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

The Tryptophan Metabolite Indole-3-Propionic Acid Raises Kynurenic Acid Levels in the Rat Brain In Vivo

Alterations in the composition of the gut microbiota may be causally associated with several brain diseases. Indole-3-propionic acid (IPrA) is a tryptophan-derived metabolite, which is produced by intestinal commensal microbes, rapidly enters the circulation, and crosses the blood-brain barrier. IPrA has neuroprotective properties, which have been attributed to its antioxidant and bioenergetic effects. Here, we evaluate an alternative and/or complementary mechanism, linking IPrA to kynurenic acid (KYNA), another neuroprotective tryptophan metabolite. Adult Sprague-Dawley rats received an oral dose of IPrA (200 mg/kg), and both IPrA and KYNA were measured in plasma and frontal cortex 90 minutes, 6 or 24 hours later. IPrA and KYNA levels increased after 90 minutes and 6 hours (brain IPrA: ~56- and ~7-fold; brain KYNA: ~4- and ~3-fold, respectively). In vivo microdialysis, performed in the medial prefrontal cortex and in the striatum, revealed increased KYNA levels (~2.5-fold) following the administration of IPrA (200 mg/kg, p.o), but IPrA failed to affect extracellular KYNA when applied locally. Finally, treatment with 100 or 350 mg IPrA, provided daily to the animals in the chow for a week, resulted in several-fold increases of IPrA and KYNA levels in both plasma and brain. These results suggest that exogenously supplied IPrA may provide a novel strategy to affect the function of KYNA in the mammalian brain.

An N-methyl-D-aspartate receptor mediated large, low-frequency, spontaneous excitatory postsynaptic current in neonatal rat spinal dorsal horn neurons

Examples of spontaneous oscillating neural activity contributing to both pathological and physiological states are abundant throughout the CNS. Here we report a spontaneous oscillating intermittent synaptic current located in lamina I of the neonatal rat spinal cord dorsal horn. The spontaneous oscillating intermittent synaptic current is characterized by its large amplitude, slow decay time, and low-frequency. We demonstrate that post-synaptic N-methyl-D-aspartate receptors (NMDARs) mediate the spontaneous oscillating intermittent synaptic current, as it is inhibited by magnesium, bath-applied d-2-amino-5-phosphonovalerate (APV), or intracellular MK-801. The NR2B subunit of the NMDAR appears important to this phenomenon, as the NR2B subunit selective NMDAR antagonist, alpha-(4-hydroxphenyl)-beta-methyl-4-benzyl-1-piperidineethanol tartrate (ifenprodil), also partially inhibited the spontaneous oscillating intermittent synaptic current. Inhibition of spontaneous glutamate release by the AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or the mu-opioid receptor agonist [D-Ala2, N-Me-Phe4, Gly5] enkephalin-ol (DAMGO) inhibited the spontaneous oscillating intermittent synaptic current frequency. Marked inhibition of spontaneous oscillating intermittent synaptic current frequency by tetrodotoxin (TTX), but not post-synaptic N-(2,6-dimethylphenylcarbamoylmethyl)triethylammonium bromide (QX-314), suggests that the glutamate release important to the spontaneous oscillating intermittent synaptic current is dependent on active neural processes. Conversely, increasing dorsal horn synaptic glutamate release by GABAA or glycine inhibition increased spontaneous oscillating intermittent synaptic current frequency. Moreover, inhibiting glutamate transporters with threo-beta-benzyloxyaspartic acid (DL-TBOA) increased spontaneous oscillating intermittent synaptic current frequency and decay time. A possible functional role of this spontaneous NMDAR-mediated excitatory postsynaptic current in modulating nociceptive transmission within the spinal cord is discussed.

