Sustained and selective block of IPSPs in brain slices from rats made epileptic by intrahippocampal tetanus toxin

A New Approach of Modified Submerged Patch Clamp Recording Reveals Interneuronal Dynamics during Epileptiform Oscillations

Highlights

Simultaneous epileptiform LFPs and single-cell activity can be recorded in the membrane chamber.

Interneuron firing can be linked to epileptiform high frequency activity.

Fast ripples, unique to chronic epilepsy, can be modeled in ex vivo tissue from TeNT-treated rats.

Traditionally, visually-guided patch clamp in brain slices using submerged recording conditions has been required to characterize the activity of individual neurons. However, due to limited oxygen availability, submerged conditions truncate fast network oscillations including epileptiform activity. Thus, it is technically challenging to study the contribution of individual identified neurons to fast network activity. The membrane chamber is a submerged-style recording chamber, modified to enhance oxygen supply to the slice, which we use to demonstrate the ability to record single-cell activity during in vitro epilepsy. We elicited epileptiform activity using 9 mM potassium and simultaneously recorded from fluorescently labeled interneurons. Epileptiform discharges were more reliable than in standard submerged conditions. During these synchronous discharges interneuron firing frequency increased and action potential amplitude progressively decreased. The firing of 15 interneurons was significantly correlated with epileptiform high frequency activity (HFA; ~100–500 Hz) cycles. We also recorded epileptiform activity in tissue prepared from chronically epileptic rats, treated with intrahippocampal tetanus neurotoxin. Four of these slices generated fast ripple activity, unique to chronic epilepsy. We showed the membrane chamber is a promising new in vitro environment facilitating patch clamp recordings in acute epilepsy models. Further, we showed that chronic epilepsy can be better modeled using ex vivo brain slices. These findings demonstrate that the membrane chamber facilitates previously challenging investigations into the neuronal correlates of epileptiform activity in vitro.

Structural and functional substrates of tetanus toxin in an animal model of temporal lobe epilepsy

The effects of tetanus toxin (TeNT) both in the spinal cord, in clinical tetanus, and in the brain, in experimental focal epilepsy, suggest disruption of inhibitory synapses. TeNT is a zinc protease with selectivity for Vesicle Associated Membrane Protein (VAMP; previously synaptobrevin), with a reported selectivity for VAMP2 in rats. We found spatially heterogeneous expression of VAMP1 and VAMP2 in the hippocampus. Inhibitory terminals in stratum pyramidale expressed significantly more VAMP1 than VAMP2, while glutamatergic terminals in stratum radiatum expressed significantly more VAMP2 than VAMP1. Intrahippocampal injection of TeNT at doses that induce epileptic foci cleaved both isoforms in tissue around the injection site. The cleavage was modest at 2 days after injection and more substantial and extensive at 8 and 16 days. Whole-cell recordings from CA1 pyramidal cells close to the injection site, made 8-16 days after injection, showed that TeNT decreases spontaneous EPSC frequency to 38 % of control and VAMP2 immunoreactive axon terminals to 37 %. In contrast, TeNT almost completely abolished both spontaneous and evoked IPSCs while decreasing VAMP1 axon terminals to 45 %. We conclude that due to the functional selectivity of the toxin to the relative sparing of excitatory synaptic transmission shifts the network to pathogenically excitable state causing epilepsy.

