Pharmacological characterization of the rat metabotropic glutamate receptor type 8a revealed strong similarities and slight differences with the type 4a receptor.

In the brain, group-III metabotropic glutamate (mGlu) receptors mGlu(4), mGlu(7) and mGlu(8) receptors play a critical role in controlling the release process at many glutamatergic synapses. The pharmacological profile of mGlu(4) receptor has been studied extensively, allowing us to propose a pharmacophore model for this receptor subtype. Surprisingly, the activity of only a few compounds have been reported on mGlu(7) and mGlu(8) receptors. In order to identify new possibilities for the design of selective compounds able to discriminate between the members of the group-III mGlu receptors, we have undertaken a complete pharmacological characterization of mGlu(8) receptor and compared it with that of mGlu(4) receptor, using the same expression system, and the same read out. The activities of 32 different molecules revealed that these two mGlu receptors subtypes share a similar pharmacological profile. Only small differences were noticed in addition to that previously reported with S-carboxyglutamate (S-Gla) being a partial agonist at mGlu(4) receptor and a full antagonist at mGlu(8) receptor. These include: a slightly higher relative potency of the agonists 1S,3R and 1S,3S-aminocyclopentane-1,3-dicarboxylic acid (ACPD), S-4-carboxyphenylglycine (S-4CPG) and S-4-carboxy-3-hydroxyphenylglycine (S-4C3HPG), and a slightly higher potency of the antagonists 2-aminobicyclo[3.1.0]hexane-2, 6-dicarboxylic acid (LY354740) and RS-alpha-methyl-4-phosphonophenylglycine (MPPG) on mGlu(8) receptor. When superimposed on the mGlu(4) receptor pharmacophore model, these molecules revealed three regions that may be different between the ligand binding sites of mGlu(8) and mGlu(4) receptors.