COR758, a negative allosteric modulator of GABA receptors.

Allosteric modulators of G protein coupled receptors (GPCRs), including GABARs (GABARs), are promising therapeutic candidates. While several positive allosteric modulators (PAM) of GABARs have been characterized, only recently the first negative allosteric modulator (NAM) has been described. In the present study, we report the characterization of COR758, which acts as GABAR NAM in rat cortical membranes and CHO cells stably expressing GABARs (CHO-GABA). COR758 failed to displace the antagonist [H]CGP54626 from the orthosteric binding site of GABARs showing that it acts through an allosteric binding site. Docking studies revealed a possible new allosteric binding site for COR758 in the intrahelical pocket of the GABA monomer. COR758 inhibited basal and GABAR-stimulated O-(3-[Sthio)-triphosphate ([S]GTPγS) binding in brain membranes and blocked the enhancement of GABAR-stimulated [S]GTPγS binding by the PAM GS39783. Bioluminescent resonance energy transfer (BRET) measurements in CHO-GABA cells showed that COR758 inhibited G protein activation by GABA and altered GABAR subunit rearrangements. Additionally, the compound altered GABAR-mediated signaling such as baclofen-induced inhibition of cAMP production in transfected HEK293 cells, agonist-induced Ca mobilization as well as baclofen and the ago-PAM CGP7930 induced phosphorylation of extracellular signal-regulated kinases (ERK1/2) in CHO-GABA cells. COR758 also prevented baclofen-induced outward currents recorded from rat dopamine neurons, substantiating its property as a NAM for GABARs. Altogether, these data indicate that COR758 inhibits G protein signaling by GABARs, likely by interacting with an allosteric binding-site. Therefore, COR758 might serve as a scaffold to develop additional NAMs for therapeutic intervention.