methyl-D-aspartic acid receptor gating by an analog of phencyclidine

We investigated inhibition of the N-methyl-D-aspartic acid (NMDA) receptor-channel complex by N-ethyl-1,4,9, 9alpha-tetrahydro-4alphaR-cis-4alphaH-fluoren-++ +4alpha-amine (NEFA), a structural analog of phencyclidine (PCP). Using the whole-cell recording technique, we demonstrated that NEFA inhibits NMDA responses with an IC50 of 0.51 microM at -66 mV. We determined that NEFA binds to the open channel, and subsequently the channel can close and trap the blocker. Once the channel has closed, NEFA is unable to dissociate until the channel reopens. Single-channel recordings revealed that NEFA reduces the mean open time of single NMDA-activated channels in a concentration-dependent manner with a forward blocking rate (k+) of 39.9 microM-1 s-1. A computational model of antagonism by NEFA was developed and constrained using kinetic measurements of single-channel data. By multiple criteria, only models in which blocker binding in the channel causes a change in receptor operation adequately fit or predicted whole-cell data. By comparing model predictions and experimental measurements of NEFA action at a high NMDA concentration, we determined that NEFA affects receptor operation through an influence on channel gating. We conclude that inhibition of NMDA receptors by PCP-like blockers involves a modification of channel gating as well as block of current flow through the open channel.

Interaction of memantine and amantadine with agonist-unbound NMDA-receptor channels in acutely isolated rat hippocampal neurons

1. Using whole-cell patch-clamp techniques, the mechanisms of NMDA channel blockade by amino-adamantane derivatives (AADs) memantine (3, 5-dimethyl-aminoadamantane, MEM) and amantadine (1-aminoadamantane, AM) have been studied in rat hippocampal neurons acutely isolated by the vibrodissociation method. A rapid concentration-jump technique was used to replace superfusing solutions. 2. The aspartate (Asp)-induced channel opening greatly accelerated but was not a prerequisite for the recovery from the block by MEM: it was able to leave the channel without agonist assistance. The co-agonist (glycine) as well as the competitive NMDA antagonist DL-2-amino-7-phosphonoheptanoic acid (APV), did not affect this recovery. Membrane depolarization accelerated it, strongly suggesting that this process proceeded via the hydrophilic pathway of the channel. 3. A comparison of the kinetics of the recovery from the block by AADs in the presence and absence of the agonist prompted a hypothesis that the blocker trapped in the channel increased the probability of its transition to the open state. 4. Both MEM and AM were able to block NMDA channels not only in the presence but also in the absence of Asp, although in the latter case the effective blocking concentrations were much higher and the rate of the block development was much smaller than in the former case. The extent of the block increased with the duration of the blocker application. Glycine enhanced this block, while APV attenuated it. The MEM-induced blockade of agonist-unbound channels was enhanced by membrane hyperpolarization and weakened by external Mg2+. These findings strongly suggested that the blocker reached its binding sites via the same hydrophilic pathway both in the presence and absence of the agonist. 5. A comparative analysis of the channel unblocking kinetics in the presence of Asp after their blockade with or without the agonist assistance led us to conclude that in the two cases AADs were bound to the same blocking sites in the channel.

Inhibition of reinforcing effects of morphine and naloxone-precipitated opioid withdrawal by novel glycine site and uncompetitive NMDA receptor antagonists

The glycine site (MRZ 2/570 and L-701,324), and uncompetitive (MRZ 2/579) NMDA receptor antagonists inhibited morphine-produced behaviors related to drug-abuse. The expression of morphine dependence was blocked by pretreatment with all three compounds (3-7.5 mg/kg); the effects of glycine/NMDA antagonists were not dose-dependent. Mice which were morphine-free for 3 days still displayed a significant severity of the withdrawal syndrome when challenged again with naloxone. This extinction of a residual morphine dependence was markedly diminished by treatment with similar doses of NMDA receptor antagonists at the test following the wash-out period. The rewarding impact of morphine was investigated in rats using the place preference (CPP) paradigm. All NMDA receptor antagonists (2.5-10 mg/kg) inhibited both the acquisition and expression of morphine-induced CPP. Once established, morphine-induced CPP was observed until 2 weeks after conditioning. NMDA receptor antagonists given for 3 days after the end of conditioning did not influence the extinction of morphine-induced CPP. Microdialysis studies revealed that the behaviorally effective doses of MRZ 2/579 resulted in a brain concentration close to its in vitro potency as an NMDA receptor antagonist. These data suggest that novel glycine site and uncompetitive NMDA receptor antagonists may have therapeutic potential in the treatment of opioid abuse.

Structural determinants of the blocker binding site in glutamate and NMDA receptor channels

Glutamate receptor channels of the NMDA-type (N-methyl-D-aspartate) and non-NMDA-type (GluR) differ in their pore properties. The N-site in the M2 transmembrane segment of NMDA receptors (NMDAR), or the corresponding Q/R-site in GluRs, is a pivotal structural determinant of their permeation and blockade characteristics. Substitutions at a second site in M2, the L-site (L577) in GluR1, drastically alter the receptor selectivity to divalent cations. Here we report that M2 mutants carrying an asparagine or a threonine residue at the Q-site of GluR1, along with a tryptophan residue at the L-site, form homomeric GluR1 channels that are highly sensitive to structurally diverse, uncompetitive NMDA antagonists such as arylcyclohexylamines, dibenzocycloheptenimines, and to morphinian and adamantane derivatives. Analysis of the voltage dependence of channel blockade locates the blocker binding site approximately 0.65 partway into the transmembrane electric field in both GluR1 mutants and NMDAR channels. Our results suggest that the homomeric GluR1 double mutants, L577W/Q582N and L577W/Q582T, fairly approximate the pore properties of the heteromeric NMDA receptor and support the structural kinship of their permeation pathways.

Budipine is a low affinity, N-methyl-D-aspartate receptor antagonist: patch clamp studies in cultured striatal, hippocampal, cortical and superior colliculus neurones

The NMDA receptor antagonistic effects of budipine were assessed using concentration- and patch-clamp techniques on cultured striatal, hippocampal, cortical and superior colliculus neurones. Inward current responses of striatal neurones to NMDA (200 microM) at -70 mV were antagonized by budipine in a concentration-dependent manner (50% inhibitory concentration (IC50) 59.4 +/- 10.7 microM, n = 17) with 24 times lower potency than memantine but similar potency to amantadine. In striatal neurones, budipine blocked outward currents at +70 mV with an IC50 of 827 microM, suggesting that the binding site is less deep in the channel (delta = 0.45) than for memantine. However, more detailed analysis of the fractional block by budipine 300 microM in hippocampal neurones gave a delta-value of 0.90, but revealed that 28% block is mediated at a voltage-independent site. This voltage-insensitive site was accessible in the absence of agonist. Budipine exhibited concentration-dependent open channel blocking kinetics (kappa(on) = 0.71 x 10(4) M(-1) s(-1)) whereas the fast offset rate was concentration-independent (kappa(off) = 0.63 s(-1)). Calculation of the ratio kappa(off)/kappa(on) revealed an apparent Kd value of 88.7 microM. Budipine, memantine and amantadine had similar effects against NMDA-induced currents in cultured hippocampal, cortical and superior colliculus neurones, although amantadine was somewhat more potent in cultured striatal neurones. The relevance of NMDA receptor antagonism to the anti-Parkinsonian effects of budipine remains to be established.

