Synthesis and Pharmacological Characterization of C4-disubstituted Analogs of 1S,2s,5r,6s-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylate: Identification of a Potent, Selective Metabotropic Glutamate Receptor Agonist and Determination of Agonist-Bound Human Mglu2 and Mglu3 Amino Terminal Domain Structures.