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

(+/-)-1-Aminocyclopentane-trans-1,3-dicarboxylic acid (t-ACPD) is an equimolar mixture of two enantiomers: (1S,3R)-1-Aminocyclopentane-1,3-dicarboxylic acid (SR-ACPD) and 1R,3S-1-Aminocyclopentane-1,3-dicarboxylic acid (RS-ACPD). t-ACPD and SR-ACPD have been commonly used as agonists for metabotropic 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 m_M and V_max of 22.8 ± 3.4 nmol/mg/min. This low-affinity transport is Na⁺-dependent and is competitively blocked by glutamate or other substrates for the glutamate transporter. RS-ACPD therefore is probably taken up by the glutamate transporter. Prolonged incubation with high levels of RS-ACPD (> 500 μM) induced significant swelling of astrocytes. At lower concentrations (100 μM), RS-ACPD reduced intracellular glutamate content ([Glu]_i) by > 50% without obvious morphological changes. The reduction in [Glu]_i was accompanied by an increase in [glutamine]_i. The RS-ACPD's effect on [Glu]_i required 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 intracellular glutamate and glutamine. By reducing [Glu]_i, RS-ACPD reduces the amount of glutamate available for release.