Synthesis and characterization of 4-methoxy-7-nitroindolinyl-D-aspartate, a caged compound for selective activation of glutamate transporters and N-methyl-D-aspartate receptors in brain tissue.
The D-isomer of aspartate is efficiently transported by high-affinity Na(+)/K(+)-dependent glutamate transporters and is an effective ligand of N-methyl-d-aspartate (NMDA) receptors. To facilitate analysis of the regulation of these proteins in their native membranes, we synthesized a photolabile analogue of D-aspartate, 4-methoxy-7-nitroindolinyl-D-aspartate (MNI-D-aspartate). This compound was photolyzed with a quantum efficiency of 0.09 at pH 7.4. Photorelease of d-aspartate in acute hippocampal slices through brief (1 ms) UV laser illumination of MNI-d-aspartate triggered rapidly activating currents in astrocytes that were inhibited by the glutamate transporter antagonist DL-threo-beta-benzyloxyaspartic acid (TBOA), indicating that they resulted from electrogenic uptake of D-aspartate. These transporter currents exhibited a distinct tail component that was approximately 2% of the peak current, which may result from the release of K(+) into the extracellular space during counter transport. MNI-D-aspartate was neither an agonist nor an antagonist of glutamate transporters at concentrations up to 500 muM and was stable in aqueous solution for several days. Glutamate transporter currents were also elicited in Bergmann glial cells and Purkinje neurons of the cerebellum in response to photolysis of MNI-D-aspartate, indicating that this compound can be used for monitoring the occupancy and regulation of glutamate transporters in different brain regions. Photorelease of D-aspartate did not activate alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptors or metabotropic glutamate receptors (mGluRs) in neurons, but resulted in the selective, but transient, activation of NMDA receptors in hippocampal pyramidal neurons; MNI-D-aspartate was not an antagonist of NMDA receptors. These results indicate that MNI-D-aspartate also may be useful for studying the regulation of NMDA receptors at excitatory synapses.