A role for synaptic and network plasticity in controlling epileptiform activity in CA1 in the kainic acid-lesioned rat hippocampus in vitro

Exposure to 1-bromopropane vapors during pregnancy enhances the development of hippocampal neuronal excitability in rat pups during lactation

OBJECTIVES

Although 1-Bromopropane (1-BP) exposure has been reported to cause neurotoxicity in adult humans and animals, its effects on the development of the central nervous system remain unclear. Recently, we reported delayed developmental neurotoxicity (DNT) upon 1-BP exposure in rats. Here we aimed to study the effect of prenatal 1-BP exposure on the hippocampal excitability in the juvenile offspring.

METHODS

Pregnant Wistar rats were exposed to vaporized 1-BP for 20 days (6 h/d) with concentrations of 0 (control), 400, or 700 ppm. Hippocampal slices were prepared from male offspring during postnatal days (PNDs) 13, 14, and 15. Field excitatory postsynaptic potential (fEPSP) and population spike (PS) were recorded simultaneously from the CA1 region.

RESULTS

In the exposed groups, the stimulation/response relationships of fEPSP slope and PS amplitude were enhanced more than in the control group at PND 14. Analysis of fEPSP-spike coupling demonstrated increased values of Top and Eslope50 in the exposed groups. Real-time PCR analysis showed a significant increase in the mRNA levels of the adult type Nav 1.1 Na+ channel subunit and the GluR1 glutamate receptor subunit in the hippocampus of the 700 ppm group at PND 14.

CONCLUSIONS

Our results provide evidence that prenatal exposure to 1-BP accelerates developmental enhancement of hippocampal excitability in the pups before eye-opening. The current study suggests that our evaluation method of DNT is applicable to the industrial chemical 1-BP.

Epileptic pilocarpine-treated rats exhibit aberrant hippocampal EPSP-spike potentiation but retain long-term potentiation

Hippocampal neuron plasticity is strongly associated with learning, memory, and cognition. In addition to modification of synaptic function and connectivity, the capacity of hippocampal neurons to undergo plasticity involves the ability to change nonsynaptic excitability. This includes altering the probability that EPSPs will generate action potentials (E‐S plasticity). Epilepsy is a prevalent neurological disorder commonly associated with neuronal hyperexcitability and cognitive dysfunction. We examined E‐S plasticity in chronically epileptic Sprague–Dawley rats 3–10 weeks after pilocarpine‐induced status epilepticus. CA1 neurons in hippocampal slices were assayed by whole‐cell current clamp to measure EPSPs evoked by Schaffer collateral stimulation. Using a weak spike‐timing‐dependent protocol to induce plasticity, we found robust E‐S potentiation in conjunction with weak long‐term potentiation (LTP) in saline‐treated rats. In pilocarpine‐treated rats, a similar degree of LTP was found, but E‐S potentiation was reduced. Additionally, the degree of E‐S potentiation was not correlated with the degree of LTP for either group, suggesting that they independently contribute to neuronal plasticity. E‐S potentiation also differed from LTP in that E‐S plasticity could be induced solely from action potentials generated by postsynaptic current injection. The calcium chelating agent BAPTA in the intracellular solution blocked LTP and E‐S potentiation, revealing the calcium dependence of both processes. These findings suggest that LTP and E‐S potentiation have overlapping but nonidentical mechanisms of inducing neuronal plasticity that may independently contribute to cognitive disruptions observed in the chronic epileptic state.

The role of TRPC6 in seizure susceptibility and seizure-related neuronal damage in the rat dentate gyrus

Transient receptor potential canonical channel-6 (TRPC6) forms Ca(2+)-permeable non-selective cation channels in neurons. Although TRPC6 plays an important role in neurite outgrowth and neuronal survival during development, TRPC6 expression profiles available to identify distinctive hippocampal neuronal damage and hippocampal excitability in epilepsy are less defined. As compared to normal animals, TRPC6 expression was down-regulated in chronic epileptic rats showing spontaneous recurrent seizures. TRPC6 knockdown increased seizure susceptibility, excitability ratio and paired-pulse inhibition in the dentate gyrus (DG) of normal animals. Furthermore, TRPC6 knockdown promoted programmed neuronal necrosis in dentate granule cells, but prevented it in CA1 and CA3 neurons following status epilepticus. The present data suggest for the first time that TRPC6 may inhibit seizure susceptibility and neuronal vulnerability in the rat DG.

