IN-VITRO BINDING CHARACTERISTICS OF A NEW SELECTIVE GROUP-II METABOTROPIC GLUTAMATE-RECEPTOR RADIOLIGAND, [H-3]LY354740, IN RAT-BRAIN

The in vitro binding of [H-3]LY354740, the first high affinity group I i-selective metabotropic glutamate (mGlu) receptor radioligand, was ch aracterized in rat cortical, hippocampal, and thalamic membranes as we ll as in rat brain sections. [H-3]LY354740 binding was saturable in al l regions investigated. Nonspecific binding (in the presence of 10 mu M DCG-IV) was approximate to 8% of the total. Ionotropic glutamate rec eptor agonists, N-methyl-D-aspartate, (R,S)-amino-3-hydroxy-5-methyl-4 -isoxazole- propionic acid/kainate, a Na+-dependent glutamate uptake b locker as well as a group I-selective mGlu receptor agonist (all up to 1 mM) did not inhibit [H-3]LY354740 binding to cortical membranes. Ho wever, several known metabotropic receptor ligands inhibited the bindi ng 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-ca rboxycyclopropyl)glycine > glutamate = (1S,3R)-1-aminocyclopentane-1,3 -dicarboxylic acid > 1'S,2'S)-2-methyl-2-(2-carboxycyclopropyl)-glycin e > quisqualate > ibotenate > L-2-amino-3-phosphonopropionic acid = (S )-alpha-methyl-4-carboxyphenylglycine > L-(+)-2-amino-4-phosphonobutyr ic acid. N-Acetyl-aspartyl-glutamate, (2S)-alpha-ethylglutamic acid, a nd (R,S)-alpha-methyl-4-phosphonophenylglycine inhibited [H-3]LY354740 binding in a biphasic manner. Guanosine-5'-O-(3-thiotriphosphate conc entration-dependently and almost completely inhibited the binding. Fin ally, in parasagittal sections of rat brain, a high density of specifi c binding was observed in the accessory olfactory bulb, cortical regio ns (layers 1-3 > 4-6), caudate putamen, molecular layers of the hippoc ampus and dentate gyrus, presubiculum, retrosplenial cortex, anteroven tral thalamic nuclei, and cerebellar granular layer, reflecting its pr eferential (perhaps not exclusive) affinity for presynaptic and postsy naptic mGlu2 receptors. Thus, the pharmacology, tissue distribution, a nd sensitivity to guanosine-5'-O-(3-thiotriphosphate show that [H-3]LY 354740 binding probably occurs to group II mGlu receptors in rat brain .