Epileptogenic properties of the mast cell degranulating peptide in CA3 hippocampal neurones

Diminished Dentate Gyrus Filtering of Cortical Input Leads to Enhanced Area Ca3 Excitability after Mild Traumatic Brain Injury

Mild traumatic brain injury (mTBI) disrupts hippocampal function and can lead to long-lasting episodic memory impairments. The encoding of episodic memories relies on spatial information processing within the hippocampus. As the primary entry point for spatial information into the hippocampus, the dentate gyrus is thought to function as a physiological gate, or filter, of afferent excitation before reaching downstream area Cornu Ammonis (CA3). Although injury has previously been shown to alter dentate gyrus network excitability, it is unknown whether mTBI affects dentate gyrus output to area CA3. In this study, we assessed hippocampal function, specifically the interaction between the dentate gyrus and CA3, using behavioral and electrophysiological techniques in ex vivo brain slices 1 week following mild lateral fluid percussion injury (LFPI). Behaviorally, LFPI mice were found to be impaired in an object-place recognition task, indicating that spatial information processing in the hippocampus is disrupted. Extracellular recordings and voltage-sensitive dye imaging demonstrated that perforant path activation leads to the aberrant spread of excitation from the dentate gyrus into area CA3 along the mossy fiber pathway. These results suggest that after mTBI, the dentate gyrus has a diminished capacity to regulate cortical input into the hippocampus, leading to increased CA3 network excitability. The loss of the dentate filtering efficacy reveals a potential mechanism by which hippocampal-dependent spatial information processing is disrupted, and may contribute to memory dysfunction after mTBI.

l-Homocysteate Preferentially Activates N-methyl-D-aspartate Receptors to CA1 Rat Hippocampal Neurons

Intracellular recordings and current and single-electrode voltage-clamp techniques were used to study the membrane responses of CA1 pyramidal neurons to bath application of l-homocysteic acid (l-HC) in the rat hippocampal slice preparation. In control artificial cerebrospinal fluid (ACSF), l-HC (25 - 250 microM) depolarized the membrane and induced a burst-like firing pattern. Both the membrane depolarization and the burst firing were blocked by the N-methyl-d-aspartic acid (NMDA) receptor antagonists d-(-)-2-amino-5-phosphonovaleric acid (AP-5, 50 microM), d-(-)-2-amino-7-phosphonoheptanoic acid (AP-7, 50 microM) and (+/-)-3-(2-carboxy-piperazin-4-yl)-propyl-1-phosphonic acid (CPP, 20 microM). In ACSF containing tetrodotoxin (1 microM), l-HC (100 - 300 microM) induced at resting membrane potential a depolarization which was associated with a small increase in input conductance. These effects were unaffected by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 - 20 microM) but were fully blocked by AP-5, AP-7 (50 microM) and CPP (10 - 20 microM). In voltage-clamp experiments, l-HC induced slow inward currents which were voltage-dependent between - 70 and - 30 mV and reversed polarity near 0 mV. The l-HC-induced inward current was unaffected by CNQX (10 - 20 microM) but was strongly reduced by AP-5 or AP-7 (50 microM). The l-HC-induced inward current was temperature-dependent. Between - 60 and - 70 mV, its amplitude increased by 320% when the temperature was lowered from 33 to 22 degrees C. The l-HC-induced current was also potentiated by the specific l-HC uptake blocker beta-p-chlorophenylglutamate (Chlorpheg, 0.5 - 2 mM). These data suggest that l-HC preferentially activates NMDA receptors in CA1 hippocampal neurons.