Endogenous inhibition of hippocampal LTD and depotentiation by vasoactive intestinal peptide VPAC1 receptors

Vasoactive intestinal peptide (VIP), an important modulator of hippocampal synaptic transmission, influences exploration and hippocampal-dependent learning in rodents. Homosynaptic long-term depression (LTD) and depotentiation are two plasticity phenomena implicated in learning of behavior flexibility and spatial novelty detection. In this study, we investigated the influence of endogenous VIP on LTD and depotentiation induced by low-frequency stimulation (1 Hz, 900 pulses) of the hippocampal CA1 area in vitro in juvenile and young adult rats, respectively. LTD and depotentiation were enhanced by the VIP receptor antagonist Ac-Tyr(1) , D-Phe(2) GRF (1-29), and the selective VPAC1 receptor antagonist, PG 97-269, but not the selective VPAC2 receptor antagonist, PG 99-465. This action was mimicked by an anti-VIP antibody, suggesting that VIP, and not pituitary adenylate cyclase-activating polypeptide (PACAP), is the endogenous mediator of these effects. Selective inhibition of PAC1 receptors with PACAP (6-38) enhanced depotentiation, but not LTD. VPAC1 receptor blockade also revealed LTD in young adult rats, an effect abolished by the GABAA antagonist bicuculline, evidencing an involvement of GABAergic transmission. We conclude that inhibition of LTD and depotentiation by endogenous VIP occurs through VPAC1 receptor-mediated mechanisms and suggest that disinhibition of pyramidal cell dendrites is the most likely physiological mechanism underlying this effect. As such, VPAC1 receptor ligands may be considered promising pharmacological targets for treatment of cognitive dysfunction in diseases involving altered GABAergic circuits and pathological saturation of LTP/LTD like Down's syndrome and temporal lobe epilepsy.

Further evidence that pathologic high-frequency oscillations are bursts of population spikes derived from recordings of identified cells in dentate gyrus

PURPOSE

To analyze activity of identified dentate gyrus granular cells and interneurons during pathologic high-frequency oscillations (pHFOs).

METHODS

Pilocarpine-treated epileptic mice were anesthetized with urethane and ketamine. Their heads were fixed in a stereotaxic frame. Extracellular unit activity was recoded with glass micropipettes, whereas multiunit and local field activity was simultaneously recorded with attached tungsten microelectrodes. After electrophysiologic experiments, recorded cells were labeled by neurobiotin and visualized by immunohistochemical methods. KEY FINDINGS AND SIGNIFICANCES: pHFOs containing more than three waves were recorded in our experiments, but pathologic single-population spikes also occurred. Identified granular cells discharged preferentially in synchrony with pHFOs and single population spikes, whereas interneurons decreased their discharge frequency during this time. These experiments provide additional confirmation that pHFOs in the dentate gyrus represent single or recurrent population spikes, which in turn reflect summated hypersynchronous discharges of principal cells.

Long-term plasticity is proportional to theta-activity

Background

Theta rhythm in the hippocampal formation is a main feature of exploratory behaviour and is believed to enable the encoding of new spatial information and the modification of synaptic weights. Cyclic changes of dentate gyrus excitability during theta rhythm are related to its function, but whether theta epochs per se are able to alter network properties of dentate gyrus for long time-periods is still poorly understood.

Methodology/Principal Findings

We used low-frequency stimulation protocols that amplify the power of endogenous theta oscillations, in order to estimate the plasticity effect of endogenous theta oscillations on a population level. We found that stimulation-induced augmentation of the theta rhythm is linked to a subsequent increase of neuronal excitability and decrease of the synaptic response. This EPSP-to-Spike uncoupling is related to an increased postsynaptic spiking on the positive phases of theta frequency oscillations. Parallel increase of the field EPSP slope and the population spike occurs only after concurrent pre- and postsynaptic activation. Furthermore, we observed that long-term potentiation (>24 h) occurs in the dentate gyrus of freely behaving adult rats after phasic activity of entorhinal afferents in the theta-frequency range. This plasticity is proportional to the field bursting activity of granule cells during the stimulation, and may comprise a key step in spatial information transfer. Long-term potentiation of the synaptic component occurs only when the afferent stimulus precedes the evoked population burst, and is input-specific.

Conclusions/Significance

Our data confirm the role of the dentate gyrus in filtering information to the subsequent network during the activated state of the hippocampus.

