Mapping and Deciphering Neural Codes of NMDA Receptor-Dependent Fear Memory Engrams in the Hippocampus
(A) An example of various memory traces being retrieved during the 1-hr contextual fear recall test (first 60 sec shown here). The black bar on the top of spike raster illustrates the non-freezing state, whereas the orange bar indicates the freezing state of the animal. Note that the two initial memory traces were recalled ∼3–4 seconds before freezing behavior once the animal returned to the conditioning chamber. Colored triangles or diamonds at the bottom the raster indicate the moments at which those memory traces were retrieved. Memory traces were detected in both freezing and non-freezing states. (B) Examples of four types of memory traces retrieved during the contextual recall. (C) Memory traces retrieved over the first 60-sec of contextual retention tests in all five control mice. Symbols: simple CS trace, blue triangle; simple US trace, red triangle; US-to-CS associative trace, red diamond; CS-to-US associative trace, blue diamond. (D) Reduced numbers of memory traces during 1-hr contextual recall test in the five knockout mice. (E) Freezing responses correlated with memory trace retrievals in a control mouse in the 5-min contextual retention test. (F) Lower freezing and lower numbers of retrieved memory traces in a mutant mouse in the 5-min contextual test. (G) Linear regression analysis shows that at group level, averaged freezing responses also correlated with their averaged numbers of total pattern retrievals (r2 = 0.73, p<0.01). Each blue dot represents the data from a single control mouse and each red triangle represents the data from a single knockout mouse. (H) The total numbers of memory traces retrieved in the control and knockout mice (Wilcoxon rank sum test, **p<0.01; error bars represent SEM). The filled bar portion represents the associative memory traces retrieved during the contextual retention test. The knockout mice had few associative memory traces retrieved (Wilcoxon rank sum test, p<0.05). (I) Inter-memory trace-time interval analysis revealed that contextual recall in the control mice has the characteristics of exponential decay distribution. (J) Memory trace retrieved in the mutant mice did not show obvious exponential decay process temporal associativeness. There is significant difference between memory trace time distribution from control and mutant mice (two-sample Kolmogorov-Smirnov test, p = 1.2E-7).