Mapping and Deciphering Neural Codes of NMDA Receptor-Dependent Fear Memory Engrams in the Hippocampus
(A) Reverberation of real-time ensemble traces during the CS/US pairings. The black bar on the top of the spike raster (100 simultaneously recorded CA1 units were selected for illustration) indicates the non-freezing state, whereas the orange bar indicates the freezing state of the animal during learning. The reverberation of various ensemble traces is shown at the bottom of the raster: CS trace, blue triangle; US trace, red triangle; US-to-CS associative trace, red diamond; CS-to-US associative trace, blue diamond. (B) Examples of reverberated CS, US, US-to-CS, and CS-to-US traces during the learning phase. (C) Increased on-line reverberations of various traces in a representative control mouse over seven CS-US pairing trials. CS and US stimulation time windows are shown by the blue bar and the vertical line, respectively. (D) Example of sporadic on-line reverberations from a knockout mouse over the seven pairing trials, with associative traces rarely showing reverberation. (E) Average on-line reverberations per trial in control and mutant groups (Wilcoxon rank sum test, ***p<0.001). Left plot for total reverberation numbers in control and mutant groups. Right panel for each type of reverberated traces (genotype difference: Wilcoxon rank sum test, *p<0.05, ***p<0.001). (F) Compositions of reverberation numbers for each trace type (CS, US, US-to-CS and CS-to-US) in control and mutant groups. (G) Trial-dependent increases in on-line reverberation of both simple and associative traces in the control but not in the mutant groups (n = 5 for each group). Student t-test, **p<0.01. (H) Increased on-line reverberation of associative traces (CS-to-US and US-to-CS traces combined) in the control, but not mutant mice (n = 5 for each group). Student t-test, *p<0.05, error bars represent SEM. (I) The increase in the numbers of learning pattern reverberations was correlated with the increase in the amounts of immediate freezing during this learning phase in control mice (r = 0.84, p<0.05). (J) Lower level of freezing responses and learning pattern reverberations in knockout mice (r = 0.28, p > 0.05).