Obesogenic diet induces circuit-specific memory deficits in mice
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Evaluation Summary:
This work is valuable for those who study how diet and metabolism impact neurological function, specifically learning and memory since it investigates the impact of high-fat diet intake during the preadolescent period on memory performances. The data convincingly showed the possibility to reverse memory deficits related to obesity by manipulating selected hippocampal circuits. The claims would benefit from additional controls and analyses.
(This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the manuscript. Reviewer #1 agreed to share their name with the authors.)
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Abstract
Obesity is associated with neurocognitive dysfunction, including memory deficits. This is particularly worrisome when obesity occurs during adolescence, a maturational period for brain structures critical for cognition. In rodent models, we recently reported that memory impairments induced by obesogenic high-fat diet (HFD) intake during the periadolescent period can be reversed by chemogenetic manipulation of the ventral hippocampus (vHPC). Here, we used an intersectional viral approach in HFD-fed male mice to chemogenetically inactivate specific vHPC efferent pathways to nucleus accumbens (NAc) or medial prefrontal cortex (mPFC) during memory tasks. We first demonstrated that HFD enhanced activation of both pathways after training and that our chemogenetic approach was effective in normalizing this activation. Inactivation of the vHPC–NAc pathway rescued HFD-induced deficits in recognition but not location memory. Conversely, inactivation of the vHPC–mPFC pathway restored location but not recognition memory impairments produced by HFD. Either pathway manipulation did not affect exploration or anxiety-like behaviour. These findings suggest that HFD intake throughout adolescence impairs different types of memory through overactivation of specific hippocampal efferent pathways and that targeting these overactive pathways has therapeutic potential.
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Author Response
Reviewer #1 (Public Review):
Bakoyiannis et al. investigated the distinct contribution of ventral hippocampal outputs to the nucleus accumbens and medial prefrontal cortex on memory in mice exposed to a high-fat diet (HFD) beginning in adolescence. The authors first characterize the hippocampal to accumbens or mPFC circuits using intersectional viral approaches. They then replicate their previous finding that adolescent HFD contributes to the overactivation of the ventral hippocampus during contextual learning via quantification of c-fos+ cells. In this manuscript, the authors further explore the distinct contribution of these two outputs from the ventral hippocampus using chemogenetics to specifically inhibit one circuit or the other. Interestingly, the authors find that inhibition of either circuit returns c-fos+ …
Author Response
Reviewer #1 (Public Review):
Bakoyiannis et al. investigated the distinct contribution of ventral hippocampal outputs to the nucleus accumbens and medial prefrontal cortex on memory in mice exposed to a high-fat diet (HFD) beginning in adolescence. The authors first characterize the hippocampal to accumbens or mPFC circuits using intersectional viral approaches. They then replicate their previous finding that adolescent HFD contributes to the overactivation of the ventral hippocampus during contextual learning via quantification of c-fos+ cells. In this manuscript, the authors further explore the distinct contribution of these two outputs from the ventral hippocampus using chemogenetics to specifically inhibit one circuit or the other. Interestingly, the authors find that inhibition of either circuit returns c-fos+ cell number to control levels, but the effects on memory are dissociable. They demonstrate that inhibition of output to the NAc rescues HFD-induced deficits on object recognition, while inhibition of mPFC outputs rescues HFD-induced deficits on object location recall. The authors further confirmed that chemogenetic manipulations resulted in alterations in c-fos+ cells that were specific to CA1, and not CA3 or DG. Behaviorally, they excluded any contribution of anxiety on recall, finding no effect on the elevated plus maze.
The strengths of this manuscript include robust behavioral findings that can be attributed to specific circuits. The conclusions of this paper are largely well supported by the data, although some of the methods could provide more detail and the statistical approaches used for analysis need improvement.
We thank the Reviewer for thoroughly summarizing the main results of the study and for providing the comments that we address below.
Reliance on only one measure of anxiety to exclude this as a confound on recall performance is a weakness of the manuscript. To be more convincing that anxiety is not a confound, more than one behavioral assay should be performed.
Reviewer #2 (Public Review):
Bakoyiannis et al. aim to analyze the impact of high-fat diet (HFD) intake during the preadolescent period on memory performances by optogenetically manipulating the circuits responsible for related memory performances. In previous work, they showed the possibility to rescue object-based memory impairments in HFD-exposed animals by silencing the ventral hippocampus (vHPC). Here they investigated further the projections to the nucleus accumbens (NAc) and medial prefrontal cortex (mPFC), 2 of the main monosynaptic targets of the vHPC.
They used a precise strategy to target and manipulate only vHPC cells that project to either NAc or mPFC. They found that preadolescent HFD can induce different types of memory deficits related to different vHPC pathways. In particular, they found that silencing vHPC-NAc, but not vHPC-mPFC, pathway restored HFD-induced object recognition memory deficit. On the other side, silencing vHPC to mPFC, but not vHPC-NAc, pathway rescued HFD-induced object location memory deficits. Moreover, these pathways do not control anxiety-like behaviours since their inactivation has no effect on anxiety levels.
