FIGURE 3.

Changes to CA1 activity patterns during demyelination and remyelination. (A) Time line of the demyelination/remyelination experiment for mice 1–4. Craniotomy surgery was performed between day-14 and -7, and baseline activity recording from CA1 and DG neurons started between day -6 and day 0 [indicated by green background in panels (B–D)]. On day 0, after an activity recording session, we switched the diet to include 0.3% cuprizone. This diet was administered until day 53 [demyelination period, indicated by magenta background in panels (B–D)], while we maintained weekly activity recordings. On day 53, after an activity recording was completed, we switched the mice back to normal diet and monitored activity until day 100 [remyelination period, indicated by cyan background in panels (B–D)]. Bottom, two example fields of view acquired from mouse 1 (cuprizone group) showing the CA1 layer and activity of two CA1 example cells on days 53 and 58. The estimated timing and number of APs is shown (red) beneath each raw data trace (blue). (B) Summary of the average firing rates from all recorded CA1 neurons from mouse 1 during 107 days of recording. Firing rates decreased significantly from 0.345 to 0.17 Hz between day 0 and 9 (P = 1.8 × 10^–37, Wilcoxon Rank Sum Test), after cuprizone diet had been introduced. Activity levels continued to significantly decrease down to zero on day 53 (P = 0.002, Mann-Kendall Trend Test), and were significantly increased to 0.205 Hz on day 58, 5 days after normal diet was resumed (P = 4 × 10^–133, Wilcoxon Rank Sum Test). Activity levels on the following days continued to increase compared to the demyelination period (P = 0.064, Mann-Kendall Trend Test), but remained lower than the pre-cuprizone period. Magenta and cyan backgrounds highlight the demyelination and remyelination periods, respectively; red lines correspond to medians, blue boxes show the 25th–75th percentile range, whisker length is the shorter of 1.5 times the 25th–75th range or the extreme data point. Outliers are not shown. ^∗∗∗P < 0.001. (C) Summary of the average firing rates from all recorded CA1 neurons from all cuprizone-treated mice. Solid lines connect the distribution medians, and the error bars indicate the 25th–75th percentile range for each recording session. For all recorded mice, there was a significant decrease in the average firing rate upon the start of cuprizone diet, between day 0 and 9 (n = 4 mice, P < 10^–13, Wilcoxon Rank Sum Test), and a significant increase upon the termination of cuprizone diet, between day 53 and 58 (n = 3 mice, P < 10^–32, Wilcoxon Rank Sum Test). Note that the rapid increase in firing rate on day 58 was followed by a second decrease in activity rate. ^∗∗∗P < 0.001. (D) Summary of the fraction of cells that fired bursts of APs from all recorded cells for all cuprizone-treated mice. This fraction decreased for all mice during the demyelination period and rapidly recovered upon the start of the remyelination period. Colors are the same as in panel (C). P-values for the decrease were 0.035, 0.108, and 0.064 for mice 1, 2, 3, respectively (Mann-Kendall Trend Test). (E) Summary of the average firing rates from all recorded CA1 neurons from control mouse 5, which received normal diet throughout the recording period, showing no similar decreases or increases in firing rate as those recorded from cuprizone-treated mice. (F) Summary of the average firing rates from all recorded CA1 neurons from all control mice. Solid lines connect the distribution medians, and the error bars indicate the 25th–75th percentile range for each recording session. None of the traces showed a significant monotonous trend (P = 0.99, 0.86, 0.14, for mice 5, 6, 7, respectively, Mann-Kendall Trend Test). (G) Summary of the fraction of cells that fired bursts of APs from all CA1-recorded neurons from all control mice. Colors are the same as in panel (F).