humans, mutations of two genes, LIS1 (located on 17p13.3) and DCX (located on Xq22.3), have been found to account for the majority of cases (Pilz et al., 1998). Both of these genes have been shown to have roles in neuronal migration by their interactions with the neuron microtubule network (Gleeson et al., 1999a; Sapir et al., 1999). X-linked lissencephaly and double cortex syndrome is a disorder of neuronal migration documented in humans. Double cortex or subcortical band heterotopias often occur in females whereas more severe lissencephaly is found in affected males. A causal mutation in a gene called doublecortin has been identified (Gleeson et al., 1998). It was suggested that doublecortin acts as an intracellular signalling molecule critical for the migration of developing neurons (Allen and Walsh, 1999; Gleeson et al., 1999b). Lissencephaly has been documented in Lhasa apsos with histopathology indicating the condition to be very similar to that seen in people (Greene et al., 1976; Saito et al., 2002). This condition has also been documented in a mixed breed dog and together with either cerebellar hypoplasia in two wire-haired fox terriers and three Irish setters, with cyclopia in one German shepherd-mixed breed dog, or with microencephaly in the Korat breed of cat (Saito et al., 2002; Lee et al., 2011).
Although these disorders are relatively rare, studying the underlying pathophysio-logical mechanisms may shed light on the pathophysiology of more common epileptic syndromes.
How Do Seizures Stop?
Most seizures are self-limited, lasting no more than a few minutes. The persistence of a seizure lasting longer than several minutes is usually a cause for alarm as physiological mechanisms terminating the seizure may have failed. Why seizures typically do not continue indefinitely, and how intrinsic anti-convulsant mechanisms in the brain lead to seizure termination, are questions that potentially offer new avenues for developing novel treatments for epilepsy, as well as offering insights into brain autoregulatory mechanisms.
Mechanisms acting at the level of single neurons
Within a single neuron, prolonged depolarizations with sustained action-potential firing may be triggered by a brief depolarizing pulse, as in the paroxysmal depolarizing shift, or may be the result of sustained excitatory synaptic input from neighbouring neurons engaged in seizure activity (Ayala, 1983). Intrinsic mechanisms of seizure termination active in a single neuron, discussed below, include: the potassium currents activated by calcium and sodium entry; the loss of ionic gradients, particularly of potassium, leading first to depolarization with increased firing, followed by depolarization blockade of membrane firing and cessation of firing; and possibly the depletion of energy substrates locally, with the decline in adenosine triphosphate (ATP), resulting in cessation of neuronal firing.
Intracellular ion-activated potassium currents
The membrane after hyperpolarization that follows bursts of action potential discharge is the result, at least in part, of potassium currents activated by the entry of calcium and sodium. Increased calcium entry during the paroxysmal depolarizing shift, or as a result of the action of glutamate at the postsynaptic membrane, activates a calcium-dependent membrane potassium conductance that allows potassium efflux, membrane hyperpolarization and cessation of firing (Alger and Nicoll, 1980; Timofeev et al., 2004). Like calcium, sodium entry may also activate a sodium-dependent potassium current that reduces neuronal excitability by hyperpolarizing the membrane and increasing shunt conductance (Schwindt et al., 1989).
Transmembrane ion gradients
The effect of extracellular potassium is multi-faceted. Sustained potassium efflux increases extracellular potassium concentration, depolarizing the membrane and moving the intra-cellular voltage toward the threshold for sodium action potential firing. As extracellular potassium continues to accumulate, there is membrane depolarization and action potential firing increases. With further accumulation, the membrane potential becomes more depolarized than the firing threshold for sodium-action potentials, sodium channels inactivate, and neuronal firing ceases. In vitro experiments by Bikson et al. (2003) illustrate these effects of extracellular potassium accumulation. Electrographic seizure-like activity triggered in hippocampal slices by exposure to low-calcium artificial cerebro-spinal fluid (aCSF) manifested as recurrent periods of population firing followed by periods of electrographic silence lasting 12–18 s. The termination of each electrographic discharge by a period of electrographic silence resulted from transient increases in extracellular potassium to plateaus of approximately 12 mM. The depolarized state was maintained by the elevation of extracellular potassium and by the presence of persistent sodium channels that did not inactivate. Depolarization blockade-terminating seizure-like discharges have also been observed in neocortical slices in which GABA-ergic inhibition is partially blocked by picrotoxin (Pinto et al., 2005). Focal or localized increases in potassium may also trigger additional potassium release beyond the initial region of potassium accumulation. Shifts in extracellular potential, and oscillations seen at the end of hippocampal after-discharges, have been attributed to a rapid rise in extracellular potassium that triggers waves of astrocyte depolarization and a propagating rise in potassium that terminates neuronal firing (Bragin et al., 1997). In addition to its direct depolarizing effects, increased extracellular potassium may also indirectly result in membrane depolarization through the action of the potassium–chloride co-transporter KCC2. The rise in extracellular potassium can increase intracellular chloride, shifting the chloride reversal potential toward membrane depolarization. In the setting of increased intracellular chloride, the action of GABA to open chloride channels could enhance membrane depolarization to the point of becoming refractory to further firing of action potentials (Jin et al., 2005; Galanopoulou, 2007).
Extracellular calcium levels also change markedly during paroxysmal neuronal firing and may affect the efficiency of neuron-to-neuron spread of activity. Focal seizure activity results in a decline in extracellular calcium activity of approximately 50% (Heinemann et al., 1977). This decline may inhibit synaptic transmission because synaptic vesicle fusion and neurotransmitter release are dependent on entry of extracellular calcium (King et al., 2001; Cohen and Fields, 2004). Decline in extracellular calcium also potentially affects gap junction function as hemichannel opening increases in low calcium (Thimm et al., 2005).
Energy failure
Sustained neuronal activation also markedly increases energy, namely ATP, utilization to restore ion gradients across the membrane. In some neurons, the presence of an ATP-gated potassium channel (KATP) reduces neuronal activity when ATP levels decline intracellularly (Yamada et al., 2001). When the ATP level falls because energy utilization outpaces energy production, potassium channels open and produce membrane hyperpolarization. Indeed, knockout mice lacking functioning KATP channels experience a myoclonic seizure on average 8.9 ± 1.1 s following onset of hypoxia, followed by generalized convulsions and death. A similar hypoxic challenge, however, does not trigger seizures in wild-type mice, indicating that KATP channels in vivo resist membrane depolarization during energy failure. Reduced levels of energy metabolites, such as glucose, may also affect seizure duration. In vitro recordings show that decreasing extracellular glucose terminates electrographic seizure-like activity in the low magnesium hippocampal slice (Kirchner et al., 2006). The effect of hypoglycaemia on seizure-like discharges in vitro was statistically significant, but not immediate. Fifty per cent fewer seizure-like discharges occurred in the 24-min period following application of low glucose artificial cerebrospinal fluid compared to the frequency of discharges in the 30 min prior to application. Low glucose also reduced the amplitude of the seizure-like discharge by 25%. These effects on the frequency and amplitude of seizure-like discharges were reversed by restoration of normal glucose levels.
