Regulation of carrier-mediated and exocytotic release of [3H]GABA in rat brain synaptosomes.

In this study we investigated the role of external monovalent cations, and of intracellular Ca2+ concentration ([Ca2+]i) in polarized and depolarized rat cerebral cortex synaptosomes on the release of [3H]-gamma-aminobutyric acid (3H-GABA). We found that potassium-depolarization, in the absence of Ca2+, of synaptosomes loaded with 3H-GABA releases 7.4 +/- 2.1% of the accumulated neurotransmitter, provided that the external medium contains Na+, and an additional 19.0 +/- 2.5% is released upon adding 1.0 mM CaCl2 to the exterior. The Ca(2+)-independent release component does not occur in a choline medium and it is only 3.4 +/- 0.8% of the 3H-GABA accumulated in a Li+ medium, but both ions support the Ca(2+)-dependent release of 3H-GABA (13.4 +/- 0.6% in choline and 15.4 +/- 1.5% in Li+), which suggests that the exocytotic release is independent of the external monovalent cation present, whereas the carrier-mediated release specifically requires Na+ outside. Furthermore, previous release of the cytosolic 3H-GABA due to predepolarization in the absence of Ca2+ does not influence the amount of 3H-GABA subsequently released by exocytosis due to Ca2+ addition (19.1 +/- 2.5% or 19.1 +/- 1.1%, respectively). In choline or Li+ medium, the value of the [Ca2+]i is raised by Na+/Ca2+ exchange to 663 +/- 75 nM or 782 +/- 54 nM, respectively, within three minutes after adding 1.0 mM Ca2+, in the absence of depolarization, and parallel release experiments show no release of 3H-GABA in the choline medium, but a substantial release (7.1 +/- 2.1%) of 3H-GABA occurs in the Li+ medium without depolarization. Subsequent K(+)-depolarization shows normal Ca(2+)-dependent release of 3H-GABA in the choline medium (14.1 +/- 2.0%) but only 8.6 +/- 1.1% release in the Li+ medium, which suggests that raising the [Ca2+]i by Na+/Ca2+ exchange, without depolarization, supports some exocytotic release in Li+, but not in choline media. The role of [Ca2+]i and of membrane depolarization in the release process is discussed on the basis of the results obtained and other relevant observations which suggest that both Ca2+ and depolarization are essential for optimal exocytotic release of GABA.