Ventral Hippocampal Temporoammonic and Schaffer Collateral Pathways Differentially Control Fear- and Anxiety-Related Behaviors

Reviewer #1 (Public review):

Summary:

The hippocampus, especially the ventral subregion, has been related to emotional processing. However, the specific circuitry involved deserves further investigation. By using a bidirectional optogenetic modulation, Kambali et al. have investigated the role of different inputs to vCA1 (i.e., from vCA3 and entorhinal cortex) in anxiety- and fear-related responses. The major findings of this work suggested that both inputs to vCA1 control fear-related responses, whereas only the projection between vCA3 and vCA1 controls anxiety-related behavior. Overall, the authors used an advanced methodological approach, which allows them to modulate specific brain circuits, to study specific hippocampal projections, providing some new information regarding the hippocampal function in anxiety and fear.

Strengths:

(1) The manuscript is well written, clear and has a detailed and specific discussion.

(2) Results from each optogenetic manipulation are clear in different anxiety- and fear-related tasks, demonstrating the robustness of the findings.

(3) The overall conclusions are very interesting and might be relevant for the field of mental health disorders accompanied by anxiety- and fear-related alterations.

Weaknesses:

(1) The major differences in basal behavioral performance in the different paradigms between the two optogenetic modulations prevent the achievement of strong conclusive results.

(2) Data presentation and representative figures need a major revision.

(3) No analysis has been performed to analyze potential sex differences in behavioral domains where sex is important.

Reviewer #2 (Public review):

Summary:

This paper uses an optogenetic approach to either activate or inhibit separate neural pathways projecting to the ventral CA1 hippocampal subregion, from either CA3 or the entorhinal cortex. The authors report that manipulation of the vCA3→vCA1 pathway affected behavioural performance on a number of tasks: elevated plus maze, open field, Vogel conflict test and freezing behaviour to both context and a trace CS cue. In contrast, optogenetic manipulation of neural activity in the EC→vCA1 pathway only affected behaviour on the trace CS/context fear memory test but had no effect on the elevated plus maze, open field or Vogel conflict test. The authors suggest different roles for these two ventral hippocampal pathways in fear versus anxiety.

Strengths:

This is an interesting study addressing an important question in a highly topical subject area. The experiments are well conducted and have generated interesting and important data.

Weaknesses:

While I am broadly sympathetic to the overall narrative of the paper, I have some questions/comments around the specific interpretation of the results presented. In my view, the authors' claims may not be completely supported by their data, but the data are interesting nonetheless.

In terms of the framework presented by the authors for interpreting their data, many would argue that freezing (or at least reduced activity/behavioural inhibition) to the context provides a readout of conditioned anxiety rather than fear. In this sense, the context is a signal of potential threat (i.e. the context becomes associated with both shock and with the absence of shock) and thus generates anxiety rather than fear. Likewise, the trace CS cue could be considered as an ambiguous predictor of shock in that the shock doesn't occur straight away. In contrast, a punctate CS cue which co-terminates with shock would be a reliable signal of imminent threat and thus generates a fear response. Thus, it might be argued that all of the assays adopted by the authors are readouts of anxiety (albeit comprising tests of both conditioned and unconditioned anxiety). For example, from the authors' perspective, it is not clear a priori why the Vogel conflict test is considered anxiety, but contextual freezing is considered fear? Indeed, in the Discussion, the authors mention another study in which the data from the Vogel conflict test align with fear assays rather than anxiety tests. Can the authors elaborate on their distinction? I appreciate that, in practice, it might be difficult to distinguish between fear and anxiety at the behavioural level in rodents (although opposing effects of fear and anxiety on pain responses might be one option). At the very least, this issue merits further discussion.