Electrophysiological study, biodistribution in mice, and preliminary PET evaluation in a rhesus monkey of 1-amino-3-[18F]fluoromethyl-5-methyl-adamantane (18F-MEM): a potential radioligand for mapping the NMDA-receptor complex

The effect of the fluorinated memantine derivative and NMDA receptor antagonist, 1-amino-3-fluoromethyl-5-methyl-adamantane (19F-MEM), at the NMDA receptor ion channel was studied by patch clamp recording. The results showed that 19F-MEM is a moderate NMDA receptor channel blocker. A procedure for the routine preparation of the 18F-labelled analog 18F-MEM has been developed using a two-step reaction sequence. This involves the no-carrier-added nucleophilic radiofluorination of 1-[N-(tert-butyloxy)carbamoyl]-3-(toluenesulfonyloxy)methyl- 5-methyl-adamantane and the subsequent cleavage of the BOC-protecting group using aqueous HCI. The 18F-MEM was obtained in 22 +/- 7% radiochemical yield (decay-corrected to EOB) in a total synthesis time including HPLC purification of 90 min. A biodistribution study after i.v. injection of 18F-MEM in mice showed a fast clearance of radioactivity from blood and relatively high initial uptake in the kidney and in the lung, which gradually decreased with time. The brain uptake was high (up to 3.6% ID/g, 60 min postinjection) with increasing brain-blood ratios: 2.40, 5.10, 6.33, and 9.27 at 5, 30, 60, and 120 min, respectively. The regional accumulation of the radioactivity in the mouse brain was consistent with the known distribution of the PCP recognition site. Preliminary PET evaluation of the radiotracer in a rhesus monkey demonstrated good uptake and prolonged retention in the brain, with a plateau from 35 min onwards p.i. in the NMDA receptor-rich regions (frontal cortex, striata, and temporal cortex). Delineation of the hippocampus, a region known to contain a high density of NMDA receptors, was not possible owing to the resolution of the PET tomograph. The regional brain uptake of 18F-MEM was changed by memantine and by a pharmacological dose of (+)-MK-801, indicating competition for the same binding sites. In a preliminary experiment, haloperidol, a dopamine D2 and sigma receptor antagonist, decreased the binding of 18F-MEM from the brain regions examined, suggesting that binding was also occurring to the sigma recognition sites.

Two blocking sites of amino-adamantane derivatives in open N-methyl-D-aspartate channels

Using whole-cell patch-clamp techniques, we studied the blockade of open N-methyl-D-aspartate (NMDA) channels by amino-adamantane derivatives (AADs) in rat hippocampal neurons acutely isolated by the vibrodissociation method. The rapid concentration-jump technique was used to replace superfusion solutions. A kinetic analysis of the interaction of AAD with open NMDA channels revealed fast and slow components of their blockade and recovery. Mathematical modeling showed that these kinetic components are evidence for two distinct blocking sites of AADs in open NMDA channels. A comparative analysis of different simplest models led us to conclude that these AAD blocking sites can be simultaneously occupied by two blocker molecules. The voltage dependence of the AAD block suggested that both sites were located deep in the channel pore.

The pharmacology of native N-methyl-D-aspartate receptor subtypes: different receptors control the release of different striatal and spinal transmitters

1. N-methyl-D-aspartate (NMDA) increases the release of radiolabelled dopamine, GABA, acetylcholine and spermidine from rat striatal slices and of noradrenaline from the dorsal cervical spinal cord. 2. These five responses show differing sensitivities to NMDA and also to a variety of competitive antagonists, NMDA channel blockers, glycine antagonists and polyamine site antagonists. 3. Inhibitory activity profiles for 20 different antagonists are presented. All compounds tested showed some degree of selectivity with regard to the different responses and each response showed particular characteristics that suggested mediation by a particular native NMDA receptor subtype. 4. Receptors controlling dopamine, GABA and noradrenaline release were generally more sensitive to most antagonists compared to those controlling acetylcholine and spermidine release. 5. Receptors controlling spermidine release were furthermore insensitive to magnesium, argiotoxin, ifenprodil and eliprodil and displayed low sensitivity to memantine, dextrorphan and dextromethorphan. 6. Receptors controlling noradrenaline release could be further discriminated from those controlling dopamine and GABA release by very high sensitivity to magnesium and MK-801 and to the glycine antagonist L-689,560 but not to other glycine antagonists (CNQX, DNQX, 7-Chlorokynurenate, HA-966). 7. Many other individual drug or receptor differences were noted. The different profiles observed suggest a wide diversity of native NMDA receptors with different properties and an unexpectedly rich pharmacopeia of subtype selective antagonists of native NMDA receptors. 8. Matching subtype selectivity to particular behavioural effects may be possible and the design of subtype selective NMDA antagonists for particular clinical applications while avoiding side effect generation seems to be feasible.

Memantine suppresses the glutamatergic neurotransmission of mammalian inner hair cells

The glutamatergic synapses between inner hair cells and afferent neurons seem to be involved in pathophysiological conditions of the cochlea. The excessive release of glutamate from inner hair cells during noise trauma and ischemia affects the afferent neurons. It is possible that in tinnitus outer hair cell or inner hair cell dysfunction or damage leads to an altered spontaneous release of glutamate from inner hair cells. Thus, the pharmacological modulation of glutamatergic neurotransmission could be of great value in the therapy of certain inner ear diseases. Recently, it has been discovered that the spasmolytic drug memantine has antiglutamatergic properties. As a possible drug for inner ear diseases, we were interested in the action of memantine on the neurotransmission of inner hair cells. With the aid of microiontophoretic techniques we were able to show a strong depressing effect on spontaneous activity as well as on glutamate-induced activity. This effect seems to be mediated by a blockade of N-methyl-D-aspartate (NMDA) receptors as memantine showed a strong inhibiting effect on NMDA-induced activity but not on AMPA-induced activity. These results recommend memantine for the treatment of inner ear diseases, e.g. especially tinnitus.

