Cloning and functional expression of GABA(B) receptors from Drosophila

The neurotransmitter GABA (gamma -aminobutyric acid) functions as the major inhibitory neurotransmitter in the central nervous system of vertebrates a nd invertebrates. In vertebrates GABA signals both through ionotropic recep tors (GABA(A), GABA(C)), which induce fast synaptic inhibitory responses, a nd through metabotropic receptors (GABA(B)), which play a fundamental role in the reduction of presynaptic transmitter release and postsynaptic inhibi tory potentials. Whilst GABA(A) and GABA(C) receptors have been cloned from vertebrates as well as invertebrates, GABA(B) receptors have only been ide ntified in vertebrate species to date, although indirect evidence suggests their existence in arthropods, too. Here we report the cloning of three put ative invertebrate GABA(B) receptor subtypes (D-GABA(B)R1, R2 and R3) isola ted from Drosophila melanogaster. Whilst D-GABA(B)R1 and R2 show high seque nce identity to mammalian GABA(B)R1 and R2, respectively, the receptor D-GA BA(B)R3 seems to be an insect-specific subtype with no known mammalian coun terpart so far. All three D-GABA(B)R subtypes are expressed in the embryoni c central nervous system. In situ hybridization of Drosophila melanogaster embryos shows that two of the D-GABA(B)Rs (D-GABA(B)R1 and R2) are expresse d in similar regions, suggesting a coexpression of the two receptors, whils t the third D-GABA(B)R (D-GABA(B)R3) displays a unique expression pattern. In agreement with these results we have only been able to functionally char acterize D-GABA(B)R1 and R2 when the two subtypes are coexpressed either in Xenopus laevis oocytes or mammalian cell lines, whilst D-GABA(B)R3 was ina ctive in any combination. The pharmacology of the coexpressed D-GABA(B)R1/2 receptor was different from the mammalian GABA(B)Rs: e.g. baclofen, an ago nist of mammalian GABA(B)Rs, showed no effect.