Excitatory amino acid-evoked release of [ 3H]GABA from hippocampal neurons in primary culture

Abstract

We investigated the release of γ-[2,3- 3H(N)]aminobutyric acid ([ 3H]GABA) from hippocampal neurons in primary cell culture. [ 3H]GABA release was stimulated by the excitatory amino acid neurotransmitter glutamate as well as by N-methyl- D-aspartate (NMDA) and kainate. Cell depolarization induced by raising [K +] 0 or by veratridine also stimulated [ 3H]GABA release. NMDA-induced release was completely blocked by 3-((±)-car☐ypiperazin-4yl)-propyl-1-phosphonic acid (CPP +), Mg 2+ and Zn 2+ whereas the release induced by glutamate and kainate was much less susceptible to inhibition by these substances. Furthermore, removal of external Ca 2+ inhibited NMDA-induced release, but not that induced by glutamate, kainate, veratridine or 50 mM K +. Removal of external Na + reduced [ 3H]GABA release evoked by all stimuli, but to different extents. All of the excitatory amino acids tested increased [Ca 2+] i within hippocampal neurons as assessed by fura-2 based microspectrofluorimetry. This increase in [Ca 2+] i was completely dependent on the presence of external Ca 2+. These results suggest that Ca 2+-dependent and -independent forms of GABA release from hippocampal interneurons may occur. [ 3H]GABA release evoked by glutamate, kainate, veratridine or 50 mM K +, appeared to be mediated by the reversal of electrogenic, Na +-coupled GABA uptake. Release was inhibited by nipecotic acid, an inhibitor of the Na +-coupled GABA uptake system. However, release induced by NMDA may also include a Ca 2+-dependent component.