In Vitro Binding Characteristics of a New Selective Group II Metabotropic Glutamate Receptor Radioligand, [3H]LY354740, in Rat Brain

The in vitro binding of [3H]LY354740, the first high affinity group II-selective metabotropic glutamate (mGlu) receptor radioligand, was characterized in rat cortical, hippocampal, and thalamic membranes as well as in rat brain sections. [3H]LY354740 binding was saturable in all regions investigated. Nonspecific binding (in the presence of 10 μm DCG-IV) was ≈8% of the total. Ionotropic glutamate receptor agonists, N-methyl-d-aspartate, (R,S)-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid/kainate, a Na+-dependent glutamate uptake blocker as well as a group I-selective mGlu receptor agonist (all up to 1 mm) did not inhibit [3H]LY354740 binding to cortical membranes. However, several known metabotropic receptor ligands inhibited the binding with the following rank order of potency: LY354740 = LY341495 > (2S,2′R,3′R)-2-(2′,3′-dicarboxycyclopropyl)glycine = (2S,1′S,2′S)-2-(2-carboxycyclopropyl)glycine > glutamate = (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid > (2S,1′S,2′S)-2-methyl-2-(2-carboxycyclopropyl)-glycine > quisqualate > ibotenate >l-2-amino-3-phosphonopropionic acid = (S)-α-methyl-4-carboxyphenylglycine >l-(+)-2-amino-4-phosphonobutyric acid.N-Acetyl-aspartyl-glutamate, (2S)-α-ethylglutamic acid, and (R,S)-α-methyl-4-phosphonophenylglycine inhibited [3H]LY354740 binding in a biphasic manner. Guanosine-5′-O-(3-thiotriphosphate concentration-dependently and almost completely inhibited the binding. Finally, in parasagittal sections of rat brain, a high density of specific binding was observed in the accessory olfactory bulb, cortical regions (layers 1–3 > 4–6), caudate putamen, molecular layers of the hippocampus and dentate gyrus, presubiculum, retrosplenial cortex, anteroventral thalamic nuclei, and cerebellar granular layer, reflecting its preferential (perhaps not exclusive) affinity for presynaptic and postsynaptic mGlu2 receptors. Thus, the pharmacology, tissue distribution, and sensitivity to guanosine-5′-O-(3-thiotriphosphate show that [3H]LY354740 binding probably occurs to group II mGlu receptors in rat brain.