Drosophila melanogaster GRD and LCCH3 subunits form heteromultimeric GABA-gated and cation channels

In addition to its action as a fast inhibitory neurotransmitter, gamma-aminobutyric acid (GABA) is thought to medial e excitatory action by activating cation currents in some cell types in invertebrates. However, to date no GABA receptor capable of mediating such action has been identified at the molecular level in insects. Using a systematic expression screening approach, we found that the Drosophila ligand-gated ion channel subunits GRD and LCCH3 combine to form cation-selective GABA-gated ion channels when coexpressed in Xenopus laevis oocytes. The heteromultimeric receptor is activated by GABA (EC50 = 4.5 mum), muscimol (EC50 = 4.8 mum) and trans-4-aminocrotonic acid (EC50 = 104.5 mum), and partially by cis-4-aminocrotonic acid (EC50 106.3 mum). Picrotoxin effectively blocked the GABA-izated channel (IC50-0.25 mum), but bicuculline, TPMTA, dieldrin and lindane did not. The benzodiazepines flunitrazepam and diazepam did not potentiate the GABA-evoked current. Our data suggest that heteromultimeric channels composed of GRD and LCCH3 subunits form GABA-gated cation channels in insects.