Distribution and abundance of metabotropic glutamate receptor subtype 2 in rat brain revealed by [3H]LY354740 binding in vitro and quantitative radioautography: Correlation with the sites of synthesis, expression, and agonist stimulation of [35S]GTPγs binding

Until recently, there was a lack of selective radioligands for the subtypes of metabotropic glutamate (mGlu) receptors. [3H]LY354740 ((+)‐2‐aminobicyclo[3,1,0]hexane‐2,6‐dicarboxylic acid), a selective agonist for group II receptors (mGlu2 and ‐3, which are negatively coupled to cAMP production), has now been used to map their brain distribution and abundance by in vitro binding and quantitative radioautography. The selective cation dependence of its binding allowed the discrimination between mGlu2 and mGlu3 receptor labeling. Thus, in the presence of Ca2+ and Mg2+ ions, the agonist bound selectively to mGlu2 receptors as evidenced by: 1) the correlative distribution and abundance of binding sites (highest in the lacunosum moleculare of the hippocampus and lowest in white matter) with mGlu2 receptor mRNA and protein revealed by in situ hybridization histochemistry and immunohistochemistry, respectively; 2) its selective pharmacology; and 3) the distribution of LY354740‐stimulated [35S]GTPγS binding (25–97% above basal, according to the brain region), revealing G protein‐coupled receptor coupling to Gi proteins. Nonspecific binding (in the presence of 10 μM DCG‐IV, a group II‐selective, mGlu2‐preferring, receptor agonist) was