Anxiolytic activity of glycine-B antagonists and partial agonists--no relation to intrinsic activity in the patch clamp

On the basis of animal models, anxiety was one of the first suggested clinical applications of partial agonists of the glycineB site coupled to the NMDA receptor. It is not certain, however, whether these findings can be extended to full glycineB antagonists and what is the relation between intrinsic activity (degree of NMDA receptor antagonism) and anxiolytic effect. In the present study several NMDA receptor antagonists, including several glycineB antagonists/partial agonists, were tested for anxiolytic activity in the Vogel conflict test and the elevated plus-maze. Additionally, the intrinsic activities of the glycineB partial agonists used [ACPC, (R,+)-HA-966 and D-cycloserine] were compared in patch-clamp experiments in cultured neurones. In the plus-maze the most striking increase in the time spent in open arms (index of anxiolytic effect) was seen after diazepam and D-cycloserine (at doses that did not change locomotion). Also reliable (dose-dependent), although weaker, anxiolytic activity was produced by the uncompetitive NMDA receptor antagonist (+)MK-801 and the competitive antagonist CGP 39551. Modest anxiolytic-like effect in the plus-maze was also observed after the glycineB antagonist L-701,324 and the partial agonist (+,R)-HA-966. Uncompetitive antagonists memantine and amantadine, the glycineB partial agonist ACPC (up to 600 mg/kg) or the full antagonists MRZ 2/570, MRZ 2/571 and MRZ 2/576 had no effect. In the Vogel conflict test neither memantine, nor any of the full glycineB antagonists tested (L-701,324 and MRZ 2/576), showed anxiolytic activity. Patch-clamp studies revealed that the intrinsic activity of (+,R)-HA-966, D-cycloserine and ACPC was 13, 57 and 92%, respectively, as compared to that of glycine itself (100%). In conclusion, for the agents tested there is no clear relation between the levels of intrinsic activity, i.e. degree of NMDA receptor inhibition, and anxiolytic activity. Moreover, L-701,324 and MRZ-type glycineB full antagonists do not exchibit anxiolytic activity in the elevated plus-maze and Vogel conflict test.

Stimulus intensity, cell excitation and the N-methyl-D-aspartate receptor component of sensory responses in the rat spinal cord in vivo

The importance of receptors for N-methyl-D-aspartate in synaptic plasticity and in triggering long-term pronociceptive changes is explained by their voltage-dependence. This suggests that their contribution to acute nociceptive responses would be determined both by the magnitude of synaptic input and by the level of background excitation. We have now examined the role of N-methyl-D-aspartate receptors in acute nociceptive transmission in the spinal cord. Drugs selectively affecting activity mediated by these receptors were tested on responses of dorsal horn neurons to noxious stimuli of different intensities and at different levels of ongoing spike discharge. The drugs used were the N-methyl-D-aspartate receptor channel blocker ketamine; the competitive antagonists, 3-((R)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (D-CPP) and D-2-amino-5-phosphonopentanoic acid (D-AP5), and the positive modulator thyrotropin-releasing hormone. The activity of dorsal horn wide dynamic range neurons was recorded extracellularly in alpha-chloralose-anaesthetized spinalized rats. Their responses to noxious stimuli (pinch, heat and electrical) were monitored in parallel with responses to iontophoretic N-methyl-D-aspartate and (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA). Drugs were given i.v. or (D-AP5) iontophoretically. At doses that selectively inhibited responses to exogenous N-methyl-D-aspartate, ketamine (4 or 8, mean 5 mg/kg i.v.) reduced the nociceptive responses of the majority of the cells in deep dorsal horn. Ketamine also reduced wind-up of the responses to repetitive electrical stimulation. Ketamine (4 or 8 mg/kg). D-CPP (2 mg/kg), D-AP5 (iontophoretically) and thyrotrophin-releasing hormone (1 mg/kg) were tested on different magnitude nociceptive responses evoked by alternating intensities of noxious heat or pinch. In percentage terms, the less vigorous responses were affected by all four drugs as much as or more than the more vigorous responses. When background activity of neurones was enhanced by continuous activation of C-fibres with cutaneous application of mustard oil, ketamine was less effective against superimposed noxious pinch responses. Ongoing background activity was affected in parallel with evoked responses. When background discharge of the cells was maintained at a stable level with continuous ejection of kainate, neither the N-methyl-D-aspartate antagonists nor thyrotrophin-relasing hormone affected the responses to noxious pinch or heat, although responses to exogenous N-methyl-D-aspartate were still blocked. The wind-up of the electrical responses was, however, reduced by ketamine irrespective of the level of background activity. The results indicate that under these conditions in vivo, N-methyl-D-aspartate receptors mediate ongoing low-frequency background activity rather than phasic high-frequency nociceptive responses. The effects of N-methyl-D-aspartate antagonists and positive modulators on nociceptive responses are evidently indirect, being secondary to changes in background synaptic excitation. These results cannot be explained simply in relation to the voltage-dependence of N-methyl-D-aspartate receptor-mediated activity; other factors, such as modulation by neuropeptides, must be involved.

Aminoadamantanes as NMDA receptor antagonists and antiparkinsonian agents--preclinical studies

Aminoadamantanes such as 1-aminoadamantane (amantadine) and 1-amino-3,5-dimethyladamantane (memantine) are N-methyl-D-aspartate (NMDA) receptor antagonists which show antiparkinsonian-like activity in animal models and in Parkinson's patients. The issue of whether NMDA antagonism plays a role in the symptomatological antiparkinsonian activity of amantadine and memantine is addressed by comparing: behaviourally effective doses, serum/brain levels, and their potency as NMDA receptor antagonists. In the case of memantine, blockade of NMDA receptors is probably the only mechanism responsible for antiparkinsonian activity, whereas for amantadine the situation is clearly far more complex. There are a number of differences between memantine and amantadine both in vitro and in vivo, and although NMDA receptor antagonism certainly participates in the antiparkinsonian activity of amantadine, other effects, some of which are elusive, also play a role. Moreover, it has been suggested that the pathomechanism of Parkinson's disease involves excitotoxic processes and that treatment with NMDA receptor antagonists might also slow the progression of neurodegeneration. If this claim is true, such an effect could be achieved with amantadine and memantine which show neuroprotective activity in animals at therapeutically relevant doses.

Uncompetitive NMDA receptor antagonists attenuate NMDA-induced impairment of passive avoidance learning and LTP

In general, N-methyl-D-aspartate (NMDA) receptor antagonists inhibit learning and long term potentiation (LTP). However, it has been suggested that direct tonic, i.e. non-temporal, activation of NMDA receptors, in contrast to learning, may lead to an increase in synaptic "noise" and, in turn, to a loss of association detection. In the present study, a two-choice passive avoidance task and LTP in vitro (CA1 hippocampal region) were used to address this issue. Dark avoidance learning was impaired by systemic NMDA administration (starting at 25 mg/kg) that was not related to either toxic effects or state-dependent learning. NMDA-induced amnesia was antagonized by ((+)-5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine maleate (MK-801) and 1-amino-3,5-dimethyladamantane (memantine), starting at low doses of 0.05 and 2.5 mg/kg, respectively, in a bell-shaped dose-response relationship. A competitive NMDA receptor antagonist CGP-39551 failed to reverse NMDA-induced amnesia. In hippocampal slices, NMDA (10 microM) depressed (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolproprionic acid (AMPA) receptor-mediated field potentials in CA1 and also caused a moderate reduction of LTP induction/expression. It was this latter effect that was antagonized by memantine (1 microM). Thus, under conditions of tonic activation of NMDA receptors, uncompetitive NMDA receptor antagonists can paradoxically reverse deficits in learning and synaptic plasticity.

