Agonist pharmacology of two Drosophila GABA receptor splice variants

1 The Drosophila melanogaster γ‐aminobutyric acid (GABA) receptor subunits, RDL ac and DRC 17‐1‐2, form functional homo‐oligomeric receptors when heterologously expressed in Xenopus laevis oocytes. The subunits differ in only 17 amino acids, principally in regions of the N‐terminal domain which determine agonist pharmacology in vertebrate ionotropic neurotransmitter receptors. A range of conformationally restricted GABA analogues were tested on the two homo‐oligomers and their agonist pharmacology compared with that of insect and vertebrate iontropic 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 of 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 these agonists on each receptor were very similar. 3 A more complete agonist profile was established for RDL ac homo‐oligomers. The most potent agonists of these receptors were GABA, muscimol and trans ‐aminocrotonic acid (TACA), which were approximately equipotent. RDL ac homo‐oligomers were fully activated by a range of GABA analogues, with the order of potency: GABA>ZAPA ((Z)‐3‐[(aminoiminomethyl)thio]prop‐2‐enoic acid) > isoguvacine > imidazole‐4‐acetic acid ≥ isonipecotic acid ≥ cis ‐aminocrotonic acid (CACA) > β‐alanine. 3‐Aminopropane sulphonic acid (3‐APS), a partial agonist of RDL ac homo‐oligomers, was the weakest agonist tested and 100 fold less potent than GABA. 4 SR95531, an antagonist of vertebrate GABA A receptors, competitively inhibited the GABA responses of RDL ac homo‐oligomers, which have previously been found to be insensitive to bicuculline. However, its potency (IC 50 500 μ m was much reduced when compared to GABA A receptors. 5 The agonist pharmacology of Drosophila RDL ac homo‐oligomers exhibits aspects of the characteristic pharmacology of certain native insect GABA receptors which distinguish them from vertebrate GABA receptors. 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 GABA A receptors. The differences in the potency of agonists on RDL ac and DRC 17‐1‐2 homo‐oligomers observed in the present study may assist in identification of further molecular determinants of GABA receptor function.