Differential Calcium Dependence of γ‐Aminobutyric Acid and Acetylcholine Release in Mouse Brain Synaptosomes

The dependence of γ‐aminobutyric acid (GABA) and acetylcholine (ACh) release on Ca 2+ was comparatively studied in synaptosomes from mouse brain, by correlating the influx of 45 Ca 2+ with the release of the transmitters. It was observed that exposure of synaptosomes to a Na + ‐free medium notably increases Ca 2+ entry, and this condition was used, in addition to K + depolarization and the Ca 2+ ionophore A23187, to stimulate the influx of Ca 2+ and the release of labeled GABA and ACh. The effect of ruthenium red (RuR) on these parameters was also investigated. Of the three experimental conditions used, the absence of Na + in the medium proved to be the most efficient in increasing Ca 2+ entry. RuR inhibited by 60–70% the influx of Ca 2+ stimulated by K + depolarization but did not affect its basal influx or its influx stimulated by the absence of Na + or by A23187. The release of ACh was stimulated by K + depolarization, absence of Na + in the medium, and A23187 in a strictly Ca 2+ ‐dependent manner, whereas the release of GABA was only partially dependent on the presence of Ca 2+ in the medium. The extent of stimulation of ACh release was related to the extent of Ca 2+ entry, whereas no such correlation was observed for GABA. In the presence of Na + , RuR did not affect the release of the transmitters induced by A23187. In the absence of Na + , paradoxically RuR notably enhanced the release of both ACh and GABA induced by A23187, in a Ca 2+ ‐dependent manner. These results are discussed in terms of a possible A23187‐induced inward transport of RuR into the synaptosomes, which would result in an inhibition of the trapping of Ca 2+ by intraterminal stores. It is concluded that Na + participates in the regulation of the permeability to Ca 2+ in synaptosomes. The release of ACh requires an increase in the concentration of cytosolic Ca 2+ , whereas a large proportion of GABA can be released, in addition to this Ca 2+ ‐dependent mechanism, by a different one that does not require this cation. The possibility that such mechanism is related to Na + is discussed.