Figure 5. Developmental strengthening of BLA excitatory synapses formed by mPFC projections. A, Summary of experimental procedure. At 7 d after stereotaxic viral infusions, acute brain slices containing the amygdala were obtained for synaptic physiology. Prefrontal tissue was retained and subsectioned to confirm target expression. Monosynaptic EPSCs were recorded from principal excitatory neurons in the anterior basal nucleus during stimulation of mPFC projections by microscope-objective coupled LEDs (460 nm). B, Response amplitude during paired-pulse stimulation was examined as a proxy for glutamate release probability of mPFC terminals. Scale bars: P15, 150 pA; P30, 80 pA; P45, 500 pA; P60, 150 pA × 100 ms. C, Comparison of paired-pulse ratio (second/first response) revealed a decrease between P21 and P30. Bonferroni post hoc test was conducted on all pairwise comparisons following two-way repeated-measures ANOVA. *[black]p < 0.05, P15 versus P60. *[purple]p < 0.01, P21 versus P30, P45, and P60. **[black]p < 0.01, P15 versus P30, P45, and P60. **[purple]p < 0.01, P21 versus P30, P45, and P60. P15, n = 8 cells (3 mice); P21, n = 11 cells (4 mice); P30, n = 10 cells (5 mice); P45, n = 6 cells (5 mice); P60, n = 22 cells (10 mice). D, Representative EPSCs resulting from optic stimulation at −70 mV and 40 mV during blockade of GABAergic transmission (100 μm picrotoxin). For calculation of AMPA/NMDA ratio, AMPAR-EPSC amplitude was defined as the peak amplitude at −70 mV, a potential at which NMDA receptors contribute negligible current. Conversely, NMDAR-EPSCs were measured at 40 mV at 100 ms after LED stimulation, a time point when AMPA receptor currents have fully decayed and responses are dominated by NMDA currents. E, Developmental increase in AMPA/NMDA ratio. P15, n = 11 cells (3 mice); P21, n = 13 cells (5 mice); P30, n = 12 cells (6 mice); P45, n = 13 cells (5 mice); P60, n = 8 cells (5 mice). Tukey's post hoc test was conducted on all pairwise comparisons following one-way ANOVA. **p < 0.01. F, Developmental increase in maximal amplitude of light-evoked EPSCs at −70 mV. Tukey's post hoc test was conducted on all pairwise comparisons following one-way ANOVA. *p < 0.05. G, No change in decay time constant of responses at 40 mV. Inset, Overlay of group-averaged peak-scaled currents at 40 mV, color coded according to scheme in B. P15, n = 10 cells (3 mice); P21, n = 12 cells (4 mice); P30, n = 11 cells (6 mice); P45, n = 13 cells (5 mice); P60, n = 8 cells (5 mice).