Activity-dependent pH shifts and periodic recurrence of spontaneous interictal spikes in a model of focal epileptogenesis

The mechanisms that control the periodicity of spontaneous epileptiform cortical potentials were investigated in the in vitro isolated guinea pig brain preparation. A brief intracortical application of bicuculline in the piriform cortex induced spontaneous interictal spikes (sISs) that recurred with high periodicity (8.5 +/- 3.1 sec, mean +/- SD). Intracellular recordings from principal neurons showed that the early phase of the inter-sIS period is caused by a GABAb receptor-mediated inhibitory potential. The late component of the interspike period correlated to a slowly decaying depolarization abolished at membrane potentials positive to -32.1 +/- 5.3 mV and was not associated with membrane conductance changes. Specific pharmacological tests excluded the contribution of synaptic and intrinsic conductances to the late inter-sIS interval. Recordings with ion-sensitive electrodes demonstrated that sISs determined both a rapid increase in extracellular K+ concentration (0.5-1 mM) and an extracellular alkalinization (0.05-0.08 pH units) that slowly decayed during the inter-sIS period and returned to control values just before a subsequent sIS was generated. These observations were not congruous with the presence of a silent period, because both extracellular increase in K+ and alkalinization are commonly associated with an increase in neuronal excitability. Extracellular alkalinization could be correlated to an sIS-induced intracellular acidification, a phenomenon that reduces cell coupling by impairing gap junction function. When intracellular acidification was transiently prevented by arterial perfusion with NH4Cl (10-20 mM), spontaneous ictal-like epileptiform discharges were induced. In addition, the gap junction blockers octanol (0.2-2 mM) and 18-alpha-glycyrrethinic acid (20 microM) applied either via the arterial system or locally in the cortex completely and reversibly abolished the sIS. The results reported here suggest that the massive cell discharge associated with an sIS induce a strong inhibition, possibly secondary to a pH-dependent uncoupling of gap junctions, that regulates sIS periodicity.

The role of long-term potentiation in persistent epileptiform burst-induced hyperexcitability following GABAA receptor blockade

Persistent hyperexcitability follows synchronized bursting induced in the CA3 region of hippocampal slices by perfusion with high concentrations (2000 IU/ml) of the GABAA antagonist, penicillin. This hyperexcitable state is characterized by: i) slow recovery from bursting following penicillin washout; ii) persistent "post-burst" field potential oscillations and iii) increased probability of spontaneous bursting with ordinarily sub-convulsant doses of GABAA antagonists. An N-methyl-D-aspartate-independent type of long-term potentiation of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate excitatory postsynaptic potentials occurred following bursting. However, similar increases in excitatory postsynaptic potential magnitude also occurred after a subconvulsant dose of penicillin (500 IU/ml) which did not produce the other features of persistent hyperexcitability. Furthermore, long-term potentiation either increased or remained stable after bursting stopped, whereas, post-burst oscillations gradually diminished with time. Low doses of the AMPA/kainate antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, which restored the potentiated excitatory postsynaptic potentials to control levels, reduced but did not eliminate the post-burst oscillation. Tetanus-induced long-term potentiation did not reproduce the hyperexcitable state seen after bursting. These findings indicate that the epileptiform bursting caused by blocking GABAA-mediated inhibition induces long-term potentiation which is partially responsible for persistent burst-induced hyperexcitability but is not sufficient to entirely explain it. The hippocampus which is critical for normal memory is also frequently the generator of intractable epileptic seizures. Seizure-like discharges in the hippocampus induced long-lasting increases in synaptic efficacy similar to those thought to underlie normal memory. This form of long-term potentiation contributed to the network oscillations characteristics of the hyperexcitable state persisting after epileptiform activity but was not sufficient to entirely explain them. Epileptic seizures may engage normal memory mechanisms which increase neuronal excitability and predispose the hippocampal network to further seizures. This may, in part, account for the propensity for hippocampal seizure foci to become intractable.

Effects of in vivo perforant path kindling on 0-Mg(2+)-induced CA1 burst activity in rat hippocampal slices

Previous studies have reported persistent alterations in the electrophysiological characteristics of the CA1 region of the hippocampus after kindling. The present study examined the effects of perforant path kindling on 0-Mg(2+)-induced epileptiform bursting in the CA1 region of hippocampal slices. The duration of evoked bursting was significantly longer in slices taken from kindled animals as compared to those taken from implanted, non-stimulated controls. No significant differences were found in spontaneous burst frequency between slices taken from kindled and control animals. These data suggest that perforant path kindling causes a persistent increase in hyperexcitability in the CA1 region of the hippocampus leading to a facilitation of evoked burst activity perhaps through an enhancement in NMDA-related excitatory neurotransmission.