The tetanus toxin model of chronic epilepsy

In experimental models of epilepsy, single and recurrent seizures are often used in an attempt to determine the effects of the seizures themselves on mammalian brain function. These models attempt to emulate as many features as possible of their human disease counterparts without many of the confounding factors such as underlying disease processes and medication effects. Numerous models have been used in the past to address different questions. Nevertheless, the basic questions are often the same: 1. Do seizures cause long-term damage? 2. Do seizures predispose to chronic epilepsy (epileptogenesis), that is long-term spontaneous repetitive seizures? 3. Are these results developmentally regulated? 4. Are the underlying mechanisms of epileptogenesis and brain damage related? In pursuing these questions, the goal is to determine how seizures exert their effects and to minimize any side effects from the methods employed to induce the seizures themselves. This requires a detailed characterization of the methods used to induce seizures. In this chapter, we will review the literature regarding the tetanus toxin model of chronic epilepsy with regard to its mechanisms of action, clinical comparisons, how it is experimentally implemented and the results obtained thus far. These results will be compared to other models of chronic epilepsy in order to make generalizations about the effects of repetitive seizures in adult and early life. At this time, it appears that repetitive seizures cause long-term changes in learning ability and may cause a predisposition to chronic seizures at all ages. In younger animals, both features of learning impairment and epilepsy are not typically associated with cell loss as they are in adult animals. At all ages, some form of synaptic reorganization has been demonstrated to occur.

A comparison of the efficacy of carbamazepine and the novel anti-epileptic drug levetiracetam in the tetanus toxin model of focal complex partial epilepsy

1. The tetanus toxin seizure model, which is associated with spontaneous and intermittent generalized and non-generalized seizures, is considered to reflect human complex partial epilepsy. The purpose of the present study was to investigate and compare the anticonvulsant effects of carbamazepine with that of levetiracetam, a new anti-epileptic drug in this model. 2. One microl of tetanus toxin solution (containing 12 mLD(50) microl(-1) of tetanus toxin) was placed stereotactically into the rat left hippocampus resulting in generalized and non-generalized seizures. 3. Carbamazepine (4 mg kg(-1) h(-1)) and levetiracetam (8 and 16 mg kg(-1) h(-1)) were administered during a 7 day period via an osmotic minipump which was placed in the peritoneal cavity. Carbamazepine (4 mg kg(-1) h(-1)) exhibited no significant anticonvulsant effect, compared to control, when the entire 7 day study period was evaluated but the reduction in generalized seizures was greater (35.5%) than that for non-generalized seizures (12.6%). However, during the first 2 days of carbamazepine administration a significant reduction in both generalized seizure frequency (90%) and duration (25%) was observed. Non-generalized seizures were unaffected. This time-dependent anticonvulsant effect exactly paralleled the central (CSF) and peripheral (serum) kinetics of carbamazepine in that steady-state concentrations declined over time, with the highest concentrations achieved during the first 2 days. Also there was a significant 27.3% reduction in duration of generalized seizures during the 7 day study period (P=0.0001). 4. Levetiracetam administration (8 and 16 mg kg(-1) h(-1)) was associated with a dose-dependent reduction in the frequency of both generalized (39 v 57%) and non-generalized (36 v 41%) seizures. However, seizure suppression was more substantial for generalized seizures. Also a significant dose-dependent reduction in overall generalized seizure duration was observed. 5. These data provide experimental evidence for the clinical efficacy of levetiracetam for the management of patients with complex partial seizures. Furthermore, levetiracetam probably does not act by preventing ictogenesis per se but acts to reduce seizure severity and seizure generalization.

Increased pyramidal excitability and NMDA conductance can explain posttraumatic epileptogenesis without disinhibition: a model

Partially isolated cortical islands prepared in vivo become epileptogenic within weeks of the injury. In this model of chronic epileptogenesis, recordings from cortical slices cut through the injured area and maintained in vitro often show evoked, long- and variable-latency multiphasic epileptiform field potentials that also can occur spontaneously. These events are initiated in layer V and are synchronous with polyphasic long-duration excitatory and inhibitory potentials (currents) in neurons that may last several hundred milliseconds. Stimuli that are significantly above threshold for triggering these epileptiform events evoke only a single large excitatory postsynaptic potential (EPSP) followed by an inhibitory postsynaptic potential (IPSP). We investigated the physiological basis of these events using simulations of a layer V network consisting of 500 compartmental model neurons, including 400 principal (excitatory) and 100 inhibitory cells. Epileptiform events occurred in response to a stimulus when sufficient N-methyl-D-aspartate (NMDA) conductance was activated by feedback excitatory activity among pyramidal cells. In control simulations, this activity was prevented by the rapid development of IPSPs. One manipulation that could give rise to epileptogenesis was an increase in the threshold of inhibitory interneurons. However, previous experimental data from layer V pyramidal neurons of these chronic epileptogenic lesions indicate: upregulation, rather than downregulation, of inhibition; alterations in the intrinsic properties of pyramidal cells that would tend to make them more excitable; and sprouting of their intracortical axons and increased numbers of presumed synaptic contacts, which would increase recurrent EPSPs from one cell onto another. Consistent with this, we found that increasing the excitability of pyramidal cells and the strength of NMDA conductances, in the face of either unaltered or increased inhibition, resulted in generation of epileptiform activity that had characteristics similar to those of the experimental data. Thus epileptogenesis such as occurs after chronic cortical injury can result from alterations of intrinsic membrane properties of pyramidal neurons together with enhanced NMDA synaptic conductances.

