Cloning and functional expression of intracellular loop variants of the honey bee (Apis mellifera) RDL GABA receptor

The insect GABA receptor, RDL (resistance to dieldrin), plays central roles in neuronal signalling and is the target of several classes of insecticides. To study the GABA receptor from an important pollinator species, we cloned Rdl cDNA from the honey bee, Apis mellifera. Three Rdl variants were identified, arising from differential use of splice acceptor sites in the large intracellular loop between transmembrane regions 3 and 4. These variants were renamed from previously, as Amel_RDLvar1, Amel_RDLvar2 and Amel_RDLvar3. When expressed in Xenopus laevis oocytes, the three variants showed no difference in sensitivity to the agonist, GABA, with EC50s of 29 μM, 20 μM and 29 μM respectively. Also, the potencies of the antagonists, fipronil and imidacloprid, were similar on all three variants. Fipronil IC50 values were 0.18 μM, 0.31 μM and 0.20 μM whereas 100 μM imidacloprid reduced the GABA response by 17%, 24% and 31%. The possibility that differential splicing of the RDL intracellular loop may represent a species-specific mechanism leading to insensitivity to insecticides is discussed.