Prolonged field potentials evoked by 1 Hz stimulation in the dentate gyrus of temporal lobe epileptic human brain slices

An abnormal electrophysiological response in brain slices of the dentate gyrus from biopsy material from patients surgically treated for intractable epilepsy (46/57), exhibited characteristics similar to the physiological hallmark of epilepsy, the paroxysmal discharge, a prolonged (30-600 ms) and often large amplitude field potential. The most striking feature of the prolonged response to a single perforant path stimulus was a predominantly biphasic field potential (23/46 cases). The biphasic response was characterized by a negative field potential of substantial duration exceeding 180 ms which followed an initial shorter duration positive field potential. Multiple population spikes occurred during both phases of the response. During a 1 Hz stimulus train applied to the perforant path, the magnitude and duration of the negative component of the field response was significantly increased. Approximately half of the cases (Group 1; 30/57) exhibited potentiation of the biphasic response, while the remaining cases (Group 2; 27/57) exhibited no negative field component during 1 Hz stimulation trains. This repetitive stimulation, in general, increased the area of the field response in a large majority of cases (44/57) regardless of the sign of the field potential. The number of population spikes following 1 Hz stimulation increased significantly for cases in both groups, although the increase was greater for those in Group 1 than in Group 2. Paired pulse depression (20 ms ISI) was reduced in cases that exhibited potentiated biphasic responses during 1 Hz stimulation (Group 1) in comparison to cases that exhibited no negative field potentials (Group 2). Paired pulse depression at a 200 ms ISI was not significantly different between the groups. During a single stimulus, bicuculline disinhibition (20 microM) resulted in either a prolonged positive or biphasic field potential. Intracellularly recorded responses to single perforant path stimuli also exhibited prolonged and large depolarizations that were comparable in time course to the duration of field potentials recorded in the same area whether generated in the absence or presence of bicuculline. The prolonged field potential after bicuculline was reduced by APV (20 microM). We suggest that the prolonged field response, whether biphasic or monophasic when generated by either 1 Hz stimulation or bicuculline disinhibition, may be due directly or indirectly to an increase in membrane depolarization mediated by activation of the NMDA receptor.

Seizure activity causes elevation of endogenous extracellular kynurenic acid in the rat brain

This study was designed to examine the effects of several classic convulsants on the extracellular concentration of the anticonvulsant and neuroprotective brain metabolite kynurenic acid (KYNA) in the rat brain. Drug effects were investigated in vivo, mostly by unilateral microdialysis in the dorsal hippocampus. Systemic administration of pentylenetetrazole (60 mg/kg, SC), pilocarpine (325 mg/kg, SC), bicuculline (6 mg/kg, SC), or kainic acid (10 mg/kg, SC) caused characteristic clonic and/or tonic convulsions. In all seizure paradigms, KYNA levels in the dialysate began to rise within 1 h and gradually reached a plateau approximately 4 h after administration of the convulsants. Peak increases were 1.5-3-fold over basal levels. The duration of the elevation in KYNA levels was significantly prolonged following kainic acid application. In the kainic acid model, extracellular KYNA was also measured and found to be increased in the ventral hippocampus, piriform cortex, and striatum. Moreover, temporary intrahippocampal infusion of the KYN synthesis inhibitor aminooxyacetic acid (1 mM) in the kainic acid- and pentylenetetrazole models attenuated the increase in extracellular KYNA levels, demonstrating that de novo production of KYNA in the brain accounts for the seizure-induced KYNA overflow. A separate group of animals received a unilateral intrahippocampal injection of the endogenous convulsant excitotoxin quinolinic acid (120 nmol) and showed long-lasting (> 24 h) bilateral increases in extracellular KYNA levels. Taken together, these data indicate that an increase in extracellular KYNA may constitute a common occurrence in response to seizures and that KYNA elevations may signify the brain's attempt to counteract seizure activity.