Bepridil-induced blockade of NMDA channels in rat hippocampal neurones

Neurones isolated from the CA1 region of rat hippocampal slices by the "vibrodissociation" method were voltage-clamped in the whole-cell configuration. The currents through N-methyl-D-aspartate (NMDA) channels were recorded in response to the rapid application (solution exchange time <30 msec) of 100 microM aspartate (ASP) in a Mg2+-free solution in the presence of 3 microM glycine. When added to the ASP solution, bepridil (BPD) caused a concentration-dependent decrease in both peak and stationary currents due to an uncompetitive open-channel blockade of NMDA channels. At -100 mV, the half-blocking concentration (IC50) for the stationary current was 14.01 +/- 0.17 microM (n = 10). The blocking and unblocking time constants were 7.4 +/- 0.3 x 10(3)/M/sec and 0.12 +/- 0.02/sec, respectively. Membrane hyperpolarization enhanced the BPD block. The equilibrium dissociation constant behaved as an exponential function of the membrane potential and increased e-fold every 37 mV.

Antagonist properties of eliprodil and other NMDA receptor antagonists at rat NR1A/NR2A and NR1A/NR2B receptors expressed in Xenopus oocytes

We have studied the effects of a variety of N-methyl-D-aspartate (NMDA) antagonists acting at different sites of the NMDA receptor complex on NMDA-induced currents in Xenopus oocytes expressing heteromeric NR1A/NR2 and NR1A/NR2B receptors. The polyamine site antagonists eliprodil (IC50 = 3.0 microM) and ifenprodil (IC50 = 0.27 microM) antagonized NMDA responses at NR1A/NR2B receptors but not at NR1A/NR2A receptors (IC50 > 100 microM). The channel blockers dizocilpine, memantine and phencyclidine (PCP) were equally potent antagonists at both receptor subtypes whereas dextromethorphan was four times more potent at NR1A/NR2A receptors. The glycine site antagonists L-689,560 and 7-Cl-kynurenate were 10 times more potent at NR1A/NR2A than at NR1A/NR2B receptor subtypes. The selectivity of eliprodil and ifenprodil for the NR1A/NR2B receptor subtype may, at least partially, explain their favorable side effects profile.

Neuroprotection of acetylcholinergic basal forebrain neurons by memantine and neurokinin B

The present study investigated whether chronic, low dose therapy with memantine could (1) prevent the loss of basal forebrain cholinergic cells induced by injection of N-methyl-D-aspartate (NMDA) into the nucleus basalis magnocellularis (NBM) of rats, and (2) attenuate impaired performance in the radial maze of rats with entorhinal cortex lesions. In addition, we investigated whether neuroprotection could be provided by neurokinin B (NKB). Following an injection of NMDA (0.015 M) into the NBM, rats were implanted with osmotic minipumps containing memantine (20 or 0.20 mg/kg/day for 2 weeks). Other rats were given unilateral NBM injections of 1.0 microliter of Solution A (0.5 microliter containing 8.26 mM NKB and 0.24 units of bacitracin and 0.5 microliter containing 0.03 M NMDA) or Solution B (0.5 microliter of PBS containing 0.24 U of bacitracin and 0.5 microliter containing 0.03 M NMDA). Two weeks later, the anterior cortex was analyzed for choline acetyltransferase (ChAT), a specific marker for the loss of acetylcholinergic neurons. Both chronic administration of memantine, and acute administration of NKB, prevented the decline in cortical ChAT activity associated with injection of NMDA into the NBM, and attenuated a reference memory deficit in the radial maze produced by entorhinal cortex lesions. Thus, memantine infusion at low doses leading to steady-state serum levels within a therapeutic range provides both neuroprotection and cognitive enhancement-an optimal combination for the treatment of neurodegenerative disorders.

Inhibition of glycine currents by dextromethorphan in neurones dissociated from the guinea-pig nucleus tractus solitarii

1. The effect of dextromethorphan (DM) on the current induced by glycine in acutely dissociated nucleus tractus solitarii (NTS) neurones of guinea-pigs was studied by use of the whole-cell patch clamp technique. The effect of DM on gamma-aminobutyric acid (GABA)-induced currents (IGABA) was also examined. 2. DM inhibited 30 microM glycine-induced current (IGly), without affecting the current caused by 30 microM GABA. The action of DM was concentration-dependent, with a maximum effect at 100 microM, and reversible. The half-maximum inhibitory concentration (IC50) of DM was 3.3 microM, about 85 times higher than that of strychnine. 3. DM 3 microM shifted the concentration-response curve for glycine to the right without affecting the maximum value. DM 10 microM shifted the curve even more to the right, although it was not a parallel shift. Strychnine at a concentration of 0.1 microM shifted the curve for glycine in a nearly parallel fashion. 4. The effect of 10 microM DM was slightly weak voltage-dependency, but the lower concentration of DM, 3 microM, inhibited IGly equally at -50 mV and +50 mV. The effect of 3 microM DM on IGly showed no use-dependence. Blockade by strychnine 0.1 microM showed no voltage- or use-dependence. 5. The results indicate that DM inhibits IGly in single neurones of NTS, and further suggest that DM at a low concentration may act on the glycine receptor-ionophore complex, but not on the Cl channel of the complex. However, a relatively high concentration of DM may at least partly affect the Cl- channel of the complex.

Psychotogenicity and N-methyl-D-aspartate receptor antagonism: implications for neuroprotective pharmacotherapy

The development of neuroprotective agents for the prevention of neuronal loss in acute conditions such as stroke and epilepsy or chronic neurodegenerative disorders including Parkinson's disease, Alzheimer's disease, Huntington's chorea, and motor neuron disease is currently focusing on drugs that inhibit excitatory amino acid neurotransmission or exhibit antioxidant properties. Unfortunately, potent antagonists at the N-methyl-D-aspartate (NMDA) type glutamate receptor, which is thought to mediate excitotoxic neuronal injury, e.g., MK-801 or phencyclidine (PCP), share a high probability of inducing psychotomimetic side effects. Further, these drugs have been associated with acute neurotoxicity in vitro and in vivo, precluding their clinical use. In contrast, low affinity NMDA receptor antagonists like amantadine and its dimethyl derivative, memantine, have been administered clinically for the management of Parkinson's disease, dementia, neuroleptic drug-induced side effects, and spasticity. These agents have only rarely induced significant psychotomimetic side effects. Recent pharmacologic advances have helped to elucidate how high drug affinity for the PCP binding site of the NMDA receptor may enhance psychotogenicity. Low affinity and associated fast voltage-dependent channel unblocking kinetics are likely to be responsible for the better tolerance of amantadine and memantine compared with MK-801 and PCP. Further factors apparently modulating psychotogenicity of glutamate receptor antagonists include differential actions on neuronal populations in various brain regions and interactions with neurotransmitter receptors other than the NMDA type glutamate receptor.

