Single channel properties of recombinant GABA(A) receptors containing γ2 or δ subtypes expressed with α1 and β3 subtypes in mouse L929 cells

Abstract

1. To determine their contributions to GABA(A) receptor (GABAR) channel properties, rat γ2L and δ subunits were acutely co-expressed with α1 and β3 subtypes in mouse L929 fibroblasts to produce α1β3, α1β3δ or α1β3γ2L GABAR isoforms. 2. With whole-cell recording, the α1β3 isoform had relatively high sensitivity to GABA (EC50 2.1 μM) and low maximum current amplitude. The α1β1δ isoform had similar sensitivity to GABA (EC50 2.8 μM) and low current amplitude. The α1β3γ2L isoform had lower sensitivity to GABA (EC50 11.6 μM) and higher maximum current amplitude. 3. The single channel conductance of α1β3 channels was low (13 pS) compared with that of α1β3δ and α1β3γ2L channels (27 pS). 4. The single channel kinetic properties of the channels also differed. The α1β3γ2L channel exhibited three open states, while the α1β3 and α1β3δ channels exhibited only two open states with mean dwell times similar to those of the two shorter open states of the α1βP3γ2L channel. All three channels exhibited at least five closed states. Bursts of α1β3δ channels consisted primarily of only one or two openings, while those of α1β3 channels contained multiple openings. α1β3γ2L channels exhibited burst kinetics typical for native GABARs with several long openings per burst. 5. These results show that the α1β3 heterodimer formed a GABA-sensitive channel with complex gating kinetics. Addition of a δ subunit to form the α1β3δ heterotrimer altered the single channel conductance and the kinetic properties of the closed components, but did not affect the GABA sensitivity of the receptor nor the open state kinetics. In contrast, addition of a γ subunit (γ2L subtype) to produce the α1β3γ2L heterotrimer affected the GABA sensitivity, channel conductance and kinetic properties of both open and closed states.