N-methylaspartate receptors mediate epileptiform activity evoked in some, but not all, conditions in rat neocortical slices

Biphasic action of dextrorphan on penicillin induced bursting in rat hippocampal slice

Effects of dextrorphan (DX), a metabolite of the over-the-counter antitussive, dextromethorphan, were investigated in rat hippocampal slices exposed to the epileptogenic agent penicillin. At 50 microM and 100 microM concentrations dextrorphan suppressed late components of the epileptiform CA1 field potential elicited by afferent electrical stimulation, and partially suppressed the intracellularly recorded paroxysmal depolarization shift. These effects were not due to non-specific changes in cell excitability, since resting cell membrane potential, input resistance, and the ability of cells to fire action potentials in response to direct depolarizing current were unaffected. The depressant effect of 100 microM dextrorphan was probably due to actions at the NMDA receptor, since pretreatment with the competitive NMDA antagonist D-APV prevented any further depressant effects of dextrorphan in this model. In contrast, at a 10 microM concentration DX enhanced the amplitude of evoked epileptiform field potentials and intracellularly recorded EPSPs. These findings support a role for dextrorphan and similar agents as anticonvulsants at high concentrations, but raise a caution regarding possible excitatory actions of dextrorphan at low concentrations.

Transient expression of polysynaptic NMDA receptor-mediated activity during neocortical development

During a restricted period of early postnatal development, rat neocortical neurons receive a powerful N-methyl-D-aspartate (NMDA) receptor-mediated synaptic input of variable onset latency and duration. These large-amplitude excitatory postsynaptic potentials are especially pronounced in supragranular layers and are generated by activities in polysynaptic circuits. Their occurrence in cortical slices from juvenile (postnatal (P) days 11-20), but not neonatal (P5-10) or adult (greater than or equal to P28) animals, appears to be in part a consequence of the relative immaturity of gamma-aminobutyric acid (GABA)-mediated inhibition, at a time when the requisite functional excitatory circuitry has been established. The transient manifestation of strong NMDA receptor-mediated potentials coincides temporally with a 'developmental window' within which there is enhanced sensitivity for epileptogenesis and for induction of long-term synaptic modifications in rat cortex.

Low magnesium epileptogenesis in the rat hippocampal slice: electrophysiological and pharmacological features

Extra- and intracellular recording techniques were used to study the epileptiform activity generated by rat hippocampal slices perfused with Mg2(+)-free artificial cerebrospinal fluid (ACSF). This procedure induced in both CA1 and CA3 subfields the appearance of synchronous, spontaneously occurring epileptiform discharges which consisted of extracellularly recorded 100-800 ms long, positive shifts with superimposed negative going population spikes. Simultaneous, extracellular recordings from CA1 and CA3 subfields revealed that the epileptiform discharges in CA3 preceded those occurring in CA1 by 5-25 ms. Surgical separation of the two areas led to the disappearance of spontaneous events in the CA1 but not in the CA3 subfield. In this type of experiment CA1 pyramidal cells still generated epileptiform discharges following orthodromic stimuli. The intracellular counterpart of both spontaneous and stimulus-induced epileptiform discharges in CA1 and CA3 pyramidal cells was a large amplitude depolarization with high frequency discharge of action potentials which closely resembled the paroxysmal depolarizing shift recorded in the experimental epileptogenic focus. A hyperpolarizing potential triggered by alvear stimuli was recorded in CA1 cells perfused with Mg2(+)-free ACSF. This hyperpolarization was blocked by bicuculline methiodide (BMI) indicating that it represented a GABAergic inhibitory postsynaptic potential (IPSP). BMI also caused a prolongation of both spontaneous and stimulus-induced Mg(+)-free epileptiform discharges. Perfusion of the slices with the N-methyl-D-aspartate (NMDA) receptor antagonist DL-2-amino-5-phosphono-valerate (APV) reduced and eventually abolished the Mg(+)-free epileptiform discharges. These effects were more pronounced in the CA1 than in the CA3 subfield. APV also reduced the amplitude and the duration of the alveus-induced IPSP. These data demonstrate that Mg(+)-free epileptiform activity is present in the hippocampal slice at a time when inhibitory GABAergic potentials are operant as well as that in the CA1 subfield this type of epileptiform activity is dependent upon NMDA-activated conductances. Our experiments also indicate that NMDA receptors might be involved in the neuronal circuit responsible for the hyperpolarizing IPSP generated by CA1 pyramidal neurons.