Expression of mRNAs encoding flip isoforms of GluR1 and GluR2 glutamate receptors is increased in rat hippocampus in epilepsy induced by tetanus toxin

The messenger RNAs encoding the flip and flop isoforms of the glutamate receptor subunits GluR1 and GluR2 were detected and quantified by in situ hybridization in the hippocampal formation of rats following intrahippocampal injection of tetanus on one side. The mRNAs encoding the flip isoforms of both GluR1 and GluR2 were significantly increased 4 weeks after injection. The GluR1 flip mRNA was significantly elevated only in the dentate gyrus, whereas significant increases in the GluR2 flip mRNA were seen in all hippocampal subfields examined. There were no significant changes in the mRNA encoding the flop isoforms of either GluR1 or GluR2. The significant changes in flip isoform mRNAs occurred on both sides.

Local circuit abnormalities in chronically epileptic rats after intrahippocampal tetanus toxin injection in infancy

In vitro slice experiments were undertaken in adult rats to investigate the physiological origins of a chronic epileptic condition that was initiated in infancy. A unilateral injection of a minute quantity of tetanus toxin into hippocampus on postnatal day 10 produced a severe convulsive syndrome characterized by brief but repeated seizures that lasted for 5-7 days. Hippocampal slices were then taken from these rats in adulthood because at this time previous studies have shown the occurrence of electrographic and behavioral seizures. Dramatic alterations in local circuit functioning were observed. In normal artificial cerebrospinal fluid (ACSF), spontaneous epileptiform network bursts were recorded in a majority (73%) of experimental rats. Network bursts occurred in area CA3 of both the injected and contralateral hippocampus. These consisted of intracellular depolarization shifts that were coincident with extracellularly recorded network bursts. Often they occurred at frequencies of 0.05-0.1 Hz and although variable in amplitude and duration, had all-or-none-like qualities. These events appeared to arise largely from local circuits in the CA3C subfield. Network bursts were rarely recorded in area CA1 and were never observed in the dentate gyrus. However in 31% of rats, a novel, higher frequency (2-8 Hz) field potential was recorded in area CA1. This was coincident with rhythmic, intracellularly recorded, inhibitory postsynaptic potentials (IPSPs). These summated IPSPs blocked action potential firing and reversed polarity near -75 mV. To understand the origins of network bursting in area CA3C, comparisons were made of the fundamental neurophysiological properties of pyramidal cells in epileptic and control rats. Of the passive and active membrane properties examined, all appeared normal. Unusually prolonged bursts of action potentials were observed in a small subset of pyramidal cells. However on average the duration of intrinsic bursts were unaltered in the CA3 neurons analyzed from experimental rats. To explore the role that alterations in CA3 recurrent excitatory network excitability may play in epileptiform discharges, picrotoxin was bath applied. On blockade of gamma-aminobutyric acid (GABAA) receptors, slices from experimental rats underwent prolonged electrographic seizures that were up to 10 s in duration. In contrast, slices from control rats produced only brief 100-ms network bursts. These results suggest that a change in excitability within CA3C recurrent excitatory networks likely contributes to seizures in chronically epileptic rats. However, at the same time, this hyperexcitability is controlled to an important degree by functional GABAA-mediated synaptic inhibition.

