Memantine restores long term potentiation impaired by tonic N-methyl-D-aspartate (NMDA) receptor activation following reduction of Mg2+ in hippocampal slices.

This study compared the ability of memantine and (+)MK-801 to counteract deficits in the induction of long term potentiation (LTP) following reduction of Mg2+ in hippocampal slices--a model of increased synaptic noise due to tonic N-methyl-D-aspartate (NMDA) receptor activation. Decreasing Mg2+ from 1 mM to 10 microM for 60 min enhanced baseline field excitatory post-synaptic potential (fEPSP) slopes (87.2 +/- 10.6% above control) and impaired LTP (-4.1 +/- 9.8% compared to pre-tetanic levels). Long pre-incubations with memantine (1 microM), a concentration achieved in the CSF of dementia patients, almost fully restored the induction of LTP (to 43.4 +/- 8.4%) without changing the enhancement of baseline fEPSP slopes (84.1 +/- 11.6%). Memantine (10 microM) fully restored the induction of LTP (61.5 +/- 5.3%) and also decreased the enhancement of baseline fEPSP slopes (30.1 +/- 4.9%). In contrast, although (+)MK-801 (0.01, 0.1 and 1 microM) caused a concentration-dependent reduction in the low Mg2+ -induced enhancement of baseline fEPSP slopes, it was not able to restore the induction of LTP (3.0 +/- 9.8%, 16.3 +/- 5.7% and 4.8 +/- 6.7% respectively). These data indicate that memantine could produce symptomatological improvement in learning under conditions of tonic NMDA receptor activation such as those occurring in chronic neurodegenerative diseases whereas (+)MK-801 is likely to have only negative effects.

Differential effects of NMDA-receptor antagonists on long-term potentiation and hypoxic/hypoglycaemic excitotoxicity in hippocampal slices.

Whole-cell patch clamp recording from cultured hippocampal neurones was used to investigate the NMDA antagonistic effects of the glycineB antagonist 5,7-DCKA and the competitive antagonist CGP 37849. Extracellular field potential recording from area CA1 of hippocampal slices was used to investigate their effects on the induction of LTP and hypoxia/hypoglycaemia-induced suppression of fEPSPs. Additionally, memantine and (+)MK-801 were tested in the later model. 5,7-DCKA inhibited NMDA-induced plateau currents (IC50=0.24+/-0.02 microM) with around nine times higher potency than against peak (IC50=2.14+/-0.17 microM). In contrast, CGP 37849 slowed the onset of NMDA-induced currents considerably and antagonized currents at the time point when the peak component occurred in control responses (IC50=0.18+/-0.01 microM) with around seven times higher potency than against plateau (IC50=1.26+/-0.19 microM). Both 5,7-DCKA and CGP 37849 inhibited the induction of LTP (IC50s=2.53+/-0.13 and 0.37+/-0.04 microM respectively) with potencies close to those inhibiting peak currents in patch clamp studies. 5,7-DCKA and CGP 37849 also blocked the hypoxia/hypoglycaemia-induced suppression of fEPSPs but CGP 37849 (EC50=4.3+/-0.33 microM) was far less potent than against the induction of LTP whilst 5,7-DCKA (EC50=1.47+/-0.04 microM) had similar potency in these two models. Memantine and (+)MK-801 also blocked hypoxia/hypoglycaemia-induced suppression of fEPSPs with EC50s of 14.1+/-0.52 and 0.53+/-0.02 microM respectively. Whereas memantine blocked this effect with similar potency as we previously reported for LTP, (+)MK-801 was four time less potent in this model. The calculated relative therapeutic indices (IC50 LTP over EC50 hypoxia/hypoglycaemia) for 5,7-DCKA, CGP 37849, memantine and (+)MK-801 were 1.72, 0.09, 0.82 and 0.24 respectively. These results show that even in a severe model of hypoxia/hypoglycaemia, glycineB site antagonists and moderate affinity channel blockers exhibit a better therapeutic index than competitive antagonists and high affinity channel blockers. It is likely that in milder forms of pathology the observed differences in therapeutic indices remain the same but the absolute values are expected to be higher.

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.

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.

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.

Trans-(+-)-1-amino-1,3-cyclopentanedicarboxylate (trans-ACPD) stimulates cAMP accumulation in rat cerebral cortical slices but not in glial or neuronal cultures.

