GABA uptake-dependent Ca ²⁺ signaling in developing olfactory bulb astrocytes

<jats:p> We studied GABAergic signaling in astrocytes of olfactory bulb slices using confocal Ca <jats:sup>2+</jats:sup> imaging and two-photon Na <jats:sup>+</jats:sup> imaging. GABA evoked Ca <jats:sup>2+</jats:sup> transients in astrocytes that persisted in the presence of GABA <jats:sub>A</jats:sub> and GABA <jats:sub>B</jats:sub> receptor antagonists, but were suppressed by inhibition of GABA uptake by SNAP 5114. Withdrawal of external Ca <jats:sup>2+</jats:sup> blocked GABA-induced Ca <jats:sup>2+</jats:sup> transients, and depletion of Ca <jats:sup>2+</jats:sup> stores with cyclopiazonic acid reduced Ca <jats:sup>2+</jats:sup> transients by approximately 90%. This indicates that the Ca <jats:sup>2+</jats:sup> transients depend on external Ca <jats:sup>2+</jats:sup> , but are mainly mediated by intracellular Ca <jats:sup>2+</jats:sup> release, conforming with Ca <jats:sup>2+</jats:sup> -induced Ca <jats:sup>2+</jats:sup> release. Inhibition of ryanodine receptors did not affect GABA-induced Ca <jats:sup>2+</jats:sup> transients, whereas the InsP <jats:sub>3</jats:sub> receptor blocker 2-APB inhibited the Ca <jats:sup>2+</jats:sup> transients. GABA also induced Na <jats:sup>+</jats:sup> increases in astrocytes, potentially reducing Na <jats:sup>+</jats:sup> /Ca <jats:sup>2+</jats:sup> exchange. To test whether reduction of Na <jats:sup>+</jats:sup> /Ca <jats:sup>2+</jats:sup> exchange induces Ca <jats:sup>2+</jats:sup> signaling, we inhibited Na <jats:sup>+</jats:sup> /Ca <jats:sup>2+</jats:sup> exchange with KB-R7943, which mimicked GABA-induced Ca <jats:sup>2+</jats:sup> transients. Endogenous GABA release from neurons, activated by stimulation of afferent axons or NMDA application, also triggered Ca <jats:sup>2+</jats:sup> transients in astrocytes. The significance of GABAergic Ca <jats:sup>2+</jats:sup> signaling in astrocytes for control of blood flow is demonstrated by SNAP 5114-sensitive constriction of blood vessels accompanying GABA uptake. The results suggest that GABAergic signaling is composed of GABA uptake-mediated Na <jats:sup>+</jats:sup> rises that reduce Na <jats:sup>+</jats:sup> /Ca <jats:sup>2+</jats:sup> exchange, thereby leading to a Ca <jats:sup>2+</jats:sup> increase sufficient to trigger Ca <jats:sup>2+</jats:sup> -induced Ca <jats:sup>2+</jats:sup> release via InsP <jats:sub>3</jats:sub> receptors. Hence, GABA transporters not only remove GABA from the extracellular space, but may also contribute to intracellular signaling and astrocyte function, such as control of blood flow. </jats:p>