Unexpected Properties of δ-Containing GABAA Receptors in Response to Ligands Interacting with the α+ β- Site.

GABAA receptors are the major inhibitory neurotransmitter receptors in the central nervous system and are the targets of many clinically important drugs, which modulate GABA induced chloride flux by interacting with separate and distinct allosteric binding sites. Recently, we described an allosteric modulation occurring upon binding of pyrazoloquinolinones to a novel binding site at the extracellular α+ β- interface. Here, we investigated the effect of 4-(8-methoxy-3-oxo-3,5-dihydro-2H-pyrazolo[4,3-c]quinolin-2-yl)benzonitrile (the pyrazoloquinolinone LAU 177) at several αβ, αβγ and αβδ receptor subtypes. LAU 177 enhanced GABA-induced currents at all receptors investigated, and the extent of modulation depended on the type of α and β subunits present within the receptors. Whereas the presence of a γ2 subunit within αβγ2 receptors did not dramatically change LAU 177 induced modulation of GABA currents compared to αβ receptors, we observed an unexpected threefold increase in modulatory efficacy of this compound at α1β2,3δ receptors. Steric hindrance experiments as well as inhibition by the functional α+ β- site antagonist LAU 157 indicated that the effects of LAU 177 at all receptors investigated were mediated via the α+ β- interface. The stronger enhancement of GABA-induced currents by LAU 177 at α1β3δ receptors was not observed at α4,6β3δ receptors. Other experiments indicated that this enhancement of modulatory efficacy at α1β3δ receptors was not observed with another α+ β- modulator, and that the efficacy of modulation by α+ β- ligands is influenced by all subunits present in the receptor complex and by structural details of the respective ligand.