Anticonvulsant action of 1,3-dimethyl-5-aminoadamantane. Pharmacological studies in rodents and baboon, Papio papio

Does Alzheimer's disease with mesial temporal lobe epilepsy represent a distinct disease subtype?

Alzheimer's disease (AD) patients have a high risk of developing mesial temporal lobe epilepsy (MTLE) and subclinical epileptiform activity. MTLE in AD worsens outcomes. Therefore, we need to understand the overlap between these disease processes. We hypothesize that AD with MTLE represents a distinct subtype of AD, with the interplay between tau and epileptiform activity at its core. We discuss shared pathological features including histopathology, an initial mesial temporal lobe (MTL) hyperexcitability followed by MTL dysfunction and involvement of same networks in memory (AD) and seizures (MTLE). We provide evidence that tau accumulation linearly increases neuronal hyperexcitability, neuronal hyper-excitability increases tau secretion, tau can provoke seizures, and tau reduction protects against seizures. We speculate that AD genetic mutations increase tau, which causes proportionate neuronal loss and/or hyperexcitability, leading to seizures. We discuss that tau burden in MTLE predicts cognitive deficits among (1) AD and (2) MTLE without AD. Finally, we explore the possibility that anti-seizure medications improve cognition by reducing neuronal hyper-excitability, which reduces seizures and tau accumulation and spread. HIGHLIGHTS: We hypothesize that patients with Alzheimer's disease (AD) and mesial temporal lobe epilepsy (MTLE) represents a distinct subtype of AD. AD and MTLE share histopathological features and involve overlapping neuronal and cortical networks. Hyper-phosphorylated tau (pTau) increases neuronal excitability and provoke seizures, neuronal excitability increases pTau, and pTau reduction reduces neuronal excitability and protects against seizures. The pTau burden in MTL predicts cognitive deficits among (1) AD and (2) MTLE without AD. We speculate that anti-seizure medications improve cognition by reducing neuronal excitability, which reduces seizures and pTau.

Fenfluramine diminishes NMDA receptor-mediated seizures via its mixed activity at serotonin 5HT2A and type 1 sigma receptors

Fenfluramine exhibits antiepileptic properties and thus diminishes epileptiform discharges in experimental animal models of Dravet syndrome. Fenfluramine is metabolized into norfenfluramine in vivo, which shows greater affinity and agonist activity at serotonin 5HT2 receptors (5HT2R) than fenfluramine. In this study, we found that fenfluramine and norfenfluramine disrupted the regulatory association of the sigma 1 receptor (σ1R) with NR1 subunits of glutamate N-methyl-D-aspartate receptors (NMDAR), an effect that was also produced by σ1R antagonists such as S1RA and prevented by σ1R agonists such as PPCC. The antagonists removed σ1R bound to NMDAR NR1 subunits enabling calcium-regulated calmodulin (CaM) to bind to those subunits. As a result, CaM may inhibit calcium permeation through NMDARs. The serotoninergic activity of fenfluramine at 5HT2AR, and likely also at 5HT2CR, collaborated with its activity at σ1Rs to prevent the convulsive syndrome promoted by NMDAR overactivation. Notably, fenfluramine enhanced the inhibitory coupling of G protein-coupled receptors such as 5HT1AR and cannabinoid type 1 receptor with NMDARs, thus allowing the more effective restrain of NMDAR activity. Thus, fenfluramine circumvents the negative side effects of direct NMDAR antagonists and may improve the quality of life of subjects affected by such proconvulsant dysfunctions.

Endocannabinoid control of glutamate NMDA receptors: the therapeutic potential and consequences of dysfunction

Glutamate is probably the most important excitatory neurotransmitter in the brain. The glutamate N-methyl-D-aspartate receptor (NMDAR) is a calcium-gated channel that coordinates with G protein-coupled receptors (GPCRs) to establish the efficiency of the synaptic transmission. Cross-regulation between these receptors requires the concerted activity of the histidine triad nucleotide-binding protein 1 (HINT1) and of the sigma receptor type 1 (σ1R). Essential brain functions like learning, memory formation and consolidation, mood and behavioral responses to exogenous stimuli depend on the activity of NMDARs. In this biological context, endocannabinoids are released to retain NMDAR activity within physiological limits. The efficacy of such control depends on HINT1/σ1R assisting in the physical coupling between cannabinoid type 1 receptors (CB1Rs) and NMDARs to dampen their activity. Subsequently, the calcium-regulated HINT1/σ1R protein tandem uncouples CB1Rs to prevent NMDAR hypofunction. Thus, early recruitment or a disproportionate cannabinoid induced response can bring about excess dampening of NMDAR activity, impeding its adequate integration with GPCR signaling. Alternatively, this control circuit can apparently be overridden in situations where bursts of NMDAR overactivity provoke convulsive syndromes. In this review we will discuss the possible relevance of the HINT1/σ1R tandem and its use by endocannabinoids to diminish NMDAR activity and their implications in psychosis/schizophrenia, as well as in NMDAR-mediated convulsive episodes.