Afferent inputs modulate the activity of a rhythmic burst generator in the rat disinhibited spinal cord in vitro

Application of strychnine and bicuculline to the isolated spinal cord of the neonatal rat induces spontaneous bursting of regular rhythmicity (cycle period approximately 30 s). This phenomenon is important because it shows that a spinal network, made up by excitatory connections only, generates a very reliable rhythmic pattern. To find out how signals from the periphery or higher centres might influence the operation of the rhythmogenic network, the present experiments examined whether synaptic inputs from dorsal root (DR) or ventrolateral (VL) afferent fibers could modulate this spontaneous rhythmicity. This issue was addressed with intracellular recording from motoneurons or extracellular recording from ventral roots after eliciting bursting with strychnine plus bicuculline. Single electrical shocks (0.1 ms; intensity 1-4 times threshold) applied to one DR reset spontaneous bursting without altering its period or duration. Repetitive stimulations at periods ranging from 20 to 2 s entrained bursts on a 1:1 basis. Burst duration was shorter at lower stimulation periods whereas burst amplitude was unchanged. The lowest stimulation period compatible with burst entrainment depended on stimulus strength. At stimulation periods <2-s entrainment was always lost and spontaneous bursts unexpectedly returned even if electrical pulses still elicited ventral root reflexes. Such spontaneous bursts had similar properties as those recorded in the absence of electrical pulses. Analogous results were obtained with VL stimulations. It is concluded that the spinal rhythmogenic network was highly susceptible to external synaptic inputs, which paced burst generation whereas burst duration was adapted to interstimulus interval. A scheme is provided to explain the modulatory role of synaptic inputs as well as the escape of bursting from fast stimulus entrainment in terms of a rhythmogenic network functionally separated from reflex pathways activated by DR or VL tracts.

The effects of anticonvulsant drugs on NMDA-EPSP, AMPA-EPSP, and GABA-IPSP in the rat hippocampus

The effects of phenobarbital, phenytoin, and valproic acid on pharmacologically isolated NMDA-EPSP, AMPA-EPSP, and GABA-IPSPs were examined in rat hippocampal slices. Phenobarbital (0.05 mg/ml) had no effect on the NMDA-EPSP, but decreased the slope of the AMPA-EPSP by 13.4% and facilitated the GABA-IPSP slope by 77.12%. Phenytoin (0.02 mg/ml) had no effects on the NMDA-EPSP, AMPA-EPSP, or GABA-IPSP. Valproic acid (0.1 mg/ml) decreased the NMDA-EPSP slope by 14.3%, increased the GABA-IPSP slope by 54.34%, and had no effect on the AMPA-EPSP. These data suggest that the mechanisms of action of these anticonvulsant drugs may be via their actions on different neurotransmitter systems or ion channels.

Review of the role of inhibitory neurons in chronic epileptic foci induced by intracerebral tetanus toxin

Blocking inhibition provides one of the most common experimental means of triggering epileptic activity in hippocampus and neocortex. However, it has proved much more difficult to show that chronic models of epilepsies are due to disinhibition. One problem is knowing how much inhibition needs to be blocked to provide a sufficient mechanism for epileptic activity. We have found that inhibitory (GABAA) transmission, estimated from evoked monosynaptic IPSCs, must be reduced to 17% of their control amplitude (by 4-7 microM bicuculline) before hippocampal slices generate all-or-none epileptic discharges. Similar estimates of inhibition in chronic epileptic foci induced by intrahippocampal injection of tetanus toxin showed that monosynaptic IPSCs dropped to 10% of control in the injected hippocampus during the first 2 weeks after injection. At all other stages of the active epileptic foci in the two hippocampi the reduction in IPSCs was not alone sufficient for epileptic activity; at 4-6 weeks IPSCs were normal despite continued epileptic activity. One likely mechanism for the late epileptic activity is a reduction of either the intrinsic excitability, or the synaptic excitation, of inhibitory interneurons so they fail to be recruited normally. Alternative mechanisms include the formation of new excitatory connections, as found at modest levels in the dentate gyrus. Several mechanisms may play a part in chronic foci such as those induced by tetanus toxin, either acting together, or sequentially during the progression of the epileptic focus.

