(A) Image of recorded interneuron (yellow) and SR101-labelled astrocytes (red) in CA1 region. Scale bar, 40 µm. (B) Left, astrocyte Ca2+ transient probability over time (zero time corresponds to interneuron depolarization) in resting conditions (white), and after evoking single (red) or bursts (blue) of interneuron APs, and somatic Ca2+ signals from four representative astrocytes in the field of view to single APs (red) and burst of interneuron APs (blue). Red and blue bars denote interneuron stimulation. Right, average values for those conditions. Single APs (red; p=0.92; paired t test), burst of interneuron APs (blue; p=0.007; 58 astrocytes, nine slices). (C) Ca2+ transient probability index after bursts of interneuron APs in control (58 astrocytes, nine slices; p<0.001; Wilcoxon rank-sum test), CGP55845 (91 astrocytes, 10 slices; p=0.824; Wilcoxon rank-sum test), thapsigargin (41 astrocytes, five slices; p=0.37; Wilcoxon rank-sum test), BAPTA-loaded astrocytes (41 astrocytes, nine slices; p=0.2622; Wilcoxon rank-sum test), and Ip3r2-/- mice (112 astrocytes, 14 slices; p=0.4319; Wilcoxon rank-sum test). (D) Left, merge DIC and fluorescence image of the CA1 pyramidal layer and stratum radiatum (s.r.) showing the location of the biocytin-filled astrocyte network (green). Right, maximal projection confocal image of the astrocytic syncytium revealed by biocytin-loading via whole-cell astrocyte recording, showing the distribution of biocytin-coupled astrocytes. Scale bars, 50 µm. Bottom, synaptic responses evoked by minimal stimulation (15 consecutive stimuli; light traces), and averaged EPSCs (50 consecutive stimuli; dark traces) before and after interneuron AP bursts in BAPTA-loaded astrocytes (blue). (E) Synaptic responses evoked by minimal stimulation (15 consecutive stimuli; light traces), and averaged EPSCs (50 consecutive stimuli; dark traces) before and after interneuron AP bursts in control (black) and Ip3r2-/- mice (red). (F) Relative changes of synaptic parameters induced by bursts of interneuron APs (filled bars), in control (n = 6; p<0.001; paired t test), thapsigargin (n = 9), BAPTA-loaded astrocytes (n = 11), and Ip3r2-/- mice (n = 8). See also Figure 3—figure supplement 1. **p<0.01, ***p<0.001; paired t test.