The activity of a glutamate uptake inhibitor, L-trans-pyrrolidine-2,4-dicaboxylate, is attenuated by low Ca²⁺ in astrocytes

Levels of extracellular glutamate were determined, after activation with the excitatory amino acid (EAA) receptor agonists N-methyl-D-aspartate (NMDA) and kainate, in cultured cerebellar granule neurons and astrocytes. Both NMDA and kainate elevated extracellular glutamate concentration in neurons but not in astrocytes after incubation for 1 h. Most applied glutamate (lOuM) was taken up (90%) into astrocytes but partially taken up (30%) into neurons without the glutamate uptake inhibitor L-t-pyrrolidine-2, 4-dicarboxylic acid (PDC). The effect of Ca²⁺ on glutamate re-uptake with PDC was determined after applying 10 μM glutamate by measuring the residual concentration of glutamate in physiological concentration of Ca²⁺ buffer and low-Ca²⁺ buffer. The glutamate uptake was strongly blocked by PDC and the blocking effect of PDC was not changed by low-Ca²⁺ buffer in neurons but was attenuated in astrocytes. These results show that PDC was able to block glutamate uptake both in neurons and astrocytes. However, the activity of PDC was three times less potent with low-Ca²⁺ than physiological concentrations of Ca²⁺ in astrocytes while the activity of PDC was not affected by Ca²⁺ concentration in the neurons.