Two distinct mechanisms, differentially affected by excitatory amino acids, trigger GABA release from fetal mouse striatal neurons in primary culture

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Abstract
The mechanisms leading to Ca2+-dependent and Ca2+-independent GABA release were studied on highly purified striatal neurons developed in primary culture. Ca2+-dependent GABA release, which represents about 75% of the 56 mM K+ effect was totally inhibited when striatal neurons were first exposed to tetanus toxin (TnTx) (10 micrograms/ml) for 24 hr. The K+ effect was potentiated when 1 mM nipecotic acid (an inhibitor of the GABA uptake system) was added during the stimulation period or when Na+ was replaced by Li+. However, no difference in the GABA release measured under high-K+ conditions was observed after a 22 min preincubation of the neurons in a medium containing nipecotic acid or Li+. Replacement of Cl- ions by SO4(2-) did not modify K+-evoked GABA release. Ca2+-independent GABA release was stimulated by veratridine (20 microM), ouabain (3 mM), and monensin (20 microM), as well as the excitatory amino acids glutamate (100 microM), N-methyl-D- aspartate (100 microM), quisqualate (10 microM), and kainate (1 mM), drugs known to increase intracellular Na+ concentration. The veratridine- or glutamate-evoked GABA release was neither inhibited when intracellular Ca2+ content was reduced by more than 90% nor by treatment of the neurons to TnTx. However, the Ca2+-independent GABA release elicited by veratridine was inhibited by preincubation of the neurons in a medium containing 1 mM nipectotic acid and in a medium containing Li+ instead of Na+ or SO4(2-) instead of Cl-. These results strongly suggest that 2 different GABA release mechanisms exist in striatal neurons.(ABSTRACT TRUNCATED AT 250 WORDS)