Functional GABAergic synaptic connection in neonatal mouse barrel cortex

Intracortical inhibition is crucial to proper functioning of the mature neocortex, yet, paradoxically, is reported to be rare or absent in the neonatal animal. We reexamined this issue by recording whole-cell postsynaptic currents (PSCs) of barrel cortex neurons in thalamocortical brain slices from neonatal mice. Monosynaptic, excitatory thalamocortical responses were elicited in layers V/VI neurons as early as postnatal day 0 (P0, the first 24 hr after birth) and in presumptive layer IV as early as P2. At very low stimulation frequencies, the monosynaptic response was invariably followed by a prolonged (up to 1 sec) synaptic barrage, which fatigued at stimulus repetition rates of 2/min or higher. This barrage consisted of postsynaptic responses to spiking activity in neighboring cortical cells, because (1) it could also be evoked by intracortical stimulation in coronal slices and (2) it was abolished by antagonists to NMDA receptors (NMDARs), even when NMDARs on the recorded cell were under a voltage-dependent block. Some of the larger polysynaptic events changed polarity at a negative reversal potential and were blocked by GABAA receptor (GABAAR) antagonists, with a concurrent enhancement of the extracellular field potential, indicating that they were GABAAR- mediated, CI-dependent inhibitory PSCs (IPSCs). We conclude that a network of functional intracortical GABAAR-mediated synaptic connections exists from the earliest postnatal ages, although it gives rise to responses that differ from mature IPSCs in reversal potential and latency.

Erosion of inhibition contributes to the progression of low magnesium bursts in rat hippocampal slices

1. Bathing slices of rat hippocampus in media containing no magnesium ions results in epileptic discharges that originate in hippocampal area CA3. These discharges increase in severity gradually over periods of hours. 2. The progression of epileptic activity was much slower than the equilibration of extracellular magnesium activity and the resulting increase in strength of monosynaptic NMDA receptor-mediated excitation. Its time course matched that of a progressive decrease in pharmacologically isolated, evoked GABAA receptor-mediated inhibitory postsynaptic current (IPSC) in the CA3 pyramidal cells. Conductance decreased to 37 +/- 6% of control values after 4 h. Responses to exogenous GABA application decreased to 52 +/- 12%. 3. Maximal IPSC conductance in 0 mM extracellular Mg2+ ([Mg2+]o) also decreased gradually when epileptic activity was abolished by bath application of 20 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 50 microM D-2-amino-5-phosphonovaleric acid (D-APV) throughout the 4 h incubation period. It reached 61 +/- 8% of control values, a significantly smaller decline than that seen after 4 h of epileptic activity. 4. The decrease in mean IPSC conductance only partially reversed when the recording electrode contained 100 mM Mg2+. Complete recovery of IPSC strength occurred when electrodes also contained either 50 mM MgATP or 20 mM BAPTA. Reintroduction of 1 mM [Mg2+]o rapidly abolished epileptic activity and caused a slow, partial increase in IPSC conductance. 5. In the presence of 1 mM [Mg2+]o, GABAA receptor-mediated inhibition had to decrease to 17 +/- 11% of control values, in the presence of 4-7 microM bicuculline, to reach threshold for epileptic activity. 6. These data demonstrate a postsynaptic decrease in GABAA receptor-mediated inhibition in the in vitro low magnesium model of epilepsy. We propose that the apparent leaching of intracellular Mg2+ ([Mg2+]i) from cells leads to loss of ATP and consequent partial dephosphorylation of the GABAA receptor and that this is exacerbated by epileptic activity.