Repeated penicillin-induced amygdala epileptic focus in freely moving cats. EEG, polysomnographic (23-h recording), and brain mapping study

The effect of repeated Na-penicillin (PCN) microinjections in the temporal lobe amygdala (AM) of free-moving cats was investigated in order to establish if kindling epileptogenesis is possible with this procedure. The cortical propagation of the PCN-induced post-discharge in AM and the sequence of behavioral changes induced by PCN were similar to those of AM electrical kindling. Nevertheless, the epileptogenic effect of PCN had a different evolution from that of electrical kindling, since some PCN habituation was observed after several doses. Repeated microinjections of PCN did not produce lasting alterations in sleep onset and organization. The only mild changes recorded in the 23 h following PCN microinjections were an increased latency of the first rapid eye movement (REM) sleep episode, a SWS II total time and percentage increase, and, with the highest PCN doses, a not very significant diminution of REM sleep total time. Another finding was the occurrence of REM sleep ponto-geniculo-occipital (PGO) waves, coinciding with a depression of the frequency and amplitude of interictal amygdaloid and cortical spikes. The results showed that a microinjection of PCN in the AM produced a reliable model of interictal spikes, paroxysms and generalized convulsive seizures. Nevertheless, long lasting kindling effect was not observed.

Electrical kindling is associated with a lasting increase in the extracellular levels of kynurenic acid in the rat hippocampus

Endogenous kynurenic acid (KYNA), an excitatory amino acid receptor antagonist with antineurotoxic and anticonvulsant activity, was assessed by microdialysis in the hippocampus of kindled rats. One week after the completion of amygdala or hippocampal kindling (stage 5), the dialysate concentration of KYNA in the hippocampus of both hemispheres was 1.7 +/- 0.1-fold higher than in shams (P < 0.01). Veratridine (50 microM), applied through the probe, reduced extracellular KYNA by 28% within 1 h in controls (P < 0.05), but was ineffective in stage 5 kindled rats. At the preconvulsive stage 2, dialysate KYNA concentration and the effect of veratridine were similar to controls. The activity of KYNA's biosynthetic enzyme, kynurenine aminotransferase, did not change in the hippocampus 1 week after stage 5 seizures. These data indicate an enhanced liberation of KYNA in teh hippocampus of fully kindled animals due to an impairment of normal regulatory mechanisms. This may be of relevance for the control of hippocampal excitability during epileptogenesis.

Low extracellular magnesium unmasks N-methyl-D-aspartate-mediated graft-host connections in rat neocortex slice preparation

The main purpose of this study was to investigate the role of N-methyl-D-aspartate receptors in host-graft synaptic transmission in the neocortex. The effects of low extracellular magnesium, the glutamate agonist N-methyl-D-aspartate and N-methyl-D-aspartate antagonists on the synaptic activation of connections between embryonic neocortical graft tissue and the surrounding host tissue were studied in 17 perfused slices of rat neocortex. In standard artificial cerebrospinal fluid, stimulation of the host white matter evoked field potentials in four of 17 grafts. However, in Mg(2+)-free medium, the same stimulation evoked field potentials in an additional six grafts, with significant increases in the mean duration of the evoked responses in the 10 responsive grafts. In five of these slices stimulation of the graft also evoked field potentials in the host tissue, suggesting reciprocal interaction between graft and host. Simultaneous extracellular recordings from graft and host tissues in Mg(2+)-free medium showed that spontaneous epileptiform discharges developed in the graft and host tissue synchronously. In Mg(2+)-free medium, application of N-methyl-D-aspartate induced a shift of the baseline with superimposed epileptiform discharges in both graft and host. Application of the non-competitive N-methyl-D-aspartate antagonist ketamine and the competitive antagonist D,L-2-amino-5-phosphonovaleric acid attenuated or reversibly blocked both the spontaneous epileptiform discharges and the evoked field potentials. Our data provides evidence that N-methyl-D-aspartate receptors are present at synapses created between fetal graft and host neocortex, and that the N-methyl-D-aspartate-activated receptor-channel complex plays an active role in mediating excitatory synaptic transmission in host-graft circuitry.