Loss of metabotropic glutamate receptor-dependent long-term depression via downregulation of mGluR5 after status epilepticus

Synaptic plasticity is thought to be a key mechanism of information storage in the CNS. Different forms of synaptic long-term potentiation have been shown to be impaired in neurological disorders. Here, we show that metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD), but not NMDA receptor-dependent LTD at Schaffer collateral-CA1 synapses, is profoundly impaired after status epilepticus. Brief application of the group I mGluR agonist (R,S)-3,5-dihydroxyphenylglycine (100 microM; 5 min) induced mGluR LTD in control, but not in pilocarpine-treated rats. Experiments in the presence of selective inhibitors of either mGluR5 [2-methyl-6-(phenylethynyl)-pyridine] or mGluR1 [7-(hydroxyimino)cyclopropachromen-carboxylate ethyl ester and (S)-(+)-alpha-amino-4-carboxy-2-methylbenzeneacetic acid] demonstrate that loss of mGluR LTD is most likely attributable to a loss of mGluR5 function. Quantitative real-time reverse transcription PCR revealed a specific downregulation of mGluR5 mRNA, but not of mGluR1 mRNA in the CA1 region. Furthermore, we detected a strong reduction in mGluR5 protein expression by immunofluorescence and quantitative immunoblotting. Additionally, the scaffolding protein Homer that mediates coupling of mGluR5 to downstream signaling cascades was downregulated. Thus, we conclude that the reduction of mGluR LTD after pilocarpine-induced status epilepticus is the result of the subtype-specific downregulation of mGluR5 and associated downstream signaling components.

Synaptic plasticity of the CA3 commissural projection in epileptic rats: an in vivo electrophysiological study

The hippocampal commissural system has recently been found to participate in the generation of mirror foci after kainate-induced epileptiform discharges. In the present study we have evaluated the electrophysiological alterations in the ventral commissural hippocampal system that originates in the pyramidal CA3 cells and connects to the contralateral CA3 pyramidal cells. The recordings were performed in epileptic rats 24 h after an early behavioural spontaneous seizure between 5 and 21 days after pilocarpine-induced status epilepticus. Epileptic animals presented a marked increase in neuronal excitability after contralateral CA3 stimulation, characterized by a shift to the left in the input-output curve and the clear appearance of a population spike. Input-output curves showed that maximum population excitatory postsynaptic potential (pEPSP) amplitude was decreased by 30%, which could be related to cell death in these regions. Using paired-pulse protocols to evaluate a fast form of synaptic plasticity (i.e. paired-pulse facilitation) we observed that, despite the similar pEPSP amplitude between control and experimental groups, only epileptic animals showed strong paired-pulse population spike facilitation up to 500 ms interstimulus intervals. Despite increased excitability and pyramidal cell death, epileptic animals presented a more robust potentiation after high-frequency stimulation than controls, a protocol used to evaluate a slow form of synaptic plasticity (i.e. long-term potentiation). The increased excitability in CA3 pyramidal neurons enhanced the probability of burst activity in these neurons; this could lead to greater CA1 synchronization. The present results might have relevance for the understanding of epileptogenesis and of learning and memory deficits seen in temporal lobe epilepsy.

Feedforward inhibition contributes to the control of epileptiform propagation speed

It is still poorly understood how epileptiform events can recruit cortical circuits. Moreover, the speed of propagation of epileptiform discharges in vivo and in vitro can vary over several orders of magnitude (0.1-100 mm/s), a range difficult to explain by a single mechanism. We previously showed how epileptiform spread in neocortical slices is opposed by a powerful feedforward inhibition ahead of the ictal wave. When this feedforward inhibition is intact, epileptiform spreads very slowly (approximately 100 microm/s). We now investigate whether changes in this inhibitory restraint can also explain much faster propagation velocities. We made use of a very characteristic pattern of evolution of ictal activity in the zero magnesium (0 Mg2+) model of epilepsy. With each successive ictal event, the number of preictal inhibitory barrages dropped, and in parallel with this change, the propagation velocity increased. There was a highly significant correlation (p < 0.001) between the two measures over a 1000-fold range of velocities, indicating that feedforward inhibition was the prime determinant of the speed of epileptiform propagation. We propose that the speed of propagation is set by the extent of the recruitment steps, which in turn is set by how successfully the feedforward inhibitory restraint contains the excitatory drive. Thus, a single mechanism could account for the wide range of propagation velocities of epileptiform events observed in vitro and in vivo.

Role of protein kinase C in modulation of excitability of CA1 pyramidal neurons in the rat

