[3H]R214127: A Novel High-Affinity Radioligand for the mGlu1 Receptor Reveals a Common Binding Site Shared by Multiple Allosteric Antagonists

R214127 was shown to be a potent and noncompetitive metabotropic glutamate 1 (mGlu1) receptor-selective antagonist. The kinetics and pharmacology of [ 3 H]1-(3,4-dihydro-2H-pyrano[2,3- b ]quinolin-7-yl)-2-phenyl-1-ethanone (R214127) binding to rat mGlu1a receptor Chinese hamster ovary (CHO)-dhfr − membranes was investigated, as well as the distribution of [ 3 H]R214127 binding in rat brain tissue and sections. Specific binding to rat mGlu1a receptor CHO-dhfr − membranes was ∼92% of total and was optimal at 4°C. Full association was reached within 5 min, and [ 3 H]R214127 bound to a single binding site with an apparent K D of 0.90 ± 0.14 nM and a B max of 6512 ± 1501 fmol/mg of protein. Inhibition experiments showed that [ 3 H]R214127 binding was completely blocked by 2-quinoxaline-carboxamide- N -adamantan-1-yl (NPS 2390), (3a S ,6a S )-6a-naphtalan-2-ylmethyl-5-methyliden-hexahydro-cyclopenta[ c ]furan-1-on (BAY 36-7620), and 7-(hydroxyimino)cyclo-propa[ b ]chromen-1a-carboxylate ethyl ester (CPCCOEt), but was not displaced by competitive mGlu1 receptor ligands such as glutamate and quisqualate, suggesting that R214127, NPS 2390, BAY 36-7620, and CPCCOEt bind to the same site or mutually exclusive sites. Experiments using rat cortex, striatum, hippocampus and cerebellum revealed that [ 3 H]R214127 labeled a single high-affinity binding site ( K D ∼ 1 nM). B max values were highest in the cerebellum (4302 ± 2042 fmol/mg of protein) and were 741 ± 48, 688 ± 125, and 471 ± 68 fmol/mg of protein in the striatum, hippocampus, and cortex, respectively. The distribution of [ 3 H]R214127 binding in rat brain was investigated in more detail by radioligand autoradiography. A high density of binding sites was detected in the molecular layer of the cerebellum. Moderate labeling was seen in the CA3 and dentate gyrus of the hippocampus, thalamus, olfactory tubercle, amygdala, and substantia nigra reticulata. The cerebral cortex, caudate putamen, ventral pallidum, and nucleus accumbens showed lower labeling. The high affinity and selectivity of [ 3 H]R214127 for mGlu1 receptors renders this compound the ligand of choice to study the native mGlu1 receptor in brain.