Synchronous discharges in the rat entorhinal cortex in vitro: site of initiation and the role of excitatory amino acid receptors

A slice preparation was used to study the spread of epileptiform activity in the rat entorhinal cortex. Interictal-like discharges were induced in the medial entorhinal cortex by blocking synaptic inhibition mediated via GABAA-receptors. Recorded intracellularly, these discharges consisted of an initial paroxysmal depolarizing shift followed by a variable number of afterdischarges. There was no apparent difference between these events whether they were recorded in isolated cortical slices or in slices where the hippocampus and subicular complex remained attached. The events were also unaffected by droplets of a xylocaine solution applied to sites in the hippocampus, subicular complex or superficial layers of the entorhinal cortex but applications to layer IV/V, lateral or medial to the recording site could reduce the number of afterdischarges without affecting the initial paroxysmal shift. Simultaneous intracellular recordings from neurons in layer IV/V and layer II of the medial entorhinal cortex showed that the paroxysmal depolarizing shift and all afterdischarges in the deeper layer always preceded those recorded in the superficial layer, and these events invariably occurred on a one-to-one basis. This was true whether the events were evoked or occurred spontaneously. The delay varied between 2 and 11 ms but was consistent for a given cell pair. A similar relationship existed between discharges recorded simultaneously in layer IV/V neurons and layer VI neurons, events in the layer IV/V cells preceding those in the deeper layer. Discharges recorded simultaneously in pairs of layer IV/V neurons showed more complex relationships. Paroxysmal depolarizing shifts were always recorded in both cells and the discharge could occur at the more medial site before the more lateral, or vice versa. For a given pair the temporal relationship was invariable. It was often the case, however, that the temporal relationship between afterdischarges was reversed with respect to the initial paroxysmal shift. This relationship was also invariable in a given pair of cells. Interictal-like discharges in layers II or IV/V neurons could be abolished by perfusion with 6-cyano-7-nitro-quinoxaline-2,3-dione which is an antagonist for the non-N-methyl-D-aspartate (i.e. quisqualate/kainate) subtype of excitatory amino acid receptor. The afterdischarges associated with the events were abolished in an all-or-none fashion whereas the blockade of the paroxysmal depolarizing shift was progressive. Antagonists of N-methyl-D-aspartate receptors also abolished afterdischarges but only reduced the initial paroxysmal shift. It is concluded that the interictal-like discharges arise intrinsically within the cortex and are not influenced by input from hippocampal or subicular structures.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of NMDA antagonists on picrotoxin-, low Mg2+- and low Ca2+-induced epileptogenesis and on evoked changes in extracellular Na+ and Ca2+ concentrations in rat hippocampal slices

The anticonvulsant properties of ketamine and 2-APV were compared on 3 types of convulsant activity in hippocampal area CA1: the 'picrotoxin-epilepsy,' the 'low magnesium epilepsy' and the 'low calcium epilepsy.' In particular the spontaneous activity, the synaptically evoked responses and the changes in [Ca2+]0 were examined, since in many cases of epilepsy, Ca2+ uptake into cells is enhanced. In normal medium, ketamine and 2-APV have nearly no effect on stimulus evoked decreases in [Ca2+]0, although they clearly depress NMDA-induced ionic changes. However, ketamine and 2-APV prevent to some extent the augmentation of stimulus-induced changes in [Ca2+]0, observed after treating slices with picrotoxin or Mg2+-free medium. This extra Ca2+ uptake is probably mediated by NMDA operated channels. Our findings also show that ketamine, like 2-APV, has a stronger anticonvulsant effect on the low Mg-than on the picrotoxin-induced epileptiform activity. Responses to iontophoretically applied NMDA are facilitated in the 'low calcium epilepsy' and can be selectively blocked by ketamine. Spontaneous epileptiform activity occurring in low calcium can be blocked by ketamine only when some synaptic transmission is still present.

The N-methyl-D-aspartate receptor complex in Alzheimer's disease: reduced regulation by glycine but not zinc

The binding of [3H]MK-801 to the N-methyl-D-aspartate receptor complex of well-washed cortical membranes from brains of examples of Alzheimer's disease and controls has been determined in incubations containing either glutamate or glycine plus glutamate. No changes were detected in the IC50 values for inhibition by zinc in the Alzheimer's samples compared to control although 'glycine-dependent' binding of the [3H]-ligand was significantly reduced in Alzheimer's disease.