Induction of seizures by the potent K+ channel-blocking scorpion venom peptide toxins tityustoxin-K(alpha) and pandinustoxin-K(alpha)

The scorpion venom peptide toxins tityustoxin-K(alpha) (TsTx-K(alpha)) and pandinustoxin-K(alpha) (PiTx-K(alpha)) are novel, highly potent and selective blockers of voltage-activated K+ channels. PiTx-K(alpha) preferentially blocks rapidly inactivating (A-type) K+ channels whereas TsTx-K(alpha) is selective for slowly inactivating (delayed rectifier-type) channels. K+ channel blockers are known to induce seizures, but the specific K channel types that can serve as convulsant targets are not well defined. To address this issue, we examined for convulsant activity the K+ channel type-specific scorpion toxins and the selective K+ channel antagonists 4-aminopyridine (4-AP), an inhibitor of A-type voltage-activated K+ channels, and paxilline, a selective blocker of large conductance (maxi K) Ca(2+)-activated K+ channels. Intracerebroventricular injection of recombinant TsTx-K(alpha) and PiTx-K(alpha) in mice produced limbic and clonic-tonic seizures. The severity of the seizures increased during the 60-min period following injection, culminating in continuous clonic seizure activity (status epilepticus), tonic hindlimb extension, and eventually in death. The estimated doses producing limbic and clonic seizures in 50% of animals (CD50) for TsTx-K(alpha) and PiTx-K(alpha) were 9 and 33 ng, respectively. 4-AP produced seizure activity similar to the toxins (CD50, 76 ng) whereas paxilline failed to induce seizures at doses up to 13.5 microg. Carbamazepine protected fully against the toxin- and 4-AP-induced seizures whereas phenytoin had variable activity against the clonic component although it was protective against tonic hindlimb extension. The AMPA receptor antagonist GYKI 52466 also conferred full protection against toxin-induced seizures, but the NMDA receptor antagonists (R)-CPP and dizocilpine failed to affect limbic and clonic seizures, although they protected against hindlimb extension. We conclude that selective blockade of delayed rectifier- or A-type voltage-activated K+ channels can produce limbic, clonic and tonic seizures, whereas blockade of maxi K-type Ca(2+)-activated K+ channels does not. The convulsant effects may be related to enhanced glutamate release and, in the case of the limbic and clonic convulsions, activation of AMPA receptors.

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)

Paroxysmal long-lasting depolarizations in cultured hippocampal neurons are generated by activation of NMDA and non-NMDA receptors

In primary cultures of hippocampal neurons from the embryonic rat, spontaneous depolarizations lasting up to 6 sec and resembling paroxysmal depolarization shifts (PDSs) appeared after 11 days in vitro. These depolarizations are presumably generated by synaptic events, because: (1) both their appearance and duration are independent of membrane potential, (2) the amplitudes of the underlying currents depend monotonically on membrane potential, and (3) they are reversed at the reversal potential of the excitatory postsynaptic potentials (EPSPs). In addition, PDSs disappeared reversibly when sodium-dependent action potentials were blocked by tetrodotoxin (10 microM) and when synaptic transmission was reduced by elevated Mg2+ (5 mM). Further, the fact that these depolarizations can appear simultaneously in two neurons in paired recordings also points to a synaptic origin. Inhibition of glutaminergic synaptic transmission by kynurenic acid (50 microM) and the NMDA-antagonist D-2-amino-5-phosphonovaleric acid (APV; 50 microM) led to a marked shortening of the depolarizations. This blocking effect of kynurenic acid and APV and comparison with the currents elicited by locally applied glutamate or NMDA provide evidence for an activation of both types of glutamate receptors to induce PDSs. The role of alteration of glutaminergic synaptic transmission in the induction and maintenance of these depolarizations is discussed in the context of results from the literature on the appearance of PDSs in cultures grown under chronic blockade of glutamate receptors.

Inactivation characteristics of a sustained, Ca(2+)-independent K+ current of rat hippocampal neurones in vitro

