Biomedical subjects

C G Parsons

Publications and source records attributed to C G Parsons.

At least 19 recordsLinked to original sources

PubMed2005

Excitatory amino acid neurotransmission.

In recent years great progress has been made in understanding the function of ionotropic and metabotropic glutamate receptors; their pharmacology and potential therapeutic applications. It should be stressed that there are already N-methyl-D-aspartate (NMDA) antagonists in clinical use, such as memantine, which proves the feasibility of their therapeutic potential. It seems unlikely that competitive NMDA receptor antagonists and high-affinity channel blockers will find therapeutic use due to limiting side-effects, whereas agents acting at the glycineB site, NMDA receptor subtype-selective agents and moderate-affinity channel blockers are far more promising. This is supported by the fact that there are several glycineB antagonists, NMDA moderate-affinity channel blockers and NR2B-selective agents under development. Positive and negative modulators of AMPA receptors such as the AMPAkines and 2,3-benzodiazepines also show more promise than e.g. competitive antagonists. Great progress has also been made in the field of metabotropic glutamate receptors since the discovery of novel, allosteric modulatory sites for these receptors. Selective agents acting at these transmembrane sites have been developed that are more drug-like and have a much better access to the central nervous system than their competitive counterparts. The chapter will critically review preclinical and scarce clinical experience in the development of new ionotropic and metabotropic glutamate receptor modulators according to the following scheme: rational, preclinical findings in animal models and finally clinical experience, where available.

Animals↗

PubMed2003

Alpha2-adrenoreceptor activation inhibits LTP and LTD in the basolateral amygdala: involvement of Gi/o-protein-mediated modulation of Ca2+-channels and inwardly rectifying K+-channels in LTD.

Activation of adrenoreceptors modulates synaptic transmission in the basolateral amygdala. Here, we investigated the effects of alpha2-adrenoreceptor activation on long-term depression and long-term potentiation in an in vitro slice preparation of the mouse basolateral amygdala. Field potentials and excitatory postsynaptic currents were evoked in the basolateral amygdala by stimulating the lateral amygdala. Norepinephrine (20 micro m) reduced synaptic transmission and completely blocked the induction of long-term potentiation and long-term depression. The alpha2-adrenoreceptor antagonist yohimbine (2 micro m) reversed this effect. The alpha2-adrenoreceptor agonist clonidine (10 micro m) mimicked the effects of norepinephrine. The Gi/o-protein inhibitor pertussis toxin (5 micro g/mL) reversed the effect of clonidine. Long-term depression was blocked in the presence of omega-conotoxin GVIA, but not omega-agatoxin IVA. Clonidine inhibited voltage-activated Ca2+ currents mediated via N- or P/Q-type Ca2+-channels. The inhibitory action of clonidine on long-term depression was reversed when inwardly rectifying K+-channels were blocked by Ba2+ (300 micro m). The present data suggest that alpha2-adrenoreceptor activation impairs the induction of long-term depression in the basolateral amygdala by a Gi/o-protein-mediated inhibition of presynaptic N-type Ca2+-channels and activation of inwardly-rectifying K+-channels.

3',5'-Cyclic-AMP Phosphodiesterases↗

PubMed2002

Amino-alkyl-cyclohexanes as a novel class of uncompetitive NMDA receptor antagonists.

Because of its widespread involvement in the physiology and pathology of the CNS, the glutamatergic system has gained considerable attention as a potential target for development of new agents for a number of therapeutic indications. In this respect, the glutamate receptor subtype of the NMDA type has been most intensively studied. The present review describes the rational for developing amino-alkyl-cyclohexanes, as new uncompetitive NMDA receptor antagonists based on our positive experience with memantine which has been used clinically for many years for the treatment of neurodegenerative dementia. Many amino-alkyl-cyclohexane derivatives have been evaluated in vitro and in animal models, and in turn, one structure, namely neramexane HCl (MRZ 2/579) was selected for further development. This agent shows some similarity to memantine e.g. channel blocking kinetics, voltage dependency, and affinity. Preclinical tests indicated particularly good activity in animal models of alcoholism (self-administration, withdrawal-induced audiogenic seizures etc.) and pain (chronic pain, inhibition of tolerance to the analgesic effects of morphine). It turn, this agent has recently entered phase II of clinical trials in alcoholism after a favourable profile seen in phase I studies.

