Synaptic defects in Hgsnat-Geo and Hgsnat ^P304L mice. (A–F) Neurotransmission is impaired in Hgsnat-Geo and Hgsnat^P304L mice. Significant decrease in the amplitude (A) and frequency (C) of mEPSCs in Hgsnat-Geo and Hgsnat^P304L mice at the ages of P14–P20 and P45–P60 compared with age-matched WT controls. (B) Representative recordings of mEPSCs from WT, Hgsnat-Geo, and Hgsnat^P304L mice at P14–P20 and P45–P60, and overlay of representative individual mEPSC events from neurons of Hgsnat-Geo, Hgsnat^P304L, and WT mice. Significant decrease in the amplitude (D) and frequency (F) of mIPSCs in Hgsnat-Geo and Hgsnat^P304L mice at the ages of P14–P20 and P45–P60 compared with age-matched WT controls. (E) Representative recording of mIPSCs from neurons of WT, Hgsnat-Geo, and Hgsnat^P304L mice at the ages of P14–P20 and P45–P60, and overlay of representative individual mIPSC events from neurons of Hgsnat-Geo, Hgsnat^P304L, and WT mice. All graphs show individual data, means, and SD of experiments performed with six or more mice per genotype. P values were calculated using one-way Kruskal–Wallis test with Dunn’s multiple comparison post hoc test. (G and H) Reduction of synaptic vesicle densities, areas of PSDs, and length of PSDs in Hgsnat-Geo and Hgsnat^P304L CA1 pyramidal neurons. Density of synaptic vesicles, length (µm), and area (µm²) of PSDs were measured in asymmetrical (G) and symmetrical (H) pyramidal neurons from the CA1 region of the hippocampus. Synaptic terminals on the TEM images are marked with black arrowheads and PSDs with red asterisks. Data show values, means, and SD of the results obtained with three mice per genotype with 10–15 neurons quantified per animal. P values were calculated by nested one-way ANOVA test with Tukey post hoc test. Scale bars equal 200 nm in all panels. *, P < 0.05; **, P < 0.01; ***, P < 0.001; ****, P < 0.0001. MO, mo.