1. Current or voltage clamp recordings from CA3 neurones of the adult rat hippocampal slice were performed to study the inactivation properties of a slow outward K+ current identified as the delayed rectifier (IK). 2. In current clamp experiments, burst firing evoked from resting membrane potential by intracellular current injection was reduced or blocked by conditioning hyperpolarizing pre-pulses of 20-40 mV amplitude. This effect was inhibited by tetraethylammonium (TEA; 20 mM) but was unaffected by Cs+ (3 mM), 4-aminopyridine (4-AP; 2 mM), carbachol (30-50 microM), mast cell degranulating peptide (MCDP; 300 nM), thyrotrophin releasing hormone (TRH; 1 microM) or by a Ca(2+)-free solution containing Mn2+ or Co2+ (2 mM). 3. Single-electrode voltage clamp experiments were carried out on neurones superfused with Ca(2+)-free solution, containing tetrodotoxin (TTX; 1 microM), Mn2+ or Co2+ (2 mM), 4-AP (2 mM), Cs+ (3 mM) and carbachol (30 microM). Step depolarizations from a holding potential of -55 mV activated an outward current which reached a plateau after 200 ms, followed by an outward tail current. Such an outward current had the characteristics of IK. 4. The outward currents were significantly potentiated by conditioning hyperpolarizing pre-pulses suggesting the IK was reduced by a voltage-dependent inactivation process. Removal of inactivation was a function of the amplitude of the conditioning hyperpolarizing pre-pulse. At a holding potential of -55 mV removal of inactivation was time dependent with a time constant of 211 ms. High K+ (12.5 or 21.5 mM) solutions did not affect the inactivation characteristics of IK. 5. Tetraethylammonium (20 mM) or low concentrations of Ba2+ (0.1 mM) readily depressed the outward current without significantly affecting the inactivation process. Dendrotoxin (200 nM) also depressed such a slow current but, in addition, increased the inactivation process of IK. 6. It is suggested that removal of inactivation of IK by hyperpolarization can modulate cell excitability by fully restoring the ability of IK to inhibit burst firing of CA3 hippocampal neurones.

Protection against dendrotoxin-induced clonic seizures in mice by anticonvulsant drugs

Various anticonvulsant drugs were evaluated for their ability to protect against clonic seizures induced in mice by intraventricular injection of the K+ channel blocking peptide dendrotoxin (DTX). Phenytoin, the phenytoin-like anticonvulsant carbamazepine and the broad spectrum drug valproate were effective in this model, whereas the GABA-enhancers diazepam and tiagabine, the NMDA antagonists (+/-)-CPP and (+)-MK-801, the AMPA antagonist NBQX, the antiabsence drug ethosuximide and the Ca2+ channel antagonist nimodipine were inactive. In contrast to the lack of activity of other NMDA antagonists, phencyclidine and ADCI [(+/-)-aminocarbonyl-10,11-dihydro-5H-dibenzo [a,d]cyclohepten-5,10-imine] were potent antagonists of DTX-induced seizures.

Cobalt blocks postsynaptic responses induced by neurotransmitters in the hippocampus in vitro

Divalent metals such as cobalt are frequently used by neurophysiologists to prevent synaptic transmission, because they are thought to selectively block presynaptic calcium conductance. Recording intracellularly from hippocampal CA3 pyramidal cells we show that Co2+ (2 mM) is not specific in this action but also diminishes postsynaptic responses mediated by agonists acting at ionotropic and metabotropic glutamatergic receptors, as well as GABAA, GABAB, adenosine, and cholinergic receptors. These findings indicate that a more selective substance should be employed for experiments where neurotransmitter release must be blocked.

Mast Cell Degranulating Peptide Increases the Frequency of Spontaneous Miniature Postsynaptic Currents in CA3 Rat Hippocampal Neurons

Mast cell degranulating peptide (MCDP) is a neurotoxic agent isolated from bee venom. It produces a long-term potentiation in the hippocampus. We now report that MCDP, at nanomolar concentrations, induces a reduction of a transient voltage-dependent potassium current (ID) in CA3 rat pyramidal neurons and a persistent (>30 min) enhancement of the frequency of spontaneous miniature excitatory and inhibitory postsynaptic currents (m.e.p.s.c.s. and m.i.p.s.c.s.). M.e.p.s.c.s. and m.i.p.s.c.s. were recorded in the presence of bicuculline (30 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM), respectively. The increased frequency of m.e.p.s.c.s. (408 +/- 60%) and m.i.p.s.c.s. (583 +/- 553%) was independent of the reduction of ID because 4-aminopyridine (4-AP, 30 microM - 2 mM) blocked ID but had no effects on m.e.p.s.c.s. and m.i.p.s.c.s. In the presence of the calcium channel blocker manganese (3 mM), MCDP still enhanced the frequency of m.e.p.s.c.s. (326 +/- 162%). It is concluded that MCDP augments the release of excitatory and inhibitory transmitter by an action, which is independent of calcium influx, through voltage-dependent channels.

