Spontaneous and gamma-aminobutyric acid (GABA)-activated GABA(A) receptor channels formed by epsilon subunit-containing isoforms

A new gamma-aminobutyric acid (GABA)(A) receptor (GABAR) subunit class, eps ilon, has recently been cloned and shown to form functional channels when c oexpressed with both alpha and beta subunits. We report that the combinatio n of alpha 1 beta 3 epsilon subunit subtypes expressed in L929 cells produc ed functional chloride ion channels that were both spontaneously active and gated by the application of extracellular GABA. When cells were voltage-cl amped at -75 mV in the whole-cell configuration, holding currents of 50 to 300 pA associated with increased noise were consistently recorded. The appl ication of pentobarbital and loreclezole, which increase GABAR currents, in creased the holding current, whereas the application of zinc and picrotoxin , which reduce GABAR currents, reduced the holding current in a concentrati on-dependent manner. Coexpression of alpha 1 beta 3 gamma 2L, alpha 1 beta 3 delta, alpha 1 epsilon, beta 3 epsilon, alpha 1 beta 3, or epsilon subtyp es did not produce holding currents that were sensitive to picrotoxin (30 m u M). Cells expressing alpha 1 b3 epsilon subtypes had concentration-depend ent GABAR currents that were potentiated by pentobarbital, loreclezole, and lanthanum and inhibited by zinc and furosemide. Spontaneous and GABAR sing le-channel currents from alpha 1 beta 3 epsilon receptors had single-channe l conductances of similar to 24 pS. The biophysical properties and the effe cts of allosteric modulators were similar for spontaneous and evoked GABAR currents, suggesting that a single GABAR isoform was responsible for both c urrents. These data extend the pharmacological characterization of epsilon- containing GABARs and demonstrate that incorporation of the epsilon subunit permits spontaneous channel gating while preserving the structural informa tion necessary for GABA sensitivity.