Hippocampal plasticity following epileptiform bursting produced by GABAA antagonists

The effects of epileptiform bursts on hippocampal excitability were examined in the CA3 region of guinea-pig hippocampal slices. Partial blockade of gamma-aminobutyric acidA (GABAA)-mediated inhibition by 500 IU/ml penicillin produced low frequency (2-4 Hz) "pro-convulsant" field potential oscillations. Normal spontaneous activity recovered less than 30 min after the penicillin was rinsed out providing bursting was prevented. Synchronized bursting rarely began on its own even after 1 h in penicillin 500 IU/ml, but could be initiated in most slices after one to eight all-or-none bursts were evoked by low-intensity, low-frequency (0.2-0.25 Hz) stimuli. Spontaneous bursting, once initiated, persisted for at least 1 h without further stimulation suggesting that a small number of bursts produced a long-lasting increase in excitability. Bursts disappeared more slowly than anticipated after convulsants were rinsed out and were followed by "post-burst" oscillations with different frequency characteristics than proconvulsant oscillations which persisted for at least 4 h. Selective augmentation of evoked N-methyl-D-aspartate excitatory postsynaptic potentials appeared to be the critical first step in the initiation of bursting. The specific N-methyl-D-aspartate antagonist, 2-amino-5-phosphonovaleric acid (50-100 microM), only partially suppressed pro-convulsant oscillations in partially disinhibited slices but completely prevented stimulus-triggered spontaneous bursting and prolonged hyperexcitability. Although N-methyl-D-aspartate receptors were necessary for the induction of bursting in partially disinhibited slices, they were not required to initiate bursting after more complete disinhibition. However, when 2-amino-5-phosphonovaleric acid was applied prior to and during perfusion with 2000 IU/ml penicillin, spontaneous bursts occurred at long, irregular intervals and lacked afterdischarges. These bursts rapidly disappeared upon penicillin washout and were not followed by persistent post-burst oscillations. N-methyl-D-aspartate antagonists applied only after bursts already established in penicillin blocked the afterdischarges but did not reduce the burst frequency. These observations indicate that epileptiform bursts can produce long-lasting, hippocampal hyperexcitability. The induction of these plastic changes requires N-methyl-D-aspartate receptor activation which then enhances both N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor mechanisms. Furthermore, N-methyl-D-aspartate excitatory postsynaptic potentials can participate in triggering spontaneous bursts but this role is masked once plasticity has occurred. Partial disinhibition produces a pro-convulsant state which does not induce long-lasting changes in hippocampal excitability but renders the neuronal network vulnerable to develop persistent epileptiform bursting with small additional excitatory inputs.(ABSTRACT TRUNCATED AT 400 WORDS)

The role of excitatory amino acids in the generation of spontaneous hippocampal oscillations

We examined the role of N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors in generating spontaneous CA3 field potential oscillations in the hippocampal slice. Non-NMDA EPSPs are responsible for a portion of the spontaneous activity recorded in standard perfusion medium. NMDA receptors are not activated when inhibition is intact, however, recurrent excitation via NMDA receptors accounts for an increasing proportion of the low frequency (2-4 Hz) rhythms produced as inhibition is progressively blocked by low concentrations of the GABAA antagonist, penicillin. Production of these rhythms involves complex interactions among NMDA, non-NMDA and GABAA receptors. NMDA EPSPs can drive the rhythm in the absence of non-NMDA receptors only when inhibition is suppressed by more than 50%. Otherwise non-NMDA EPSPs appear to be necessary to depolarize neurons before NMDA EPSPs can be activated.

Excitatory amino acids in epilepsy and potential novel therapies

Evidence that an abnormality of excitatory neurotransmission may contribute to the epileptic phenomena in various animal and human syndromes is reviewed. Altered glutamate transport or metabolism may be a contributory factor in some genetic syndromes and enhanced responsiveness to activation of NMDA receptors may be significant in various acquired forms of epilepsy. Decreasing glutamatergic neurotransmission provides a rational therapeutic approach to epilepsy. Potent anticonvulsant effects are seen with the acute administration of NMDA antagonists in a wide range of animal models. Some competitive antagonists acting at the NMDA/glutamate site show prolonged anticonvulsant activity following oral administration at doses free of motor side effects and appear suitable for clinical trial.

