Single-channel properties of neuronal GABA(A) receptors from mice lacking the gamma 2 subunit

1. The aim of this study was to define the biophysical properties contribut ed by the gamma 2 subunit to native single GABA(A) receptors. 2. Single-channel activity was recorded from neurones of wild-type (gamma 2 +/+) mice and compared with that from mice which were heterozygous (gamma 2 +/-) or homozygous (gamma 2-/-) for a targeted disruption in the gamma 2 su bunit gene of the GABA(A) receptor. Unitary currents were evoked by low con centrations of GABA (0.5-5 mu M) in membrane patches from acutely isolated dorsal root ganglion (DRG) neurones (postnatal day 0) and by 1 mu M GABA in patches from embryonic hippocampal neurones which were cultured for up to 3 weeks. 3. GABA(A) receptors from DRG and hippocampal neurones of gamma 2+/+ and ga mma 2+/- mice displayed predominantly a conductance state of 28 pS and less frequently 18 and 12 pS states. In gamma 2-/- mice, conductance states mai nly of 12 pS and less frequently of 24 pS were found. 4. The mean open duration of the 28 pS state in gamma 2+/+ GABA(A) receptor s (1.5-2.8 ms) was substantially longer than for the 12 pS state of gamma 2 -/- GABA(A) receptors (0.9-1.2 ms) at all GABA concentrations. For gamma 2/+ and gamma 2-/- channels, the mean open duration was increased at higher GABA concentrations. 5. Open duration frequency distributions of 28 and 12 pS receptors revealed the existence of at least three exponential components. Components with sh ort mean durations declined and components with long mean durations increas ed in relative frequency at higher GABA concentration indicating at least t wo binding sites of GABA per 28 and 12 pS receptor. 6. Shut time frequency distributions revealed at least four exponential com ponents of which two were identified as intraburst components in 28 pS and one in 12 pS GABA(A) receptors. 7. The mean burst duration and the mean number of openings per burst increa sed in 28 and 12 pS GABA(A) receptors with increasing GABA concentration. A t least two burst types were identified: simple bursts consisting of single openings and complex bursts of five to six openings in 28 pS but only two to three openings in 12 pS GABA(A) receptors. 8. We conclude that the gamma 2 subunit enhances the efficacy of GABA by de termining open conformations of high conductance and long lifetime, and by prolonging the time receptors remain in the activated bursting state.