Homomeric RDL and heteromeric RDL/LCCH3 GABA receptors in the honeybee

24 25 Gamma-aminobutyric acid (GABA) – gated chloride channel receptors are abundant in the 26 central nervous system where their physiological role is to mediate fast inhibitory 27 neurotransmission. In insects, this inhibitory transmission plays a crucial role in olfactory 28 information processing. In an effort to understand the nature and properties of the ionotropic 29 receptors involved in these processes in the honeybee Apis mellifera, we performed a 30 pharmacological and molecular characterization of GABA-gated channels in the primary 31 olfactory neuropile of the honeybee brain, the antennal lobe (AL), using whole-cell patch32 clamp recordings coupled with single-cell RT-PCR. Application of GABA onto AL cells at 33 110 mV elicited fast inward currents, demonstrating the existence of ionotropic GABA-gated 34 chloride channels. Molecular analysis of the GABA-responding cells revealed that both 35 subunits RDL and LCCH3 were expressed out of the three orthologs of Drosophila 36 melanogaster GABA receptor subunits encoded within the honeybee genome (RDL, resistant 37 to dieldrin; GRD, GABA/glycine-like receptor of Drosophila; LCCH3, ligand-gated chloride 38 channel homologue 3), opening the door to possible homoand/or heteromeric associations. 39 The resulting receptors were activated by insect GABA receptor agonists muscimol and 40 CACA and blocked by antagonists fipronil, dieldrin and picrotoxin but not bicuculline, 41 displaying a typical RDL-like pharmacology. Interestingly, increasing the intracellular 42 calcium concentration potentiated GABA-elicited currents, suggesting a modulating effect of 43 calcium on GABA receptors possibly through phosphorylation processes which remain to be 44 determined. 45 These results indicate that adult honeybee AL cells express typical RDL-like GABA receptors 46 whose properties support a major role in synaptic inhibitory transmission during olfactory 47 information processing. 48 49