10/3 Ramirez 2013 and 2015

Ramirez et al. (2013) optogenetically activated memory engrams in the dentate gyrus (DG) of the hippocampus in c-fos-tTA mice. Optic fibers were implanted into the DG and an AAV encoding TRE-ChR2-mCherry was injected into the DG or CA1.

The mice were taken off dox after surgery and exposed to context A for labeling. While the animals were being fear conditioned in context B, the labeled cells from context A were optically stimulated.  When the ChR2-mCherry mice were placed in context A for the second time, they froze significantly more than the mCherry only mice. In neutral and novel context C, the freezing levels were not significantly different than the controls. This showed that the previously neutral context A was successfully linked with the fear memory from being shocked, creating a false memory. A part of the method that I questioned was that the mice were taken off dox for 42 hours to label neurons associated with context A, and then immediately put back on the dox diet. Since the mice were only in context A for the final 10 minutes of the 42 hours, it seems like it would be possible that other neurons active during the rest of the time might be labeled as well. It might be interesting to compare labeled neurons from the context A exposure to labeled neurons while the mice stayed in the home cage for 42 hours.

Ramirez et al. used the same technique to label memory engram cells in the 2015 paper. One of the experiments in the paper started with the c-fos-tTA rats on dox during surgery and recovery, and then off dox to label either a positive or neutral memory and immediately put back on dox afterwards. Then, all groups except the control were subjected to ten-day immobilization stress. The labeled DG cells were reactivated for zero, one, or five days before the TST and SPT, which are used to measure anhedonia. Chronic (5 day) reactivation of DG positive memory engram cells was shown to rescue depression-like behavior after chronic immobilization stress. The results of the TST and SPT showed no difference between the anhedonic behaviors of the non-stressed controls and the stressed chronic positive memory reactivated group. Furthermore, both groups had similar levels of neurogenesis in the DG. The neutral memory and one day positive memory stimulation groups did not exhibit a rescue effect. Exposing the mice to a real positive memory for five days also did not have a rescue effect or increase neurogenesis. It would be interesting to see how long the rescue effects lasted in the mice. If the effects are long lasting, it could be beneficial to find a way to use the technique in treating depression in humans.