Synaptic plasticity in glutamatergic and GABAergic neurotransmission following chronic memantine treatment in an in vitro model of limbic epileptogenesis

Chronic N-methyl-D-aspartate receptor (NMDAR) blockade with high affinity competitive and uncompetitive antagonists can lead to seizure exacerbation, presumably due to an imbalance in glutamatergic and GABAergic transmission. Acute administration of the moderate affinity NMDAR antagonist memantine in vivo has been associated with pro- and anticonvulsive properties. Chronic treatment with memantine can exacerbate seizures. Therefore, we hypothesized that chronic memantine treatment would increase glutamatergic and decrease GABAergic transmission, similar to high affinity competitive and uncompetitive antagonists. To test this hypothesis, organotypic hippocampal slice culture were treated for 17-21 days with memantine and then subjected to electrophysiological recordings. Whole-cell recordings from dentate granule cells revealed that chronic memantine treatment slightly, but significantly increased sEPSC frequency, mEPSC amplitude and mEPSC charge transfer, consistent with minimally increased glutamatergic transmission. Chronic memantine treatment also increased both sIPSC and mIPSC frequency and amplitude, suggestive of increased GABAergic transmission. Results suggest that a simple imbalance between glutamatergic and GABAergic neurotransmission may not underlie memantine's ictogenic properties. That said, glutamatergic and GABAergic transmission were assayed independently of one another in the current study. More complex interactions between glutamatergic and GABAergic transmission may prevail under conditions of intact circuitry.

The NMDA receptor complex as a therapeutic target in epilepsy: a review

A substantial amount of research has shown that N-methyl-D-aspartate receptors (NMDARs) may play a key role in the pathophysiology of several neurological diseases, including epilepsy. Animal models of epilepsy and clinical studies demonstrate that NMDAR activity and expression can be altered in association with epilepsy and particularly in some specific seizure types. NMDAR antagonists have been shown to have antiepileptic effects in both clinical and preclinical studies. There is some evidence that conventional antiepileptic drugs may also affect NMDAR function. In this review, we describe the evidence for the involvement of NMDARs in the pathophysiology of epilepsy and provide an overview of NMDAR antagonists that have been investigated in clinical trials and animal models of epilepsy.

Memantine reduces mania-like symptoms in animal models

Memantine, a selective antagonist of the N-methyl-D-aspartate receptor, is approved for the treatment of moderate to severe Alzheimer's disease. Ion dysregulation is thought to be involved in the pathophysiology of bipolar illness, suggesting that memantine may be effective in treating bipolar manic and/or depressive episodes. We utilized two preclinical models of mania that mimic pathophysiologic changes seen in bipolar illness to examine the potential efficacy of memantine in the treatment of this disorder. Locomotor hyperactivity of male Sprague-Dawley rats in an open field was induced with intracerebroventricular (ICV) administration of 10(-3) M ouabain. Memantine (2.5, 5 or 7.5mg/kg), lithium (6.75 mEq/kg), or vehicle were administered acutely via intraperitoneal injection immediately prior to ouabain, then chronically for 7 days (oral memantine 20, 30, and 40 mg/kg/day in water; lithium 2.4 g/kg food). In a second model of bipolar disorder, cycling between population spikes and epileptiform bursts was investigated in rat hippocampal slices treated with ouabain (3.3 μM) alone or in combination with memantine (0.5, 1.0, and 5.0 μM). Ouabain-induced hyperlocomotion was normalized with acute and chronic lithium and chronic use of memantine. Memantine delayed the onset of ouabain-induced-cycling in hippocampal slices. Memantine may have antimanic properties.