No protective effect of the NMDA antagonist memantine in experimental spinal cord injuries

We have investigated the effect of memantine, a clinically used NMDA receptor antagonist, in two experimental animals models of spinal cord injury. The lesions were laser-induced photothrombosis to induce focal spinal cord ischemia and clip compression to mimic traumatic spinal cord injury. Pre- or posttreatment of rats with a dose of memantine (20 mg/kg ip) previously shown to be neuroprotective in cerebral ischemia, failed to affect both the neurological and morphological outcome of ischemic spinal cord injury. Likewise, memantine had no effects on neurological and morphological outcome after experimental traumatic injury. In view of the regional heterogeneity of NMDA receptors, the affinity of memantine for spinal cord NMDA receptors was also determined by studying displacement of [3H] (+)-5-methyl-10,11-dihydro-5-H-dibenzo[a,d]cyclohepten-5-10-imine (MK-801) to rat and human spinal cord homogenates. We found that memantine had an affinity for NMDA receptors in the spinal cord (Ki = 0.58 microM) that was significantly lower compared to that of the cerebral cortex (Ki = 0.23 microM) and that the affinity for NMDA receptors in human spinal cord was even lower. We conclude that in view of available data, memantine should not be chosen for clinical studies on neuroprotection in spinal cord injuries and that the lack of protective effect is most likely due to insufficient affinity of memantine for spinal cord NMDA receptors.

Study of potency, kinetics of block and toxicity of NMDA receptor antagonists using fura-2

NMDA receptor antagonists reduced NMDA-triggered increases in [Ca2+]i measured by fura-2 and showed qualitative differences in the potency and kinetics of block. High potency antagonists produced a slow block which allowed an initial increase in [Ca2+]i that was greater than the steady-state level, while compounds with moderate to low potency produced a rapid block that was at steady-state from the first measurement. The more potent antagonists showed the slowest unblocking rates. Using this simple method, novel NMDA antagonists can be screened to ascertain potency, kinetics of block and relative toxicity, prior to animal testing.

Memantine induces heat shock protein HSP70 in the posterior cingulate cortex, retrosplenial cortex and dentate gyrus of rat brain

High-affinity N-methyl-D-aspartate (NMDA) receptor antagonists like MK-801 are known to induce the heat shock protein, HSP70, in the posterior cingulate cortex and retrosplenial cortex of rat brain. Memantine, which is a low affinity uncompetitive NMDA receptor antagonist, has been used in the treatment of Parkinson's disease in Europe. The faster kinetics of memantine in blocking and unblocking the NMDA receptor-operated ion channel as opposed to high-affinity NMDA antagonists like MK-801 has been thought to account for the safety of memantine. The present study evaluated the neurotoxic potential of memantine and amantadine using the induction of HSP70 immunoreactivity in rat brain. Memantine (25, 50, 75 mg/kg) induced HSP70 in the posterior cingulate, retrosplenial cortex and dentate gyrus of rat brain. In contrast, amantadine (50, 100, 200 mg/kg) did not induce HSP70 in the rat brain. These results suggest that memantine has an antagonistic effect at NMDA receptor in vivo, and raises the possibility that high doses of memantine may cause neuronal damage similar to those observed with other high-affinity NMDA receptor antagonists.

Evidence for native NMDA receptor subtype pharmacology as revealed by differential effects on the NMDA-evoked release of striatal neuromodulators: eliprodil, ifenprodil and other native NMDA receptor subtype selective compounds

NMDA increases the release of [14C]acetylcholine and [3H]spermidine or of [14C]GABA and [3H]dopamine from rat striatal slices. The pharmacology of these responses suggests that release of dopamine and GABA, acetylcholine, and spermidine is mediated, respectively, by three distinct NMDA receptor subtypes. IC50 values of compounds for the inhibition of dopamine and GABA release were closely matched, suggesting mediation by the same subtype. This receptor was generally more sensitive to all NMDA antagonists tested relative to that controlling acetylcholine or spermidine release (channel blockers, glycine antagonists, competitive antagonists and polyamine antagonists). The receptors controlling acetylcholine and spermidine release were characterised by lower antagonist sensitivity in general, and that controlling spermidine release was further defined by a marked insensitivity to ifenprodil, eliprodil, magnesium, dextromethorphan, dextrorphan, memantine, desipramine and polyamine spider toxins. In binding studies in which the displacement of 2 nM [3H]MK801 was studied in membranes prepared from a number of brain regions (in the presence of saturating concentrations of glutamate, glycine and spermidine) small regional differences in IC50 values were observed for a number of channel blockers, but no compound generated biphasic displacement curves that would allow masking of a particular subtype and it was not possible to detect binding components that were insensitive to memantine, dextrorphan dextromethorphan or desipramine. Ifenprodil produced biphasic displacement curves in the 1-day-old rat cortex and midbrain (with IC50 values of approximately 2 and 70 microM) and both ifenprodil and eliprodil displaced a small proportion (18%) of [3H]MK-801 with high affinity in the adult rat spinal cord. Displacement of [3H]MK801 by these compounds in all other adult brain regions (cortex, striatum, hippocampus, thalamus, pons, medulla, cerebellum) was monophasic and of low affinity. In general the subtype selectivity suggested by the release studies was not mirrored in the binding experiments, probably because of excessive heterogeneity of sites in the membrane preparations and to the subtype selectivity of [3H]MK801 itself.

Inhibition by memantine of the development of persistent oral dyskinesias induced by long-term haloperidol treatment of rats

1. Tardive dyskinesia (TD) is a serious side-effect of long-term treatment with neuroleptics. To investigate if neuroleptic-induced excessive stimulation of striatal glutamate receptors may underlie TD development, the effect of the NMDA antagonist, memantine (1-amino-3,5-dimethyladamantane), was studied in a rat model of TD. 2. In an acute experiment, six groups of rats were treated daily for 1 week with either vehicle or memantine 20 or 40 mg kg-1 day-1, and on the seventh day they received one injection of either haloperidol 1.0 mg kg-1 i.p. or saline i.p. In a subsequent long-term experiment lasting 20 weeks, the same treatment was continued, except that haloperidol was injected i.m. as decanoate (38 mg kg-1 every 4 weeks) and control rats received sesame oil. The behaviour was videotaped and scored at intervals during both experiments, and for 16 weeks after cessation of the long-term treatment. 3. In the acute experiment, haloperidol decreased motor activity and memantine increased moving and tended to attenuate the immobility induced by haloperidol. Memantine also enhanced the haloperidol-induced increase in the putative TD-analogue vacuous chewing movements (VCM). 4. In the long-term experiment, the most marked effect of haloperidol was a gradual increase in VCM and the increase persisted significantly for 12 weeks after cessation of treatment. Memantine dose-dependently increased VCM and moving during long-term treatment. However, only one week after stopping treatment, both these effects of memantine disappeared. In contrast to rats previously treated with haloperidol alone, rats co-treated with memantine (both doses) and haloperidol had VCM at the level of controls two weeks after stopping treatment. The blood levels of drugs were within the therapeutic range achieved in human subjects. 5. These results suggest that long-lasting changes induced by haloperidol are prevented by memantine, which supports the theory that excessive NMDA receptor stimulation may be a mechanism underlying the development of persistent VCM in rats and maybe also TD in human subjects.