A new method for the measurement of endogenous transmitter release in localized regions of hippocampal slices

We describe here a method that allows measurement of the release of endogenous amino acids from localized regions of brain slices combined with conventional electrophysiological experiments. Hippocampal slices were placed in fully submerged chambers and a cannula was positioned just above the dendritic layers of CA1. The cannula was connected to a peristaltic pump and the content of amino acids in the perfusate was measured by HPLC. Extracellular field potentials were concomitantly recorded. Stable levels of aspartate and glutamate were found above the stratum radiatum of CA1. No detectable release was found when the cannula was located above the alveus, the fimbria or in the effluent of the slice. A pulse of K+ (50 mM) produced a brief 3-fold increase in glutamate, aspartate and a detectable release of GABA in CA1. Brief high frequency trains (10 Hz) also increased significantly the release. This method will be useful in determining alterations in transmitter release in the slice in relation to anoxia, epilepsy and long term potentiation.

Brief seizure episodes induce long-term potentiation and mossy fibre sprouting in the hippocampus

Much of our present understanding of the cellular mechanisms of learning and memory derives from studies on the hippocampus in which long-term potentiation (LTP) of synaptic transmission is produced by a train of high-frequency electrical stimulation or by potassium channel blockers. The hippocampus is also a seizure-prone region and recent studies have revealed that brief seizure episodes produce remarkably long-lasting changes which are reminiscent of 'classical' LTP. A brief seizure episode also sets in motion a cascade of events that includes changes in gene expression, sprouting of fibres and the establishment of new synaptic contacts. This paper reviews this use-dependent structural rearrangement of the neuronal network and discusses its possible role in epilepsy and as a model of plasticity in the adult nervous system.

Mast cell degranulating peptide: a multi-functional neurotoxin

This review discusses our present knowledge of the structure and activities of the mast cell degranulating peptide (MCDP). This peptide is a basic, 22 amino acid residue component of honey bee venom with striking immunological and pharmacological activities. MCDP is a potent anti-inflammatory agent, but at low concentrations it is a strong mediator of mast cell degranulation and histamine release. MCDP is also an epileptogenic neurotoxin, an avid blocker of the potassium channels and can cause a significant lowering of the blood pressure in rats. Some of the biological activities of MCDP appear to have distinct mechanisms and may represent a good illustration of the structure-function relationship.

Long-term enhancement of synaptic transmission by synthetic mast cell degranulating peptide and its localization of binding sites in hippocampus

A bee venom, mast-cell-degranulating (MCD) peptide, was synthesized by stepwise formation of the two disulfide bridges. This synthetic MCD peptide induced long-term potentiation (LTP) in the CA1 region of a hippocampus slice at concentrations ranging from 10(-7) to 10(-5) M. The potentiating effect of MCD was not lost by biotinylation of its N-terminus, and thus it became possible to investigate localization of its binding site at a cellular level in the hippocampus slice at a LTP-inducible concentration (1 microM). We found that even after brief application of N-biotinyl-MCD (1 microM) to perfusate, its presence was observed in pyramidal cell bodies in the CA1 and CA3 regions, but not in granule cells in the dentate gyrus of the hippocampus slice. This observation does not coincide with the localization of its high-affinity binding sites revealed by 125I-MCD binding, suggestive that another type(s) of MCD binding site, such as low-affinity binding sites, are present in the hippocampus.

Regional expression of a MCD-peptide and dendrotoxin I-sensitive voltage-dependent potassium channel in rat brain

In situ hybridization histochemistry has been used to analyze the regional expression of a class of voltage-dependent K+ channel that is sensitive to two polypeptide toxins (MCD peptide and dendrotoxin I) that produce spectacular effects on brain function. A heterogeneous expression of this K+ channel was observed throughout the brain. High mRNA contents were observed in the granule cells of the gyrus dentatus as well as in pyramidal cells of the Ammon horn (CA3 greater than CA1) and in the cerebellum. Conversely, low levels of expression were found in basal ganglia (caudate putamen, globus pallidus, and ventral pallidum).