Different effects of two N-methyl-D-aspartate receptor antagonists on seizures, spontaneous behavior, and motor performance in immature rats

Typical N-methyl-D-aspartate (NMDA) receptor antagonists exhibit anticonvulsant action and unwanted effects, even in developing rats. Therefore, we studied the actions of the low-affinity, noncompetitive antagonist memantine and the NR2B-specific antagonist ifenprodil. Seizures (minimal clonic and generalized tonic-clonic) were elicited with pentylenetetrazol (100mg/kg subcutaneously) in rats 7, 12, 18, and 25 days old pretreated with memantine (2.5-40 mg/kg intraperitoneally) or ifenprodil (10-60 mg/kg intraperitoneally). The effects of both drugs were studied in open field and motor performance tests in 12-, 18-, and 25-day-old rats. Memantine suppressed generalized tonic-clonic seizures in all age groups; minimal seizures were potentiated. Ifenprodil abolished the tonic phase of generalized tonic-clonic seizures in 7-, 12-, and 18-day-old rats only; minimal seizures remained untouched. Memantine induced locomotor hyperactivity and compromised motor performance in all age groups. Ifenprodil exerted these effects only in 12-day-old rats; older animals were less active in open field tests. Memantine exhibits both anti- and pro-convulsant and behavioral effects typical of NMDA antagonists. Ifenprodil exerted the same effects in 12-day-old rats, but its anticonvulsant action in 18-day-old rats was accompanied by a decrease in locomotion.

Laparophobia: a cognitive perspective on appetite control in anorexia nervosa

Decades of research have demonstrated that anorexia nervosa (AN) may be associated with aberrant cognition, yet, its role in maintaining stringent dieting has received relatively little attention from mainstream researchers of eating disorders. The purpose of the present article is to highlight cognitive ('top-down') factors that are considered responsible for anticipatory anxiety of stoutness and frank fat-phobia (laparophobia). A cognitive model proposed departs from the formulation suggesting that phobia of over-eating is superimposed on avoidant tendencies ('environmental autonomy syndrome'), whereas excessive exercising becomes a natural coping strategy with laparophobia, an instrument of reward. AN ideation involves complex neuronal circuitries and multiple neurochemical components that may conceivably represent a mirror image of those underlying obesity. The emphasis on phobia and aberrant membrane excitability akin to channelopathies behoves the clinicians to be aware of potential uses of drugs acting at the gamma-aminobutyric acid and the N-methyl-D-aspartate/AMPA [2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl) propionic acid] receptors sites as the adjuncts to conventional agents in managing AN.

Atypical antipsychotics and serotoninergic antidepressants in patients with epilepsy: pharmacodynamic considerations

PURPOSE

To discuss the pharmacodynamic aspects of the administration of atypical antipsychotics (APs) and serotoninergic antidepressants (SSRIs) to patients with epilepsy.

METHODS

This article represents an overview of all studies concerning the administration of APs and SSRIs to people with epilepsy. In particular, it deals with the relationship between neuroleptics (NLTs), APs, SSRIs, serotonin, and dopamine, with special focus on the possible epileptogenic role of psychoactive drugs.

RESULTS

NLTs may induce seizures by blocking D2, H1, and.1 receptors, or by sexual hormone activation or a pharmacologic kindling mechanism. The difference among APs in their ability to induce seizures is related mainly to the percentage of D2-receptor occupancy and possibly also to their action on neurosteroids. Seizures occur at SSRIs therapeutic doses, with a 0.1-4% incidence. Coversely, in animal studies fluoxetine was claimed to exert an anticonvulsant action.

CONCLUSIONS

The study of the pharmacodynamic aspects of the administration of APs and SSRIs to patients with epilepsy can help to evaluate the importance of some mechanisms of action of several psychoactive drugs in relation to their pro- or anticonvulsant activity.

Saving the nerve from glaucoma: memantine to caspaces

Three recent discoveries regarding the pathophysiology of human glaucoma are that retinal ganglion cells die by apoptosis, that nitric oxide synthase levels are altered, and that glutamate is elevated in the vitreous. These findings provide encouraging new avenues for the development of neuroprotective strategies to alleviate ganglion cell loss and blindness that accompanies this disease. In this article, we discuss some of these data, as well as potential therapies that may arise from these findings.

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.

