Authors: M D’Amico I Samengo Maria Martire
Publish Date: 2009/09/25
Volume: 117, Issue: 1, Pages: 27-
Abstract
We analyzed the effects of extracellular pH reductions on the release of 3Hdaspartate 3HdASP and 3Hnoradrenaline 3HNA from cerebrocortical synaptosomes isolated from rats Synaptosomes were superfused with standard medium at a physiologic pH of 74 and with acidified medium with a pH of 600 550 or 50 Medium acidification produced pHdependent stimulation of 3HdASP release The increase amounted to 202 ± 126 when the pH was reduced to 55 The 3HdASP release evoked by low pH 550 was still observed in the absence of Ca2+ ions but it was abolished by dlthreoβbenzyloxyaspartate dlTBOA 100 μM which inhibits neuronal glutamate/aspartate transport Exposure to 5NNhexamethyleneamiloride EIPA 30–100 μM a selective inhibitor of Na+/H+ exchange caused concentrationdependent stimulation of 3HdASP release the increase observed with EIPA 30 μM was 160 ± 12 The EIPAinduced release was not dependent on the presence of Ca2+ ions in the medium but it was abolished when synaptosomes were pretreated with 100 μM dlTBOA Reduction of the extracellular pH 550–50 also stimulated the release of 3HNA from rat cortical synaptosomes Exposure to medium with a pH of 550 increased basal release by 136 ± 95 The releasestimulating effect of this medium was calciumindependent and abolished by 3 μM desipramine DMI 3HNA release was also stimulated by EIPA The increase induced by a concentration of 30 μM amounted to 136 ± 950 and this effect was calciumindependent and abolished by pretreatment with DMI 3 μM These findings suggest that reduction of the extracellular pH stimulates release of 3HDASP and 3HNA by reversing neurotransmitter transport in the nerve terminal This reversal might be activated by increased cytosolic concentrations of the transmitters caused by reduction of the pH gradient between the cytoplasm and the synaptic vesicles that take up the transmitters This hypothesis is confirmed by the results of experiments conducted with EIPA Selective blockade of Na+/H+ exchange with this compound induces accumulation of H+ in the nerve terminals and intracellular acidification which leads to calciumindependent transportermediated release of 3HdASP and 3HNA
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