**4.1 GABA receptors**

196 Novel Aspects on Epilepsy

While it seems like the mild phenotype of GEFS+ mostly is associated with missense mutations that alter the biophysical properties of the channels, the more severe SMEI phenotype is usually caused by nonsense or frameshift mutations that prevent production of functional channels (Claes et al., 2003; Claes et al., 2001; Nabbout et al., 2003; Ohmori et al., 2002). But how can loss-of-function mutations in a sodium channel cause epilepsy when reduced sodium current should lead to hypoexcitability rather than hyperexcitability? The answer seems to be related to the expression pattern of the channels. NaV1.1 is predominantly found in inhibitory interneurons and is thought to conduct most of the sodium current in these cells, whereas excitatory pyramidal neurons express only negligible levels of NaV1.1 (Ogiwara et al., 2007). Catterall and co-workers showed that haploinsufficiency of NaV1.1 channels in heterozygous knock-out mice led to a phenotype resembling that of SMEI (Oakley et al., 2009; Yu et al., 2006). In these mice, sodium currents in GABAergic interneurons in the hippocampus were substantially reduced, whilst the effect in pyramidal cells was much less severe. Loss of one SCN1A copy led to a reduction in action potential number, frequency and amplitude in the interneurons (Yu et al., 2006). Similarly, studies in several animal models carrying nonsense or missense mutations in SCN1A show impaired interneuron function (Martin et al., 2010; Mashimo et al., 2010; Ogiwara et al., 2007; Tang et al., 2009). These studies indicate that functional loss of one copy of SCN1A reduces the inhibitory function of GABAergic interneurons and enhances the excitability of downstream synaptic targets, thereby predisposing to epileptic seizures. But if this is true, how does the predicted changed NaV1.1 function in many of the patients lead to hyperexcitability when the consequence should be increased GABA action? One possibility is that enhanced sodium current in the interneurons causes too much inhibition, and that this leads to synchronization of the downstream synaptic targets, as has been suggested in the pathogenesis of autosomal dominant nocturnal frontal lobe epilepsy (ADNFL) (Klaassen et al., 2006) (discussed later). Another possibility is that the functional consequences of the mutations in vivo are different from that predicted after in vitro characterization of the mutant channels, and that all of the mutations actually cause a reduction of sodium current in inhibitory neurons. This is supported by studies on knockout mice lacking the β1 subunit (Chen et al., 2004). These mice show downregulated NaV1.1 expression, indicating that β1 function might be necessary for normal expression of NaV1.1. As the inhibitory interneurons seem to be most affected by a reduction in NaV1.1, the consequences of the β1 mutations might be reduced sodium current in interneurons rather

than, or in addition to, increased NaV1.2 and NaV1.3 function.

As mutations in SCN1A most often are associated with febrile seizures the mutations seem not to be sufficient to cause spontaneous seizure themselves. Why are the seizures triggered by fever? Why are the seizures most prevalent in young children? And what is the reason for the age-specific onset of SMEI? It is known that an increase in body temperature leads to an increase in the rate of respiration, especially in young children (Gadomski et al., 1994). This increased respiration can cause respiratory alkalosis in the immature brain, and alkalosis of brain tissue can lead to enhanced neuronal activity and to epileptoform activity (Lee et al., 1996). Studies on rat pups showed that seizure activity induced by hyperthermia had a well-defined pH threshold and that a rise in brain pH to the threshold level by injection of bicarbonate could provoke seizures (Schuchmann et al., 2006). By suppressing the alkalosis with a moderate elevation of ambient CO2 to 5%, seizures could be abolished within 20 seconds without affecting body temperature. Bicarbonate-induced pH changes and seizures could also be blocked by elevation of ambient CO2. In older rats, hyperthermia GABA is the major inhibitory neurotransmitter in the central nervous system. There are three types of GABA receptors: GABAA, GABAB, and GABAC. GABAA and GABAC receptors are ionotropic while GABAB receptors are G-protein coupled and often act by activating potassium channels. Most of the cortical inhibitory effects of GABA are mediated by GABAA receptors (Chebib and Johnston, 1999).

The GABAA receptors are pentameric chloride channels formed by various combinations of different types of α (α1 to α6), β (β1 to β3), γ (γ1 to γ3), δ, ε, π, θ, and ρ (ρ1 to ρ3) subunits, that each have four transmembrane segments, M1 to M4 (Benarroch, 2007). The most prevalent subunit combination consists of α1β2γ2 (McKernan and Whiting, 1996). The subunit composition determines the functional and pharmacological characteristics of the receptors (Meldrum and Rogawski, 2007; Sieghart and Sperk, 2002). Binding of GABA to the receptor triggers opening of the chloride channel, allowing rapid influx of chloride that hyperpolarizes the neuron and thereby decreases the probability of generation of an action potential.
