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

摘要

We studied GABAergic signaling in astrocytes of olfactory bulb slices using confocal Ca~(2+) imaging and two-photon Na~+ imaging. GABA evoked Ca~(2+) transients in astrocytes that persisted in the presence of GABAa and GABA_B receptor antagonists, but were suppressed by inhibition of GABA uptake by SNAP 5114. Withdrawal of external Ca~(2+) blocked GABA-induced Ca~(2+) transients, and depletion of Ca~(2+) stores with cyclopiazonic acid reduced Ca~(2+) transients by approximately 90%. This indicates that the Ca~(2+) transients depend on external Ca~(2+), but are mainly mediated by intracellular Ca~(2+) release, conforming with Ca~(2+)-induced Ca~(2+) release. Inhibition of ryanodine receptors did not affect GABA-induced Ca~(2+) transients, whereas the lnsP_3 receptor blocker 2-APB inhibited the Ca~(2+) transients. GABA also induced Na~+ increases in astrocytes, potentially reducing Na~+/Ca~(2+) exchange. To test whether reduction of Na~+/Ca~(2+) exchange induces Ca~(2+) signaling, we inhibited Na~+/Ca~(2+) exchange with KB-R7943, which mimicked GABA-induced Ca~(2+) transients. Endogenous GABA release from neurons, activated by stimulation of afferent axons or NMDA application, also triggered Ca~(2+) transients in astrocytes. The significance of GABAergic Ca~(2+) 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~+ rises that reduce Na~+/Ca~(2+) exchange, thereby leading to a Ca~(2+) increase sufficient to trigger Ca~(2+)-induced Ca~(2+) release via lnsP_3 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.