Animals↗

PubMed2001

NMDA receptors as targets for drug action in neuropathic pain.

Hyperalgesia and allodynia following peripheral tissue or nerve injury are not only due to an increase in the sensitivity of primary afferent nociceptors at the site of injury but also depend on NMDA receptor-mediated central changes in synaptic excitability. Functional inhibition of NMDA receptors can be achieved through actions at different recognition sites such as the primary transmitter site (competitive), strychnine-insensitive glycine site (glycine(B)), polyamine site (NR2B selective) and phencyclidine site located inside the cationic channel. Unfortunately, most agents which completely block NMDA receptors cause numerous side effects such as memory impairment, psychotomimetic effects, ataxia and motor incoordination. There is now, however, considerable evidence that moderate affinity channel blockers, glycine(B) and NR2B selective antagonists show a much better profile in animal models than high affinity channel blockers and competitive NMDA receptor antagonists. These "therapeutically" safe NMDA receptor antagonists are also able to slow or prevent the development of opioid tolerance, indicating the utility of their combination with opioids in the treatment of chronic pain. The antinociceptive effects of NMDA receptor antagonists and opioids could be predicted to be synergistic and the presence of an NMDA receptor antagonist should block both the development of chronic pain states and inhibit the development of tolerance to the analgesic effects of morphine. Peripheral NMDA receptors offer a very attractive target for NMDA receptor antagonists that do not cross the blood brain barrier in inflammatory and visceral pain. Such agents might be predicted to be devoid of CNS side effects at doses producing powerful antinociception at peripheral NMDA receptors.

Animals↗

PubMed2001

The N-methyl-D-aspartate receptor channel blockers memantine, MRZ 2/579 and other amino-alkyl-cyclohexanes antagonise 5-HT(3) receptor currents in cultured HEK-293 and N1E-115 cell systems in a non-competitive manner.

The type 3 serotonin (5-HT(3)) receptor is a ligand-gated ion channel. In concentration-clamp experiments, we investigated the effects of the uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists memantine, amantadine and MRZ 2/579 on 5-HT receptors stabley expressed in HEK-293 cells and on native 5-HT(3) receptors in the N1E-115 cell line. All agents antagonized serotonin (10 microM)-induced inward currents with similar potency to that reported for NMDA receptors. This effect was characterized by inducing a pronounced receptor desensitization, and was probably non-competitive and voltage-independent. In contrast, (S)-ketamine was much weaker as an antagonist of 5-HT(3) receptors than NMDA receptors. Similar effects on 5-HT(3) receptors have been reported previously for a variety of anti-depressants and it is possible that the clinical anti-depressant effects reported for both memantine and amantadine are mediated, at least in part, by antagonistic effects at 5-HT(3) receptors.

Animals↗

PubMed2001

The in vivo relevance of the varied channel-blocking properties of uncompetitive NMDA antagonists: tests on spinal neurones.

The voltage dependence and channel-blocking kinetics of uncompetitive NMDA receptor antagonists have been well-described using in vitro techniques, but there is little evidence concerning the functional significance of these properties in vivo. We have now compared the effects of NMDA antagonists that display varied profiles of voltage-dependent block in vitro, on responses of spinal neurones in anaesthetised rats. The compounds examined were the uncompetitive channel blockers memantine, ketamine and MK-801 and, for comparison, an antagonist that acts at the strychnine-insensitive glycine binding site (MRZ 2/502). Using frequency of spike discharge as an indicator of somatic depolarisation, we have compared the effects of these antagonists on responses evoked by iontophoretic NMDA application and on synaptic responses evoked by pinch or electrical stimulation (the latter eliciting "wind-up"). The effectiveness of the antagonists was directly but variably related to the discharge frequency of the test response. The rank order of dependence on firing rate matched the rank order of voltage dependence reported in vitro, namely: memantine > ketamine > MK-801> or = MRZ 2/502. Doses that reduced responses to iontophoretic application of NMDA were less effective at reducing responses to pinch, perhaps due to the major non-NMDA component of the synaptic response. Memantine preferentially reduced "wind-up" relative to responses to pinch, whereas ketamine and MK-801 reduced both types of synaptic responses in parallel. This "filtering" by low affinity, voltage-dependent NMDA antagonists such as memantine, of non-physiological activity whilst leaving normal synaptic events relatively untouched, may contribute to their more favourable clinical profile.