We thank the Reviewer for summarizing the findings of the study and for their positive comments on our manuscript.
The conclusions of the manuscript are mostly supported by the results, but there are some points and controls that need to be addressed and clarified:
- While identifying the relevance of hippocampal cells projecting to NAc and mPFC, a missing control is to verify the activity of vHPC not projecting to these 2 regions in normal conditions or when the investigated pathways are manipulated. This control is essential to refine and bring novel results related to their previous discovery that vHPC overall is involved in the process.
- A downstream effect of their optogenetic manipulation on NAc and mPFC cellular populations should be shown if they want to claim that their chemogenetic inhibition decrease the activation of the pathway and not only of vHPC projecting neurons.
New c-Fos experiments were performed. Please see our response to points 4-5-6 in the “Essential Revision” section.
Reviewer #3 (Public Review):
"Obesogenic diet induces circuit-specific memory deficits in mice" by Bakoyiannis et al., investigates the role of specific ventral hippocampal circuits (specifically to nucleus accumbens and mPFC) in high-fat diet-induced memory deficits. The authors had previously shown that increases in activity in the ventral hippocampus accompany high-fat diet-induced memory deficits, and that inhibition of activity thereby normalizes those memory deficits. In this manuscript, the authors extend these findings to specific projections, showing that they normalize different types of memories by inhibiting the two different pathways.
The strengths of the paper include the pathway-specific manipulations that reveal a difference between the two types of memory. The results are a modest step forward for the field of feeding and learning and memory and would be of interest to that subgroup of neuroscientists. However, the paper also has a number of weaknesses which I detail below.
We thank the Reviewer for summarizing the finding of our study and for the positive feedback.
- First, the authors show an effect of cfos from both pathways in Figure 2 on object learning. However, the inactivation studies show a pathway-specific effect on object recognition and object location, with no experiments to delineate how this divergence occurs. The authors do not specify whether they compared cfos in the control group between NAc and mPFC projections (presumably they did some controls with each injection), which might reveal differences.
We have added new groups and presented/analyzed the results for each pathway (either vHPC-NAc pathway or vHPC-mPFC pathway) separately for c-Fos (new Figure 2 and Figure 2-Figure Supplement 1) or behaviours (new Figure 3 and Figure 3-Figure Supplement 1). Please see our responses to points 2, 4-5-6 and 9 in the “Essential Revision” section.
- Related to this, it is unclear how the pathways end up diverging for memory if they do not show any differences in cfos during training. Perhaps there are pathway-specific differences in cfos following the ORM and OLM tests? It is difficult to support the claim that there are pathway differences in memory following inactivation if we do not see any pathway-specific change in activity.
We thank the Reviewer for this comment. Please see our answer to point 7 in the “Essential Revision” section above.
- Figure 2 and Figure 3 are also hard to interpret because of the usage of a 1-way ANOVA which is not the appropriate statistical test when there are two independent variables (HFD and DREADD manipulation). Indeed, noticing the statistical test also reveals that a critical control missing: HFD -, hM4di+CNO +. It is possible that inactivation simply brings down cfos levels regardless of diet. While this might benefit memory in the case of HFD, it is critical to know whether the manipulation is specific to the overactivation caused by HFD or just provides a general decrease in activity.
Based on this comment we added new HFD-hM4di+CNO+ groups and modified statistical analyses accordingly. Indeed, inactivation of each pathway (vHPC-NAc or vHPC-mPFC) decreases c-Fos in both HFD+ and HFD- (CD+) groups (new Figure 2) whereas it has opposite effect on behaviors, improving memory performance in HFD+ groups but impairing or having no effect in HFD- (CD+) groups (new Figure 3). We have corrected this in the manuscript (please see our responses to points 2 and 9 of “Essential Revision” section).
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Evaluation Summary:
This work is valuable for those who study how diet and metabolism impact neurological function, specifically learning and memory since it investigates the impact of high-fat diet intake during the preadolescent period on memory performances. The data convincingly showed the possibility to reverse memory deficits related to obesity by manipulating selected hippocampal circuits. The claims would benefit from additional controls and analyses.
(This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the manuscript. Reviewer #1 agreed to share their name with the authors.)
