(1R,3S)-1-aminocyclopentane-1,3-dicarboxylic acid (RS-ACPD) reduces intracellular glutamate levels in astrocytes

(+/-)-1-Aminocyclopentane-trams-1,3-dicarboxylic acid (t-ACPD) is an equimo lar mixture of two enantiomers: (1S,3R)-1-Amino-cyclopentane-1,3-dicarboxyl ic acid (SR-ACPD) and 1R,3S-1-Aminocyclopentane-1,3-dicarboxylic acid (RS-A CPD). t-ACPD and SR-ACPD have been commonly used as agonists for metabotrop ic glutamate receptors (mGluR). Here we demonstrated that RS-ACPD, but not SR-ACPD, is transported into astrocytes with a K-m of 6.51 +/- 2.38 mm and V-max of 22.8 +/- 3.4 nmol/mg/min. This low-affinity transport is Na ' depe ndent and is competitively blocked by glutamate or other substrates for the glutamate transporter. RS-ACPD therefore is probably taken up by the gluta mate transporter. Prolonged incubation with high levels of RS-ACPD (> 500 m uM) induced significant swelling of astrocytes. At lower concentrations (10 0 mum), RS-ACPD reduced intracellular glutamate content ([Glu](i)) by > 50% without obvious morphological changes. The reduction in [Glu], was accompa nied by an increase in [glutamine],. The RS-ACPD's effect on [Glu], require d glutamine and high levels of phosphate, suggesting that RS-ACPD inhibited phosphate-activated glutaminase (PAG). These data suggest that astrocytic PAG is actively involved in determining the equilibrium between intracellul ar glutamate and glutamine. By reducing [Glu] RS-ACPD reduces the amount of glutamate available for release.