Another question is whether rather than representing a qualitative difference between the contributions of the vCA3→vCA1 and EC→vCA1 pathways to different aspects of fear/anxiety behaviours, the different results reflect a quantitative difference between the magnitude of effects in vCA1 that are generated from optogenetic manipulation of the two pathways, coupled with the possibility that behaviour on the trace CS/context fear memory task is more sensitive to manipulation than the "anxiety tests". The possibility that vCA3→vCA1 stimulation is more effective is potentially supported by the c-fos measurements in vCA1. vCA3→vCA1 stimulation produced a much bigger vCA1 c-fos response (approx. 350% c-fos cell activation; see Figure 1E) compared to activation of the EC→vCA1 pathway (approx. 170% c-fos cell activation; see Figure 4E).

Furthermore, in some studies, there seem to be quite large differences between the laser OFF conditions for the different groups (which presumably one would not expect to be different). For example, compare laser OFF for the Inhibition group for time in open arms of EPM in Figure 5C (> 40%) versus laser OFF for the Inhibition group for time in open arms of EPM in Fig. 2C (< 20%). This could potentially result in ceiling effects, such that it is very hard to see a further increase in time in the open arms from a level already above 40% when the laser is then switched on. This could complicate the interpretation of the laser ON condition.

Likewise, there is a big difference between the behavioral performance of the two SHAM groups in Figure 3 (compare SHAM in 3 B, C and SHAM in 3 D, E). How is this explained? Could this generate a ceiling effect? This may also merit some discussion. More details on the SHAM procedure(s) in the main manuscript may also be helpful.

According to Figure 3A, the test of freezing response to the trace Tone CS is conducted in a different context from the conditioning context. The data presented in Figure 3 for tone fear are the levels of freezing during the presentation of this cue in the different contexts. It would be important to present both pre-CS and CS freezing levels here to determine how much of the freezing is actually driven by the punctate tone CS. The pre-CS freezing levels in this different context would also provide a nice control for the contextual fear conditioning.

Reviewer #3 (Public review):

Summary:

In their paper entitled "Ventral hippocampal temporoammonic and Schaffer collateral pathways differential control fear- and anxiety-related behaviors" the authors use a bidirectional optogenetic approach to elucidate the role of temporammonic (TA) and Schaffer collateral (SC) inputs to the ventral hippocampus (CA1) in modulating both fear and anxiety-related behaviors. While fear and anxiety behaviors are often considered on a continuous spectrum, identifying neural pathways that are differentially activated represents an important open question in the field. The authors find that optogenetic stimulation or inhibition of the Schaffer Collateral pathway in the ventral hippocampus (CA3-CA1) bidirectionally modulates both fear-related and anxiety-related behavioral paradigms. More specifically, optogenetic excitation of the CA3-CA1 pathway using ChR2-expressing viral constructs increases anxiety-like behaviors in numerous behavioral paradigms (elevated plus maze, open field, Vogel conflict test). Conversely, optogenetic inhibition using halorhodopsin reduced anxiety-like behaviours. To examine fear behaviors, the authors examined contextual and trace fear conditioning. Similar to their results with anxiety-like behaviors, the authors observed bidirectional fear modulation following optogenetic stimulation of the vCA3-vCA1 pathway. The authors next examined the temporammonic pathway originating from the lateral entorhinal cortex to vCA1. Unlike with SC stimulation, stimulation of the TA pathway had no effect on anxiety-like behaviors but did bidirectionally modulate contextual fear conditioning. Together, these results differentiate the SC and TA pathways in the ventral hippocampus as distinct regulators of affective behavior.

Strengths:

The paper has numerous technical strengths, including dissecting the role of both excitation and inhibition of both pathways and the use of behavioral measures of anxiety and fear. This balanced and internally controlled design allows readers to evaluate the effects of both pathways in a single study, thereby reducing technical complications from experiments being completed across laboratories and experimental conditions.

Weaknesses:

There are a few limitations of the study, however, which bear discussion.