Tetanus toxin-induced seizures in infant rats and their effects on hippocampal excitability in adulthood

A new experimental model of developmental epilepsy is reported. Behavioral and EEG features of seizures produced by unilateral intrahippocampal injection of tetanus toxin in postnatal day 9-11 rats, are described. Within 24-72 h of tetanus toxin injection, rat pups developed frequent and often prolonged seizures which included combinations of repetitive wet dog shakes, and wild running-jumping seizures. Intrahippocampal and cortical surface EEG recordings showed that coincident with these behaviors, electrographic seizures occurred not only in the injected hippocampus, but also in the contralateral hippocampus and bilaterally in the neocortex. Analysis of the interictal EEG revealed multiple independent spike foci. One week following tetanus toxin injection, the number of seizures markedly decreased; however, interictal spiking persisted. After injection rats were allowed to mature some were observed to have unprovoked behavioral seizures and/or epileptiform EEG activity. Mature animals were also studied using in vitro slice techniques. Recordings from hippocampal slices demonstrated spontaneous epileptiform burst discharges in the majority of rats which had tetanus toxin induced seizures as infants. These events occurred in area CA3 and consisted of interictal spikes and intracellularly recorded paroxysmal depolarization shifts (PDSs). On rarer occasions, electrographic seizures were recorded. The use of the tetanus toxin model in developing rats may facilitate a better understanding of the unique features of epileptogenesis in the developing brain and the consequences early-life seizures have on brain maturation and the genesis of epileptic conditions in later life.

Epileptic activity outlasts disinhibition after intrahippocampal tetanus toxin in the rat

1. A single dose of tetanus toxin, injected under anaesthesia into one dorsal hippocampus of the rat, produces chronic epileptic foci involving both hippocampi. Generalized seizures occurred 1-6 weeks after injection and epileptic discharges were found in hippocampal slices in vitro. Here we measured the time course of decay of epileptic activity and the level of GABAA receptor-mediated inhibition in hippocampal slices 1-16 weeks after toxin injection in vivo. 2. Epileptic activity peaked in the dentate granule cell and CA3 pyramidal cell layers 2 weeks after toxin injection and at 4 weeks in CA1. Thresholds for evoking epileptic activity were lowest in the suprapyramidal blade of the dentate gyrus and area CA3c. Recovery from epileptic activity occurred more rapidly in the contralateral hippocampus. Polyspike activity ceased by 8 weeks and interictal activity by 16 weeks. Epileptic discharges could still be evoked from CA1 16 weeks after toxin injection. 3. The maximal monosynaptic fast inhibitory postsynaptic current (IPSC) conductance changes (gIPSC) decreased to < 10% of control values at the time of peak epileptic activity and remained lower than controls for 4 weeks ipsilaterally. In the contralateral hippocampus, gIPSC fell to ca 50% of control values for the first 2 weeks. Responses to exogenous GABA remained unchanged. 4. After 8 weeks dentate granule cells had gIPSC significantly larger than controls. No increase in gIPSC occurred in CA3. Epileptic activity persisted 8-10 weeks after recovery from disinhibition ipsilaterally and 4 weeks contralaterally. 5. Epileptic activity was seen when monosynaptic GABAA receptor-mediated IPSCs were normal or supranormal. At these times polysynaptic inhibition was still profoundly reduced. These observations provide strong evidence for long-term changes in the pattern of synaptic excitation contributing to a chronic epileptic syndrome syndrome following disinhibitory insult, and are consistent with weakened excitation of inhibitory neurones.

Injection of tetanus toxin into the neocortex elicits persistent epileptiform activity but only transient impairment of GABA release