NMDA antagonists differentiate epileptogenesis from seizure expression in an in vitro model

In an electrographic model of seizures in the hippocampal slice, both of the N-methyl-D-aspartate (NMDA) antagonists 2-amino-5-phosphonovaleric acid and 5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine maleate (MK-801) prevented the progressive development of seizures but did not block previously induced seizures. Thus, a process dependent on the NMDA receptor-ionophore complex establishes a long-lasting, seizure-prone state; thereafter the seizures depend on non-NMDA receptor-ionophore mechanisms. This suggests that there is an important distinction between epileptogenesis and seizure expression and between antiepileptogenic and anticonvulsant pharmacological agents.

Effect of changing extracellular levels of magnesium on spontaneous activity and glutamate release in the mouse neocortical slice

1. The mouse neocortical slice preparation, maintained in a two compartment, grease gap bath, exhibits spontaneous depolarizing activity (with or without rhythmic after potentials) after perfusion with magnesium-free artificial cerebrospinal fluid. 2. If the magnesium concentration is decrementally lowered over an extended time period, then incrementally raised following a similar time course, the spontaneous depolarizing shift activity shows a hysteresis (with regard to both frequency and amplitude), the depolarizing shifts being more resistant to magnesium during the incremental period. 3. The amino acid content of the perfusing fluid was analysed by high performance liquid chromatography (h.p.l.c.). Although a basal efflux of 6 amino acids was quantifiable, only glutamate levels increased following superfusion of the preparation with magnesium-free, artificial cerebrospinal fluid. 4. Glutamate release increased to 266% of the resting release in the presence of magnesium within the first 12 min of the change into magnesium-free artificial cerebrospinal fluid. This increase in release preceded the onset of spontaneous depolarising activity. The release of glutamate remained elevated at 182% of control up to 60 min after perfusion with magnesium-free buffer, when depolarizing activity was well established. 5. A model is presented and discussed for the genesis and maintenance of the spontaneous depolarizing shifts. It is suggested that the maintenance of this spontaneous activity reflects a long term enhancement of neocortical neurone excitability which may be related to long term potentiation in the hippocampus.

NMDA receptor antagonists CPP and MK-801 partially suppress the epileptiform discharges induced by the convulsant drug bicuculline in the rat neocortex

Intracellular recordings were obtained from neurons located in the superficial layers of rat neocortical slices maintained in vitro. In the presence of 50 microM of bicuculline methiodide, epileptiform discharges were evoked by extracellular local stimuli. Bath applications of the NMDA receptor antagonists CPP or MK-801 (3-5 microM) produced the following effects: (i) prolongation of the burst latency; (ii) attenuation of the burst duration, mainly its late phase; (iii) increase in the threshold of burst activation. These effects were not accompanied by any change in membrane potential, input resistance and repetitive firing evoked by intracellular pulses of depolarizing current. Our results indicate the involvement of conductances mediated through NMDA receptors in the genesis of epileptiform activities recorded in the neocortex upon blockade of GABA receptors.

Zinc and glycine do not modify low-magnesium-induced epileptiform activity in the immature neocortex in vitro

Exposure of neocortical slices from immature rats to saline containing no added magnesium induced spontaneous epileptiform activity that consisted of bursts of low-amplitude isolated discharges lasting 50-90 sec, recurring every 90-300 sec. Bath application of the N-methyl-D-aspartate (NMDA) receptor antagonist DL-2-amino-7-phosphonoheptanoic acid led to a rapid, reversible suppression of epileptiform activity, indicating involvement of NMDA receptors. Perfusion with zinc or glycine, putative modulators of the NMDA receptor, with suppressive and enhancing properties, respectively, had no effect on the frequency or duration of the epileptiform discharges. These results indicate that in the immature neocortex in vitro, application of zinc or glycine does not modulate NMDA receptor-mediated, low-magnesium-induced epileptiform discharges.

Excitatory amino acid blockers differentially affect bursting of in vitro hippocampal neurons in two pharmacological models of epilepsy