(1) The authors use halorhodopsin to achieve optogenetic inhibition. Halorhodopsin is generally considered a first-generation optogenetic actuator, as it is a Cl- pump rather than an ion channel. This limits the degree of inhibition (i.e. by preventing shunting inhibition) and can result in altered chloride gradients in the period immediately following optogenetic stimulation. This is of particular concern in this paper as the stimulation parameters and behavioral analysis are not temporally correlated, therefore confounds of disrupted chloride cannot be experimentally accounted for or controlled.

(2) The authors use an AAV-CaMKII-eGFP as a control (Sham) throughout the dataset; however, in the trace fear conditioning experiments, there are no AAV-CaMKII-ChR2-eYFP or AAV-CaMKII-eNpHR3.0-eYFP controls without optogenetic stimulation. Therefore, it is unclear the extent to which viral expression of optogenetic actuators impacts behavior. Additionally, the authors only provided optogenetic stimulation during contextual fear recall and tone fear recall. Additional experiments disrupting each pathway during trace conditioning would have provided additional insight into the role of each pathway in the initial encoding of fear memories.

(3) The location and extent of viral expression across animals were not systematically quantified.

Overall, however, these weaknesses do not significantly detract from the main conclusions of the paper. The authors' data convincingly demonstrates that disruption of the trisynaptic circuit bidirectionally modulates both fear- and anxiety-like behaviors while disruption of the temporammonic pathway has no effect on anxiety-like behaviors but disrupts fear-related behaviors. It is interesting to note, however, that the TA activation had no effect on tone-related fear conditioning, suggesting a potential specialized role of the temporammonic pathway specifically in contextual fear memory.

Author response:

Public Reviews:

Reviewer #1 (Public review):

Summary:

The hippocampus, especially the ventral subregion, has been related to emotional processing. However, the specific circuitry involved deserves further investigation. By using a bidirectional optogenetic modulation, Kambali et al. have investigated the role of different inputs to vCA1 (i.e., from vCA3 and entorhinal cortex) in anxiety- and fear-related responses. The major findings of this work suggested that both inputs to vCA1 control fear-related responses, whereas only the projection between vCA3 and vCA1 controls anxiety-related behavior. Overall, the authors used an advanced methodological approach, which allows them to modulate specific brain circuits, to study specific hippocampal projections, providing some new information regarding the hippocampal function in anxiety and fear.

Strengths:

(1) The manuscript is well written, clear and has a detailed and specific discussion.

(2) Results from each optogenetic manipulation are clear in different anxiety- and fear-related tasks, demonstrating the robustness of the findings.

(3) The overall conclusions are very interesting and might be relevant for the field of mental health disorders accompanied by anxiety- and fear-related alterations.

Weaknesses:

(1) The major differences in basal behavioral performance in the different paradigms between the two optogenetic modulations prevent the achievement of strong conclusive results.

The two projections of ventral CA1 were studied independently in different cohorts of animals tested at different times during the study. This difference in timing may have contributed to variations in the basal behavioral performance between the two projections. Importantly we found that within each cohort – control and optogenetic manipulation, the basal performance within each set of experiments (i.e., corresponding to projections) is highly consistent, e.g., basal cued and contextual freezing responses and responses to OFF conditions in Vogel conflict test. Moreover, the ANOVA statistics conducted across the baseline and ON conditions for each task revealed robust significant effects of bidirectional optogenetic modulation for each cohort. In case of the fear responses, a point to note is that the freezing levels in SHAM controls differ between projections but are consistent between two types of assessments (tone and context) within each projection. We will mention these limitations in the revised manuscript.

(2) Data presentation and representative figures need a major revision.

The figures will be rearranged according to the projections. The anxiety-related figures and fear response related figures will be grouped for each projection to improve clarity and readability. The revised manuscript will include representative heat maps for each behavioral task for both projections in addition to population quantification data.

(3) No analysis has been performed to analyze potential sex differences in behavioral domains where sex is important.

This assessment was not done in the original submission. We will perform statistical analysis for male and female mice separately and if the results are sex-dependent, we will present separate figures. Otherwise, the combined data presentation will be followed.