Animals↗

PubMed2000

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.

2-Amino-5-phosphonovalerate↗

PubMed2000

A novel series of 2-carboxytetrahydroquinolines provides new insights into the eastern region of glycine site NMDA antagonists.

A series of potent 4-substituted tetrahydroquinolines has been synthesized and biologically tested in order to refine the eastern region of the pharmacophore model for glycine site NMDA antagonists concerning the assessment of lipophilicity, flexibility, and hydrogen bonding. Displacement studies on rat cortical membranes using [3H]-5,7-dichlorokynurenic acid as a radioligand indicated that binding affinities are markedly enhanced when additional hydrogen-accepting groups are introduced into the eastern region of the 2-carboxytetrahydroquinolines. Among the most potent ligands were some urea, sulfonylurea, and crown ether compounds as interesting leads for new diagnostics, especially for the evaluation of PET tracers, which allow biodistribution studies and NMDA receptor studies in the living organism.

Animals↗

PubMed2000

Neuroprotective and symptomatological action of memantine relevant for Alzheimer's disease--a unified glutamatergic hypothesis on the mechanism of action.

The involvement of glutamate mediated neurotoxicity in the pathogenesis of Alzheimer's disease is finding increasingly more acceptance in the scientific community. Central to this hypothesis is the assumption that in particular glutamate receptors of the N-methyl-D-aspartate (NMDA) type are overactivated in a tonic rather than a phasic manner. Such continuous mild activation leads under chronic conditions to neuronal damage. Moreover, one should consider that impairment of plasticity (learning) may result not only from neuronal damage per se but also from continuous activation of NMDA receptors. To investigate this possibility we tested whether overactivation of NMDA receptors using either non-toxic doses/concentrations of a direct NMDA agonist or through an indirect approach--decrease in magnesium concentration--produces deficits in plasticity. In fact NMDA both in vivo (passive avoidance test) and in vitro (LTP in CA1 region) impaired learning and synaptic plasticity. Under these conditions memantine which is an uncompetitive NMDA receptor antagonist with features of "improved magnesium" (voltage dependence, affinity) attenuated the deficit. The more direct proof that memantine can act as a surrogate for magnesium was obtained in LTP experiments under low magnesium conditions. In this case as well, impaired LTP was restored in the presence of therapeutically relevant concentrations of memantine (1 microM). In vivo, doses leading to similar brain/serum levels produce neuroprotection in animal models relevant for neurodegeneration in Alzheimer's disease such as neurotoxicity produced by inflammation in the NBM or beta-amyloid injection to the hippocampus. Hence, we postulate that if in Alzheimer's disease overactivation of NMDA receptors occurs indeed, memantine would be expected to improve both symptoms (cognition) and slow down disease progression because it takes over the physiological function of magnesium.

Journal Article↗

PubMed2000

Memantine and the amino-alkyl-cyclohexane MRZ 2/579 are moderate affinity uncompetitive NMDA receptor antagonists--in vitro characterisation.