Biochemical and in situ hybridization studies demonstrated that the levels of protein kinase C variants were significantly increased in the hippocampus of the experimental models of epilepsy in rats. In addition it has been demonstrated that protein kinase C plays an important role in modulating synaptic transmission in the hippocampus. We examined the effects of activating of protein kinase C on the excitability of CA1 pyramidal neurons and synaptic transmission, using whole-cell current-clamp and extracellular field potential recording techniques. Indolactam V (1 microM) a novel protein kinase C activator, increased the excitability of CA1 neurons acting at both pre- and post-synaptic sites. Indolactam V, acting postsynaptically, significantly reduced the threshold for initiation of action potential from -42+/-3.8 mV to -51+/-3.1 mV and selectively inhibited the slow afterhyperpolarizing potential. Indolactam V also altered the neuronal firing properties in response to prolonged depolarizing pulse by eliminating the spike frequency accommodation. Our data indicate that indolactam V potentiated both amplitudes of Shaffer-collateral stimulation evoked excitatory postsynaptic currents and disynaptically evoked inhibitory evoked postsynaptic currents. However, the potentiation of inhibitory evoked postsynaptic currents amplitudes was not observed after blockade of NMDA and AMPA/kainate currents suggesting it was due to excitatory activity driving inhibitory neurons. The results indicate that the potentiation of pharmacologically isolated excitatory postsynaptic currents (215% of control) and amplitudes of population spikes (290% of control) was due to action of indolactam V presynaptically since the agonist reduced the paired-pulse ratio and the potentiating effect was not blocked by dialyzing the postsynaptic neuron through the recording electrode with a specific protein kinase C inactivator calphostin C. These findings suggest that protein kinase C increases the amplitude of epileptiform activity by causing potentiation of excitatory synaptic transmission, increasing the excitability of postsynaptic neurons and reducing negative feed back provided by slow afterhyperpolarizing potential.

Electrical and Chemical Long-term Depression Do Not Attenuate Low-Mg2+-induced Epileptiform Activity in the Entorhinal Cortex

PURPOSE

Low-frequency electrical and magnetic stimulation of cortical brain regions has been shown to reduce cortical excitability and to decrease the susceptibility to seizures in humans and in vivo models of epilepsy. The induction of long-term depression (LTD) or depotentiation of a seizure-related long-term potentiation has been proposed to be part of the underlying mechanism. With the low-Mg(2+)-model of epilepsy, this study investigated the effect of electrical LTD, chemical LTD, and depotentiation on the susceptibility of the entorhinal cortex to epileptiform activity.

METHODS

The experiments were performed on isolated entorhinal cortex slices obtained from adult Wistar rats and mice. With extracellular recording techniques, we studied whether LTD induced by (a) three episodes of low-frequency paired-pulse stimulation (3 x 900 paired pulses at 1 Hz), and by (b) bath-applied N-methyl-D-aspartate (NMDA, 20 microM) changes time-to-onset, duration, and frequency of seizure-like events (SLEs) induced by omitting MgSO(4) from the artificial cerebrospinal fluid. Next we investigated the consequences of depotentiation on SLEs themselves by applying low-frequency stimulation after onset of low-Mg(2+)-induced epileptiform activity.

RESULTS

LTD, induced either by low-frequency stimulation or by bath-applied NMDA, had no effect on time-to-onset, duration, and frequency of SLEs compared with unconditioned slices. Low-frequency stimulation after onset of SLEs did not suppress but induced SLEs that lasted for the time of stimulation and were associated with a simultaneous increase of the extracellular K(+) concentration.

CONCLUSIONS

Our study demonstrates that neither conditioning LTD nor brief low-frequency stimulation decreases the susceptibility of the entorhinal cortex to low-Mg(2+)-induced epileptiform activity. The present study does not support the hypothesis that low-frequency brain stimulation exerts its anticonvulsant effect via the induction of LTD or depotentiation.

Neuronal hyperexcitability induced by repeated brief episodes of hypoxia in rat hippocampal slices: involvement of ionotropic glutamate receptors and L-type Ca(2+) channels

Repeated exposures of rat hippocampal slices to short episodes of hypoxia induce a sustained decrease in the threshold of the development of stimulus-evoked epileptiform discharges in CA1 pyramidal neurons. We have previously demonstrated that the K(+)(o)-induced hyperexcitability required functional L-type voltage-dependent Ca(2+) channels and NMDA-receptors, but was independent of AMPA/kainate-receptor activation. As hypoxia/ischaemia can lead to increased K(+)(o), the epileptiform activity observed after exposure to these challenges could also result from high K(+)(o). The purpose of this study was: (i) to determine whether ionotropic glutamate receptors and L-type Ca(2+) channels are involved in the development of epileptiform activity induced by repeated exposures of hippocampal slices to hypoxia; and (ii) to compare the properties of hypoxia- and high K(+)(o)-induced hyperexcitability. Population spike of presynaptic fibres with field excitatory postsynaptic potential from the stratum radiatum, and population spike of CA1 pyramidal neurons from the stratum pyramidale, were recorded simultaneously in the CA1 area of rat hippocampal slices in response to electrical stimulation of the Schaffer collateral/commissural fibres. Repeated, brief hypoxic episodes induced a sustained decrease in the threshold for development of evoked epileptiform discharges that was associated with long-term potentiation of the CA3-CA1 synapses, but without EPSP-spike potentiation (i.e. in contrast to high K(+)(o)-induced hyperexcitability). The selective antagonist of NMDA receptors, D-APV (25 microM), and the selective blocker of L-type Ca(2+) channels, nifedipine (10 microM) depressed the development of hypoxia-induced hyperexcitability. However, in contrast to high K(+)(o)-induced hyperexcitability, hypoxia-induced hyperexcitability was also blocked by the AMPA/kainite-receptor antagonist, CNQX (5 microM). The present findings confirm that repeated, brief episodes of hypoxia, like exposure to high extracellular K(+), can induce a pro-epileptic state in the CA1 neuronal network, but that the mechanisms leading to hyperexcitability are different for the two stimuli.

