Afterpotentials following penicillin-induced paroxysmal depolarizations in rat hippocampal CA1 pyramidal cells in vitro

The Paroxysmal Depolarization Shift: Reconsidering Its Role in Epilepsy, Epileptogenesis and Beyond

Paroxysmal depolarization shifts (PDS) have been described by epileptologists for the first time several decades ago, but controversy still exists to date regarding their role in epilepsy. In addition to the initial view of a lack of such a role, seemingly opposing hypotheses on epileptogenic and anti-ictogenic effects of PDS have emerged. Hence, PDS may provide novel targets for epilepsy therapy. Evidence for the roles of PDS has often been obtained from investigations of the multi-unit correlate of PDS, an electrographic spike termed “interictal” because of its occurrence during seizure-free periods of epilepsy patients. Meanwhile, interictal spikes have been found to be associated with neuronal diseases other than epilepsy, e.g., Alzheimer’s disease, which may indicate a broader implication of PDS in neuropathologies. In this article, we give an introduction to PDS and review evidence that links PDS to pro- as well as anti-epileptic mechanisms, and to other types of neuronal dysfunction. The perturbation of neuronal membrane voltage and of intracellular Ca²⁺ that comes with PDS offers many conceivable pathomechanisms of neuronal dysfunction. Out of these, the operation of L-type voltage-gated calcium channels, which play a major role in coupling excitation to long-lasting neuronal changes, is addressed in detail.

Increased discharge threshold after an interictal spike in human focal epilepsy

It is commonly assumed that interictal spikes (ISs) in focal epilepsies set off a period of inhibition that transiently reduces tissue excitability. Post-spike inhibition was described in experimental models but was never demonstrated in the human epileptic cortex. In the present study post-spike excitability was retrospectively evaluated on intracerebral stereo-electroencephalographic recordings performed in the epileptogenic cortex of five patients suffering from drug-resistant focal epilepsy secondary to Taylor-type neocortical dysplasias. Patients typically presented with highly periodic interictal spiking activity at 2.33 +/- 0.87 Hz (mean +/- SD) in the dysplastic region. During the stereo-electroencephalographic procedure, low-frequency stimulation at 1 Hz was systematically performed for diagnostic purposes to identify the epileptogenic zone. The probability of evoking an IS during the interspike period in response to 1-Hz stimuli delivered close to the ictal-onset zone was examined. Stimuli that occurred early after a spontaneous IS (within 70% of the inter-IS period) had a very low probability of generating a further IS. On the contrary, stimuli delivered during the late inter-IS period had the highest probability of evoking a further IS. The generation of stimulus-evoked ISs is occluded for several hundred milliseconds after the occurrence of a preceding spike discharge. As previously shown in animal models, these findings suggest that, during focal, periodic interictal spiking, human neocortical excitability is phasically controlled by post-spike inhibition.

Involvement of GABA(B) receptors in convulsant-induced epileptiform activity in rat neocortex in vitro

The role of gamma-aminobutyric acid B (GABA(B)) receptors in the generation and maintenance of bicuculline-induced epileptiform activity in rat neocortical slices was studied using electrophysiological methods. A block of GABA(B) receptors in the presence of functional GABA(A) receptor-mediated inhibition was not sufficient to induce epileptiform activity. In the presence of the GABA(A) receptor antagonist bicuculline (10 microM) and at suprathreshold stimulation, the GABA(B) receptor antagonist CGP 35348 (10-300 microM) significantly potentiated epileptiform activity. With stimulation at threshold intensity, low concentrations of CGP 35348 (10-30 microM) potentiated bicuculline-induced activity, whereas higher concentrations (100-300 microM) invariably led to a reversible suppression of stimulus-evoked epileptiform discharges. CGP 35348 also enhanced picrotoxin-induced epileptiform activity, but at higher concentrations it was considerably less effective in suppressing such epileptiform discharges. The GABA uptake inhibitor nipecotic acid partially mimicked the actions of CGP 35348: with stimulation at threshold intensity, it reversibly suppressed bicuculline-induced epileptiform field potentials, but it did not influence epileptiform activity induced by picrotoxin. We conclude that a postsynaptic blockade of GABA(B) receptors induces an amplification of epileptiform activity in neocortical slices disinhibited by GABA(A) receptor antagonists. An additional blockade of presynaptic GABA(B) receptors, especially under conditions of weak stimulation of the neurons, reduces the inhibitory auto-feedback control of GABA release, leading to a displacement of competitive antagonists from the postsynaptic GABA(A) receptor and hence, to a suppression of epileptiform activity induced by competitive GABA(A) receptor antagonists.

