Subunit composition determines picrotoxin and bicuculline sensitivity of Drosophila gamma-aminobutyric acid receptors.

Few gamma-aminobutyric acid (GABA) receptor subunits have been cloned from insects. These include Resistance to dieldrin, or Rdl, and a homologue of the vertebrate GABAA receptor beta subunit. Unlike most vertebrate GABAA receptor subunits, Rdl forms a highly functional homomultimeric receptor. This receptor is picrotoxin (PTX) sensitive but bicuculline (BIC) insensitive and cannot be readily classified within the known GABAA receptor subtypes. In contrast, functional expression of the beta subunit homologue has not been reported. We report that coinfection of cells with recombinant baculoviruses containing Rdl plus beta subunits induces GABA receptors with distinct pharmacological and kinetic properties. Coinfection produces two separate receptor populations: one highly sensitive to PTX but BIC insensitive (Rdl homomultimers) and the other PTX insensitive and BIC sensitive (Rdl plus beta heteromultimers). Putative Rdl plus beta channels also show reduced GABA sensitivity, slow desensitization, rapid bursting, and shorter mean open time. These studies not only localize PTX and BIC sensitivity to two distinct GABA receptor subunits but also demonstrate assembly of two highly divergent GABA receptor subunits. Furthermore, the difference in channel conductance and gating between in vivo and recombinant channels implies the existence of uncharacterized GABA receptor subunits in Drosophila.