Mechanism of memantine block of NMDA-activated channels in rat retinal ganglion cells: uncompetitive antagonism

1. N-methyl-D-aspartic acid (NMDA)-activated currents were recorded from dissociated rat retinal ganglion cells using whole-cell recording. The NMDA open-channel blocking drug memantine was evaluated for non-competitive and/or uncompetitive components of antagonism. A rapid superfusion system was used to apply various drugs for kinetic analysis. 2. Dose-response data revealed that memantine blocked 200 microM NMDA-evoked responses with a 50% inhibition constant (IC50) of approximately 1 microM at -60 mV and an empirical Hill coefficient of approximately 1. The antagonism followed a bimolecular reaction process. This 1:1 stoichiometry is supported by the fact that the macroscopic blocking rate of memantine (kon) increased linearly with memantine concentration and the macroscopic unblocking rate (koff) was independent of it. The estimated pseudo-first order rate constant for macroscopic blockade was 4 x 10(5) M-1 S-1 and the rate constant for unblocking was 0.44 s-1. Both the blocking and unblocking actions of memantine were well fitted by a single exponential process. 3. The kon for 2 microM memantine decreased with decreasing concentrations of NMDA. By analysing kon behaviour, we estimate that memantine has minimal interaction with the closed-unliganded state of the channel. As channel open probability (Po) approached zero, a small residual action of memantine may be explained by the presence of endogenous glutamate and glycine. 4. Memantine could be trapped within the NMDA-gated channel if it was suddenly closed by fast washout of agonist. The measured gating process of channel activation and deactivation appeared at least 10-20-fold faster than the kinetics of memantine action. By combining the agonist and voltage dependence of antagonism, a trapping scheme was established for further kinetic analysis. 5. With low agonist concentrations, NMDA-gated channels recovered slowly from memantine blockade. By analysing the probability of a channel remaining blocked, we found that memantine binding appeared to stabilize the open conformation of the blocked channel and did not affect ligand affinity. Validity of the "trapping model' and stabilization of the open conformation were further suggested by agreement between the predicted dose-response curve for NMDA in the presence of 2 microM memantine and the empirically derived dose-response relationship. 6. Based on simple molecular schemes, the degree of blockade at various concentrations of agonist for "pure' non-competitive vs. uncompetitive inhibition was computer simulated. The measured degree of blockade by 6 microM memantine was close to ideal for pure uncompetitive antagonism. Taken together, we conclude that the predominant mechanism of open-channel blockade by memantine is uncompetitive. In general, the relative magnitude of the dissociation rate of an open-channel blocker from the open but blocked channel (the apparent off-rate) compared with the rate of leaving the closed and blocker-trapped state (the leak rate) will determine the contribution of uncompetitive vs. non-competitive actions, respectively. 7. Millimolar internal Cs+ competed with memantine for binding in the NMDA-gated channel, and reduced the association rate of memantine, but had no effect on the voltage dependence of the dissociation rate. After removal of Cs+, the calculated Ki for memantine remained voltage dependent. These observations would be difficult to reconcile with models in which memantine binds to a site outside the channel pore and instead strongly support the supposition that the blocking site for memantine is within the permeation pathway.

Effects of N-methyl-D-aspartate receptor antagonists on cisplatin-induced emesis in the ferret

Glutamate may be a key transmitter in the emetic reflex arc. The present investigation focussed on the involvement of the NMDA subtype of glutamate receptors in cisplatin-induced emesis. Ferrets were injected with cisplatin (10 mg/kg i.v.) and either of the non-competitive NMDA receptor antagonists dextromethorphan or memantine, or the competitive receptor antagonist CGS 19755. In order to determine whether there is a synergism between NMDA blockers and 5-HT3 receptor antagonists, a submaximal dose of granisetron (0.05 mg/kg) was given alone or in combination with either dextromethorphan or memantine. The latency for the onset of emesis as well as the total number of vomits and retches over 3 hr were determined. In controls, the latency for emesis was 73 +/- 6 min and the total number of vomits and retches 143 +/- 17. The corresponding figures for animals treated with dextromethorphan, 10 and 20 mg/kg, were 89 +/- 19 min (p > 0.05) and 50 +/- 17 (p = 0.008), and 113 +/- 18 min (p > 0.05) and 22 +/- 9 (p = 0.004), respectively. At 10 mg/kg, dextromethorphan failed to enhance the antiemetic effect of granisetron which by itself provided 90% inhibition. While memantine (2.5 or 5.0 mg/kg) did not have an effect per se, it tended to reduce the antiemetic effect of granisetron. CGS 19755 (10 mg/kg) provided a partial protection against cisplatin-induced emesis (latency: 111 +/- 23, number of vomits and retches 30 +/- 11). None of the NMDA receptor antagonists was free of behavioural effects (e.g. some sedation) at antiemetic doses. It is concluded that NMDA receptor antagonists may afford protection against cisplatin-induced emesis but the specificity of this effect is uncertain since it may relate to general CNS depression.