Effects of memantine on recombinant rat NMDA receptors expressed in HEK 293 cells

1. The actions of the uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists, memantine (1-amino-3,5-dimethyladamantane) and (+)-MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate, dizocilpine), on recombinant NMDA receptors has been studied by use of the whole-cell patch clamp technique. 2. Human embryonic kidney (HEK) 293 cells were transiently transfected with different NMDA receptor subunit combinations (NR1a/NR2A, NR1a/NR2B and NR1a/NR2D). A mutant form of the green fluorescent protein (GFP) cotransfected with the NMDA receptor subunits to enable the visualization of transfected cells. 3. Memantine (0.3-30 microM) blocked L-glutamate (100 microM)-mediated currents in a concentration-dependent manner in NR1a/NR2A, NR1a/NR2B and NR1a/NR2D transfected cells with IC50 values (at -70 mV) of 0.93 +/- 0.15 microM, 0.82 +/- 0.12 microM and 0.47 +/- 0.06 microM (mean +/- s.c. mean), respectively. 4. The memantine-induced block was strongly voltage-dependent. Alteration of the holding potential from -70 mV to +60 mV resulted in an e-fold increase in the IC50 values per 30-33 mV change in membrane potential, for all 3 subunit combinations investigated. 5. The kinetics of the actions of memantine (30 microM) were investigated for the NR1a/2A combination, in 6 cells (13-15 determinations). At -70 mV, the block and recovery from block were both best described by two exponentials with time-constants of 201 +/- 23 ms (81 +/- 2%) and 3.9 +/- 0.6 s and 597 +/- 94 ms (18 +/- 1%) and 18.6 +/- 2.4 s, respectively. The predominant effect of depolarization was to increase the weight of the faster recovery time-constant. Kinetic analysis suggests that these results are consistent with previously proposed Markov models. 6. (+)-MK-801 was studied briefly for comparative purposes. (+)-MK-801 (200 nM) preferentially blocked NMDA receptor currents (at -70 mV) in NR1a/NR2A and NR1a/NR2B (82 +/- 10% and 93 +/- 2% depressions) compared to NR1a/NR2D (38 +/- 7%) transfected cells. (+)-MK-801 appeared to be less voltage-dependent than memantine on all three receptor combinations. 7. In conclusion, memantine was a voltage-dependent antagonist of recombinant rat NMDA receptors expressed in HEK 293 cells but showed little selectivity between the subunits investigated. Its actions on these recombinant receptor combinations are similar to its actions on native NMDA receptors.

Memantine abrogates neurological deficits, but not CNS inflammation, in Lewis rat experimental autoimmune encephalomyelitis

Memantine, a clinically employed drug with N-methyl-D-aspartate (NMDA) receptor antagonistic effects, dose-dependently ameliorates neurological deficits in Lewis rat experimental autoimmune encephalomyelitis (EAE). Interestingly, this therapeutic effect was not due to dampened CNS inflammation, as assessed by immunohistochemical evaluation of spinal cord tissue. Furthermore, numbers of interferon gamma (IFN gamma) mRNA expressing cells were not decreased, as assessed by in situ hybridization. Systemic immunity in terms of numbers of IFN gamma secreting cells in response to immunodominant myelin basic protein (MBP) peptides ex vivo was not reduced, and non-toxic doses of memantine did not affect lymphocyte proliferation or IFN gamma secretion in vitro. Considering these findings, we hypothesize that effector mechanisms responsible for reversible neurological deficits in EAE may involve NMDA receptors, and this highlights neurons as targets during autoimmune neuroinflammation.

Comparative patch-clamp studies with freshly dissociated rat hippocampal and striatal neurons on the NMDA receptor antagonistic effects of amantadine and memantine

Patch- and concentration-clamp techniques were used to compare the effects of the uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists (+)-MK-801 (dizocilpine, (+)-5-methyl-10, 11-dihydro-5H-dibenzocyclohepten-5, 10-imine maleate), ketamine, memantine (1-amino-3,5-dimethyladamantane) and amantadine (1-amino-adamantane) on agonist-induced inward currents in freshly dissociated rat hippocampal and striatal neurons. In hippocampal neurons, ketamine (5 microM), menantine (10 microM) and amantadine (100 microM) selectively antagonized inward current responses to NMDA (500 microM plus glycine 5 microM) in a voltage-dependent manner without affecting responses to (s)-alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (100 microM) or gamma-aminobutyric acid (10 microM). The NMDA receptor antagonistic effect of all four agents was typical of open channel blockade. The kinetics of blockade/unblockade was inversely related to antagonist affinity. In hippocampal neurons amantadine was the least potent NMDA receptor antagonist (IC50 18.6 +/- 0.9 microM) and showed the fastest blocking kinetics, whereas (+)-MK-801 was the most potent (IC50 0.12 +/- 0.01 microM) and showed the slowest blocking kinetics. Memantine (IC50 1.04 +/- 0.26 microM) and ketamine (IC50 0.43 +/- 0.10 microM) were almost equipotent and had similar, intermediate blocking kinetics. In striatal neurons recorded under identical conditions (+)-MK-801, ketamine and memantine were 3- to 4-fold less potent whereas amantadine was somewhat more potent than on hippocampal neurons. This could offer an explanation for the better clinical profile of amantadine in Parkinson's disease, as therapeutically relevant concentrations of amantadine are likely to be more active in the striatum whereas memantine is likely to be more active in other structures.

Effects of the uncompetitive NMDA receptor antagonist memantine on hippocampal long-term potentiation, short-term exploratory modulation and spatial memory in awake, freely moving rats

Chronic treatment of adult male F-344 rats (9-12 months old) with therapeutically relevant doses of memantine (30 mg/kg/day in chow for > 8 weeks) increased the maintenance of long-term potentiation of field excitatory postsynaptic potentials from perforant path-granule cell hippocampal synapses recorded in the fascia dentata in vivo. In contrast, there was no effect of memantine on baseline synaptic responses or population spikes. Likewise, short-term exploratory modulation of these hippocampal evoked responses was not different between memantine-treated and control rats. Both groups of rats were able to learn the spatial version of the Morris water task equally well, but the memantine-treated group showed a strong tendency to show more selective spatial search patterns in the training quadrant of the water pool during a final probe trial. As such, these studies provide the first electrophysiological evidence that memantine can increase the durability of synaptic plasticity and provide preclinical confirmation of the cognitive improvement seen with memantine in the treatment of demented patients.