Assessment of epileptogenic potential: experimental, clinical and epidemiological approaches

There are experimental, clinical and epidemiological methods of assessing the epileptogenic potential of psychotropic drugs. In the laboratory it has been shown that there is a range of cellular and synaptic processes in the cerebral cortex and hippocampus that give rise to epileptiform neuronal activity. In addition to the classical suppression of GABA-mediated inhibitory synaptic mechanisms, in vitro studies in animal models of epilepsy and on human tissue suggest a prominent role for the N-methyl D-aspartate (NMDA) subtype of excitatory amino acid receptors. Any mechanism that leads to the depolarization of the neurones is likely to result in a facilitation of the NMDA-receptor involvement in excitatory neurotransmission. This is particularly true in the cortex and hippocampus where the densities of the NMDA-receptor are highest. Data are presented in this paper on how this epileptogenic mechanism can be studied in vitro. In humans, the importance of an accurate diagnosis is stressed and the advantages and disadvantages of routine EEG recordings and ambulatory monitoring discussed. Descriptions of large-scale systems of drug safety monitoring and their application to the assessment of the epileptogenic properties of psychotropic drugs are given.

Epileptiform activity induced by low extracellular magnesium in the human cortex maintained in vitro

Extracellular field potentials and [K+]o were recorded in slices of human epileptogenic neocortex maintained in vitro during perfusion with Mg(2+)-free artificial cerebrospinal fluid (ACSF). The human neocortex was obtained during neurosurgical procedures for the relief of seizures that were resistant to medical treatment. Spontaneous epileptiform activity and episodes of spreading depression appeared within 1.5 to 2 hours of perfusion with Mg(2+)-free ACSF. The epileptiform discharges consisted of negative field potential shifts (amplitude, 0.8-10 mV) that lasted 2.5 to 80 seconds and recurred at intervals ranging between 4 and 160 seconds. Both duration and frequency of occurrence of epileptiform events were not significantly different when measured in slices obtained from spiking tissue compared with those gathered from nonspiking neocortical areas. Transient increases in [K+]o of up to 10.5 mM were associated with each epileptiform discharge; these changes were maximal and fastest in the middle neocortical layers. Spreading depression episodes were characterized by 20 to 30-mV negative shifts that lasted up to 200 seconds and were accompanied by increases in [K+]o of approximately 100 mM. Epileptiform discharges and spreading depressions did not occur during perfusion with Mg(2+)-free ACSF that contained either competitive or noncompetitive antagonists of the N-methyl-D-aspartate (NMDA) receptor subtype. In contrast, pharmacological blockade of non-NMDA receptors did not influence the epileptiform activity observed in Mg(2+)-free ACSF. These findings demonstrate that decreasing [Mg2+]o leads to the appearance of both spontaneous epileptiform discharges and spreading depression in the human epileptogenic neocortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Actions of ketamine, phencyclidine and MK-801 on NMDA receptor currents in cultured mouse hippocampal neurones

