Stimulation-Evoked Ca²⁺ Signals in Astrocytic Processes at Hippocampal CA3–CA1 Synapses of Adult Mice Are Modulated by Glutamate and ATP

Abstract

To date, it has been difficult to reveal physiological Ca²⁺ events occurring within the fine astrocytic processes of mature animals. The objective of the study was to explore whether neuronal activity evokes astrocytic Ca²⁺ signals at glutamatergic synapses of adult mice. We stimulated the Schaffer collateral/commissural fibers in acute hippocampal slices from adult mice transduced with the genetically encoded Ca²⁺ indicator GCaMP5E driven by the glial fibrillary acidic protein promoter. Two-photon imaging revealed global stimulation-evoked astrocytic Ca²⁺ signals with distinct latencies, rise rates, and amplitudes in fine processes and somata. Specifically, the Ca²⁺ signals in the processes were faster and of higher amplitude than those in the somata. A combination of P2 purinergic and group I/II metabotropic glutamate receptor (mGluR) antagonists reduced the amplitude of the Ca²⁺ transients by 30–40% in both astrocytic compartments. Blockage of the mGluRs alone only modestly reduced the magnitude of the stimulation-evoked Ca²⁺ signals in processes and failed to affect the somatic Ca²⁺ response. Local application of group I or I/II mGluR agonists or adenosine triphosphate (ATP) elicited global astrocytic Ca²⁺ signals that mimicked the stimulation-evoked astrocytic Ca²⁺ responses. We conclude that stimulation-evoked Ca²⁺ signals in astrocytic processes at CA3–CA1 synapses of adult mice (1) differ from those in astrocytic somata and (2) are modulated by glutamate and ATP.