Ion channels belong to a superfamily of transmembrane proteins that regulate intracellular and extracellular concentrations of cations and anions essential for triggering action potentials that propagate information or elicit biological responses. 1,2
This superfamily of receptors can further be divided into two main classes, the Voltage-Gated Ion Channels (VGIC) and the Ligand-Gated Ion Channels (LGIC).1,2 VGICs respond to changes in the membrane potential, while LGICs respond to binding of an endogenous neurotransmitter or an exogenous molecule. Ion channels play a crucial role in electrophysiological events pertaining to epilepsy and it is therefore not surprising that they are targeted by numerous AEDs.76-78
1.3.1.1. Voltage-gated sodium channels
Voltage-gated sodium channels (VGSCs) are large Na+-specific ion channels that respond to changes in membrane potential. They are composed of one α
subunit (~260 kDa) consisting of 4 domains (I–IV) that form a central pore, each containing 6 membrane-spanning α helical segments (S1–S6) (Figure 5), with intracellular N- and C-termini.76 The S4 segment of each domain contains one
positively charged amino acid every three residues.79 These residues control the channel gating and respond to depolarization by moving across the membrane and initiating the activation of the receptor.80 The ionic selectivity of the channel is determined by repetitive amino acid motifs in the membrane-reentrant loop between S5 and S6 of each domain, and a mutation of three residues in the S5S6 loop of each domain is enough to turn the Na+ channel into a Ca2+ channel.81
Figure 5. Schematic representation of a voltage-gated Na+ channel α-subunit
Native VGSCs are associated with one or more β1, β2, or β3 subunits (~30 kDa) that can influence the gating properties of the channel, the kinetics of channel activation, as well as protein interactions with cell adhesion molecules.82-84 In humans, 9 different types of voltage-gated Na+ channels exist and they are designated Nav1.1–Nav1.9.80 Among these, Nav1.1–Nav1.3 and Nav1.6 are the main VGSCs found in the CNS.80 Mutations in Scn1A, Scn2A and Scn1B, the genes that respectively code for the α subunits of Nav1.1 and Nav1.2, and the β1 subunit, are associated with severe forms of infant epilepsy.85-87,27
open (activated) state to the inactivated state. Two distinct inactivation pathways exist, fast and slow, from which the Na+ channel can recover. The fast inactivation pathway happens within a few milliseconds of channel opening, and involves a “hinged lid” mechanism.76 The short intracellular region between domains III and IV physically obstructs the pore by docking a conserved hydrophobic peptide sequence isoleucine, phenylalanine, methionine, threonine (IFMT) into domain IV S6 segment (Scheme 2).88 Recovery from the fast inactivated state depends on the membrane potential and time. This mechanism is responsible for the refractory period allowing APs to propagate in only one direction (Scheme 1).
Scheme 2. Schematic representation of the fast inactivation hinged lid mechanism of VGSCs.
The VGSC slow inactivation pathway is a biological process independent of the fast inactivation.89 Although its precise mechanism is subject to controversy, it is likely to involve a rearrangement of the pore, as well as specific domains of the channel.89-92 The “slow” terminology refers to the prolonged time required for the channel to recover from its inactivated state, ranging from hundreds of milliseconds to a second.76 This pathway is a cellular mechanism that only takes place under
conditions that may be relevant to an epileptic seizure such as high frequency depolarizations, or under sustained membrane depolarization (membrane potential at -60 or -55 mV). This type of inactivation acts as a protective way to lessen excessive neuronal excitability.76
1.3.1.2. Voltage-gated calcium channels
Voltage-gated Ca2+ channels (VGCCs), also termed voltage-dependent calcium channels (VDCCs) are transmembrane receptors that mediate calcium influx into the cell. Ten different types of VGCCs exist, Cav1.1–1.4, Cav2.1–2.3, and Cav3.1–3.3, that are comprised of four to five subunits, the largest and most important of which is the α1 subunit (190–250 kDa).93 Analogous to the VGSCs α
subunit, the VGCC α1 subunit is made of four domains (I–IV) each containing 6 α- helical membrane-spanning segments (S1–S6) where the S4 segment senses changes in membrane potential. Ion selectivity is controlled by the loop between S5 and S6 segments of domains I, III and IV.93 VGCCs additionally possess an intracellular β subunit, as well as a disulfide-linked transmembrane δ subunit, and an extracellular α2 subunit that forms a α2δ complex, and less frequently an intracellular
γ subunit.94 The pharmacological and electrophysiological properties of the Ca2+ channel are primarily determined by the different types of α1 subunits whereas β, α2δ and γ subunits serve as modulators.94 The alphabetical designation of VGCCs comes from the distinct types of currents they elicit and their sensitivity to established channel blockers: L-type for “long-lasting” (Cav1.1–Cav1.4), N-type for
“neural” (Cav2.2), P/Q-type for “Purkinje neuron”95 (Cav2.1), R-type for “resistant to the other blockers” (Cav2.3), and T-type for “transient” (Cav3.1–Cav3.3).93
The VGCC-mediated influx of Ca2+ ions has several consequences. Electrophysiologically, the increase of intracellular calcium leads to a depolarization of the neuron (ECa = +120 mV).1 Pharmacologically, Ca2+ is an important second messenger signaling molecule that can start a cascade of cellular downstream events ranging from activation of protein kinase C (PKC), calmodulin (CaM) and Ca2+-dependent proteases to triggering the transcription of pro-apoptotic factors.96- 101