Ontogeny of altered synaptic function in a rat model of chronic temporal lobe epilepsy

In the limbic status model of chronic temporal lobe epilepsy, hippocampal stimulation induces acute status epilepticus in rats; recurrent, spontaneous seizures develop following an asymptomatic silent period lasting several weeks. Previous work has shown increased excitability and decreased inhibition in CA1 pyramidal neurons in chronically epileptic animals. To determine the relationship of altered cellular responses to seizure onset, in vitro intracellular recording was used to follow the evolution of changes in synaptic physiology occurring during the seizure-free silent period. Pyramidal cells displayed increasing epileptiform activity throughout the period investigated, 3-14 days following status; the mean number of evoked action potentials from 1.1+/-0.05 in control cells to 2.4+/-0.4 early (3 days after status) and 4. 3+/-0.7 late (14 days) in the silent period. Monosynaptic inhibitory postsynaptic potentials mediated by gamma-aminobutyric acid-A receptors in silent period cells differed markedly from controls. Area, rise time, and duration of these potentials decreased by 40-60% within 3 days following status and to values commensurate with chronically epileptic animals in 7 to 10 days. gamma-Aminobutyric acid-B receptor-mediated IPSPs diminished more gradually in the silent period, reaching a minimum at day 14. In contrast, presynaptic gamma-aminobutyric acid-B receptor function showed maximum impairment 3 days after status. The benzodiazepine type 1 receptor agonist zolpidem reduced hyperexcitability in both silent period and chronically epileptic cells, but was more effective at unmasking the underlying IPSP in silent period neurons. The results indicate that changes in different components of pyramidal cell inhibitory synaptic physiology associated with chronic epilepsy in this model evolve individually at different rates, but are all complete before seizure onset. Although the results do not imply causality, they do suggest that the development of physiological changes in CA1 pyramidal cells may contribute to the lag period preceding the onset of chronic seizures.

Induction of FOS and JUN proteins during focal epilepsy: congruences with and differences to [14C]deoxyglucose metabolism

fos and jun belong to multigene families coding for transcription factors. These cellular immediate-early genes (IEGs) are thought to be involved in coupling neuronal excitation to changes of target gene expression. Immunocytochemistry with specific antisera was used to assess regional levels of five IEG-encoded proteins (c-FOS, FOS B, c-JUN, JUN B and JUN D) in a rat model of penicillin-induced focal epilepsy. To assess whether brain regions with post-ictal de novo transcription factor synthesis correspond to those areas with increased glucose metabolism, IEG expression patterns were compared with [14C]deoxyglucose autoradiography performed in a subset of animals. The results demonstrated marked induction of c-FOS, FOS B, c-JUN and JUN B but not JUN D in the cortical epileptic focus. Thereby, individual IEG-encoded proteins exhibited differential temporal and spatial expression patterns. Within the epileptic focus, IEG expression correlated with increased glucose metabolism. In contrast, IEG induction was not observed in brain areas distant from the epileptic focus that also demonstrated increased glucose metabolism, such as homotopic contralateral motor cortex and ipsilateral thalamic nuclei. These findings indicate that in focal epilepsy changes of the genetic programme are restricted to neurons of the epileptic focus. In contrast, the increased [14C]deoxyglucose metabolism in contralateral motor cortex and ipsilateral thalamus seems to indicate functional changes.

Cellular activity underlying altered brain metabolism during focal epileptic activity

Demonstration of focal alterations of brain metabolism with positron emission tomography has become a widely used method for identifying epileptic foci. Here we investigated how neuronal and glial cell activity relates to alterations of brain metabolism. Acutely induced epileptic activity in the motor cortex of rat brain increased metabolism in the focus and homotopic contralateral areas, and decreased metabolism in the ipsilateral somatosensory area. Increases and decreases of deoxyglucose uptake did not directly correlate with excitations and inhibitions; instead, deoxyglucose uptake was related to the overall strength of synaptic activity, and both strong excitations and strong inhibitions increased brain metabolism. Reduction of metabolism below normal values was associated with reduced synaptic activity and with tonic hyperpolarization of the cells. Our results show that in the absence of structural abnormalities, hypometabolism indicates functional disturbances which may be both reversible and remote from the epileptogenic focus.