Changes in neuronal excitability and synaptic function in a chronic model of temporal lobe epilepsy

Long-term potentiation and depression of glutamatergic synaptic responses are accompanied by an increased firing probability of neurons in response to a given excitatory input. This property, named excitatory postsynaptic potential/spike potentiation, has also been described in epileptic tissue and has pro-epileptic consequences. In this study, we show that excitatory postsynaptic potential/spike potentiation can be reversed in the kainic acid lesioned rat hippocampus, a chronic model of temporal lobe epilepsy. Simultaneous in vitro extracellular recordings in stratum radiatum and stratum pyramidale were performed in the CA1 area of the kainic acid lesioned rat hippocampal slices. Fifteen minutes, application of the K(+) channel blocker tetraethylammonium resulted in excitatory postsynaptic potential/spike potentiation (measured 90min after the start of the washout period) which could be reversed by subsequent low-frequency or tetanic stimuli. Excitatory postsynaptic potential/spike potentiation and its subsequent reversal by an electrical conditioning stimulus were found to have a N-methyl-D-aspartate receptor-independent component. Tetraethylammonium treatment also resulted in excitatory postsynaptic potential/spike potentiation of pharmacologically isolated N-methyl-D-aspartate receptor-mediated responses which could be reversed by subsequent low-frequency or tetanic stimuli. We conclude that excitatory postsynaptic potential/spike potentiation can be reversed in epileptic tissue, even in the absence of synaptic plasticity. These results suggest the presence of endogenous regulatory mechanisms which are able to decrease cell excitability.

In vivo hippocampal kindling occludes the development of in vitro kindling-like state in CA1 area of rat hippocampal slices

In our previous work (Semyanov and Godukhin, 1997), we showed that the repeated short-term extracellular K+ (K0+) increases induced long-lasting reduction of the threshold of evoked epileptiform discharges in CA1 hippocampal slices isolated from normal (nonkindled) rats. This state had some features characteristic of traditional in vivo kindling and was described as in vitro kindling-like state (VKLS). The aim of the present investigations was to determine the features of the VKLS development in CA1 slices isolated from electrical hippocampal kindled (nonepileptic) rats and from genetically-prone to audiogenic seizures audiokindled rats. We found that both forms of in vivo kindling occluded the VKLS development induced by the repeated K& increases in CA1 slices. These data provide more evidence that the in vivo kindling and VKLS developments in CA1 region of hippocampus are based on activation of similar cellular mechanisms. It is suggested that the described model of in vitro kindling can be useful for further studies of the cellular-molecular mechanisms of plastic alterations in neurons associated with the start of kindling induced epileptogenesis.

Newly formed excitatory pathways provide a substrate for hyperexcitability in experimental temporal lobe epilepsy

Temporal lobe epilepsy (TLE) in humans and animals is associated with axonal sprouting of glutamatergic neurons and neosynaptogenesis in the hippocampal formation. We examined whether this plasticity of excitatory pathways contributes to an increased level of glutamatergic excitation in the CA1 region of rats experiencing chronic spontaneous limbic seizures following kainic acid or pilocarpine treatment. In chronic cases, we report an extensive axonal sprouting of CA1 pyramidal neurons, with many axonal branches entering the pyramidal cell layer and stratum radiatum, regions that are not innervated by axonal collaterals of CA1 pyramidal neurons in control animals. Concurrently with this anatomical reorganization, a large increase of the spontaneous glutamatergic drive is observed in the dendrites and somata of CA1 pyramidal cells. Furthermore, electrical activation of the reorganized CA1 associational pathway evokes epileptiform bursts in CA1 pyramidal cells. These findings suggest that reactive plasticity could contribute to the hyperexcitability of CA1 pyramidal neurons and to the propagation of seizures in these two models of TLE.

Reversal of excitatory postsynaptic potential/spike potentiation in the CA1 area of the rat hippocampus

In the CA1 area of the hippocampus, low frequency and tetanic conditioning stimuli are known to trigger long-term depression and potentiation of synaptic responses respectively and to produce irreversible excitatory postsynaptic potential/spike potentiation, i.e. an increase of the probability of discharge of the neurons. Using simultaneous extracellular recordings in stratum radiatum and stratum pyramidale in the CA1 area of the rat hippocampus, brief application of the K+ channel blocker tetraethylammonium resulted both in long-term potentiation of synaptic responses and in excitatory postsynaptic potential/spike potentiation that could be reversed by subsequent low frequency or tetanic stimuli. Excitatory postsynaptic potential/spike potentiation and its subsequent reversal by an electrical conditioning stimulus were found to have an N-methyl-D-aspartate receptor-independent component. We conclude that the reversal of excitatory postsynaptic potential/spike potentiation can occur and that it does not require the induction of long-term modification of synaptic responses.

