New phenylglycine derivatives with potent and selective antagonist activity at presynaptic glutamate receptors in neonatal rat spinal cord

The depression of the monosynaptic excitation of neonatal rat motoneurones produced by the metabotropic glutamate receptor (mGluR) agonists (1S,3S)-1-aminocyclopentane-1, 3-dicarboxylate (ACPD) or L-2-amino-4-phosphonobutyrate (L-AP4) was antagonized by three novel phenylglycine analogues: (RS)-alpha-methyl-4-sulphonophenylglycine (MSPG), (RS)-alpha-methyl-4-phosphonophenylglycine (MPPG) and (RS)-alpha-methyl-4-tetrazolylphenylglycine (MTPG). The potencies of all the new compounds were greater than that of the previously reported (RS)-alpha-methyl-4-carboxyphenylglycine (MCPG). For L-AP4-sensitive presynaptic mGluRs, the order of antagonist potency found was MPPG > MSPG > MTPG > MCPG. In contrast, the order of antagonist potency found for (1S,3S)-ACPD-sensitive presynaptic mGluRs was MTPG > MPPG > MSPG > MCPG. To date, MPPG (KD 9.2 microM) is the most potent L-AP4-sensitive receptor antagonist yet tested on the neonatal rat spinal cord. In addition, MTPG (KD 77 microM) is the most potent antagonist yet tested for (1S,3S)-ACPD-sensitive receptors in this preparation.