Focal injection of a minute quantity of tetanus toxin into the rat neocortex induces chronic epileptogenesis. Within a day, spontaneous and stimulus-evoked paroxysmal discharges appear in widespread regions of both hemispheres and this lasts for at least nine months. Tetanus toxin blocks transmitter release, apparently by catalysing the breakdown of synaptobrevin, a synaptic protein. It specifically binds to neuronal membranes but its potent epileptogenic properties have been ascribed to a higher affinity for inhibitory neurons. Following focal injection of tetanus toxin into the hippocampus a long-lasting epileptic syndrome also develops. During the early part of the syndrome GABA release is depressed in slices from the injected side, but not in slices from the contralateral, secondary focus. In the present experiments on neocortex, release of radiolabelled GABA was measured from primary and secondary epileptic foci induced by unilateral focal injection of tetanus toxin into the parietal cortex. By four weeks after the injection, no differences were detected in GABA release from any neocortical site in control or toxin-injected animals, despite the persistence of profound epileptic activity in slices from the latter. At earlier times (1.5 days) after the toxin injection, however, release was significantly depressed in both hemispheres. The results indicate that at first, the toxin induces focal neocortical epileptogenesis by directly impeding GABAergic synaptic transmission but that with time there is a recovery from this initial effect. We propose, as has also been suggested for other models, that the initial epileptogenesis leaves in its wake a long-lasting change in the local functional connectivity, such that the neocortex is rendered permanently epileptic.

Synaptic inhibition in primary and secondary chronic epileptic foci induced by intrahippocampal tetanus toxin in the rat

1. Injecting twelve mouse minimum lethal doses of tetanus toxin into one hippocampus of a rat leads to the development of chronic epileptic foci in both hippocampi. These generate intermittent epileptic discharges for 6-8 weeks. Here we compare GABAergic inhibition, 10-18 days after injection, in slices prepared from the injected and contralateral hippocampi (respectively the primary and the secondary or 'mirror' foci), using both neurochemical and electrophysiological methods. 2. Epileptic activity was recorded from slices of both hippocampi from all tetanus toxin-injected rats. Evoked epileptic discharges were similar on the two sides, but spontaneous epileptic discharges were more common contralaterally. 3. Ca(2+)-dependent, K(+)-stimulated (synaptic) release of radiolabelled GABA was depressed in slices from the injected hippocampus, compared with vehicle-injected controls. In contrast, slices from the contralateral hippocampus had normal levels of Ca(2+)-dependent, K(+)-stimulated GABA release, even though adjacent slices were epileptogenic. 4. Intracellular recordings revealed that both fast and slow stimulus-evoked inhibitory postsynaptic potentials (IPSPs) were abolished in CA3 pyramidal cells in the primary focus. In the secondary focus, however, fast IPSPs were seen in seven of twenty-five cells, and slow IPSPs were seen in all cells if the stimulus was strong enough. 5. Monosynaptic IPSPs were isolated pharmacologically by blocking glutamatergic excitatory postsynaptic potentials (EPSPs) with 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and D(-)-2-amino-5-phosphopentanoic acid (AP-5). No monosynaptic IPSPs were uncovered in cells from the primary focus at any stimulus strength. Monosynaptic IPSPs were evoked in all cells from both the secondary focus and control slices. The estimated conductances of monosynaptic fast IPSPs were similar in cells from the secondary focus and from the controls, although the former required twice the stimulus strength. 6. Slow IPSPs were found in the secondary focus and in controls, but not in the primary focus. They were sensitive to 3-amino-2-(4-chlorophenyl)-2-hydroxy-propylsulphonic acid (2-OH saclofen). The estimated conductances of slow IPSPs evoked by weak stimuli in the secondary focus were much smaller than in the controls. However, stimuli that could trigger epileptic discharges in the secondary focus, evoked 2-OH saclofen-sensitive slow IPSPs with estimated conductances approaching the controls. This marked increase in the slow IPSP did not occur when EPSPs, and epileptic bursts, were blocked with CNQX and AP-5, suggesting that a strong barrage of excitation is needed to generate full-sized slow IPSPs in the secondary focus.(ABSTRACT TRUNCATED AT 400 WORDS)

Synaptic and intrinsic conductances shape picrotoxin-induced synchronized after-discharges in the guinea-pig hippocampal slice