Inhibitory mechanisms in epileptiform activity induced by low magnesium

In rat hippocampal slices epileptiform activity was induced by superfusion with Mg(2+)-free artificial cerebrospinal fluid (ACSF). Paroxysmal depolarization shifts (PDS) were evoked by electrical stimulation of Schaffer collaterals. To investigate the afterpotentials that follow PDS, intracellular recordings were made from CA1 pyramidal cells. The experiments revealed that several components are engaged in the generation of PDS afterpotentials in Mg(2+)-free ACSF. A long lasting component which determined the overall duration of the PDS afterhyperpolarization was blocked by intracellular application of ethylenebis(oxonitrilo)-tetraacetate (EGTA); concomitantly, the afterhyperpolarizations following depolarizing current injections were blocked. This indicated that the long lasting component was due to a slow Ca(2+)-activated K+ current. The block of Ca(2+)-activated K+ current uncovered a depolarizing PDS afterpotential with an N-shaped voltage dependence, suggesting that this depolarizing afterpotential component may be due to an N-methyl D-aspartate (NMDA) conductance. Intracellular injection of Cl- revealed that the PDS were followed by Cl- currents lasting about 500 ms. This component could be blocked by application of bicuculline suggesting that it is due to a synaptically GABA-mediated (i.e. gamma-aminobutyric acid) Cl- current. A comparison of PDS afterpotentials in Mg(2+)-free ACSF and those in other models of epileptiform activity suggests that similar sequences of inhibitory components are activated in spite of different pharmacological alterations of membrane conductances which induce the epileptiform discharges.

Refractory periods following interictal spikes in acute experimentally induced epileptic foci

Under epileptic conditions, interictal epileptic events are followed by large inhibitions which prevent the transition to ictal discharges. In the present experiments the refractory period following interictal epileptic spikes was investigated in animal experiments. Interictal epileptic activity was elicited by application of penicillin onto the motor cortex of anesthetized rats. Interictal epileptic discharges were followed by an absolute refractory phase lasting 200-300 msec, in which no epileptic event could be elicited by epicortical stimulation. This was followed by a relative refractory period up to 900 msec after onset of the conditioning spike; spikes elicited with intervals between 300 and 900 msec were smaller than those with greater intervals and required higher stimulation intensities. This period ends by a sharp drop of threshold. In two-thirds of the experiments, spikes were favoured in intervals of 300-500 msec due to a sag of the threshold, which possibly indicates recurrent neuronal excitations. Stimulations with frequencies of about 1/sec favoured a transition from a pattern with spikes appearing in an irregular sequence every 2-3 sec, to a discharge pattern with spikes appearing with regular intervals of about 1 sec. This change of firing pattern was associated with a drop of the spike threshold. It is concluded that interictal epileptic events are followed by a refractory period comprising different components. Alterations of the neuronal inhibitions responsible for these refractory phases may be critical for the activity of the focus and may determine the transition from interictal to ictal discharges.

Epileptic activity can induce both long-lasting potentiation and long-lasting depression

Epileptic seizures are associated with massive changes of intracellular ion concentrations which could cause persistent changes in efficacy of afferent excitation. Such alterations were investigated in the CA1 area of hippocampal slices using the acute high potassium model of epilepsy which allows rapid wash of the epileptogenic solution. Field potentials elicited by stimulation of Schaffer collaterals were recorded in stratum pyramidale and stratum radiatum. The experiments revealed that long-lasting potentiation as well as long-lasting depression of field potentials could result from superfusion with high potassium. These changes were NMDA-dependent, but did not depend on the discharge type (interictal vs. ictal) of the slice. It is concluded that epileptic activity cannot only lead to an enhancement of responses to afferent excitation, but also to a long-lasting depression. This depression could represent a self-protective mechanism of the brain which may also be involved in post-ictal depression of cerebral activity.

