Dicarboxyphenylglycines antagonize AMPA- but not kainate-induced depolarizations in neonatal rat motoneurones

Ionotropic glutamate receptors have been categorized into three main groups according to the selective agonists that activate them, the N-methyl-D-aspartate (NMDA), (S)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propanoic acid (AMPA) and (2S,3S,4S)-3-carboxymethyl-4-isopropenylpyrrolidine-2-carboxylic acid (kainate) receptors. Both AMPA and kainate induce depolarizations in neonatal rat spinal motoneurones. However, selective antagonists capable of discriminating between the effects of these two antagonists are not widely available. As part of a search for such compounds we report the actions of (RS)-3,4-dicarboxyphenylglycine (DCPG) and (RS)-3,5-dicarboxyphenylglycine on agonist-induced motoneuronal depolarizations in the neonatal rat spinal cord preparation. In addition, the actions of (R)- and (S)-3,4-DCPG are also described. (RS)-3,4-DCPG and (RS)-3,5-DCPG antagonized AMPA-induced depolarizations (apparent Kd = 137 microM (n = 3) and 167 microM (n = 5), respectively). However, (RS)-3,5-DCPG (1 mM) potentiated responses due to kainate (n = 5) while (RS)-3,4-DCPG (1 mM) displayed weak antagonism of these responses (apparent Kd > 12 mM, n = 3). (RS)-3,4- and (RS)-3,5-DCPG at 500 microM both displayed antagonism at the NMDA Receptor (apparent Kd = 472 microM and 346 microM, respectively) and a postsynaptic subgroup I metabotropic glutamate receptor activated by (1S,3R)-ACPD. The AMPA Receptor Antagonist activity of (RS)-3,4-DCPG was shown to reside in the (R)-enantiomer (apparent Kd = 77 microM, n = 3). The same isomer was responsible for the NMDA Receptor antagonism while showing little or no antagonism of kainate-induced depolarizations (apparent Kd > 3 mM, n = 3), and a weak antagonistic effect at (1S,3R)-ACPD receptors. (S)-3,4-DCPG (500 microM) was unable to antagonize kainate-induced depolarizations, showed weak or no antagonism of NMDA- and AMPA-induced depolarizations, but antagonized (1S,3R)-ACPD-induced depolarizations. Thus (RS)-3,4-, (RS)-3,5- and (R)-3,4-DCPG demonstrate useful discrimination of responses due to AMPA and kainate, strongly suggesting that pharmacologically distinct AMPA and kainate receptors exist in motoneurones in the neonatal rat spinal cord.