Interneurones are not so dormant in temporal lobe epilepsy: a critical reappraisal of the dormant basket cell hypothesis

One axiom at the basis of epilepsy research is that there exists an imbalance between excitation and inhibition. This abnormality can be achieved by an increase of excitation on principal cells, a decreased inhibition (i.e. disinhibition) or both. This review focuses on dysfunction of inhibition, and in particular on the 'dormant basket cell hypothesis'. This hypothesis states that, (1) interneurones are functionally disconnected from excitatory afferents, resulting in hyperexcitability of principal neurones and loss of paired pulse inhibition, (2) when properly activated, interneurones can still perform their task, i.e. suppress epileptiform activity and restore paired pulse inhibition. The aim of this review is to discuss the evidence in support of the 'dormant basket cell hypothesis'. We will first discuss the rationale underlying the hypothesis and the criteria needed to validate the hypothesis. We will then show that, (1) the key experimental data offered in support of the hypothesis (Bekenstein and Lothman, 1993. Dormancy of inhibitory interneurones in a model of temporal lobe epilepsy. Science 259, 97-100; Sloviter, 1991. Permanently altered hippocampal structure, excitability, and inhibition after experimental status epilepticus in the rat: the 'dormant basket cell' hypothesis and its relevance to temporal lobe epilepsy. Hippocampus 1, 41-66) are difficult to interpret, and (2) recent recordings from interneurones in epileptic tissue argue against the hypothesis. The 'dormant basket cell hypothesis' is then discussed in the broader context of disinhibition.

Molecular mechanisms that underlie structural and functional changes at the postsynaptic membrane during synaptic plasticity

The synaptic plasticity that is addressed in this review follows neurodegeneration in the brain and thus has both structural as well as functional components. The model of neurodegeneration that has been selected is the kainic acid lesioned hippocampus. Degeneration of the CA3 pyramidal cells results in a loss of the Schaffer collateral afferents innervating the CA1 pyramidal cells. This is followed by a period of structural plasticity where new synapses are formed. These are associated with changes in the numbers and shapes of spines as well as changes in the morphometry of the dendrites. It is suggested that this synaptogenesis is responsible for an increase in the ratio of NMDA to AMPA receptors mediating excitatory synaptic transmission at these synapses. Changes in the temporal and spatial properties of these synapses resulted in an altered balance between LTP and LTD. These properties together with a reduction in the inhibitory drive increased the excitability of the surviving CA1 pyramidal cells which in turn triggered epileptiform bursting activity. In this review we discuss the insights that may be gained from studies of the underlying molecular machinery. Developments in one of the collections of the cogs in this machinery has been summarized through recent studies characterizing the roles of neural recognition molecules in synaptic plasticity in the adult nervous systems of vertebrates and invertebrates. Such investigations of neural cell adhesion molecules, cadherins and amyloid precursor protein have shown the involvement of these molecules on the morphogenetic level of synaptic changes, on the one hand, and signal transduction effects, on the other. Further complex cogs are found in the forms of the low-density lipoprotein receptor (LDL-R) family of genes and their ligands play pivotal roles in the brain development and in regulating the growth and remodelling of neurones. Evidence is discussed for their role in the maintenance of cognitive function as well as Alzheimer's. The molecular mechanisms responsible for the clustering and maintenance of transmitter receptors at postsynaptic sites are the final cogs in the machinery that we have reviewed. Postsynaptic densities (PSD) from excitatory synapses have yielded many cytoskeletal proteins including actin, spectrin, tubulin, microtubule-associated proteins and calcium/calmodulin-dependent protein kinase II. Isolated PSDs have also been shown to be enriched in AMPA, kainate and NMDA receptors. However, recently, a new family of proteins, the MAGUKs (for membrane-associated guanylate kinase) has emerged. The role of these proteins in clustering different NMDA receptor subunits is discussed. The MAGUK proteins are also thought to play a role in synaptic plasticity mediated by nitric oxide (NO). Both NMDA and non-NMDA receptors are highly clustered at excitatory postsynaptic sites in cortical and hippocampal neurones but have revealed differences in their choice of molecular components. Both GABAA and glycine (Gly) receptors mediate synaptic inhibition in the brain and spinal cord. Whilst little is known about how GABAA receptors are localized in the postsynaptic membrane, considerable progress has been made towards the elucidation of the molecular mechanisms underlying the formation of Gly receptors. It has been shown that the peripheral membrane protein gephyrin plays a pivotal role in the formation of Gly receptor clusters most likely by anchoring the receptor to the subsynaptic cytoskeleton. Evidence for the distribution as well as function of gephyrin and Gly receptors is discussed. Postsynaptic membrane specializations are complex molecular machinery subserving a multitude of functions in the proper communication between neurones. Despite the fact that only a few key players have been identified it will be a fascinating to watch the story as to how they contribute to structural and functional plasticity unfold.

