Figure 6.
Summary of all plasticity outcomes at P3–P4 and P6–P7. All plasticity outcomes are expressed as percentage of baseline, placed into 10% bins, and plotted as frequency histograms. LTP was defined as >120% of baseline, and no change was defined as 80–120% of baseline. The vertical dashed line represents 120% threshold.
A
, Summary of all control P3–P4 experiments, with 7 of 10 showing LTP.
B
, Summary of all β2−/− P3–P4 experiments, with 4 of 7 showing LTP.
C
, Summary of all control P6–P7 experiments, with 2 of 12 showing LTP.
D
, Summary of all β2−/− P6–P7 experiments, with 9 of 16 showing LTP. Using a Mann–Whitney U test, we find that the control P3–P4 distribution is different from the control P6–P7 distribution (p < 0.05), whereas the β2−/− distributions at P3–P4 and P6–P7 are not significantly different (p = 0.6). The β2−/− P6–P7 distribution is, however, different from the control P6–P7 group (p < 0.05). Using χ2 statistics, we also tested whether the frequency of LTP differs across ages and genotypes. The probability of finding LTP is greater in control P3–P4 cells than in control P6–P7 cells (control P3–P4, 7 of 10; control P6–P7, 2 of 12; p < 0.05), whereas the frequency of LTP remains similar across ages for β2−/− cells (β2−/− P3–P4, 4 of 7; β2−/− P6–P7, 9 of 16; p = 0.9). Accordingly, the frequency of LTP is greater in β2−/− P6–P7 cells than in control P6–P7 cells (control P6–P7, 2 of 12; β2−/− P6–P7, 9 of 16; p < 0.05). These results confirm that the plasticity outcomes differ among the immature and mature synaptic populations, and that the probability of inducing LTP decreases after synaptic strengthening and maturation have occurred.