ARACHIDONIC-ACID INHIBITS UPTAKE OF AMINO-ACIDS AND POTENTIATES PKC EFFECTS ON GLUTAMATE, BUT NOT GABA, EXOCYTOSIS IN ISOLATED HIPPOCAMPAL NERVE-TERMINALS

Arachidonic acid (AA), the putative retrograde messenger in long-term potentiation, enhanced extracellular aspartate, glutamate, and GABA le vels in rat hippocampus synaptosomes. Whether this effect was determin ed by stimulating the release and/or inhibiting the uptake of amino ac ids was further investigated using different experimental conditions. To approach physiological conditions, a static incubation assay was us ed where both release and uptake occur. Under these conditions, AA dos e-dependently (10-25 mu M) enhanced basal extracellular amino acid lev els in a completely Ca2+-independent way. AA still exerted this effect in the presence of inhibitors of PKC or of AA metabolism. When using the superfusion release assay, in which amino acid uptake cannot occur , no potentiating effect of AA on superfusate amino acid levels was ob served. Therefore, AA possibly enhances the extracellular levels of as partate, glutamate and GABA by inhibiting the uptake of these amino ac ids and not their efflux. Indeed, AA reduced the Na+-dependent uptake of endogenously released amino acids, which were labelled with traces of tritiated D-aspartate and GABA. When stimulating hippocampus synapt osomes with 4-aminopyridine, AA (2 mu M) potentiated the Ca2+-dependen t release of glutamate, but not of GABA, synergistically with PKC acti vation by 4 beta-phorbol-12,13-dibutyric acid. In rat hippocampus, AA exerts different presynaptic effects to regulate extracellular amino a cid levels, by inhibiting carrier-mediated uptake and, for glutamate, by stimulating exocytosis. (C) 1997 Elsevier Science B.V.