Spread of excitation in chronically lesioned mouse hippocampus determined by laser scanning microscopy

Fast optical recordings by means of laser scanning microscopy in conjunction with a voltage-sensitive dye (RH 414) were performed to monitor the spatio-temporal spread of neuronal activity in CA3/CA4-lesioned C57BL6 mouse hippocampal slices prepared approximately 3 months after intracerebroventricular kainic acid (KA) injection. The aim of our study was to assess the effects of a circumscribed neuronal loss on the propagation of electrical activity along the trisynaptic hippocampal circuit. Both in physiological bathing solution and in bicuculline (10 microM), hilar stimulation failed to activate the downstream pathway, so that, under these conditions, the chronically disinhibited CA1 region appeared to be effectively isolated from burst activity arising upstream; however, epileptiform discharges evoked in zero Mg2+ solution were reliably transmitted from the dentate gyrus to the CA1 region. That these bursts were indeed spreading across the lesion, and not along newly formed connections (e.g., between dentate gyrus and CA1), was confirmed by acute transection experiments of the Schaffer collateral/commissural pathway, which completely abolished translesional burst propagation. The fact that the surviving CA3-CA1 connections are unable to trigger epileptiform bursts after suppression of GABAergic inhibition suggests that the lesioned region might serve as a filter that shields hyperexcitable CA1 neurons from epileptic activity arising upstream, in particular from chronically disinhibited granule cells of the dentate gyrus. An impaired GABAergic inhibition will thus only have minor facilitating effects on seizure propagation in the hippocampus of CA3-lesioned animals.

Pro-epileptic changes in synaptic function can be accompanied by pro-epileptic changes in neuronal excitability

Repetitive sensory input, stroboscopic lights or repeated sounds can induce epileptic seizures in susceptible individuals. In order to understand the process we have to consider multiple factors. The output of a set of neurones is determined by the amount of excitatory synaptic input, the degree of positive feedback and their inherent electrical excitability, which can be modified by synaptic inhibition. Recent research has shown that it is possible to separate these phenomena, and that they do not always behave in unison.

CA3-driven hippocampal-entorhinal loop controls rather than sustains in vitro limbic seizures

Continuous application of 4-aminopyridine (4-AP, 50 microM) to combined slices of hippocampus-entorhinal cortex obtained from adult mice induces (1) interictal discharges that initiate in the CA3 area and propagate via the hippocampal regions CA1 and subiculum to the entorhinal cortex and return to the hippocampus through the dentate gyrus; and (2) ictal discharges that originate in the entorhinal cortex and propagate via the dentate gyrus to the hippocampus proper. Ictal discharges disappear over time, whereas synchronous interictal discharges continue to occur throughout the experiment. Lesioning the Schaffer collaterals abolishes interictal discharges in CA1, entorhinal cortex, and dentate gyrus and discloses entorhinal ictal discharges that propagate, via the dentate gyrus, to the CA3 subfield. Interictal discharges originating in CA3 also prevent the occurrence of ictal events generated in the entorhinal cortex during application of Mg2+-free medium. In both models, ictal discharge generation recorded in the entorhinal cortex after Schaffer collateral cut is prevented by mimicking CA3 neuronal activity through rhythmic electrical stimulation (0.25-1.5 Hz) of the CA1 hippocampal output region. Our findings demonstrate that interictal discharges of hippocampal origin control the expression of ictal epileptiform activity in the entorhinal cortex. Sectioning the Schaffer collaterals may model the chronic epileptic condition in which cell damage in the CA3 subfield results in loss of CA3 control over the entorhinal cortex. Hence, we propose that the functional integrity of hippocampal output neurons may represent a critical control point in temporal lobe epileptogenesis.

