Extended Data Figure 3: Physiological properties of GCaMP6m-expressing CA3 neurons.

a, To ensure that expression of GCaMP6m did not alter Ca²⁺-related physiological processes, we tracked a form of endocannabinoid-mediated short-term plasticity known as depolarization-induced suppression of inhibition (DSI). Schematic diagram of DSI is shown; DSI is dependent on the increase of postsynaptic intracellular Ca²⁺ to trigger the synthesis and release of endocannabinoids, which then signal in a retrograde fashion to suppress GABA release from presynaptic inhibitory neurons expressing cannabinoid receptors (adapted with permission from ref. 65). Intrinsic membrane properties of the GCaMP6m-expressing CA3 cells were similar to previously reported values for CA3 (ref. 66); mean resting potential: −72.1 ± 1.6 mV; mean input resistance: 161.8 ± 26.4 ΜΩ; n = 7. b, Sample trace illustrating DSI of sIPSCs in a GCaMP6m-expressing CA3 cell following application of a depolarizing current step (from −65 mV to 0 mV for 500 ms). c, Sample trace illustrating lack of DSI of sIPSCs with inclusion of the intracellular calcium chelator BAPTA (1,2-bis(o-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid) in the patch pipette. d, Summary graph of normalized charge transfer in GCaMP6m-expressing cells with standard intracellular solution (left, normalized charge transfer of sIPSCs following DSI compared with pre-pulse baseline over the same fixed interval: charge reduced to 46.9 ± 6.7% of baseline charge; n = 7; comparable to charge transfer reported for non-GCaMP-expressing cells⁶⁷) and with addition of intracellular BAPTA (right, n = 6; error bars represent standard error of the mean (s.e.m.); P < 0.05, paired t-test). e, Spontaneous event rate (detection described in Methods) of GCaMP6m-expressing neurons as a function of baseline GCaMP6m fluorescence intensity (arbitrary units spanning the range over which event-rate population data could be reliably quantified) in each cell (pooling all neurons with ≥1 significant transient, from all mice, over all fields of view). Event rates were not observed to change significantly as a function of GCaMP6m expression level (Spearman’s rank correlation coefficient: 0.48, P = 0.3). f, Behavioural scores from mice before GCaMP6m virus injection and implantation of cannulae above hippocampus; lick rates for the first 2 min in the fear (black) versus neutral (grey) contexts are provided. The level of learning assessed by lick suppression on day 3 retrieval (mean 0.5 ± 0.3 for day 3 fear versus 2.7 ± 0.3 for day 3 neutral; n = 10, P < 0.001, paired t-test) pre-injection/implantation was comparable to levels corresponding to post-injection/implantation (compare with Fig. 3b). *P < 0.05, **P < 0.01, ***P < 0.001.