The role of dopamine in epilepsy

The clinical benefits of dopamine agonists in the management of epilepsy can be traced back over a century, whilst the introduction of neuroleptics into psychiatry practice 40 years ago witnessed the emergence of fits as a side effect of dopamine receptor blockade. Epidemiologists noticed a reciprocal relationship between the supposed dopaminergic overactivity syndrome of schizophrenia and epilepsy, which came to be regarded as a dopamine underactivity condition. Early pharmacological studies of epilepsy employed nonselective drugs, that often did not permit dopamine's antiepileptic action to be clearly dissociated from that of other monoamines. Likewise, the biochemical search for genetic abnormalities in brain dopamine function, as predeterminants of spontaneous epilepsy, proved largely inconclusive. The discovery of multiple dopamine receptor families (D1 and D2), mediating opposing influences on neuronal excitability, heralded a new era of dopamine-epilepsy research. The traditional anticonvulsant action of dopamine was attributed to D2 receptor stimulation in the forebrain, while the advent of selective D1 agonists with proconvulsant properties revealed for the first time that dopamine could also lower the seizure threshold from the midbrain. Whilst there is no immediate prospect of developing D2 agonists or D1 antagonists as clinically useful antiepileptics, there is a growing awareness that seizures might be precipitated as a consequence of treating other neurological disorders with D2 antagonists (schizophrenia) or D1 agonists (parkinsonism).

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.

Chronic treatment with the uncompetitive NMDA receptor antagonist memantine influences the polyamine and glycine binding sites of the NMDA receptor complex in aged rats

Receptor binding studies on rat cortical membranes were used to characterize the NMDA receptor in aged rats (22 months) treated for 20 months with a memantine containing diet delivering 30 mg/kg/day in comparison to aged and young/adult rats treated with control-diet. Spatial memory impairing effects of (+)-MK-801 (0.16 mg/kg) in the radial maze was not altered within the course of memantine-treatment (up to 16 months). However, chronic memantine-treatment significantly increased the number of [3H]MK-801 binding sites and the affinity of [3H]glycine. A non-significant trend to such changes was also seen in aged-control rats. Glycine-dependent [3H]MK-801 binding (functional binding under non-equilibrium conditions at a fixed L-glutamate concentration) revealed that a decreased ability of glycine to stimulate channel opening in aged rats was partially attenuated by the long-term memantine treatment. Furthermore, an increased ability of spermidine to enhance [3H]MK-801 binding in aged-control rats was even more pronounced in the aged memantine-treated group. Together these findings may indicate that changes in functional receptor-channel properties during the process of aging occur prior to a detectable loss of binding sites and that memantine enhances an endogenous compensatory mechanism triggered by glutamatergic hypofunction which is suggested to take place in aging.

Anticonvulsant effects of memantine and MK-801 in guinea pig hippocampal slices

The anticonvulsant properties of memantine (Mem) were compared to those of MK-801. Extracellular field recordings were obtained from area CA1 of guinea pig hippocampal slices in a total submersion chamber at 32 degrees C in normal oxygenated artificial cerebrospinal fluid (ACSF). Evoked responses were elicited by 0.07 Hz stimulation of the Schaffer collateral and commissural fibers. Bath perfusion of slices with Mg(2+)-free ACSF and N-methyl-D-aspartate (NMDA)-containing ACSF induced epileptiform afterdischarges following evoked responses. Pretreatment of slices by bath application of 100 microM Mem for 18-20 min prevented epileptiform afterdischarges under both convulsant conditions. Perfusion with 100 microM Mem alone for up to 50 min had no discernible effect on evoked responses. MK-801 was as effective at < or = 10 microM and required application for over 15 min to suppress afterdischarges completely. Both Mem and MK-801 suppressed epileptiform activity when applied after such activity was induced by NMDA or MG(2+)-free ACSF. The EC50 of Mem was 16.6 microM and that of MK-801 was 0.19 microM for blocking NMDA-induced evoked response suppression. Thus, in the guinea pig hippocampal slice preparation, Mem appeared to have anticonvulsant properties qualitatively similar to those of MK-801, but was 10-100 fold less potent.

