Figure 6.

Ca²⁺ elevations in astrocytes evoked NMDAR-mediated SICs in pyramidal neurons. A, Infrared differential interference contrast image. Note the recording pipettes to the right of the astrocyte in the stratum oriens (top) and to the left of the neuron in the pyramidal layer (bottom). Scale bar, 25 μm. B, Representative EPSC and SIC recorded from a CA1 pyramidal neuron. Note their different time courses. C, Mean frequency of a spontaneous SIC recorded for 3-10 min in an Mg²⁺-free condition (control) and in 1 μm TTX, 10 μm CNQX, 50 μm AP-5, or 2 mm Mg²⁺ (n ≥ 5 for each bar). D, Representative Ca²⁺ levels (top traces) and whole-cell currents (bottom traces) simultaneously recorded in astrocytes and neurons, respectively. As in 69.2 ± 4.8% of the cases, the neuronal SIC (expanded in the bottom trace) occurred in parallel with a Ca²⁺ elevation in an adjacent astrocyte. A concentration of 50 μm AP-5 abolished the SIC but did not affect astrocytic Ca²⁺ elevations (n=5). E, Representative astrocyte Ca²⁺ signal and neuronal current obtained from paired whole-cell recordings in controls and after astrocyte stimulation (top horizontal bar) with 180 mV depolarizing pulses (duration, 100-500 ms) delivered every 1 s. The asterisks indicate neuronal SICs. Some SICs have been truncated, and three are expanded in the bottom traces. Note the frequency increase of both astrocyte Ca²⁺ elevation and neuronal SIC after astrocyte stimulation. F, Mean frequency of astrocytic Ca²⁺ elevations and neuronal SICs recorded spontaneously (□) or during astrocytic depolarization (▪) in control conditions or in the presence of 20 mm BAPTA in the recording pipette of the astrocyte (n = 6). Significant differences from control values were established by the Student's t test at ^**p < 0.01 and ^#p < 0.001. G, Plot of the neuronal SIC amplitude versus the fluorescence amplitude recorded in the soma of the astrocyte (n = 39 from 6 pairs of cells). The fluorescence amplitude was normalized to the highest amplitude for each astrocyte. Only SICs that coincided with Ca²⁺ elevations were considered. Points were fitted to a linear regression (straight line; r² = 0.20).