Inhibitory mechanisms terminating paroxysmal depolarization shifts in hippocampal neurons of rats

The mechanisms responsible for the termination of paroxysmal depolarization shifts (PDS) were studied with intracellular recordings on CA1 neurons of rat hippocampal slices. Epileptiform activity was induced by application of penicillin, bicuculline or Mg-free artificial cerebrospinal fluid (ACSF). PDS in penicillin-containing and Mg-free ACSF were markedly prolonged when GABAA-dependent IPSPs were blocked by bicuculline. PDS in bicuculline-containing ACSF were furthermore prolonged after block of potential dependent K+ conductances by TEA. TEA also exerted some effect on PDS induced by penicillin containing or Mg-free ACSF. Block of GABAB-dependent IPSPs or Ca(2+)-dependent K+ currents did not affect PDS duration in any of the three models. It is concluded that PDS termination is due to active inhibitory processes which comprise different components. If one of these components is blocked another inhibitory component governs PDS repolarization resulting in PDS with a slightly different duration but otherwise unchanged features.

Afterpotentials of penicillin-induced epileptiform neuronal discharges in the motor cortex of the rat in vivo

Interictal spikes and sharp waves in the EEG are followed by intervals in which the excitability of the brain seems to be normal or decreased. Often interictal spikes even appear in rhythmical patterns with intervals in the order of 0.5-2 s. These observations suggest that intrinsic and synaptic inhibitory and excitatory processes are activated which outlast the duration of the interictal discharge. In the present study such afterpotentials were analyzed in penicillin foci of the rat motor cortex in vivo using intracellular recording techniques. Paroxysmal depolarizations (PDS) of neurons within the focus were followed by afterpotentials comprising several components. Fast afterpotentials with a duration of 640 ms were associated with a sevenfold increase in membrane conductance. The fast afterpotentials were depolarizing in the majority of recordings and had an average equilibrium potential of -62 mV. This equilibrium potential was Cl(-)-dependent and was not affected by intracellular EGTA or Cs+. It is suggested that these afterpotentials represent GABAA responses. In 38% of the neurons slow afterhyperpolarizations with a twofold increase in membrane conductance and a duration of 2 s were observed. These afterhyperpolarizations had a reversal potential of -79 mV, were blocked by intracellular Cs+, were reduced in duration and amplitude by intracellular EGTA, and are suggested to present a combination of a GABAB response and a calcium-dependent potassium current. In addition, slow afterdepolarizations with a duration of about 1900 ms were registered in 16% of the recordings. It is concluded that afterpotentials with several intrinsic and synaptic components follow penicillin-induced PDS. Among these are giant Cl(-)-dependent potentials which probably represent GABAA responses, GABAB responses and a slow calcium-dependent potassium current. It is suggested that the depolarizing equilibrium potential of the Cl(-)-dependent component is due to intracellular Cl- accumulation which might favor transition to ictal discharges.

Separation of different interictal discharge patterns in acute experimentally induced epileptic foci of the rat in vivo

Epileptic discharge patterns in an acute experimental model of epilepsy were analyzed. Epileptic foci were induced by epicortical application of penicillin on the rat motor cortex in vivo. Patterns with regular 1/s discharges, patterns with irregular discharges of about 0.5/s as well as compound patterns comprising discharges with intervals of about 300 ms could be differentiated by means of interval histograms and autocorrelation functions. These patterns occurred in an ordered sequence indicating that the different rhythms are activated by a progressive enlargement of the focus and duration of focal activity. The experiments suggest that different interictal discharge patterns can occur within the same brain regions and are not specific for a certain etiology; instead they seem to represent 'resonance' frequencies characteristic for the brain tissue which are disclosed under pathophysiological conditions.

In vivo kindling does not alter afterhyperpolarizations (AHPs) following action potential firing in vitro in basolateral amygdala neurons

Kindling in vivo results in enhanced glutamatergic synaptic transmission and epileptiform bursting in vitro in neurons of the basolateral amygdala (BLA). We tested the hypothesis that reduction of intrinsic inhibitory mechanisms, such as the slow- and medium-afterhyperpolarizations (s-AHPs, m-AHPs), contributes to the enhanced neuronal excitability observed in kindling-induced epileptogenesis using intracellular recording methodology. In these studies, neurons were recorded from the BLA contralateral to the kindling site. AHPs following depolarizing current-induced (100 ms, 1 nA) action potentials were recorded from BLA neurons of control and kindled animals. We found no difference in the amplitude of the s-AHP and m-AHP, or the duration of the s-AHP between control and kindled neurons. In addition, kindling did not alter the distribution of accommodating/non-accommodating BLA neurons (as assessed from neuronal responses during long (500 ms) depolarizing current injection). It is concluded that an alteration in the neuronal network within the BLA rather than a blockade of an intrinsic inhibitory mechanism underlies the enhanced excitability recorded in BLA neurons following kindling.