Alterations in monoamine levels in discrete regions of rat brain after chronic administration of carbamazepine

Carbamazepine (25 mg/kg body weight) was administered intraperitoneally to adult male Wistar rats for 45 days and norepinephrine (NE), dopamine (DA) and serotonin (5-HT) levels were simultaneously assayed in discrete brain regions by high performance liquid chromatographic (HPLC) method. Experimental rats displayed no behavioral abnormalities. Body and brain weights were not significantly different from control group of rats. After exposure it was observed that norepinephrine levels were elevated in motor cortex (P < 0.01) and cerebellum (P < 0.05), while dopamine levels were decreased in these two regions (P < 0.001, P < 0.05). However, dopamine levels were increased in hippocampus (P < 0.01). Serotonin levels were significantly decreased in motor cortex (P < 0.001) and hypothalamus (P < 0.001) but increased in striatum-accumbens (P < 0.001) and brainstem (P < 0.001). These results suggest that carbamazepine may mediate its anticonvulsant effect by differential alterations of monoamine levels in discrete brain regions particularly in motor cortex and cerebellum.

Therapeutic potential of excitatory amino acid antagonists: channel blockers and 2,3-benzodiazepines

NMDA and non-NMDA (AMPA/kainate) antagonists have potential in the treatment of a diverse group of neurological disorders associated with excessive activation of excitatory amino acid receptors. Here Michael Rogawski reviews recent progress in the development of therapeutically useful NMDA receptor channel blockers and a new class of selective AMPA/kainate receptor antagonists, the 2,3-benzodiazepines. Research on these novel noncompetitive excitatory amino acid antagonists has opened promising new avenues for the development of drugs to treat epilepsy, ischaemia, neurodegeneration and Parkinson's disease.

Kindled rats are more sensitive than non-kindled rats to the behavioural effects of combined treatment with MK-801 and valproate

The effects of combined treatment with low doses (0.025-0.05 mg/kg i.p.) of the non-competitive NMDA receptor antagonist, MK-801 (dizocilpine), and the antiepileptic drug, valproate, were studied in amygdala-kindled and non-kindled rats. MK-801, 0.05 mg/kg, did not exert anticonvulsant effects in fully kindled rats but increased the anticonvulsant potency of valproate, 100 mg/kg i.p. However, the increase in anticonvulsant activity was paralleled by a marked increase in adverse effects such as motor impairment and hyperactivity, resulting in a considerable reduction of the therapeutic index of the combined treatment compared to valproate alone. Furthermore, MK-801 potentiated the adverse effects but not the anticonvulsant activity of 50 mg/kg valproate. Combined treatment with MK-801 and valproate induced much less marked adverse effects in non-kindled rats than in kindled rats. The competitive NMDA receptor antagonist, CGP 37849 1 mg/kg i.p., did not alter the effects of valproate in kindled rats. The data on combined treatment with MK-801 and valproate substantiate the conclusion that kindling alters the susceptibility to manipulations of NMDA receptor-mediated events.

Prophylactic and therapeutic efficacy of memantine against seizures produced by soman in the rat

Male Sprague-Dawley rats injected sc with a single sublethal dose of the organophosphate nerve agent, soman (100 micrograms/kg), had motor limbic seizures within 5-15 min. Pretreatment with a single dose of memantine HCl (MEM, 18 mg/kg, sc), alone or in combination with atropine sulfate (ATS, 16 mg/kg, sc), before soman prevented seizures without sedation or ataxia. Rats appeared normal or demonstrated increased exploratory activity. Excessive salivation, a peripheral manifestation of soman intoxication, was decreased by ATS, but pretreatment with ATS alone did not prevent seizures. After seizure onset, MEM +/- ATS, but not ATS, abolished seizures. Acetylcholinesterase (AChE) activity in several brain regions (cortex, stem, striatum, and hippocampus) was markedly reduced by soman, but not by MEM, ATS, or MEM + ATS. Preadministration of MEM + ATS in vivo significantly protected AChE from inhibition by soman. Memantine reduced inhibition of AChE activity in crude brain homogenates by soman, but not by edrophonium (anionic site inhibitor) or decamethonium (peripheral site inhibitor). Thus, MEM may bind to a different modulatory site, not yet characterized, to protect AChE. When given after onset of soman-induced seizures, treatment with MEM +/- ATS did not reactivate AChE although seizures were controlled, suggesting additional anticonvulsant mechanisms of action. At concentrations (10(-4) to 5 x 10(-4) M) which did not significantly alter the spontaneous firing of action potentials (APs), MEM limited sustained high frequency repetitive firing (SRF) induced by depolarization of spinal cord (mouse and rat) and neocortical (mouse) neurons in monolayer-dissociated cell culture. In the same range of concentrations, ATS both limited SRF and suppressed spontaneous activity, suggesting toxicity. In addition, MEM and ATS reversibly produced use-dependent block of depolarizing responses to acetylcholine (ACh) applied by pressure ejection to spinal cord neurons. Thus, the anticonvulsant efficacy of MEM, with or without ATS, may have resulted from a combination of actions, including protection of AChE from inhibition by soman, limitation of high frequency firing of APs, and blockade of excitatory postsynaptic responses to ACh.