Reviewer #2 (Public review):

Summary:

This paper uses an optogenetic approach to either activate or inhibit separate neural pathways projecting to the ventral CA1 hippocampal subregion, from either CA3 or the entorhinal cortex. The authors report that manipulation of the vCA3→vCA1 pathway affected behavioural performance on a number of tasks: elevated plus maze, open field, Vogel conflict test and freezing behaviour to both context and a trace CS cue. In contrast, optogenetic manipulation of neural activity in the EC→vCA1 pathway only affected behaviour on the trace CS/context fear memory test but had no effect on the elevated plus maze, open field or Vogel conflict test. The authors suggest different roles for these two ventral hippocampal pathways in fear versus anxiety.

Strengths:

This is an interesting study addressing an important question in a highly topical subject area. The experiments are well conducted and have generated interesting and important data.

Weaknesses:

While I am broadly sympathetic to the overall narrative of the paper, I have some questions/comments around the specific interpretation of the results presented. In my view, the authors' claims may not be completely supported by their data, but the data are interesting nonetheless.

In terms of the framework presented by the authors for interpreting their data, many would argue that freezing (or at least reduced activity/behavioural inhibition) to the context provides a readout of conditioned anxiety rather than fear. In this sense, the context is a signal of potential threat (i.e. the context becomes associated with both shock and with the absence of shock) and thus generates anxiety rather than fear. Likewise, the trace CS cue could be considered as an ambiguous predictor of shock in that the shock doesn't occur straight away.

In contrast, a punctate CS cue which co-terminates with shock would be a reliable signal of imminent threat and thus generates a fear response. Thus, it might be argued that all of the assays adopted by the authors are readouts of anxiety (albeit comprising tests of both conditioned and unconditioned anxiety).

We agree with the reviewer that context and trace fear conditioning do not represent an “imminent” threat as severe as would likely be internalized in delay fear conditioning. However, the goal of the study was to probe hippocampal dependent processes (contextual and trace fear conditioning are strongly modulated by the hippocampus while delay conditioning is not). Consistent with several other studies, we believe the conditional nature of the task (context and trace are invariably linked to shock) provides support for a “non-ambiguous” relationship that is conducive for measuring the assessment of fear-based behavior.

Several studies show clear differences in the involvement of amygdala and hippocampus in delay vs. trace fear conditioning. Inactivating amygdala led to deficits in contextual and delay conditioning but had no effect on trace conditioning. In contrast, inactivating hippocampus led to deficits in trace and contextual but not delay fear conditioning. These findings suggest that a temporal gap between the CS and US can generate amygdala-independent but hippocampal-dependent fear conditioning (Raybuck J. D., Lattal K. M 2011, PMID: 21283812). Lesions of the entorhinal cortex impair the acquisition of trace fear conditioning but not the acquisition of delay fear conditioning (Raybuck J. D., Lattal K. M 2011, PMID: 21283812) . Further, using single unit recording during fear retention tests after delay or trace fear conditioning, the study showed that entorhinal neurons specifically respond after trace but not after delay fear conditioning (Kong et al 2023, PMID: 36919333). These findings demonstrate that trace fear conditioning and delay fear conditioning may involve overlapping but largely different neuronal circuits. A knockdown of the expression of the α5-subunit–containing GABA_𝐴 receptors in the CA1 region (α5CA1KO mice) leads to improved spatial learning and enhanced trace fear conditioning memory, actually to the level of delay fear conditioning, suggesting that α5GABA_𝐴Rs in CA1 pyramidal neurons normally constrain hippocampus-dependent memory processes and that trace fear conditioning in the absence of a5-GABA_𝐴 receptors in CA1 has the same effect size as delay fear conditioning (Engin et al 2020, PMID: 32934095), supporting the view that trace fear conditioning is not “ambiguous”.

