We previously reported that co-expression of the Gi-coupled metabotropic glutamate receptor 2 (mGlu2R) and the Gq-coupled serotonin (5-HT) 2A receptor (2AR) in Xenopus oocytes (Fribourg et al. Cell 147:1011–1023, 2011) results in inverse cross-signaling, where for either receptor, strong agonists suppress and inverse agonists potentiate the signaling of the partner receptor. Importantly,...