Effects of the 1-amino-adamantanes at the MK-801-binding site of the NMDA-receptor-gated ion channel: a human postmortem brain study

Recent studies from our laboratory have provided evidence that the 1-amino-adamantane derivative memantine (1-amino-3,5-dimethyl-adamantane) binds to the MK-801-binding site of the N-methyl-D-aspartate (NMDA)-receptor-gated ion channel. This action has been suggested to account for the antiparkinsonian and antispastic activity of the drug. In the present investigation we have extended our work by testing a series of 1-amino-adamantanes, including amantadine (1-amino-adamantane) and memantine, for their ability to compete with [3H]MK-801 binding in membrane homogenates of postmortem human frontal cortex. The most potent substance (1-amino-3,5-diethyl-adamantane) had a Ki-value of 0.19 +/- 0.06 microM while the weakest substance (1-N-methyl-amino-adamantane) had a Ki-value of 21.72 +/- 1.63 microM. The Ki-value of amantadine was 10.50 +/- 6.10 microM. In agreement with our earlier investigation, the Ki-value of memantine was 0.54 +/- 0.23 microM. The results indicate that 1-amino-adamantanes, in general, may produce their pharmacological effects through an interaction with the NMDA-receptor-gated ion channel. The displacement of [3H]MK-801 binding thus may provide the basis to predict the antiparkinsonian and antispastic activity of novel substituted 1-amino-adamantanes and possibly of other drugs.

Inhibition by thyrotropin-releasing hormone of epileptic seizures in spontaneously epileptic rats

The effects of thyrotropin-releasing hormone (TRH) were investigated on absence-like seizures, which are characterized by the sudden appearance of 5-7 Hz spike-wave-like complexes in the cortical and hippocampal EEG, and on tonic convulsions of spontaneously epileptic rats (SER; zi/zi, tm/tm), a double mutant obtained by mating zitter homozygote (zi/zi) with tremor heterozygote rats (tm/+). TRH (5 and 10 mg/kg i.v.) inhibited the appearance of both absence-like seizures and tonic convulsions of SER without inducing obvious changes in the background EEG. The inhibitory effects were seen 5-20 min after injection of 10 mg/kg TRH and were antagonized by pretreatment with haloperidol (0.5 and 1.0/kg i.p.), although haloperidol alone did not affect the seizures. These results suggest that TRH has an antiepileptic effect in the genetically defined animal model, SER, and that the effect is mediated by the central dopaminergic system.

Memantine reduces repetitive action potential firing in spinal cord nerve cell cultures

(1) The anticonvulsant effects of memantine were examined and compared with those of baclofen in monolayer primary cultures of murine nerve cells using conventional intracellular recordings. (2) Memantine and baclofen (each 10-100 microM) decreased spontaneous synaptic activity when action potential frequencies exceeded 6 Hz. Neurons firing action potentials at frequencies below 6 Hz (about 90% of all impaled cells), however, were not affected by the drugs. (3) Memantine reduced the duration of strychnine-elicited bursts and the firing rate of action potentials within a burst. In contrast, baclofen lowered the frequency of the bursts without reducing intra-burst firing. The duration of the bursts was increased. (4) Memantine, but not baclofen, reduced the extent of sustained repetitive firing evoked by pulses of depolarizing current. (5) In the presence of memantine, the second of two electrically evoked action potentials increasingly failed to appear as the intervals between successive stimulating pulses were shortened. Such an effect was not seen when baclofen was applied. Thus, both antispastic agents, memantine and baclofen, reduce hyperactivity of spinal cord neurons in culture, although their mechanisms of action are different.

