The experiments presented in this study illustrate that the same area of the brain can trigger different electrographic ictal onset patterns depending on specific disruptions to the existing balance between excitatory and inhibitory components of the neuronal network. These data are important because they show that the disturbance that determines the epileptiform electrographic pattern is functional, not structural.
Both bicuculline and kainic acid are widely used as convulsants to mimic seizure activity in in vitro
and in vivo
experiments. These compounds bind to completely different types of receptors in the brain. The action of bicuculline is primarily on the ionotropic GABAA
receptors, which are ligand-gated ion channels. It only blocks the early component of IPSPs (Edwards et al., 1990
; De Koninck and Mody, 1994
). The late component of IPSP is bicuculline resistant and determined by the activity of GABAB
receptors via a calcium-dependent potassium current (Newberry and Nicoll, 1984
). Kainic acid is a specific agonist for non-NMDA receptors which mimics the effect of glutamate. In addition to its excitation, a major effect of kainic acid is a depression of inhibitory synaptic potentials. Kainic acid reduces both fast GABA-mediated IPSPs and slow, non-GABA-mediated (GABAB
) late hyperpolarizing potentials. IPSP depression correlates closely with the onset of burst potential firing in response to synaptic stimulation (Scharfman, 1994a
). According to Fisher and Alger (1984)
, epileptiform activity in hippocampus evoked by kainic acid is caused, in part, by presynaptic depolarization blockage of IPSP pathways.
Interictal epileptiform patterns induced by other GABAA
antagonists such as penicillin have been studied in vivo
and in vitro
(Dichter and Spencer, 1969a
; Traub and Wong, 1981
; Schneiderman et al., 1990
). At the cellular level, these interictal events consist of an initial depolarization shift with superimposed action potentials (Dichter and Spencer, 1969b
; Schwartzkroin and Prince, 1977
; Dingledine and Gjerstad, 1980
; Swann et al., 1986
; Scharfman, 1994b
; de Curtis and Avanzini, 2001
). At the network level, epileptiform EEG spikes evoked by bicuculline are similar in recordings from different brain areas. They consist of initial population EPSPs with superimposed bursts of population spikes followed by powerful hyperpolarization. This pattern becomes transformed into a regular sinusoidal 5–20 Hz rhythm, before the occurrence of the clinical seizure. This transformation occurs because of a gradual decrease of the afterhyperpolarization and appearance of afterdepolarization because of accumulation of extracellular potassium (Matsumoto and Marsan, 1964a
In in vitro
slice preparations during the first few minutes of perfusion with bicuculline, IPSPs transiently and paradoxically increase in amplitude and as IPSPs increase, the reversal potential and latency to onset remain the same (Scharfman, 1994a
). The pattern of epileptiform events which was observed in our experiments in freely moving rats was similar to that observed earlier in in vitro
conditions in hippocampus (Scharfman, 1994a
) and piriform cortex (De Curtis et al., 1994
; Forti et al., 1997
The weakening of early GABAA
inhibition “unleashes” excitation via local excitatory collaterals and generation of hypersynchronous neuronal discharges (Miles et al., 1984
). The burst events in CA3 after blockage of GABAA
receptors occur because of an increase in firing of pyramidal cells via local excitatory connections (Traub and Miles, 1991
; de la Prida et al., 2006
These hypersynchronous bursts of action potentials rapidly spread from the area of generation to remote areas of the brain, leading to the occurrence of a clinical seizure. A remarkable feature of epileptiform events that occur as a result of blockade of GABAA
receptors (in our case by bicuculline) is that they quickly spread from the area of generation to other adjacent and remote brain areas. This suggests that the synchrony of neuronal discharges during the initial phase of bicuculline induced epileptiform events is so high that existing feed forward inhibition (Buzsáki, 1984
; Trevelyan et al., 2006
), which controls the level of synchrony of the incoming signal, is not strong enough to prevent their propagation from the area of injection to other brain areas. A similar situation is frequently observed during strong electrical stimulation, when an initial burst of action potentials spreads from the area of stimulation through many brain areas causing reverberation of electrical activity.
inhibition is not fast enough to interrupt the initial synchronization of electrical activity triggered by local excitatory connections, but it is crucial for termination of neuronal firing within the epileptiform burst (de la Prida et al., 2006
inhibition then serves as an additional synchronizing mechanism. Its later inhibition promotes rebound excitation resulting in hypersynchronous discharges which progressively increase in amplitude after each event. A similar mechanism was proposed by McCormick and Contreras (2001)
for generation of epileptiform events in the thalamus.