CA3-driven hippocampal-entorhinal loop controls rather than sustains in vitro limbic seizures

Continuous application of 4-aminopyridine (4-AP, 50 microM) to combined slices of hippocampus-entorhinal cortex obtained from adult mice induces (1) interictal discharges that initiate in the CA3 area and propagate via the hippocampal regions CA1 and subiculum to the entorhinal cortex and return to the hippocampus through the dentate gyrus; and (2) ictal discharges that originate in the entorhinal cortex and propagate via the dentate gyrus to the hippocampus proper. Ictal discharges disappear over time, whereas synchronous interictal discharges continue to occur throughout the experiment. Lesioning the Schaffer collaterals abolishes interictal discharges in CA1, entorhinal cortex, and dentate gyrus and discloses entorhinal ictal discharges that propagate, via the dentate gyrus, to the CA3 subfield. Interictal discharges originating in CA3 also prevent the occurrence of ictal events generated in the entorhinal cortex during application of Mg2+-free medium. In both models, ictal discharge generation recorded in the entorhinal cortex after Schaffer collateral cut is prevented by mimicking CA3 neuronal activity through rhythmic electrical stimulation (0.25-1.5 Hz) of the CA1 hippocampal output region. Our findings demonstrate that interictal discharges of hippocampal origin control the expression of ictal epileptiform activity in the entorhinal cortex. Sectioning the Schaffer collaterals may model the chronic epileptic condition in which cell damage in the CA3 subfield results in loss of CA3 control over the entorhinal cortex. Hence, we propose that the functional integrity of hippocampal output neurons may represent a critical control point in temporal lobe epileptogenesis.

Operative GABAergic inhibition in hippocampal CA1 pyramidal neurons in experimental epilepsy

Patch-clamp recordings of CA1 interneurons and pyramidal cells were performed in hippocampal slices from kainate- or pilocarpine-treated rat models of temporal lobe epilepsy. We report that gamma-aminobutyric acid (GABA)ergic inhibition in pyramidal neurons is still functional in temporal lobe epilepsy because: (i) the frequency of spontaneous GABAergic currents is similar to that of control and (ii) focal electrical stimulation of interneurons evokes a hyperpolarization that prevents the generation of action potentials. In paired recordings of interneurons and pyramidal cells, synchronous interictal activities were recorded. Furthermore, large network-driven GABAergic inhibitory postsynaptic currents were present in pyramidal cells during interictal discharges. The duration of these interictal discharges was increased by the GABA type A antagonist bicuculline. We conclude that GABAergic inhibition is still present and functional in these experimental models and that the principal defect of inhibition does not lie in a complete disconnection of GABAergic interneurons from their glutamatergic inputs.

Epileptiform activity but not synaptic plasticity is blocked by oxidation of NMDA receptors in a chronic model of temporal lobe epilepsy

Simultaneous extracellular recordings were performed in stratum radiatum and stratum pyramidale of hippocampal slices 7 days following unilateral intracerebroventricular injections of kainic acid. In this ex vivo experimental model of human temporal lobe epilepsy, stimulation of the surviving commissural fibres in stratum radiatum produced graded epileptiform activity in the CA1 area. The oxidizing reagent 5,5'-dithiobis (2-nitrobenzoic acid) (DTNB) acting at NMDA receptors redox sites decreases NMDA receptor-mediated responses by half and suppresses evoked epileptiform discharges. We have examined the effect of DTNB on NMDA-dependent bidirectional synaptic plasticity and EPSP/spike coupling. DTNB treatment did not prevent either long-term potentiation induced by tetanic stimulation or long-term depression induced by low frequency stimulation of field EPSPs. Application of DTNB alone did not induce EPSP/spike dissociation. However, both high and low frequency stimulations induced EPSP/spike potentiation indicating that neurons had a high probability to discharge in synchrony. These results suggest that oxidizing reagents may provide novel antiepileptic treatments since they decrease NMDA-dependent evoked epileptiform activity but do not interfere with either NMDA-dependent synaptic plasticity or the probability of synchronous discharge.

Redox sites of NMDA receptors can modulate epileptiform activity in hippocampal slices from kainic acid-treated rats

Using an animal model of temporal lobe epilepsy, the kainic acid lesioned rat hippocampus, we have evaluated the possibility of modulating glutamate N-methyl-D-aspartate (NMDA) receptor-dependent evoked epileptiform activity through the manipulation of NMDA receptor redox sites. Epileptiform activity was recorded extracellularly from hippocampal slices, in the stratum pyramidale of the CA1 area, and the effects of the oxidizing reagent 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) and the reducing agent Tris(2-carboxy ethyl)phosphine (TCEP) on these responses were quantified. Epileptiform activity was substantially reduced in the presence of DTNB but was fully reinstated with the application of TCEP. The effects of both drugs persisted even after wash. Epileptiform activity was totally abolished in the presence of the NMDA receptor antagonist D-2-amino-5-phosphonovaleric acid. These results suggest that epileptiform activity can be controlled by manipulation of the redox sites of NMDA receptors and raise the possibility of developing new anticonvulsant drugs which do not fully block NMDA receptor-mediated synaptic transmission.