Seizure promotion and protection by D-1 and D-2 dopaminergic drugs in the mouse

Mice injected with pilocarpine (100-400 mg/kg plus 1 mg/kg methylscopolamine), picrotoxin (0.75-6 mg/kg) or strychnine (0.75-6 mg/kg) exhibited clonic or clonic/tonic convulsions. Pretreatment with the D-1 agonist CY 208-243 (0.375-1.5 mg/kg) dose-dependently potentiated the convulsions elicited by 100 mg/kg pilocarpine, but had neither a convulsant nor anticonvulsant effect in mice receiving picrotoxin (3 or 6 mg/kg) or strychnine (0.75 or 1.5 mg/kg). This facilitatory effect of CY 208-243 was abolished by the D-1 antagonist SCH 23390 (0.2 mg/kg). SCH 23390 by itself (0.05-0.8 mg/kg) dose-dependently protected mice against pilocarpine (400 mg/kg) seizures. Stimulating D-2 receptors with LY 171555 (0.167-4.5 mg/kg) dose-dependently protected mice against seizure activity induced by pilocarpine, but neither protected nor sensitised mice given picrotoxin or strychnine. The neuroleptics haloperidol (1-4 mg/kg), sulpiride (10-50 mg/kg), metoclopramide (1.25-6.25 mg/kg), thioridazine (0.5-2 mg/kg) and clozapine (0.5-2 mg/kg) had no effect on the seizure threshold to 100 mg/kg pilocarpine by themselves, although 10 mg/kg thioridazine and clozapine caused 100% convulsions, possibly through a toxic action. When administered in conjunction with a minimally effective quantity of CY 208-243 (0.375 mg/kg), however, all five neuroleptics interacted synergistically with the D-1 agonist to promote convulsions to pilocarpine (100 mg/kg). No such interaction occurred between submaximally protective doses of the D-1 blocker SCH 23390 (0.05 and 0.2 mg/kg) and a wide range of doses of the D-2 stimulant LY 171555 (0.167-4.5 mg/kg).(ABSTRACT TRUNCATED AT 250 WORDS)

Memantine inhibits serotonin-induced rise of cytosolic Ca2+ activity and of cyclic GMP level in a neuronal cell line

Serotonin (5-HT) evoked a rise of cytosolic Ca2+ activity in neuroblastoma X glioma hybrid cells, most probably due to the entry of extracellular Ca2+; cyclic GMP synthesis was also stimulated. The rise of both cytosolic Ca2+ activity and of cyclic GMP level was blocked by memantine (1-amino-3,5-dimethyladamantane). Memantine inhibited the rise of the cyclic GMP level non-competitively (Ki about 50 microM). Thus, memantine suppresses the effects of 5-HT in the neuronal cell line, most likely by blocking Ca2+-permeable ion channels. This interpretation is in line with the previously reported finding that memantine suppressed the 5-HT-induced depolarizing response in the same cell line.

Toxins from the venom of the green mamba Dendroaspis angusticeps that inhibit the binding of quinuclidinyl benzilate to muscarinic acetylcholine receptors

Two protein toxins that displace the muscarinic antagonist quinuclidinyl benzilate from rat cortex synaptosomal membranes have been isolated from the green mamba (Dendroaspis angusticeps) venom by gel filtration on sephadex G-50, chromatography on the ion-exchangers Bio-Rex 70 and Sulphopropyl-Sephadex C-25 and reversed-phase HPLC. Toxin 1 has 64 amino acids and four disulfides and a formula weight of 7200 and the corresponding values for toxin 2 are 63, 4 and 6840, respectively. Ultracentrifugation gave a molecular weight of 6900 for toxin 1 and 6700 for toxin 2, Quinuclidinyl benzilate that binds to all types of muscarinic cholinergic receptor was displaced to about 50% by both toxins. This partial displacement indicates that the toxins might be specific for one subtype of receptor.

Memantine (1-amino-3,5-dimethyladamantane) blocks the serotonin-induced depolarization response in a neuronal cell line

The influence of memantine on several properties of a neuronal cell line was tested. The aim was to get some insight into possible mechanisms of action of this drug which is therapeutically applicable in treatment of spasticity, Parkinson's disease, and cerebral coma. In neuroblastoma X glioma hybrid cells, memantine, at micromolar concentrations, blocked the depolarization induced by iontophoretically applied serotonin (5-hydroxytryptamine, 5-HT). In the hybrid cells, receptors of the 5-HT3 type mediated the depolarization, which was frequently accompanied by a series of action potentials. The inhibition by memantine of the serotonin response occurred fast and was completely reversible, irrespective of whether the cell showed a stable membrane potential or spontaneous action potentials. However, memantine did not alter spontaneous or electrically evoked action potential activity in the hybrid cells, and apparently did not block the underlying ionic conductances. Furthermore memantine did not affect either the cation permeability activated by substance P in the hybrid cells or the K+ channel triggered by bradykinin in a glioma cell line. Thus, memantine appears specifically to suppress the ion channel opened by serotonin in the hybrid cells. The interaction of memantine with serotonin receptors and the associated ion channels reported here, might give an important clue, as to a site of action of memantine in the nervous system.