Long-term potentiation in the rat hippocampus induced by the mast cell degranulating peptide: analysis of the release of endogenous excitatory amino acids and proteins

Using a push-pull device, we have analysed, in vivo, the release of endogenous excitatory amino acids and proteins induced by the mast cell degranulating peptide in the CA1 region of the hippocampus. Local application of the mast cell degranulating peptide (20 microM) for 5 or 10 min produced a long-term potentiation of the slope of the field excitatory postsynaptic potential (70 +/- 40%, 3 h after the drug application). This long-term potentiation was associated with (i) a transient increase (10 min) in the release of endogenous glutamate and aspartate and (ii) a late transient enhanced release of proteins and newly secreted proteins. In cases in which the mast cell degranulating peptide induced recurrent interictal activity, there was a sustained enhanced release of glutamate. These observations suggest that mast cell degranulating peptide induced long-term potentiation is not associated with a sustained enhanced release of excitatory amino acids.

Giant synaptic potentials in immature rat CA3 hippocampal neurones

1. Intracellular recordings were made from rat CA3 hippocampal neurones in vitro during the first eighteen days of postnatal life. The cells had resting membrane potentials more negative than -51 mV, action potentials greater than 55 mV and membrane input resistances of 117 +/- 12 M omega. An unusual characteristic of these cells was the presence of spontaneous giant depolarizing potentials (GDPs) which were observed during the first eight postnatal (P) days in over 85% of neurones. They were less frequent between P9 and P12 (48%) and disappeared after P12. 2. The GDPs were synchronously generated by a population of neurones; they reversed polarity at -27 mV when recorded with KCl-containing electrodes and at -51 mV with potassium acetate- or potassium methylsulphate-filled electrodes. 3. The GDPs were blocked by bath application of bicuculline (10 microM) or picrotoxin (100-200 microM). Exogenously applied gamma-aminobutyric acid (GABA; 0.2-1 mM) induced at resting membrane potential a bicuculline-sensitive membrane depolarization which reversed polarity at -25 and -51 mV when recorded with KCl- or potassium methylsulphate-filled electrodes respectively. 4. The GDPs were reduced in frequency or blocked by the N-methyl-D-aspartate (NMDA) receptor antagonists DL-2-amino-7-phosphonoheptanoate (AP-7; 50 microM), D(-)2-amino-5-phosphonovalerate (AP-5, 10-50 microM) and (+-)3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP, 10-50 microM) or NMDA channel blockers phencyclidine (2 microM) and ketamine (20 microM). 5. Stimulation of the hilus during the first week of life evoked a GDP followed by a hyperpolarization. The GDPs were generated by a population of synchronized neurones and reversed polarity at -27 mV with KCl-filled electrodes and at -52 mV with potassium acetate- or potassium methylsulphate-containing electrodes. 6. Bath application of bicuculline (1-10 microM) or picrotoxin (100-200 microM) reversibly blocked the evoked GDPs in the majority of cells. The NMDA receptor antagonists AP-5 (50 microM), AP-7 (50 microM) and CPP (30 microM) usually reduced the amplitude and the duration of the evoked GDPs. In neurones in which evoked GDPs were blocked by bicuculline, a NMDA-mediated component was revealed by increasing the strength or the frequency of stimulation. 7. During the second week of postnatal life, when spontaneous GDPs were extremely rare or absent, superfusion with bicuculline (10 microM) induced, as in adult slices, interictal discharges. These reversed polarity near 0 mV with KCl- or potassium acetate-containing electrodes and were reduced in amplitude and duration by AP-5 (50 microM).(ABSTRACT TRUNCATED AT 400 WORDS)

Endogenous and network bursts induced by N-methyl-D-aspartate and magnesium free medium in the CA3 region of the hippocampal slice

