Kinetics and Spontaneous Open Probability Conferred by the ϵ Subunit of the GABAAReceptor

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Abstract
GABAAreceptors mediate synaptic and extrasynaptic inhibition. Native receptors consist of α and β subunits, which are required for function, and another “modulatory” subunit, for example, γ, δ, or ϵ. Of these, the ϵ subunit has the most restricted distribution, confers resistance to neurosteroid and anesthetic modulation, and causes spontaneous channel opening. Little is known, however, about how ϵ affects receptor kinetics, which in turn shape responses to both ambient and synaptic GABA exposure. Here, we expressed human α2β1, α2β1γ2, or α2β1ϵ subunit combinations in human embryonic kidney 293 cells and used rapid solution exchange to study receptor kinetics in outside-out patches. The ϵ subunit greatly slowed deactivation and recovery after brief GABA pulses. During long, saturating GABA pulses, the rate of desensitization was slower for α2β1ϵ and α2β1γ2 than for α2β1. However, in α2β1ϵ, the final extent of desensitization was large compared with that of α2β1γ2. Responses in α2β1ϵ, but not the others, were often followed by an “overshoot” above the baseline, suggesting that a fraction of channels are spontaneously open and are transiently silenced by receptor activation and subsequent desensitization. The baseline current and associated noise were reduced by picrotoxin, revealing that ϵ-containing channels are open ∼4% of the time in the absence of GABA. These results suggest that, if ϵ-containing receptors are expressed at synapses, the synaptic currents would be long-lasting but may rundown quickly under high-frequency activation. In addition, silencing of spontaneous openings by desensitization raises the possibility that tonic inhibition mediated by ϵ-containing receptors may be regulated by phasic inhibition.