For example, from the authors' perspective, it is not clear a priori why the Vogel conflict test is considered anxiety, but contextual freezing is considered fear? Indeed, in the Discussion, the authors mention another study in which the data from the Vogel conflict test align with fear assays rather than anxiety tests. Can the authors elaborate on their distinction? I appreciate that, in practice, it might be difficult to distinguish between fear and anxiety at the behavioral level in rodents (although opposing effects of fear and anxiety on pain responses might be one option). At the very least, this issue merits further discussion.

We will make this distinction clearer in the revisions. Briefly, behavioral actions in the Vogel conflict test are generally considered to be most pertinent to general anxiety disorders in humans and anxiolytics have high predictive validity in animals in this task. In particular, the robust actions of benzodiazepines and 5-HT_1A partial agonists parallel their clinical efficacy in patients (McMillan and Brocco, 2003, PMID: 12600703).

Our previous study (Engin et al 2016, PMID: 26971710) used global diazepam-induced neuronal inhibition and identified that positive modulation of α2-GABA_𝐴Rs in dentate gyrus granule cells and CA3 pyramidal neurons is required to reduce anxiety-like behaviors while inhibition of positive modulation of α2-GABA_𝐴Rs in CA1 pyramidal neurons is required to reduce fear-related behaviors. The effects were absent when α2-GABA_𝐴Rs was knocked out in the respective subregions. These results indicate that these intrahippocampal subregions can modulate fear and anxiety-like behaviors independently of the amygdala. In the previous study we used conditional α2-GABA_𝐴R knockouts in hippocampal subregions and subjected these mice to systemic diazepam. In these experiments, diazepam still acts on α1-, α3- and α5-_𝐴Rs in the hippocampal subregions and cell types in which when α2-GABA_𝐴Rs are lacking. Therefore, for example when α2CA1KO mice were administered diazepam, diazepam still led to inhibition of pyramidal neurons in CA3 and DG via α1-, α2-, α3- and α5- GABA_𝐴Rs, and in addition, diazepam also inhibited α1-, α3- and α5- GABA_𝐴Rs in CA1 itself. Diazepam also acted on GABA_𝐴Rs in amygdala or other brain regions. These are fundamentally different experimental conditions compared to the optogenetic experiment described in this paper. Moreover, in contrast to the current paper, the previous work did not examine projections but used global diazepam-induced neuronal inhibition as a baseline. Moreover, whereas the previous paper examined whether a specific neuronal cell type was required for anxiolytic-like or fear-like actions, the current manuscript examined whether activation or inhibition of neuronal projections is sufficient to modulate anxiety- and fear-related behaviors. Overall, one cannot easily compare the results in the Vogel conflict test in both papers.

Another question is whether rather than representing a qualitative difference between the contributions of the vCA3→vCA1 and EC→vCA1 pathways to different aspects of fear/anxiety behaviours, the different results reflect a quantitative difference between the magnitude of effects in vCA1 that are generated from optogenetic manipulation of the two pathways, coupled with the possibility that behaviour on the trace CS/context fear memory task is more sensitive to manipulation than the "anxiety tests". The possibility that vCA3→vCA1 stimulation is more effective is potentially supported by the c-fos measurements in vCA1. vCA3→vCA1 stimulation produced a much bigger vCA1 c-fos response (approx. 350% c-fos cell activation; see Figure 1E) compared to activation of the EC→vCA1 pathway (approx. 170% c-fos cell activation; see Figure 4E).

Furthermore, in some studies, there seem to be quite large differences between the laser OFF conditions for the different groups (which presumably one would not expect to be different). For example, compare laser OFF for the Inhibition group for time in open arms of EPM in Figure 5C (> 40%) versus laser OFF for the Inhibition group for time in open arms of EPM in Fig. 2C (< 20%). This could potentially result in ceiling effects, such that it is very hard to see a further increase in time in the open arms from a level already above 40% when the laser is then switched on. This could complicate the interpretation of the laser ON condition.