Infusion of (+) -MK-801 and memantine -- contrasting effects on radial maze learning in rats with entorhinal cortex lesion

(+)-5-Methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine maleate ((+)-MK-801) and 1-amino-3,5-dimethyladamantane (memantine), two uncompetitive antagonists of the NMDA receptor were tested in an allocentric version of the radial maze test (with four out of eight arms reinforced) both in normal rats and after quinolinic acid-induced entorhinal cortex lesions. Both agents were infused s.c. using Alzet osmotic minipumps in order to assure steady state drug levels in the serum and brain during the experiment. In non-lesioned rats, (+)-MK-801 (0.312 mg/kg per day) produced disturbances in learning of spatial information dependent on reference memory but not that involving working memory. In contrast, memantine (20 mg/kg per day) had no effect in normal rats. In rats with entorhinal cortex lesions, (+)-MK-801 enhanced the lesion-induced deficit in reference memory. In contrast, memantine reversed the lesion-induced increase in reference memory errors. The divergent effects of those two uncompetitive NMDA receptor antagonists could, at least partially, be due to the differences reported in their channel blocking kinetics and voltage dependence. The results indicate that under conditions of pathological impairment of brain structures such as entorhinal cortex lesion, memantine might produce beneficial effects on cognitive functions.

Effects of memantine and MK-801 on NMDA-induced currents in cultured neurones and on synaptic transmission and LTP in area CA1 of rat hippocampal slices

The effects of the uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists, memantine (1-amino-3,5-dimethyladamantane) and MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenzocyclo-hepten-5,10-imin e maleate) were compared on synaptic transmission and long-term potentiation (LTP) in hippocampal slices and on NMDA-induced currents in cultured superior collicular neurones. 2. Memantine (10-100 microM) reversibly reduced, but did not abolish, NMDA receptor-mediated secondary population spikes recorded in area CA1 of hippocampal slices bathed in Mg(2+)-free artificial cerebrospinal fluid. 3. Memantine (100 microM) antagonized NMDA receptor-mediated excitatory postsynaptic currents recorded in area CA1 in a strongly voltage-dependent manner i.e. depressed to 11 +/- 4% of control at -35 mV and 95 +/- 5% of control at +40 mV (n = 9), with no apparent effect on response kinetics. 4. The effects of MK-801 and memantine on the induction of LTP were assessed after prolonged pre-incubations with these antagonists. When present for 6.6 +/- 0.4 h prior to tetanic stimulation, memantine blocked the induction of LTP with an IC50 of 11.6 +/- 0.53 microM. By comparison, similar long pre-incubations with MK-801 (6.4 +/- 0.4 h) blocked the induction of LTP with an IC50 of 0.13 +/- 0.02 microM. 5. Memantine and MK-801 reduced NMDA-induced currents in cultured superior colliculus neurones recorded at -70 mV with IC50s of 2.2 +/- 0.2 microM and 0.14 +/- 0.04 microM respectively. The effects of memantine were highly voltage-dependent and behaved as though the affinity decreased epsilon fold per 50 mV of depolarization (apparent delta = 0.71). In contrast, under the conditions used, MK-801 appeared to be much less voltage-dependent i.e. affinity decreased epsilon fold per 329 mV of depolarization (apparent delta = 0.15). 6. Depolarizing steps from -70 mV to +50 mV in the continuous presence of memantine (10 microM) caused a rapid relief of blockade of NMDA-induced currents from 83.7 +/- 1.9% to 21.8 +/- 1.8% (n = 5). This relief was best fitted by a double exponential function (17.2 +/- 11.7 and 698 +/- 204 ms), the faster component of which was most pronounced. 7. In conclusion, whereas MK-801 is equipotent in blocking NMDA-induced currents (at - 70 mV) and the induction of LTP, memantine is relatively less potent in blocking the induction of LTP. This is due to its rapid relief of blockade upon depolarization; a property which might explain its promising clinical profile in the treatment of chronic neurodegenerative diseases.

Learning deficits induced by chronic intraventricular infusion of quinolinic acid--protection by MK-801 and memantine

The NMDA receptor agonist quinolinic acid (9 mM) was infused i.c.v. via ALZET osmotic minipumps for 2 weeks. This treatment produced a persistent, short-term memory deficit in the T-maze. Autoradiography revealed a decrease in the density of choline uptake sites in the hippocampus. Parallel s.c. infusion by another minipump of the uncompetitive NMDA receptor antagonist memantine (1-amino-3,5-dimethyladamantane, 20 mg/kg per day) or (+)-5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine maleate ((+)-MK-801, 0.31 mg/kg day) prevented the learning deterioration induced by quinolinic acid. The treatment with memantine resulted in steady-state serum levels of 1.2 mu M which, based on in vitro data, should assure inhibition of NMDA receptors and are similar to levels seen in the serum of demented patients treated with this agent. In naive animals this treatment had no effect on either learning or on ex vivo induction of long-term potentiation, indicating that under chronic conditions it is possible to obtain neuroprotective effects with NMDA receptor antagonists without negative effects on memory processes. This contrasts to some acute insults (e.g. ischaemia) where high doses of NMDA receptor antagonists that produce side effects are required.

MK-801, memantine and amantadine show neuroprotective activity in the nucleus basalis magnocellularis

The activation of glutamate receptors by endogenuous glutamate has been implicated in the processes that underlie cell loss associated with ischemia and trauma and in the development of some neurodegenerative diseases. The antagonism of NMDA-sensitive glutamate receptors may therefore have therapeutic applications. The present study compared the side effects and neuroprotective potency of 1-aminoadamantane hydrochloride (amantadine), 1-amino-3,5-dimethyladamantane hydrochloride (memantine), and (+)-5-methyl-10,11-dihydro-5H-debenzocyclhepten-5,10-imine maleate ((+)-MK-801) against NMDA injected directly into the nucleus basalis magnocellularis of rats. Each drug significantly attenuated the loss of nucleus basalis magnocellularis cholinergic cells. The ED50s were respectively 0.077, 2.81 and 43.5 mg/kg for (+)-MK-801, memantine and amantadine, giving a relative potency ratio of 1:36:565. The ratio of the ED50 for the side effects observed, including ataxia, myorelaxation and stereotypy, and the ED50 for neuroprotective ability, was highest for memantine and the lowest for (+)-MK-801. The results suggest that a potential neuroprotective action of NMDA receptor antagonists, memantine and amantadine in particular, can be seen at low doses lacking side effects.

