Astrocyte Calcium Signaling in Hippocampal LTP

Whole-cell recordings from passive astrocytes (n = 146) were made in stratum radiatum, area CA1 in acute transverse hippocampal slices prepared from adult rats. Cells (30-100 μm deep inside the slice) were loaded with a bright morphological tracer Alexa Fluor 594 and the high-affinity Ca²⁺ indicator Oregon Green BAPTA (OGB-1) and imaged in two-photon excitation mode (λ^2p_x = 800 nm). Field EPSPs were recorded using either an extracellular recording electrode placed in the immediate vicinity of the visualised astrocyte dendritic arbour or through the astrocytic patch pipette, as described. Whole-cell EPSCs were recorded from CA1 pyramidal cells. Electric stimuli were applied to Schaffer collateral fibres. LTP was induced by a standard high-frequency stimulation protocol (three 100-pulse trains at 100 Hz, 60 or 20 seconds apart). Inside the recorded astrocyte, conditions of Ca²⁺ homeostasis were altered using intracellular solutions containing EGTA, Oregon Green BAPTA-1, and CaCl₂; the exocytosis machinery was suppressed using light-chain tetanus toxin; synthesis of D-serine was inhibited with serine racemase inhibitor L-erythro-3-hydroxyaspartate (HOAsp).