AGONIST PHARMACOLOGY OF 2 DROSOPHILA GABA RECEPTOR SPLICE VARIANTS

1 The Drosophila melanogaster gamma-aminobutyric acid (GABA) receptor subunits, RDL(ac) and DRC 17-1-2, form functional homo-oligomeric rece ptors when heterologously expressed in Xenopus laevis oocytes. The sub units differ in only 17 amino acids, principally in regions of the N-t erminal domain which determine agonist pharmacology in vertebrate iono tropic neurotransmitter receptors. A range of conformationally restric ted GABA analogues were tested on the two homo-oligomers and their ago nist pharmacology compared with that of insect and vertebrate iontropi c GABA receptors. 2 The actions of GABA, isoguvacine and isonipecotic acid on RDL(ac) and DRC 17-1-2 homo-oligomers were compared, by use of two-electrode voltage-clamp. All three compounds were full agonists o f both receptors, but were 4-6 fold less potent agonists of DRC 17-1-2 homo-oligomers than of RDL(ac). However, the relative potencies of th ese agonists on each receptor were very similar. 3 A more complete ago nist profile was established for RDL(ac) homo-oligomers. The most pote nt agonists of these receptors were GABA, muscimol and trans-aminocrot onic acid (TACA), which were approximately equipotent. RDL(ac) homo-ol igomers were fully activated by a range of GABA analogues, with the or der of potency: GABA>ZAPA ((Z)-3-[(aminoiminomethyl)thio]prop-2-enoic acid)>isoguvacine > imidazole-4-acetic acid greater than or equal to i sonipecotic acid greater than or equal to cis-aminocrotonic acid (CACA )> beta-alanine. 3-Aminopropane sulphonic acid (3-APS), than GABA. 4 S R95531, an antagonist of vertebrate GABA(A) receptors, competitively i nhibited the GABA responses of RDL(ac) homo-oligomers, which have prev iously been found to be insensitive to bicuculline. However, its poten cy (IC50 500 mu M) was much reduced when compared to GABA(A) receptors . 5 The agonist pharmacology of Drosophila RDL(ac) homo-oligomers exhi bits aspects of the characteristic pharmacology of certain native inse ct GABA receptors which distinguish them from vertebrate GABA receptor s. The high potency and efficacy of isoguvacine and ZAPA distinguishes RDL(ac) homo-oligomers from bicuculline-insensitive vertebrate GABA(C ) receptors, while the low potency of SR95531 and 3-APS distinguishes them from GABAA receptors. The differences in the potency of agonists on RDL,, and DRC 17-1-2 homo-oligomers observed in the present study m ay assist in identification of further molecular determinants of GABA receptor function.