MICRODIALYSIS AS A TOOL FOR IN-VIVO INVESTIGATION OF GLUTAMATE TRANSPORT CAPACITY IN RAT-BRAIN

The role of glutamate as a possible mediator of neurodegeneration is w ell described, and the homeostasis of extracellular glutamate is consi dered of major importance when addressing the pathogenesis of excitato ry neurodegeneration. Applying the 'indicator diffusion' method to the microdialysis technique, we present a method that is suitable for the in vivo investigation of the capacity of cellular uptake of glutamate . Using C-14-mannitol as reference, we measured the cellular extractio n and the cell membrane permeability of the test substance H-3-D-aspar tate in the corpus striatum of the rat brain. The cellular extraction fraction of 3H-D-aspartate was 0.29, and the cell membrane permeabilit y 2.24 x 10(-4) cm/s. In the presence of the glutamate-uptake blocker DL-threo-beta-hydroxyaspartate (THA) the extraction of 3H-D-aspartate was completely abolished, indicating that extraction of 3H-D-aspartate was due to cellular uptake by glutamate transporters. The cell membra ne permeability towards 3H-D-aspartate was reduced by approximate to 9 8% due to THA, indicating that the cell membranes per se are highly re sistent to diffusion of H-3-D-aspartate. It is concluded that the pres ent method can be used in studying the capacity of the glutamate trans porters in vivo.