1. Stable N-methyl-D-aspartic acid (NMDA) receptor-mediated currents in cultured mouse hippocampal neurons were evoked by 20 ms pressure pulse applications of L-aspartate, repeatedly applied at 30 or 40 s intervals, to the cell body region of the neurone. We have characterized the voltage- and use-dependent blockade of the currents by three dissociative anaesthetics: ketamine, phencyclidine (PCP) and MK-801 in mouse hippocampal neurones grown in dissociated tissue culture. 2. We have used a simple model of the blockade, based on the 'guarded receptor hypothesis' to interpret our data. The model assumes that receptors are maximally activated at the peak of the response with an open probability (Po) approaching 1, that there is no desensitization and that the blocking drug only associates with, or dissociates from, receptor channels which have been activated by agonist (e.g. open channels). 3. The model allows us to estimate forward and reverse rate constants for binding of the blockers to open channels from measurements of the steady-state level of blockade and the rate of change of the current amplitude per pulse during onset and offset of blockade. As predicted by the model, the estimated reverse rate was independent of blocker concentration while the forward rate increased with concentration. Changing the level of positively charged ketamine (pKa 7.5) tenfold by changing pH from 6.5 to 8.5 caused a corresponding change in the forward rate while having no effect on the reverse rate. Most of the voltage dependence of the blockade could be accounted for by reduction of the reverse rate by depolarization. 4. Estimated forward rate constants for ketamine, PCP and MK-801 were similar to one another when measured under similar conditions and were 3 x 10(4) - 3 x 10(5) M-1 S-1. Most of the differences in potency of the three blockers could be accounted for by differences in the reverse rate constants which were approximately 0.2, 0.03 and 0.003 s-1 for ketamine, PCP and MK-801, respectively. The estimated rate constants actually are the product of the rate constants and 1/Po. Suggestions that maximum Po is much less than 1 for NMDA channels imply that both forward and reverse rate constants of blockade may in fact be larger than we have calculated. However, their magnitudes, relative to one another, are unaffected by this consideration. 5. The reverse rate constant of blockade increased at positive potentials. This increase was prevented when the neurone was loaded with N-methyl-D-glucamine, an impermeant cation which prevented outward currents.(ABSTRACT TRUNCATED AT 400 WORDS)

Different changes in spontaneous field potential oscillations precede epileptiform bursting in hippocampal slices perfused with penicillin or reduced magnesium

Power spectra were used to analyse spontaneous field potentials (SFPs) recorded in the CA3 distal apical dendritic region of guinea pig hippocampal slices perfused with either penicillin or reduced Mg2+. High concentrations of penicillin (2000 IU/ml) progressively converted the low amplitude, irregular oscillations observed in control medium to higher amplitude, low frequency, rhythmic oscillations at approximately 2-3 Hz just prior to the onset of spontaneous, synchronized bursting. Low concentrations (50-300 IU/ml) increased the power of frequencies below 10 Hz and suppressed higher frequencies in a dose-dependent fashion. Although Mg2(+)-free medium also increased the magnitude of the SFPs prior to the onset of synchronous bursting, the changes were smaller than with penicillin and the frequency distribution was completely different. Low concentrations of Mg2+ (0.0-0.5 mM) increased the power across all frequencies, however, the maximal effect was on frequencies between 5 and 25 Hz. The transition from normal to epileptiform activity may proceed through at least 2 distinct intermediate states. When recurrent inhibition is blocked (penicillin), synchronous synaptic activity precedes the onset of bursting, whereas non-specific increases in excitability and activation of NMDA receptors (reduced Mg2+) produce an asynchronous transition state.

Epileptiform activity induced by 4-aminopyridine in rat amygdala neurons: the involvement of N-methyl-D-aspartate receptors

The involvement of the N-methyl-D-aspartate (NMDA) receptor in the epileptiform activity induced by 4-aminopyridine (4-AP) was studied in rat amygdala slices using intracellular recording techniques. Stimulation of the ventral endopyriform nucleus evoked an excitatory postsynaptic potential (EPSP). After exposure to 4-AP (200 microM) the amygdala slices usually exhibited spontaneous and evoked epileptiform activity. The epileptiform events had an average duration of 522 +/- 78 ms with a frequency of 0.5-8.5 bursts/min. Superfusion of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a selective non-NMDA receptor antagonist, practically abolished the epileptiform bursting. However, there remained a residual depolarizing component in 13 out of 18 neurons. This CNQX-resistant component was markedly enhanced both in amplitude and duration when extracellular Mg2+ was removed and could be reversibly blocked by the specific NMDA receptor antagonist, DL-2-amino-5-phosphonovaleate (DL-APV). Compared with the CNQX-sensitive component, the APV-sensitive component had a much smaller amplitude shorter duration. These data suggest that the NMDA receptor is likely to play only a minor role, and activation of the NMDA receptor may contribute to but is not required, for the generation of these bursts.