Benzodiazepine modulation of the rat GABAA receptor α4β3γ2L subtype expressed in Xenopus oocytes

The effects of benzodiazepines on GABA(A) receptors are dependent largely on the particular α subunit isoform that is present in the receptor pentamer. The inclusion of either the α4 or α6 subunit is generally thought to render the receptor insensitive to classical benzodiazepines. We expressed the rat α4β3γ2L subtype in Xenopus oocytes and observed that both diazepam and flunitrazepam significantly potentiated GABA-gated currents. This potentiation occurred at nanomolar concentrations similar to those seen at the most abundant "diazepam-sensitive" receptor i.e., the α1β2γ2 subtype. In the α4β3γ2L receptor, the effects of diazepam and flunitrazepam were inhibited by nanomolar concentrations of the benzodiazepine site antagonists, Ro15-1788 and ZK93426. The presence of the β3 subunit appears to be important for this modulation since diazepam did not affect GABA responses mediated by recombinant α4β1γ2L or α4β2γ2L receptors. Interestingly, when the α4β3γ2L receptor was expressed in HEK293 cells, diazepam and flunitrazepam displaced the relatively non-selective benzodiazepine site ligand, [(3)H]Ro15-4513, only at high concentrations (>10 μM) demonstrating a lack of high affinity binding for these classical benzodiazepines. Functional studies of the cell-expressed receptors using whole cell recording techniques showed that neither diazepam nor flunitrazepam potentiated GABA-evoked currents although currents were enhanced by nanomolar concentrations of Ro15-4513. These results suggest that the observed benzodiazepine modulation of the α4β3γ2L subtype depends on the expression system used and may be specific for expression in Xenopus oocytes.