COR758, a negative allosteric modulator of GABA B receptors

Allosteric modulators of G protein coupled receptors (GPCRs), including GABA Rs (GABA Rs), are promising therapeutic candidates. While several positive allosteric modulators (PAM) of GABA Rs 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 GABA R NAM in rat cortical membranes and CHO cells stably expressing GABA Rs (CHO-GABA ). COR758 failed to displace the antagonist [ H]CGP54626 from the orthosteric binding site of GABA Rs 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 GABA R-stimulated O-(3-[ Sthio)-triphosphate ([ S]GTPγS) binding in brain membranes and blocked the enhancement of GABA R-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 GABA R subunit rearrangements. Additionally, the compound altered GABA R-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 GABA Rs. Altogether, these data indicate that COR758 inhibits G protein signaling by GABA Rs, likely by interacting with an allosteric binding-site. Therefore, COR758 might serve as a scaffold to develop additional NAMs for therapeutic intervention.