1. A computer model was constructed of the guinea-pig hippocampal region in vitro, containing 100 pyramidal neurones. This approach has contributed to the understanding of brief (usually less than 100 ms) epileptic events known as 'interictal spikes'. The present study addresses the cellular mechanisms of more prolonged epileptic events, lasting 200 ms and more, that may represent short-duration seizures. Each neurone was simulated with a nineteen-compartment model using six voltage-dependent ionic conductances. The neurones were randomly interconnected with excitatory synapses, each synapse exerting a fast voltage-independent alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) component and a slower voltage-dependent N-methyl-D-aspartate (NMDA) component. Each neurone received input from twenty other neurones. 2. This model was able to generate, in response to synaptic noise or to stimulation of one neurone, a series of synchronized population bursts, the initial (primary) burst being longer than later (secondary) bursts, terminating in a prolonged after-hyperpolarization. The simulated after-discharge potentials resemble those recorded experimentally from pyramidal neurones during perfusion of the hippocampal slice with media containing picrotoxin, a blocker of synaptic inhibition mediated by GABAA receptors. 3. Simulated after-discharges agree with the following experiments: over a certain range of total NMDA conductance, blockade of AMPA receptors will prevent the occurrence of synchronized firing, whereas, blockade of NMDA receptors will, in contrast, abolish the secondary bursts, leaving a shortened and somewhat smaller primary burst. Dendritic potential oscillations occur in phase with somatic oscillations. When interneurones (some generating GABAA-mediated IPSPs, others generating GABAB IPSPS) are included in the model, the occurrence of synchronized events was suppressed, the most significant suppressant effect coming from GABAA IPSPS. 4. The model predicts that: a dendritic calcium spike occurs during each secondary burst; AMPA receptors serve to maintain the synchrony of secondary bursts, as well as to initiate the primary burst; and that with sufficient total NMDA conductance, synchronized firing can occur even with AMPA receptors blocked. 5. The model suggests, in addition, that the duration of the initial burst is determined in part by the experimentally observed delay between Ca2+ entry and peaking of the after-hyperpolarization (AHP) conductance, and hence reflects properties of the individual pyramidal neurones. Specifically, a pattern of a long initial burst followed by brief secondary bursts is elicited in single-cell simulations by injection of a steady depolarizing current into the apical dendrite. The same pattern is produced when the single-cell model includes only calcium and calcium-dependent conductances.(ABSTRACT TRUNCATED AT 400 WORDS)

Increased expression of GAD mRNA during the chronic epileptic syndrome due to intrahippocampal tetanus toxin

A few mouse minimum lethal doses (MLD) of tetanus toxin injected into rat hippocampus triggers prolonged changes in neuronal function. Spontaneously recurring epileptic discharges arise in both the injected and the contralateral, uninjected hippocampus. The seizures remit after about 6 weeks, to be succeeded by a permanent depression of hippocampal neuronal responses. There is no evidence of any loss of pyramidal cells at this low dose of toxin. Here we studied presumptive inhibitory, GABAergic neurons, using in situ hybridization (ISH) with a probe directed against the mRNA encoding glutamic acid decarboxylase (GAD), at each of 1, 2, 4 and 8 weeks after injection of tetanus toxin. Epileptic activity was recorded from hippocampal slices prepared from both injected and contralateral hippocampi of rats at each time point, unexpectedly persisting until 8 weeks. There were no significant differences in the numbers of neurons containing GAD mRNA between toxin- and vehicle-injected and control rats in any hippocampal subfield, at any survival time, except for an apparently transient loss of hilar signal in vehicle-injected rats at 1 and 2 weeks which we attribute to a significant, transient loss of neuronal GAD mRNA to below the threshold for detection by ISH using this probe. In contrast there was a marked increase in GAD mRNA in the toxin-injected group, which reached a peak at 4 weeks, and returned to control levels by 8 weeks. The changes were bilateral and were most marked in the hilus of the dentate area, but were also significant in CA3 and CA1. Upregulation of GAD mRNA was preceded by an increase in the levels of the mRNA for the alpha subunit of the GTP binding protein, Gs (Gs alpha), at 2 weeks which affected the GABAergic neurons selectively, and not the pyramidal or granule cells. These marked changes in GAD mRNA may contribute to putative adaptive responses within GABAergic neurons, which would help contain epileptic activity in these chronic foci. The changes in GAD expression may be due to mechanisms acting through an increase in mRNA encoding Gs alpha.