Recent cloning experiments indicate that multiple metabotropic receptors for excitatory amino acids (EAAs) exist, which are coupled to adenylate cyclase. Trans-(+-)-1-amino-1,3-cyclopentanedicarboxylate (trans-ACPD) is a selective agonist of metabotropic receptors for EAAs. One of the effects of trans-ACPD is stimulation of cAMP accumulation. In the present experiments, cAMP accumulation was measured using a [3H]adenine-prelabelling technique. It has been found that trans-AC-PD was able to induce significant stimulation of cAMP accumulation in rat cerebral cortical slices, with ED50 of 47.8 microM, which value is similar to that described earlier for hippocampal slices. However, trans-ACPD had no effect on cAMP accumulation either in primary neuronal or glial cell cultures. The reason for the lack of effects of trans-ACPD on cAMP accumulation in primary cultures from glial cells and neurons is discussed.

Middle cerebral artery occlusion produces secondary, remote impairment in hippocampal plasticity of rats - involvement of N-methyl-D-aspartate receptors?

There is increasing evidence that focal cerebral ischaemia produces remote functional alterations that may substantially contribute to the post-stroke neurological outcome. Changes initially limited to peri-infarct areas may evolve and spread via transneuronal connections to other structures. In the present study we investigated whether focal ischaemia produced by 2-h occlusion of the middle cerebral artery (MCAo) in SD rats may influence the physiological function of the hippocampus. Three days later in vitro long-term potentiation (LTP) was studied in hippocampal slices from ipsi- and contralateral hemispheres. In rats with MCAo LTP was not-inducible in the ipsilateral hippocampus, while the contralateral side expressed stable potentiation (6.6+/-4.1 vs. 35. 0+/-8.0%, respectively). Treatment with 6-h i.v. infusion of an uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist MRZ 2/579 starting at reperfusion not only preserved but additionally enhanced ipsilateral LTP, while a slight insignificant decrease was observed in the contralateral side (77.0+/-18.4 vs. 20.8+/-6.5%). The study demonstrates post-stroke functional changes in the hippocampus that can be modulated by NMDA receptor antagonists.

The effect of interaction between (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R)-ACPD and noradrenaline on cyclic AMP accumulation: different actions in brain slices, primary glial and neuronal cell cultures.

The existence of multiple metabotropic receptors for excitatory amino acids is firmly established by recent cloning experiments. Several subtypes of those receptors are coupled to adenylate cyclase. (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R)-ACPD) is a selective agonist for metabotropic glutamate receptors, it influences cyclic AMP accumulation in brain and is able to enhance the effect of other substances which stimulate cyclic AMP accumulation. Here we present data showing that (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid enhanced the action of noradrenaline on cyclic AMP accumulation in rat brain cortical slices (EC50 40 microM). On the contrary, it inhibited the action of noradrenaline in primary glial cell cultures (EC50 50 microM), being without any effect on noradrenaline-induced cyclic AMP accumulation in primary neuronal cell cultures. The differences may reflect stimulation of different types of metabotropic glutamate receptors in various preparations from rat brain. The nature of interaction between noradrenaline and (1S,3R)-ACPD remains to be studied.

Pharmacological action of zotepine and other antipsychotics on central 5-hydroxytryptamine receptor subtypes.

The effect of the atypical neuroleptic zotepine (CAS 26615-21-4), in comparison with clozapine, risperidone and haloperidol, on the responsiveness of different 5-hydroxytryptamine (5-HT1) receptor subtypes to their agonists was examined in rats and mice. The above antipsychotics were investigated in the following behavioural tests: 8-OH-DPAT (8-hydroxy-dipropylaminotetralin)-induced behavioural syndrome in rats, mCPP (mchlorophenylpiperazine)-induced hypothermia in mice and mCPP-induced hypoactivity measured in the open field in rats. Zotepine, clozapine and haloperidol did not affect the behavioural syndrome induced by 8-OH-DPAT (the selective agonist of 5-HT1A, receptor), only risperidone (used in higher doses) attenuated the effect of 8-OH-DPAT. The mCPP-induced hypothermia in mice (a 5-HT1B effect) was affected by neither zotepine nor clozapine, risperidone and haloperidol, all of them used in low doses which did not influence per se the body temperature of mice. All the tested antipsychotics given at high doses induced hypothermia in control mice; at the same time, zotepine, clozapine and risperidone attenuated the hypothermic effect of mCPP. mCPP decreases the exploratory activity of rats, this effect being considered to be mediated by 5-HT1C receptors. The tested antipsychotics, used in low doses, influenced neither the exploratory activity nor the hypoactivity induced by mCPP. When used at higher doses, they induced hypoactivity in control rats; the hypoactivity after joint administration of zotepine, risperidone or haloperidol and mCPP was significantly greater than after mCPP alone, whereas clozapine slightly attenuated the effect of mCPP.(ABSTRACT TRUNCATED AT 250 WORDS)