Positive allosteric modulation of the human metabotropic glutamate receptor 4 (hmGluR4) by SIB-1893 and MPEP

We have identified 2-methyl-6-(2-phenylethenyl)pyridine (SIB-1893) and 2-methyl-6-phenylethynyl pyridine hydrochloride (MPEP) as positive allosteric modulators for the hmGluR4. SIB-1893 and MPEP enhanced the potency and efficacy Of L-2-amino-4-phophonobutyrate (L-AP4) in guanosine 5'-O-(3-[S-35]thiotriphosphate ([S-35]GTPgammaS) binding and efficacy in cAMP studies. These effects were fully blocked by the mGluR4 competitive antagonist (RS)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG), indicating a dependency on receptor activation. Although SIB-1893 and MPEP had no effects alone in GTPTS binding, effects were observed in the cell-based cAMP assay due to mediaderived activation as indicated by CPPG inhibition. Positive modulation of the mGluR4 was a receptor-specific effect since SIB-1893 and MPEP had neither effects on mGluR2-expressing cells nor on the parent BHK cell line. In [H-3]L-AP4 binding, a two-fold decrease in K-D but not in B-max was observed with 100 mum SIB-1893, whereas MPEP affected neither parameter. Finally, SIB-1893 and MPEP failed to displace [H-3]L-AP4 binding. Taken together, these data identify positive allosteric modulators for the hmGluR4.