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

GABA(A) 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 alpha+ beta- 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 alpha beta, alpha beta gamma and alpha beta delta receptor subtypes. LAU 177 enhanced GABA-induced currents at all receptors investigated, and the extent of modulation depended on the type of alpha and beta subunits present within the receptors. Whereas the presence of a gamma 2 subunit within alpha beta gamma 2 receptors did not dramatically change LAU 177 induced modulation of GABA currents compared to alpha beta receptors, we observed an unexpected threefold increase in modulatory efficacy of this compound at alpha 1 beta 2,3 delta receptors. Steric hindrance experiments as well as inhibition by the functional alpha+ beta- site antagonist LAU 157 indicated that the effects of LAU 177 at all receptors investigated were mediated via the alpha+ beta- interface. The stronger enhancement of GABA-induced currents by LAU 177 at alpha 1 beta 3 delta receptors was not observed at alpha 4,6 beta 3 delta receptors. Other experiments indicated that this enhancement of modulatory efficacy at alpha 1 beta 3 delta receptors was not observed with another alpha+ beta- modulator, and that the efficacy of modulation by alpha+ beta- ligands is influenced by all subunits present in the receptor complex and by structural details of the respective ligand.