(A) Anatomical location of neurobiotin-filled CA1PNs recorded from acute horizontal slices in voltage-clamp experiments. DG: dentate gyrus, Sub: subiculum, SR: stratum radiatum, SP: stratum pyramidale, SO: stratum oriens. (B1–B2) Representative traces of spontaneous excitatory synaptic currents (s-EPSCs) and inhibitory synaptic currents (s-IPSCs) from CA1PNs located respectively in the deep, middle, and superficial portion of the pyramidal layer (SP). Black circles represent detected synaptic events. Note that the occurrence of s-EPSCs increases from top (superficial) to bottom (deep), whereas no apparent change in s-IPSC frequency can be identified among different locations. (C1) s-EPSC amplitude recorded in E12.5, E14.5, and E16.5 CA1PNs. On the left panel, violin plot of E12.5, E14.5, and E16.5 PNs s-EPSC amplitude (E12.5 vs E14.5, Padj: 0.3228; E12.5 vs E16.5, Padj:0.4012; E14.5 vs E16.5, Padj:0.3228). On the right panel, color-coded scatterplot of the same data plotted against the radial position of each cell. No linear correlation between s-EPSC amplitude and location (p: 0.4495,CI95% [–0.45; –0.44]) was found. (C2) s-EPSC frequency recorded in E12.5, E14.5, and E16.5 CA1PNs. On the left panel, violin plot of the three birthdate groups (E12.5 vs E14.5, Padj:0.3386, E12.5 vs E16.5, Padj:0.4845, E14.5 vs E16.5, Padj:0.4845). On the right panel, scatterplot of the same data against the radial position. S-EPSC frequency increases linearly with the cell location, the two being positively correlated (p: 0.017; CI95% [0.186; 0.710]). The deeper, the more frequent spontaneous excitatory events a cell receives. (D1) s-IPSC amplitude recorded in E12.5, E14.5 and E16.5 CA1PNs. On the left panel, violin plot of the three birthdate groups (E12.5 vs E14.5, Padj:0.8238; E12.5 vs E16.5, Padj>0.999; E14.5 vs E16.5, Padj>0.999). On the right panel, scatterplot of the same data against the radial position. No linear correlation between s-IPSC amplitude and location was found (p: 0.12, CI95% [–0.177; –0.492]). (D2) s-IPSC frequency recorded in E12.5, E14.5, and E16.5 CA1PNs. On the left panel, violin plot of the three birthdate groups (E12.5 vs E14.5, Padj:0.282; E12.5 vs E16.5, Padj:0.4526; E14.5 vs E16.5, Padj:0.1575). On the right panel, scatterplot of the same data against the radial position. No linear correlation between s-IPSC frequency and location was found (p: 0.067, CI95% [–0.019; –0.485]). (E1) Ratio between EPSC and IPSC (E/I) amplitude recorded in E12.5, E14.5, and E16.5 CA1PNs. On the left panel, violin plot of the three birthdate groups. E14.5 PNs display a lower ratio than E12.5 PNs (Padj: 0.010; CI95% [0.031; 0.210]) and E16.5 PNs (Padj: 0.006; CI95% [–0.193; –0.057]), suggesting that their overall synaptic drive leans more toward inhibition than the other two groups (E12.5 vs E16.5, Padj:0.2061). On the right panel, scatterplot of the same data against the radial position. No linear correlation between E/I amplitude ratio and location was found (p:0.4615, CI95% [–0.38; 0.33]). (E2) E/I frequency ratio recorded in E12.5, E14.5, and E16.5 CA1PNs. On the left panel, violin plot of the three birthdate groups (E12.5 vs E14.5, Padj: 0.61, E12.5 vs E16.5; Padj: 0.61; E14.5 vs E16.5, Padj: 0.59). On the right panel, scatterplot of the same data against the radial position. E/I ratio linearly correlates with the cell location, similarly to s-EPSC frequency (p: 0.0043; CI95% [0.126; 0.654]). Violin plots present medians (center), interquartile ranges (bounds), minima and maxima. Color-code: E12.5: light blue, E14.5: dark blue, E16.5: magenta. The gray shaded area in scatterplots represents the thickness of the stratum pyramidale. *p < 0.05.