There is general agreement that moderate affinity uncompetitive NMDA receptor antagonists combine good efficacy and tolerability in animal models of disturbances in glutamatergic transmission. There are several theories on which properties are important for this profile including 1, rapid access to the channel at the start of pathological overactivity 2, rapid, voltage-dependent relief of blockade during physiological synaptic activation and 3, partial untrapping. Merz has developed a series of novel uncompetitive NMDA receptor antagonists based on the cyclohexane structure. In cultured hippocampal neurones MRZ 2/579 (1-amino-1,3,3,5,5-pentamethylcyclohexane) shows similar blocking kinetics to memantine (Kon 10.7 * 10(4)M(-1)sec(-1), Koff 0.20sec(-1) at -70mV) and binds at the same depth in the NMDA receptor channel (delta = 0.8). The potency of MRZ 2/579 assessed as Kd = Koff/Kon = 1.87microM agrees well with the IC50 of 1.29microM against steady-state currents in cultured hippocampal neurones (at -70mV) and with the Ki in [3H]-MK-801 binding of 0.65microM. MRZ 2/579 protected cultured cortical neurones against glutamate toxicity with an IC50 of 2.16microM and was also effective in protecting hippocampal slices against hypoxia/hypoglycaemia-induced reduction of fEPSP amplitude in CA1 with an EC50 of 7.01microM. MRZ 2/579 has similar potency and bio-availability to memantine in vivo assessed using microdialysis, microiontophoresis and MES-induced seizures. Initial characterization in animal models provides strong support for the assumption that MRZ 2/579 could be a useful therapeutic in morphine/alcohol dependence, inhibition of morphine tolerance, chronic pain and as a neuroprotective agent.

Animals↗

PubMed2000

Synthesis and structure-affinity relationships of 1,3, 5-alkylsubstituted cyclohexylamines binding at NMDA receptor PCP site.

A series of 1,3,5-alkylsubstituted cyclohexylamines 2 were synthesized as ligands for the N-methyl-D-aspartate (NMDA) receptor phencyclidine (PCP) binding site. Pure diastereomers with defined configuration of amino group 2-ax and 2-eq were obtained. The optimal size of 1,3,5-substituents was determined for cyclohexylamines 2 with an equatorial amino group in the lowest energy conformation using Hansch analysis. According to the data, the lipophilic part of cyclohexylamines 2 does not discriminate between hydrophobic regions of the PCP binding site but rather recognizes this site as a whole lipophilic pocket.

Amantadine↗

PubMed2000

NMDA channel blockers: memantine and amino-aklylcyclohexanes--in vivo characterization.

The previous overviews provided the basis for better therapeutic efficacy/tolerability of low to moderate affinity NMDA channel blockers. This prediction finds support in in vitro studies comparing protective and plasticity impairing effects (therapeutic vs. side-effect) of memantine and (+)MK-801. In fact it turned out that memantine had a far better therapeutic index. But can it be confirmed in the in vivo situation?

Animals↗

PubMed1999

Brain distribution of an uncompetitive NMDA receptor antagonist; comparison to its in vitro potency in electrophysiological studies.

Although the concentration of drugs in brain homogenates is relatively easy to determine, such data are sometimes misleading due to accumulation in intracellular compartments. This is apparent for uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists where concentrations assessed in this manner are much higher than those sufficient to block the NMDA channel from the extracellular space. The aim of the present study was to determine whether free brain concentrations (extracellular fluid - ECF) of a new uncompetitive NMDA receptor antagonist MRZ 2/579 (1-amino-1,3,3,5,5-pentamethyl-cyclohexane hydrochloride) following administration of doses effective in animal models are sufficient to block NMDA receptors based on its potency in vitro. This issue was addressed using brain microdialysis corrected for in vivo recovery and patch clamp experiments.MRZ 2/579 blocked steady-state inward current responses of cultured hippocampal neurones to NMDA with an IC50 of 1.11 microM at -70 mV. Much higher concentrations of MRZ 2/579 blocked voltage-activated Ca2+ channels with an IC50 of 340 microM. MRZ 2/579 (10 microM) reduced peak inward current responses of neuronal nicotinic receptors only to 72.3% of control. MRZ 2/579 (10-100 microM) had little or no effect at AMPA and GABA(A) receptors. Following chronic s.c. infusion of MRZ 2/579 (40 mg/kg day for 7 days) brain ECF (2.15 microM) and cerebro-spinal fluid (CSF) levels (2.16 microM) were twofold lower than free plasma levels (4.3 microM). MRZ 2/579 showed pronounced accumulation in brain tissue compared to free plasma (28-fold) and ECF (58-fold). After acute i.p. administration (5, 10 and 20 mg/kg) peak concentrations in ECF were 0.70, 0.96 and 2.53 microM, respectively. In conclusion, MRZ 2/579 is indeed strongly accumulated in brain tissue compared to brain ECF, CSF and plasma. However, the brain ECF levels attained following administration of behaviourally effective doses are sufficient for selective NMDA receptor blockade.