The role of excitatory amino acid neurotransmitters in generating two distinct types of epileptiform discharge in the CA3 region of hippocampal slices was examined in the guinea-pig using a variety of different excitatory amino acid blockers. In magnesium-free medium the N-methyl-D-aspartate receptor antagonist, DL-aminophosphonovaleric acid, and the putative N-methyl-D-aspartate channel blockers phencyclidine and ketamine, reduced the amplitude and duration of spontaneous bursts and blocked their afterdischarges. In marked contrast none of these compounds significantly depressed spontaneous bursts recorded in the presence of 2 mM Mg2+ and penicillin. This was confirmed for individual slices where Mg2+-free bursts had previously been suppressed by the same concentration of blocker. Therefore, the N-methyl-D-aspartate receptor mechanism does not contribute significantly to the generation of this type of epileptiform discharge. The channel blockers lost their effect on Mg2+-free bursts in the presence of penicillin; however, the receptor blockers did not. This is probably due to the unique mechanisms of action of the channel blockers. This implies that different types of N-methyl-D-aspartate blockers might be effective anticonvulsants only in specific conditions. While the non-specific excitatory amino acid blocker, kynurenic acid, was effective against both burst types it appeared to suppress them by different mechanisms. Kynurenic acid depressed the amplitude and duration of Mg2+-free bursts but its only effect on penicillin bursts was to reduce their frequency. This suggests that neither N-methyl-D-aspartate nor non-N-methyl-D-aspartate receptors play a major role in the production of the paroxysmal depolarizing shift resulting from the block of GABA-mediated inhibition by penicillin. However, these receptors may be involved in generating the spontaneous activity which triggers the bursts.

The NMDA-receptor antagonist, MK-801, suppresses limbic kindling and kindled seizures

Excitatory amino acid receptors in the mammalian central nervous system (CNS) are divided into 3 receptor subtypes: kainate, quisqualate, and N-methyl-D-aspartate (NMDA). MK-801 is a selective, non-competitive antagonist of excitatory amino acid transmitters at the NMDA receptor site. The role of excitatory amino acid neurotransmission in electrical kindling was examined in animals stimulated daily in the amygdala following i.p. administration of low dosages of MK-801 (0.1 and 0.5 mg/kg). A second experiment evaluated the anticonvulsant properties of MK-801 in rats kindled in the hippocampus and amygdala, and contrasted its efficacy with the antiepileptic agents, diazepam, phenobarbital and phenytoin, and the dissociative anesthetics phencyclidine and ketamine. MK-801 (0.5 mg/kg) retarded the development of amygdala kindling and reduced mean AD duration over the first 10 stimulation sessions. The low dosage reduced total AD accrued during each kindling stage but failed to alter kindling rate. MK-801 blocked motor seizures induced by stimulation of hippocampal or amygdala kindled foci, but was more effective in reducing seizure severity and AD duration resulting from stimulation of the hippocampal focus. All other drugs tested, with the exception of phenytoin, protected against amygdaloid kindled seizures. It was concluded that excitatory amino acid transmission contributes in an important, but non-critical way to amygdala kindling.

Epileptiform bursts elicited in CA3 hippocampal neurons by a variety of convulsants are not blocked by N-methyl-D-aspartate antagonists

Intracellular and extracellular recordings from CA3 hippocampal neurons in vitro were used to study the ability of several NMDA (N-methyl-D-aspartate) receptor antagonists to suppress epileptiform bursts induced by NMDA and convulsants not thought to act at NMDA receptors. The antagonists, APV (D-2-amino-5-phosphonovalerate), AP-7 (D,L-2-amino-7-phosphonohepatanoate) and CPP (D,L-3[(+/-)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid), blocked the spontaneous and evoked bursts induced by NMDA. CPP, but not APV or AP-7, prevented the development of bursts induced by Mg-free medium. The NMDA antagonists failed to block bursting induced by kainate, 7 mM K+, mast cell degranulating peptide, anoxia or spontaneous bursting. In some cases the NMDA antagonists induced spontaneous bursts or enhanced burst frequency, a proconvulsant effect. It is concluded that activation of NMDA receptors is sufficient but not necessary for burst generation in the CA3 region.

A1 adenosine receptor-mediated block of epileptiform activity induced in zero magnesium in rat neocortex in vitro

It has been suggested that endogenous chemical substances such as adenosine, released during a seizure attack, may act as anticonvulsants in vivo. To further investigate this putative role, we have tested adenosine and stable adenosine analogues for anticonvulsant activity in vitro against ictal-like epileptiform activity induced by the removal of magnesium ions from medium superfusing wedges and slices of rat neocortex. Purinoceptor agonists attenuated such burst activity with a potency profile of L-phenylisopropyl-adenosine greater than 2-chloroadenosine greater than adenosine, suggesting that their anticonvulsant actions were mediated via the A1 adenosine receptor sub-type. Adenosine exerted no apparent effect on responses to agonists acting at glutamate receptor sub-types, implying no direct postsynaptic activity at glutamatergic synapses. Adenosine receptor antagonists, the methylxanthines (3-isobutyl-1-methylxanthine greater than theophylline) markedly enhanced established epileptiform activity and reversed the anticonvulsant action of adenosine. The selectivity of this reversal was demonstrated by the lack of effect of methylxanthines on pentobarbitone-induced inhibitions of epileptiform bursts. When added to a normal medium containing 1 mM magnesium, the methylxanthines were unable to induce long-lasting ictal-like epileptiform activity.