The epileptogenic properties of N-methyl-D-aspartate and magnesium-free medium were investigated in the CA3 region of the hippocampal slice preparation in the rat. Bath application of N-methyl-D-aspartate (5-10 microM) or magnesium-free medium induced both spontaneous and stimulus-evoked bursts. Both endogenous and network bursts were generated, the former always preceding the latter. The paroxysmal depolarizing shift underlying the network bursts generated by N-methyl-D-aspartate and magnesium-free medium resembled a giant excitatory postsynaptic potential with a reversal potential near 0 mV and a synaptic input in the apical dendrites above the mossy fibre zone. In the presence of N-methyl-D-aspartate or magnesium-free medium, population bursts were synchronized by activating single CA3 neurons. N-methyl-D-aspartate receptor antagonists prevented the development of N-methyl-D-aspartate-induced spontaneous and stimulus-evoked bursts. However, the only N-methyl-D-aspartate receptor antagonist effective in preventing such bursts in magnesium-free medium was DL-3-[(+/-)-2-carboxypiperazin-4-yl-]-propyl-1-phosphonic acid. Endogenous bursting in the CA3 region has not been observed with other convulsants and thus may reflect the novel voltage dependence of the N-methyl-D-aspartate receptor gated ionic channel. N-methyl-D-aspartate receptors may also partially contribute to the excitatory interaction between CA3 neurons and thereby account for the synchronization of the population observed when activating single CA3 neurons.

Long-lasting modification of the synaptic properties of rat CA3 hippocampal neurones induced by kainic acid

1. The action of a short bath application of kainic acid (KA, 200-250 nM, 3-5 min) on the CA3 region of rat hippocampal slices has been studied with intracellular and extracellular recording techniques. 2. KA evoked bursts which persisted for 10-15 min. In addition, after KA, electrical stimulation of various inputs to CA3 which elicited an EPSP-IPSP sequence in control conditions evoked an EPSP followed by a burst. This evoked response persisted for several hours after removal of KA suggesting the occurrence of a long-lasting modification of the synaptic properties of CA3 neurones. 3. Intracellular recordings showed the spontaneous and evoked bursts to consist of five to ten action potentials riding on a depolarizing shift 10-25 mV in amplitude and 40-100 ms in duration. Both spontaneous and evoked bursts were followed by a long-lasting hyperpolarization 15-25 mV in amplitude and 1-1.5 s in duration. 4. We propose that both spontaneous and evoked synchronized bursts are generated by a polysynaptic network since: (a) intracellularly recorded bursts were synchronized with the bursts in extracellular field recording; (b) bursts disappeared when synaptic transmission or Na+ action potential were blocked by cobalt (1 mM) or TTX (1 microM) respectively; (c) bursts were suppressed by elevated divalent cation concentration; (d) burst occurrence was independent of the membrane potential of the cell; (e) the depolarization shift that underlies the bursts was a linear function of the membrane potential and reversed in polarity at 0 mV. In addition, the evoked bursts were all-or-none events with a variable latency. 5. Laminar profile analysis of the spontaneous and evoked bursts suggests that they were generated by synapses located on the distal apical segments of the dendrites of CA3 pyramidal cells. 7. The persistence of the evoked bursts was neither due to a persistent change in cell excitability nor to a long-lasting reduction in GABAergic synaptic inhibition. 8. Bath application of a high concentration of potassium (7 mM) also induced spontaneous and evoked bursts; the latter also persisted several hours after return to control medium. 9. The N-methyl-D-aspartate (NMDA) antagonist, D-APV (D(-)-2-amino-5-phosphonovaleric acid) (30 microM), did not block the spontaneous discharges induced by KA or high potassium, but prevented the long-lasting effects on the synaptic responses.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Brief anoxic episodes induce long-lasting changes in synaptic properties of rat CA3 hippocampal neurons

The effects of brief anoxic episodes on rat CA3 hippocampal neurons were studied with intracellular and extracellular techniques in the in vitro slice preparation. After repeated (3-7 times), brief (2-6 min duration each) applications of artificial cerebrospinal fluid (ACSF) saturated with 95% N2 and 5% CO2, electrical stimulation of various inputs to CA3 neurons, evoked an excitatory postsynaptic potential (EPSP) followed by an all-or-none burst. This response which persisted for several hours after the last anoxic episode, is reminiscent of the bursts induced by various convulsive agents. Post anoxic bursts are generated by a polysynaptic network which converge on the apical distal segment of CA3 neurons. It is concluded that a repetitive impairement of metabolism produces long lasting changes in the synaptic properties of CA3 neurons.