2006). The subunit composition (two α, two β and another) determines the pharmacological and kinetic characteristics of the receptor. Certain combinations, such as α6βδ, α4βδ and α5βγ2, are expressed extrasynaptically, have a high affinity for GABA and incompletely desensitize (Semyanov et al. 2004; Mohler, 2006). These receptors are able to detect low ambient concentrations of extracellular GABA and so mediate a tonic current. Such tonic currents are developmentally regulated, depend on synaptic and non-synaptic GABA release and are modulated by GABA uptake (Semyanov et al. 2004). Tonic currents play critical roles in regulating neuronal excitability, information processing, cognition and memory. These currents are modified in conditions such as epilepsy, where up-regulation of tonic currents may act as an adaptive mechanism protecting the brain from seizure activity (Scimemi et al. 2005; Zhang et al. 2007). α5 β1/3 γ2 α4 β2 δ α β1/3 δ α2 β3 γ2