Peer review of: Preventing and treating PTSD-like memory by trauma contextualization (2020)

Reviewers' Comments: 
 
Reviewer #1: 
Remarks to the Author: 
The authors tested a hypothesis that hippocampal hypofunction-associated contextual fear amnesia contributes to PTSD-
like ‘decontextualized’ (generalized) emotional hypermnesia. To test this, a series of experiments were conducted in mice 
subjected to unpaired tone and footshock stimuli followed by systemic corticosterone injection (to produce PTSD-like 
memory generalization to the tone), and then their contextual memory strengths were altered via and optogenetic 
inhibition/activation of the dorsal CA1 hippocampus. The main findings are that (i) optogenetic inhibition of CA1 during 
(but not prior to) fear conditioning produced generalized fear to the tone stimulus (akin to posttraining corticosterone 
injection), (ii) optogenetic stimulation of CA1 neutralized corticosterone-induced tone fear memory, and (iii) retrieving 
generalized tone fear memory in the fear conditioned context (‘re-contextualization’) restored normal fear memory. Based 
on these findings, the authors conclude that “dCA1 inhibition during a stressful event is sufficient to induce both contextual 
amnesia and hypermnesia components of PTSD-like memory” and that “promoting memorization of the context through 
dCA1 activation prevents the development of PTSD-like memory.” In my view, the findings are novel, well presented and 
possibly translationally significant. However, I do have some comments and questions. 
 
1. The pre-tone freezing levels during the tone test in a familiar chamber are comparably high in all groups (e.g., Fig. 
1a,b; Fig. 3), which would indicate generalized fear between trained and familiar chambers (both are round with 20 cm 
diameter). If so, shouldn’t there be some re-contextualization going on during the tone test? 
 
2. The post-training cort injection producing PTSD-like effects require some explanation. For example, is the cort effect due 
to its action on dCA1 cells since the dCA1 inhibition during fear conditioning mimics post-training cort effect, whereas the 
dCA1 activation offsets post-training cort effect? If so, this should be tested by applying optogenetic inhibition, not only 5 
minutes before and during fear conditioning (as shown in Fig 1b,c) but also right after fear condition. 
 
3. Related to comment #2, there should be some mentioning of how cort affects CA1 neurons, and thereby the formation 
of tone fear memory, which is presumably either hippocampal-independent or ventral hippocampal-dependent. 
 
4. (Fig 1a, Tone Test) For the Laser OFF/Veh group, the % freezing during the Tone period is lower than % freezing during 
the Pre-T period. If so, why is the Tone Ratio shown as being greater than 0? This seems to be the case for all Tone Ratio 
figures shown in Figure 1. 
 
5. Are the n sizes for the four experiments before or after histological verification? Proving both before and after histology 
n size information would be helpful. 
 
6. How was the freezing behavior quantified by a blind observer? Time sampling or duration? 
 
7. Shouldn’t the vehicle be β-cyclodextrin dissolved in .9% NaCl, rather than NaCl alone, to confirm that β-cyclodextrin is 
inert? 
 
 
 
Reviewer #2: 
Remarks to the Author: 
Alice Shaam Al Abed et al investigated roles of hippocampus in generation of PTSD-like memory and switching this 
memory to normal fear memory by trauma-contextualization and re-contextualization using optogenetics. The authors 
showed that optogenetic inactivation of hippocampus during the exposure to the cue is sufficient to generate PTSD -like 
memory while activation of hippocampus with CORT injection blocks the formation of PTSD-like memory. Interestingly, 
reactivation of traumatic memory by re-exposure to the context with the cue switched the PTSD-like memory to normal 
fear memory and this normalization was hippocampus-dependent. Furthermore, the authors showed the persistence of 
PTSD-like and normalized memories. Importantly, the findings support the importance of exposure therapy for PTSD that 
may normalize PTSD-like memory through reactivation of this memory. However, the novelty of findings in this manuscript 
seems to be not so much because the authors have already shown that the generation of PTSD-like memory is 
hippocampus-dependent using microinfusion of CORT into hippocampus (ref15). 
My other major concerns are as follows. 
 
1) Optogenetic activation of hippocampal CA1 in the presence of CORT blocked generation of PTSD-like memory. This 
observation is interesting. However, it still remains unclear why this artificial hippocampal activation blocks generation of 
PTSD-like memory by CORT. The authors should show or suggest how this activation could block the effect of CORT. 
2) In figure1, several control experiments are required such as mice expressing GFP with laser and mice expressing 

ChR2/ArchT without laser. 
3) Normalization of PTSD-like memory must be mediated by memory updating processes such as reconsolidation, 
extinction and/or re-learning. Although it is interesting to examine these mechanisms, at least, the authors should discuss 
about these possibilities.