Orphenadrine is an uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist: binding and patch clamp studies

Orphenadrine has been used as an antiparkinsonian, antispastic and analgesic drug for many years. Here we show that orphenadrine inhibits [3H]MK-801 binding to the phencyclidine (PCP) binding site of the N-methyl-D-aspartate (NMDA)-receptor in homogenates of postmortem human frontal cortex with a Ki-value of 6.0 +/- 0.7 microM. The NMDA receptor antagonistic effects of orphenadrine were assessed using concentration- and patch-clamp techniques on cultured superior colliculus neurones. Orphenadrine blocked open NMDA receptor channels with fast kinetics and in a strongly voltage-dependent manner. The IC50-value against steady state currents at -70 mV was 16.2 +/- 1.6 microM (n = 6). Orphenadrine exhibited relatively fast, concentration-dependent open channel blocking kinetics (Kon 0.013 +/- 0.002 10(6) M-1S-1) whereas the offset rate was concentration-independent (Koff 0.230 +/- 0.004 S-1). Calculation of the ratio Koff/Kon revealed an apparent Kd-value of 17.2 microM which is nearly identical to the IC50 calculated at equilibrium.

Comparison of the potency, kinetics and voltage-dependency of a series of uncompetitive NMDA receptor antagonists in vitro with anticonvulsive and motor impairment activity in vivo

The amino-adamantane derivatives memantine (1-amino-3,5-dimethyladamantane) and amantadine (1-amino-adamantane) are relatively low affinity, uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists which have been used clinically in the treatment of dementia and Parkinson's disease respectively for several years without serious side effects. The aim of this study was to test whether memantine, amantadine and other low affinity uncompetitive NMDA receptor antagonists also have better therapeutic indices than high affinity antagonists in preclinical models of epilepsy by assessing the potency, kinetics and voltage-dependency of open channel blockade for a series antagonists in vitro and comparing these effects to anticonvulsive and motor impairment activity in vivo. The compounds tested were memantine, amantadine, 14 other amino-adamantanes, (+)-MK-801, ketamine, dextrorphan, dextromethorphan and phencyclidine. The offset kinetics of open-channel blockade assessed with whole cell patch clamp recordings from cultured superior colliculus neurones were highly correlated to potency i.e. the less potent antagonists showed faster unblocking kinetics (Koff, r = 0.904). Although, onset kinetics as assessed by Kon were not correlated to potency (r = 0.023), tau on estimated at IC50 is perhaps a more meaningful measure of onset kinetics at equieffective concentrations and was also well correlated to potency (r = -0.863). All amino-adamantanes tested were strongly voltage-dependent. There was also a good correlation between the in vitro potencies of uncompetitive NMDA receptor antagonists assessed with patch clamp recordings and displacement of equilibrium [3H](+)-MK-801 binding and their in vivo activity against maximal electroshock (MES) and pentylenetetrazol (PTZ) induced tonic convulsions and NMDA-induced lethality in mice. Memantine and four other amino-adamantanes with somewhat lower potency and faster blocking kinetics had better therapeutic indices (ED50 rotarod and traction reflex over ED50 in MES-induced convulsions; TI = 2-4) than substances with higher affinity such as ketamine, dextrorphan and (+)-MK-801 (TI < 2). However, amantadine and several other amino-adamantanes with lower potency than memantine actually had poorer therapeutic indices (TI < or = 0.5) which may have been due to additional actions at other ion channels or receptors at the doses necessary to protect against seizures. In fact, ED50 in the MES test was negatively-correlated to therapeutic indices (traction r = -0.790, rotarod r = -0.797) i.e. the less potent uncompetitive antagonists had worse therapeutic indices. The data from the present study do not lend support to the idea that low affinity, open channel NMDA receptor blockers are also effective in models of epilepsy at doses having little effect on physiological processes. It should be stressed that these data do not contradict the known therapeutic safety of memantine and amantadine in dementia and Parkinson's disease respectively. Thus the good clinical profile of memantine in dementia has been attributed not only to its fast blocking/unblocking kinetics but also to its strong voltage-dependency. These biophysical properties may allow therapeutically-relevant concentrations to block chronic, low level pathological activation of NMDA receptors whilst leaving their synaptic activation intact. Precisely these properties may also underlie the poor therapeutic indices seen in the present study on antiepileptic activity due to the synaptic nature of both seizures and normal glutamatergic transmission.

Cerebrospinal fluid and serum concentrations of the N-methyl-D-aspartate (NMDA) receptor antagonist memantine in man

Memantine is an uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist with therapeutic potential in dementia, spasticity and Parkinson's disease. The Ki-value of memantine at the phencyclidine (PCP) binding site of the NMDA receptor is 0.5 microM in human frontal cortex. We investigated whether concentrations of memantine in cerebrospinal fluid (CSF) and serum samples under therapeutic conditions are in the range of its Ki-value at the PCP binding site. The serum levels ranged from 0.025 to 0.529 microM with daily doses between 5 and 30 mg. CSF levels were highly correlated to serum levels and were below serum levels in each patient with a mean CSF/serum ratio of 0.52. Serum and CSF levels were correlated to the daily dose, but not to the duration of treatment. At the concentrations reported here, memantine is expected to specifically interact with the PCP binding site of the NMDA receptor.

Therapeutic brain concentration of the NMDA receptor antagonist amantadine

Amantadine (1-amino-adamantane) is clinically used for the management of Parkinson's disease and drug-induced extrapyramidal symptoms. It has previously been shown that amantadine is a low-affinity uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist with rapid blocking and unblocking channel kinetics (Ki-value at the PCP binding site = 10 microM). The aim of the present studies was to estimate concentrations of amantadine in the central nervous system under therapeutic conditions. In homogenates of postmortem human brain tissue the amantadine concentration appeared to be homogeneously distributed over a wide range of brain areas. Amantadine concentration increased with duration of treatment and decreased wit drug-free time. When the duration of treatment was > or = 10 days and drug-free time < or = 3 days, mean amantadine concentrations in postmortem brain tissue ranged from 48.2 to 386 microM. In contrast to brain tissue, amantadine concentration in cerebrospinal fluid (CSF) and serum was in the low micromolar range ( < 17 microM). CSF and serum total values were highly correlated to each other and were always lower in CSF. The mean CSF/serum ratio for total amantadine was 0.76. To further estimate the extracellular concentration, amantadine was determined in microdialysates in the rat striatum. At behaviorally active doses, amantadine concentration in striatal microdialysates ranged between 6 and 21 microM. These results indicate that extracellular concentrations of amantadine (CSF and serum values in patients, striatal microdialysates in the rat) are in the range of its Ki-value at the PCP binding site. Amantadine concentrations in brain tissue are much higher, probably due to intralysosomal accumulation.