Epileptiform events induced by GABA-antagonists in entorhinal cortical cells in vitro are partly mediated by N-methyl-D-aspartate receptors

The effects of the GABA-antagonists, picrotoxin and bicuculline on responses of medial entorhinal cortical cells to subicular stimulation were tested in vitro. On every cell tested either antagonist caused a profound enhancement of synaptically evoked depolarizations to the point where paroxysmal depolarizing shifts (PDS) were recorded, although only a minority of cells showed evidence of inhibitory potentials in the control situation. The initial PDS was followed by either a long afterdepolarization or a series of afterdischarges. The afterpotentials were always reduced or blocked by the N-methyl-D-aspartate (NMDA)-receptor antagonist, 2-amino-5-phosphonovalerate (2-AP5). The amplitude of the initial PDS in many cells was also reduced by 2-AP5. Thus, entorhinal cortical cells are susceptible to epileptogenesis induced by a reduction of GABAergic inhibition and the paroxysmal events contain a large, NMDA-receptor mediated component.

Morphine induces a spontaneous and evoked bursting activity in rat cortical neurons by adding a postsynaptic active mechanism to the synaptic input: an intracellular study in vivo

The action of morphine on spontaneous and stimulus-evoked postsynaptic potentials was investigated in rat cortical neurons recorded intracellularly in vivo. Iontophoretically applied, morphine increased supra-threshold evoked depolarizing postsynaptic potentials inducing bursts of spikes, but only slightly increased weak (subthreshold) potentials. Spontaneous excitatory postsynaptic potentials were affected in a similar way, but their frequency did not change. Inhibitory postsynaptic potentials were only subsequently modified. Membrane hyperpolarization, induced by negative current injection, counteracts the morphine-induced burst generation. We suggest that the action of this alkaloid on threshold postsynaptic events involves a voltage-dependent mechanism, which may be triggered by synaptic currents.

Validation of a neocortical slice preparation for the study of epileptiform activity

A simple slice chamber was designed to achieve easy manipulations of temperature, ionic composition and drug concentrations. Spontaneous and evoked extracellular potentials could be recorded with glass microelectrodes from 500 micron thick slices of rat frontal neocortex. In the absence of magnesium ions in the superfusing medium or in the presence of convulsant agents, epileptiform activity was seen. The amplitude of this activity was greatest in layer II/III, each burst consisting of a long-lasting negative potential on the decay phase of which were superimposed many afterpotentials. There were multiple foci from which spontaneous epileptiform bursts spread to other ipsi- and contralateral parts of the cortex via both the grey and white matter. Although such bursts were observed between 23 and 37 degrees C, optimal recording of discrete epileptiform activity was achieved at 29 +/- 1 degrees C. Decreasing extracellular calcium or increasing extracellular concentrations of potassium enhanced burst discharges. Proconvulsant agents initiated both interictal and ictal epileptiform events. This, together with the reduction of epileptiform activity by standard anticonvulsant drugs such as carbamazepine and phenobarbitone suggested that this in vitro model may be useful for studying the pharmacology of epileptogenesis and for developing new therapeutic strategies for epilepsy.

Dextrorphan and dextromethorphan, common antitussives, are antiepileptic and antagonize N-methyl-D-aspartate in brain slices

The antitussive, dextromethorphan (DM), and its metabolite, dextrorphan (DX), were evaluated for antiepileptic properties in vitro. Interictal bursts and prolonged ictal epileptiform afterdischarges, induced by perfusion of guinea pig neocortical brain slices with Mg2+-free solution, were blocked by DX (1-250 microM) or DM (100 microM). Intracellular records showed that these agents blocked N-methyl-D-aspartate (NMDA)-induced depolarizations without altering intrinsic membrane properties. DX blocked NMDA but not quisqualate-evoked multi-unit excitatory responses. DM is a widely available, orally effective drug with low toxicity in antitussive doses, which has antiepileptic and NMDA-antagonist properties in vitro. Its toxicity and effectiveness as an anticonvulsant should be expeditiously examined in clinical trials.