Mechanisms acting on a local network of neurons
While seizure initiation is driven at least in part by the burst-firing properties of the individual neurons, the evolution and spread of the seizures also requires amplification and synchronization among neurons within susceptible networks. Seizure amplification occurs through the action of recurrent excitatory collaterals that form feedback loops, returning excitatory synaptic activity to the neurons within the seizure onset zone (Rutecki et al., 1989; Coulter and DeLorenzo, 1999). Seizure spread depends on the propagation and synchronization of the seizure discharge across synapses that separate neurons in the seizure onset zone from ‘normal’ neurons synaptically connected to the seizure onset zone (MacVicar and Dudek, 1980; Miles and Wong, 1983).
Glutamate depletion
Decrease in synaptic efficacy results in milder postsynaptic excitation, and consequently diminished amplification and spread of the seizure discharge. One mechanism limiting synaptic transmission during a sustained seizure discharge is the depletion of synaptic vesicles containing neurotransmitter. Staley et al. (1998) investigated the effects of synaptic depletion in vitro using a model of CA3 electrographic seizure discharges produced by hyperkalaemia. CA3 discharges consist of recurrent neuronal depolarizations with bursts of action-potential firing separated by period of electrographic silence. Staley et al. found that the duration of the seizure burst was proportional to the duration of the silent period preceding the burst, consistent with the hypothesis that the seizure burst duration depended on the renewed availability of immediately releasable glutamate. If glutamate-containing synaptic vesicles are replaced at a steady rate, longer inter-burst periods allow a greater resupply of immediately releasable glutamate, and an increased duration of the subsequent electro-graphic seizure discharge. Inter-burst intervals of 2–3 s or longer were necessary to achieve the longest burst durations (up to 420 ms). Thus, as the seizure discharge develops, it consumes the supply of readily releasable glutamate needed to sustain the seizure, potentially acting as a governor on excitatory drive. As the glutamate reservoir is replenished continuously, however, additional control mechanisms are necessary to prevent re-initiation of seizure activity.
The intra- and extracellular environments
Prolonged neuronal activity during seizure discharges may also have the effect of increasing CO2 or increasing the by-products of anaerobic metabolism, and produce extra-cellular acidosis or intracellular acidosis associated with extracellular alkalinosis (Chesler and Kaila, 1992). Glial cells may also contribute to acidification of the extracellular space in response to increases in the extracellular potassium concentration (Chesler and Kraig, 1987). In the hippocampal slice in vitro, acidification of the extracellular space to pH 6.7 terminated seizure-like burst firing facilitated by low-magnesium in the artificial CSF. The attenuation of epileptiform activity began within minutes of lowering pH (Velisek et al., 1994; Velisek, 1998). The mechanisms of action – at least in part – included decreased NMDA receptor function and loss of synaptic long-term potentiation (LTP). A milder reduction of pH to 7.1 also produced milder synaptic impairment with continued loss of LTP (Velisek, 1998). Inhibition of carbonic anhydrase, which alters extracellular pH, has some anticonvulsant benefit. In humans, the carbonic anhydrase inhibitor acetozolamide has a mild anticonvulsant effect (Thiry et al., 2007). Knockout mice deficient in carbonic anhydrase are severely acidotic and are resistant to seizures produced by flurothyl gas compared to wild-type mice (Velisek et al., 1993). Intracellular acidification may also contribute to termination of seizure discharges. Spontaneous interictal spiking following focal application of bicuculline in the piriform cortex in an in vitro whole brain preparation was associated with periodic abrupt alkanization of the extracellular space followed by a slow return to baseline pH (de Curtis et al., 1998). These observations were interpreted as evidence of intracellular acidification. Application of ammonium chloride in the perfusing medium to prevent intracellular acidification increased neuronal excitability and resulted in after-discharges following each spike, and in seizure-like discharges. The investigators hypothesized that the intracellular acidification reduced excitability by reducing gap-junction function. Application of octanol, a nonspecific gap junction blocker, abolished spontaneous interictal spiking (de Curtis et al., 1998).
Glial buffering of glutamate
Glial uptake of perisynaptic glutamate is the major mechanism forestalling accumulation of glutamate at the synapse (Benarroch, 2005). Astrocytes have an equally important role in the regulation of extracellular potassium. Astrocytic buffering of potassium maintains extracellular levels below a ceiling of 12 mM (Benarroch, 2005). In some cases, such as the epileptic brain, glia may also release glutamate, thereby prolonging post-synaptic excitation. Tian et al. (2005) recently showed that glial release of glutamate contributed to the maintenance of the paroxysmal depolarizing shift that is the hallmark of ‘epileptic’ neurons. Failure of glia to buffer extracellular glutamate, let alone glutamate release from glia, can be expected to result in prolonged excitatory drive and seizure maintenance.
Increased GABA-ergic inhibition
A basic mechanism to control focal seizure activity is GABA-ergic synaptic inhibition mediated by local interneurons. Seizure discharges within the seizure onset zone produce recurrent inhibition within the seizure initiation zone, thus reducing excitatory output (Kostopoulos et al., 1983; Dorn and Witte, 1995). Early investigations of the spike and wave components of ‘spike-wave’ discharges showed that the spike component is associated with a burst of rapid action-potential firing, while the wave component is associated with a pause in action-potential firing (Dichter and Spencer, 1969). The pause in neuronal firing results from synaptic inhibition produced by local inhibitory inter-neurons activated by the volley of excitatory activity comprising the ‘spike’ component, an example of feedback inhibition. Feed-forward inhibition is a fundamental feature of cortical processing (Swadlow, 2003). Feed-forward inhibition may also play an important role; an interneuron activated by a principal cell sends inhibitory signals to principal cells outside the focus, inhibiting the propagation of the seizure (Trevelyan et al., 2007). Recent evidence indicates that a principal cell axon may synapse on the presynaptic terminal of an inhibitory interneuron, bypassing somatic activation of the interneuron altogether by causing transmitter release directly from the inhibitory synaptic terminal (Connors and Cruikshank, 2007).
Synaptic inhibition is mediated by the presynaptic release of the neurotransmitter GABA, which acts on the postsynaptic neuron via receptors located on the soma, dendrites, or presynaptic terminals. GABA receptors are present in two major varieties, GABAA and GABAB. GABAB receptors are metabotropic acting through G-protein second messengers. The pre- and postsynaptic distribution of GABAB receptors, along with mixed evidence of anti- and proconvulsant effects of GABAB activation, makes it difficult to determine their role in seizure termination (Chen et al., 2004). GABAA receptors are chloride-conducting membrane channels that open rapidly in response to GABA. Desensitization of GABAA receptors during status epilepticus likely contributes to the failure of seizure termination (Chen et al., 2007). Desensitization of GABAA receptors is also the basis of the loss of efficacy of benzodiazepine medications used to treat status epilepticus. Multiple mechanisms appear to contribute to GABAA receptor desensitization. Increased internalization of GABAA receptors during status epilepticus reduces the effect of GABA-ergic stimulation (Goodkin et al., 2007). Changes in subunit composition may also contribute to GABAA receptor desensitization, although this process acts over many minutes to hours, and appears to affect long-term neuronal excitability and epileptogenesis rather than seizure termination. Nonsynaptic GABAA receptors, in contrast, do not desensitize and instead are capable of tonic inhibition, which produces long-lasting changes in neuronal reactivity. These tonic GABA receptors typically contain particular subunits –
