Group III mGlu Receptor Antagonists Of the three groups of mGlu receptors, the group III receptors have been less studied. Hence fewer selective antagonists exist for these receptors. MAP4 and MSOP, the a-methyl derivatives of the group III receptor agonists, L-AP4 and L-SOP, are moderately potent competitive antagonists of these receptors, although MAP4 may actually have agonist activity at mGlu4 and mGlu6 receptors. In addition, one report has demonstrated significant agonist activity at mGlu2 receptors for both MSOP and MAP4. However, in the neonatal rat spinal cord assay, neither MAP4 or MSOP antagonised group II mGlu receptor-mediated depression of synaptic transmission. Two analogues of (S)-MCPG, MPPG and CPPG, have also been shown to be antagonists of the group III receptors with a higher potency and greater selectivity than MAP4 or MSOP. However, data on the ability of these compounds to antagonise individual receptor subtypes is scarce.