Inhibitory Transmission in Locus Coeruleus Neurons Expressing GABAA Receptor Epsilon Subunit Has a Number of Unique Properties

Abstract

Fast inhibitory synaptic transmission in the brain relies on ionotropic GABAA receptors (GABAAR). Eighteen genes code for GABAAR subunits, but little is known about the ε subunit. Our aim was to identify the synaptic transmission properties displayed by native receptors incorporating ε. Immunogold localization detected ε at synaptic sites on locus coeruleus (LC) neurons. In situ hybridization revealed prominent signals from ε, and θ mRNAs, some low β1 and β3 signals, and no γ signal. Using in vivo extracellular and in vitro patch-clamp recordings in LC, we established that neuron firing rates, GABA-activated currents, and mIPSC charge were insensitive to the benzodiazepine flunitrazepam (FLU), in agreement with the characteristics of recombinant receptors including an ε subunit. Surprisingly, LC provided binding sites for benzodiazepines, and GABA-induced currents were potentiated by diazepam (DZP) in the micromolar range. A number of GABAAR ligands significantly potentiated GABA-induced currents, and zinc ions were only active at concentrations above 1 μM, further indicating that receptors were not composed of only α and β subunits, but included an ε subunit. In contrast to recombinant receptors including an ε subunit, GABAAR in LC showed no agonist-independent opening. Finally, we determined that mIPSCs, as well as ensemble currents induced by ultra-fast GABA application, exhibited surprisingly slow rise times. Our work thus defines the signature of native GABAAR with a subunit composition including ε: differential sensitivity to FLU and DZP and slow rise time of currents. We further propose that α3, β1/3, θ and ε subunits compose GABAAR in LC.