A2a Adenosine Receptor Mediates PKA-Dependent Glutamate Release from Synaptic-like Vesicles and Ca2+ Efflux from an IP3- and Ryanodine-Insensitive Intracellular Calcium Store in Astrocytes
Background/Aims: The mechanism underlying transmitter release from astrocytes is not fully understood. The present study examined A2a adenosine receptor-mediated glutamate release and intracellular Ca2+ rise in cultured rat hippocampal astrocytes. Methods: Intracellular amino acids were measured with HPLC. Glutamate release from astrocytes and intracellular Ca2+ mobilizations were monitored in the NADH imaging, FM1-43 imaging, and fura-2 imaging. The siRNA to silence the A2a adenosine receptor-targeted gene was constructed and transfected into cells. Results: Glutamate was condensed in ‘synaptic-like vesicle’ fractions. In the NADH imaging, CGS21680, an agonist of A2a adenosine receptors, increased NADH fluorescent signals, that reflects glutamate release, and the effect was inhibited by DMPX, an inhibitor of A2a adenosine receptors, H-89, a PKA inhibitor, vesicular transport inhibitors, or botulinum toxin-A, an exocytosis inhibitor. In the FM1-43 imaging to see vesicular recycling, CGS21680 decreased FM1-43 fluorescent signals, that was also prevented by DMPX, H-89, vesicular transport inhibitors, or botulinum toxin-A. CGS21680 increased intracellular Ca2+ concentrations both in Ca2+-containing and -free extracellular solution. The Ca2+ rise was inhibited by DMPX, H-89, or the vesicular transport inhibitor brefeldin A, but it was not affected by inhibitors for phospholipase C, IP3 receptor, and ryanodine receptor. CGS21680-induced glutamate release and intracellular Ca2+ rise were prevented by knocking-down A2a adenosine receptor. Conclusion: The results of the present study show that A2a adenosine receptor/PKA promotes glutamate release from synaptic-like vesicles and stimulates Ca2+ efflux from an IP3- and ryanodine-insensitive intracellular calcium store.