Animals↗

PubMed1999

Expression of early hippocampal CA1 LTP does not lead to changes in AMPA-EPSC kinetics or sensitivity to cyclothiazide.

We have analysed whether the expression of long-term potentiation (LTP) in rat hippocampal CA1 neurons involves a change in the kinetics of (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated excitatory postsynaptic currents (EPSCs) (AMPA-EPSCs) or their susceptibility to the AMPA receptor modulator cyclothiazide. AMPA-EPSCs in the CA1 region were evoked by alternate stimulation of two independent Schaffer collateral-commissural inputs of slices of adult rat hippocampus. In the current-clamp mode a strong tetanus (100 Hz, 1 s) applied to one input (input I) induced stable LTP of AMPA-EPSCs in this input, while the control input (input II) remained unaffected. For neither input were EPSC rise time and decay kinetics significantly changed. The application of cyclothiazide prolonged the rise time and the decay time constants of the AMPA-EPSCs in both control and potentiated inputs to the same extent (Input I-rise time: 198+/-8%, decay: 148+/-12%; input II-rise time: 212+/-14%, decay: 144+/-19%; n=8). Furthermore, when present during tetanization cyclothiazide did not occlude LTP, suggesting that cyclothiazide and tetanic stimulation enhance AMPA-EPSCs via independent mechanisms. Our findings argue against changes in (de-)activation or desensitization of AMPA receptors as the molecular basis for the expression of LTP.

Animals↗

PubMed1999

Flupirtine shows functional NMDA receptor antagonism by enhancing Mg2+ block via activation of voltage independent potassium channels. Rapid communication.

The spectrum of action of flupirtine includes analgesia, muscle relaxation and neuroprotection. N-methyl-D-aspartate (NMDA) receptor antagonism has been discussed as a possible mechanism of action of this compound with little direct evidence. The objective of the present study was to develop a plausible model to explain flupirtine's spectrum of action. A four-stage strategy was selected for this purpose: Firstly, the serum concentration of flupirtine under therapeutic conditions was determined on the basis of the current literature. The second stage involved assessing the known in-vitro effects in light of the therapeutic active concentration. Using whole cell patch clamp recordings from cultured rat superior colliculus neurones interactions between flupirtine and NMDA receptors were assessed. Only very high concentrations of flupirtine antagonized inward currents to NMDA (200 microM) at -70 mV with an lC50 against steady-state responses of 182.1+/-12.1 microM. The effects of flupirtine were voltage-independent and not associated with receptor desensitization making actions within the NMDA receptor channel or at the glycine modulatory site unlikely. NMDA receptor antagonism probably has little relevance for the clinical efficacy of flupirtine as the concentrations needed were far higher than those achieved in clinical practice. However, the activation of a G-protein-regulated inwardly rectifying K+ channel was identified as an interesting molecular target site of flupirtine. In the next stage, the central nervous spectrum of action of experimental K+ channel openers (PCO) was considered. As far as they have been studied, experimental K+ channel openers display a spectrum of action comparable to that of flupirtine. In the final stage, a global model was developed in which flupirtine stabilizes the resting membrane potential by activating inwardly rectifying K+ channels, thus indirectly inhibiting the activation of NMDA receptors. The model presented here reconciles the known functional NMDA receptor antagonism of flupirtine with the activation of K+ channels that occurs at therapeutic concentrations, thus providing an understanding of flupirtine's spectrum of action. This makes flupirtine the prototype of a clinically applicable substance group with analgesic, muscle-relaxant and neuroprotective properties.

Aminopyridines↗

PubMed1999

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.

Alkylation↗

PubMed1999

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.

Animals↗

PubMed1999

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.

Animals↗