In contrast to bicuculline, kainic acid effects both principal cells and interneuronal networks. In low doses (10−8
M), it evokes a small depolarization of the pyramidal cells with an increase in the frequency of discharges, and at concentrations of 10−5
M, it causes depolarization block (Robinson and Deadwyler, 1981
). At the same time, kainic acid triggers an increase in the frequency of interneuron discharges and, as a result, increases both the frequency and amplitude of IPSCs on interneurons and decreases the threshold for antidromic action potentials (Semyanov and Kullmann, 2001
In most previous publications, the epileptiform activity evoked by kainic acid was analyzed under steady conditions after the transition period was over. Inhibition remains strong during the steady epileptiform events evoked by kainic acid application. Population activity in the frequency range 30–40 Hz, which was observed during seizures evoked by kainic acid, consisted of epileptic population spikes where pyramidal cells fired exclusively during rebound from rhythmic GABAA
-mediated inhibitory postsynaptic events (Khazipov and Holmes, 2003
), which was confirmed in our experiments where discharges of units were phase locked with local gamma activity ().
In our experiments, we focused on the network patterns which occurred preceding the clinical seizure. Basically, we mimicked a situation, for example, when the level of glutamate in the extracellular space increases at the site of spontaneous seizure generation, because of impairment of glia buffering functions or other mechanisms.
The overall electrographic pattern of the transition period before the clinical seizure was recorded in our experiments was similar to what was described by Medvedev et al. (2000)
after systemic kainic acid injection and by Akaike et al. (2001)
after intrahippocampal kainic acid injection. A simultaneous effect on the excitatory and inhibitory networks leads to an increase in the power of beta–gamma frequencies of field activity representing firing of population spikes at these frequencies () during the transition period.
The process of propagation of epileptiform activity after kainic acid injection could be a function of the initial increase in the volume of tissue involving in spike discharges, at the area of injection, as a result of increased discharges of principal cells and interneurons, which evoke a similar process in target areas. This process increases gradually and breaks down into generalized activity, which triggers the occurrence of the behavioral abnormalities causing the clinical seizure.
The latency of occurrence of epileptiform EEG events is longer after kainic acid injection than after bicuculline, which correspond to earlier data on differences in the latency of seizure occurrence after systemic and intraventricular injection of these compounds (Ben Ari et al., 1981
; Gruenthal et al., 1986
). The longer latent period after kainic acid injection may indicate that feedforward inhibition is more effective in prevention of the spread of pathological activity from one area to another, than in case of the sharp epileptiform events which occur with bicuculline injection. This suggests ways in which disturbances in the excitatory–inhibitory balance can be more or less modulated by intrinsic seizure suppressing mechanisms. However, additional studies are required to better understand the means by which the brain prevents the spread of epileptiform activity from one area to another.
Relevance of observed patterns of seizure activity to clinical practice
This manuscript is one of many studies on seizures induction by pharmacological manipulation with the long-term goal of application of results to clinical practice. At present, it is premature to assume that the clinical reality is so simple as to be completely explained by the results from the experiments described in this study. However, if systematic studies that include analysis of electrographic seizure onset patterns evoked by different receptor agonists and antagonists acting on ligands of different ion channels are carefully classified, such data might produce a basis for clinical trials to determine if electrographic seizure onset patterns in human could be a principal factor for prescribing specific antiepileptic drugs (AEDs). Given that different types of epileptic seizures have different electrographic onset patterns, differential AED responses related to these patterns could inform specific selection of appropriate AED treatment.
To date, except for the classical generalized 3 per second spike and wave, electrographic seizure onset patterns are not considered useful for prescribing AEDs, because these patterns are believed to contain little information about the mechanisms triggering the seizure activity. It is reasonable to assume, however, that different electrographic patterns reflect different triggering mechanisms. Elucidation of the glia–neuronal mechanisms responsible for the generation of unique seizure onset patterns could lead to more precise therapy targeting specific functional components of the epileptogenic network.