The magnitude of activation as evidenced by c-fos measurements differs between the two projections. This might reflect different levels of modulations of CA1 neuronal activity. The fact that the two projections were studied at different time points (see response to reviewer 1) may also have contributed to the difference. The revised manuscript will include a formal discussion about magnitude of modulation that could contribute to differential sensitivity for the modulation of anxiety-like behaviors. However, the inputs from these two projections systems target different regions of CA1 pyramidal neurons and each pathway has distinct roles in other processes (sensory versus memory-based completion) – thus a dissociation may also be present for other types of behavior as well including the modulation of anxiety-like behaviors.

While it is possible that ceiling effects could impact our interpretation, we believe ceiling effects would only impact one direction of the optogenetic manipulation and there was no effect of activation (Fig. 5C) or bidirectional modulation of anxiety-related behavior in the novel open field test (Fig. 5F) which has levels of behavior comparable to Figure 2F.

Likewise, there is a big difference between the behavioral performance of the two SHAM groups in Figure 3 (compare SHAM in 3 B, C and SHAM in 3 D, E). How is this explained? Could this generate a ceiling effect? This may also merit some discussion. More details on the SHAM procedure(s) in the main manuscript may also be helpful.

With respect to contextual fear, ceiling effects are not a major factor as we still see enhanced freezing in the activation condition. With tone fear, we cannot formally exclude a ceiling effect, and this will be addressed as a potential confound in the manuscript.

According to Figure 3A, the test of freezing response to the trace Tone CS is conducted in a different context from the conditioning context. The data presented in Figure 3 for tone fear are the levels of freezing during the presentation of this cue in different contexts. It would be important to present both pre-CS and CS freezing levels here to determine how much of the freezing is actually driven by the punctate tone CS. The pre-CS freezing levels in this different context would also provide a nice control for the contextual fear conditioning.

We agree and will analyze and report the pre-CS freezing data in the revision.

Reviewer #3 (Public review):

Summary:

In their paper entitled "Ventral hippocampal temporoammonic and Schaffer collateral pathways differential control fear- and anxiety-related behaviors" the authors use a bidirectional optogenetic approach to elucidate the role of temporammonic (TA) and Schaffer collateral (SC) inputs to the ventral hippocampus (CA1) in modulating both fear and anxiety-related behaviors. While fear and anxiety behaviors are often considered on a continuous spectrum, identifying neural pathways that are differentially activated represents an important open question in the field. The authors find that optogenetic stimulation or inhibition of the Schaffer Collateral pathway in the ventral hippocampus (CA3-CA1) bidirectionally modulates both fear-related and anxiety-related behavioral paradigms. More specifically, optogenetic excitation of the CA3-CA1 pathway using ChR2-expressing viral constructs increases anxiety-like behaviors in numerous behavioral paradigms (elevated plus maze, open field, Vogel conflict test). Conversely, optogenetic inhibition using halorhodopsin reduced anxiety-like behaviours. To examine fear behaviors, the authors examined contextual and trace fear conditioning. Similar to their results with anxiety-like behaviors, the authors observed bidirectional fear modulation following optogenetic stimulation of the vCA3-vCA1 pathway. The authors next examined the temporammonic pathway originating from the lateral entorhinal cortex to vCA1. Unlike with SC stimulation, stimulation of the TA pathway had no effect on anxiety-like behaviors but did bidirectionally modulate contextual fear conditioning. Together, these results differentiate the SC and TA pathways in the ventral hippocampus as distinct regulators of affective behavior.

Strengths:

The paper has numerous technical strengths, including dissecting the role of both excitation and inhibition of both pathways and the use of behavioral measures of anxiety and fear. This balanced and internally controlled design allows readers to evaluate the effects of both pathways in a single study, thereby reducing technical complications from experiments being completed across laboratories and experimental conditions.

Weaknesses:

There are a few limitations of the study, however, which bear discussion.

(1) The authors use halorhodopsin to achieve optogenetic inhibition. Halorhodopsin is generally considered a first-generation optogenetic actuator, as it is a Cl- pump rather than an ion channel. This limits the degree of inhibition (i.e. by preventing shunting inhibition) and can result in altered chloride gradients in the period immediately following optogenetic stimulation. This is of particular concern in this paper as the stimulation parameters and behavioral analysis are not temporally correlated, therefore confounds of disrupted chloride cannot be experimentally accounted for or controlled.

Choice of halorhodopsin was in part influenced by a report that spontaneous archaerhodopsin activation was paradoxically associated with increased spontaneous release of neurotransmitter from presynaptic terminals, whereas activation of chloride-reducing halorhodopsin triggered neurotransmitter release upon light onset (Mahn et al., PMID: 26950004), suggesting that halorhodospin may be advantageous in studies inhibiting presynaptic nerve terminals. Halorhodpsin has been used in several studies to effectively silence activity and had substantial influence on behavioral in our studies that was inversely proportional to ChR2 stimulation. While perhaps not optimal out of an abundance of caution, we chose it over Archaerhodopsin based on the cited literature.

(2) The authors use an AAV-CaMKII-eGFP as a control (Sham) throughout the dataset; however, in the trace fear conditioning experiments, there are no AAV-CaMKII-ChR2-eYFP or AAV-CaMKII-eNpHR3.0-eYFP controls without optogenetic stimulation. Therefore, it is unclear the extent to which viral expression of optogenetic actuators impacts behavior. Additionally, the authors only provided optogenetic stimulation during contextual fear recall and tone fear recall. Additional experiments disrupting each pathway during trace conditioning would have provided additional insight into the role of each pathway in the initial encoding of fear memories.

Thank you for your observation. We have used a SHAM control that was injected with the AAV vector without any opsins. In fear conditioning experiments we performed optogenetic manipulations only during the fear response either with context or cue recall. This aligned well with our hypothesis to test whether the intrahippocampal projections play any role in fear response modulation. Investigating the role of each pathway during acquisition of trace and/or contextual fear conditioning is also highly relevant; however, evaluating these projections in fear memory formation was beyond the scope of this study. The observation that we can bidirectionally modulate fear responses with light is consistent with (although it does not prove) a light-specific modulation. In any case, even if there were baseline effects without light, they would still be suggestive of the effects observed being mediated by the optogenetic actuators.

(3) The location and extent of viral expression across animals were not systematically quantified.Overall, however, these weaknesses do not significantly detract from the main conclusions of the paper. The authors' data convincingly demonstrates that disruption of the trisynaptic circuit bidirectionally modulates both fear- and anxiety-like behaviors while disruption of the temporammonic pathway has no effect on anxiety-like behaviors but disrupts fear-related behaviors. It is interesting to note, however, that the TA activation had no effect on tone-related fear conditioning, suggesting a potential specialized role of the temporammonic pathway specifically in contextual fear memory.

Thank you for your thoughtful description of the present study. It is true that TA pathway is distinct from vCA3 to vCA1 pathway in various ways, one being the synapse formation of these two projections are at different locations or layers on vCA1 neurons i.e., the TA pathway synapses on the stratum lacunosum-moleculare (LMol) layer while the vCA3 to vCA1 pathway synapses at stratum radiatum (Rad), close to the CA1 pyramidal cell layer, which is in line with differential functions of the two projections They modulate the pyramidal cell activity in a different way, with TA pathway synapses being distinct from vCA3 to vCA1 synapses on the pyramidal cell layer, which may result in different computational properties of the two projections. Additionally, TA projections are modulated by dopamine while projections from vCA3 are not, but the projections from vCA3 receive inputs from various sources including collaterals, and entorhinal via dentate gyrus. These distinct features of the two projections may contribute to differential modulation of vCA1 activity. We note that cue-related fear is not affected by the TA activation, however even in this case, the TA pathway activation by channelrhodopsin or inhibition by halorhodopsin results in a decrease or an increase of the contextual fear response, respectively.