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Reviewer #1 (Public Review):
Bakoyiannis et al. investigated the distinct contribution of ventral hippocampal outputs to the nucleus accumbens and medial prefrontal cortex on memory in mice exposed to a high-fat diet (HFD) beginning in adolescence. The authors first characterize the hippocampal to accumbens or mPFC circuits using intersectional viral approaches. They then replicate their previous finding that adolescent HFD contributes to the overactivation of the ventral hippocampus during contextual learning via quantification of c-fos+ cells. In this manuscript, the authors further explore the distinct contribution of these two outputs from the ventral hippocampus using chemogenetics to specifically inhibit one circuit or the other. Interestingly, the authors find that inhibition of either circuit returns c-fos+ cell number to …
Reviewer #1 (Public Review):
Bakoyiannis et al. investigated the distinct contribution of ventral hippocampal outputs to the nucleus accumbens and medial prefrontal cortex on memory in mice exposed to a high-fat diet (HFD) beginning in adolescence. The authors first characterize the hippocampal to accumbens or mPFC circuits using intersectional viral approaches. They then replicate their previous finding that adolescent HFD contributes to the overactivation of the ventral hippocampus during contextual learning via quantification of c-fos+ cells. In this manuscript, the authors further explore the distinct contribution of these two outputs from the ventral hippocampus using chemogenetics to specifically inhibit one circuit or the other. Interestingly, the authors find that inhibition of either circuit returns c-fos+ cell number to control levels, but the effects on memory are dissociable. They demonstrate that inhibition of output to the NAc rescues HFD-induced deficits on object recognition, while inhibition of mPFC outputs rescues HFD-induced deficits on object location recall. The authors further confirmed that chemogenetic manipulations resulted in alterations in c-fos+ cells that were specific to CA1, and not CA3 or DG. Behaviorally, they excluded any contribution of anxiety on recall, finding no effect on the elevated plus maze.
The strengths of this manuscript include robust behavioral findings that can be attributed to specific circuits. The conclusions of this paper are largely well supported by the data, although some of the methods could provide more detail and the statistical approaches used for analysis need improvement.
Reliance on only one measure of anxiety to exclude this as a confound on recall performance is a weakness of the manuscript. To be more convincing that anxiety is not a confound, more than one behavioral assay should be performed.
-
Reviewer #2 (Public Review):
Bakoyiannis et al. aim to analyze the impact of high-fat diet (HFD) intake during the preadolescent period on memory performances by optogenetically manipulating the circuits responsible for related memory performances. In previous work, they showed the possibility to rescue object-based memory impairments in HFD-exposed animals by silencing the ventral hippocampus (vHPC). Here they investigated further the projections to the nucleus accumbent (NAc) and medial prefrontal cortex (mPFC), 2 of the main monosynaptic targets of the vHPC.
They used a precise strategy to target and manipulate only vHPC cells that project to either NAc or mPFC. They found that preadolescent HFD can induce different types of memory deficits related to different vHPC pathways. In particular, they found that silencing vHPC-NAc, but not …
Reviewer #2 (Public Review):
Bakoyiannis et al. aim to analyze the impact of high-fat diet (HFD) intake during the preadolescent period on memory performances by optogenetically manipulating the circuits responsible for related memory performances. In previous work, they showed the possibility to rescue object-based memory impairments in HFD-exposed animals by silencing the ventral hippocampus (vHPC). Here they investigated further the projections to the nucleus accumbent (NAc) and medial prefrontal cortex (mPFC), 2 of the main monosynaptic targets of the vHPC.
They used a precise strategy to target and manipulate only vHPC cells that project to either NAc or mPFC. They found that preadolescent HFD can induce different types of memory deficits related to different vHPC pathways. In particular, they found that silencing vHPC-NAc, but not vHPC-mPFC, pathway restored HFD-induced object recognition memory deficit. On the other side, silencing vHPC to mPFC, but not vHPC-NAc, pathway rescued HFD-induced object location memory deficits. Moreover, these pathways do not control anxiety-like behaviours since their inactivation has no effect on anxiety levels.
The conclusions of the manuscript are mostly supported by the results, but there are some points and controls that need to be addressed and clarified:
- While identifying the relevance of hippocampal cells projecting to NAc and mPFC, a missing control is to verify the activity of vHPC not projecting to these 2 regions in normal conditions or when the investigated pathways are manipulated. This control is essential to refine and bring novel results related to their previous discovery that vHPC overall is involved in the process.
- A downstream effect of their optogenetic manipulation on NAc and mPFC cellular populations should be shown if they want to claim that their chemogenetic inhibition decrease the activation of the pathway and not only of vHPC projecting neurons.
-
Reviewer #3 (Public Review):
"Obesogenic diet induces circuit-specific memory deficits in mice" by Bakoyiannis et al., investigates the role of specific ventral hippocampal circuits (specifically to nucleus accumbens and mPFC) in high-fat diet-induced memory deficits. The authors had previously shown that increases in activity in the ventral hippocampus accompany high-fat diet-induced memory deficits, and that inhibition of activity thereby normalizes those memory deficits. In this manuscript, the authors extend these findings to specific projections, showing that they normalize different types of memories by inhibiting the two different pathways.
The strengths of the paper include the pathway-specific manipulations that reveal a difference between the two types of memory. The results are a modest step forward for the field of feeding …
Reviewer #3 (Public Review):
"Obesogenic diet induces circuit-specific memory deficits in mice" by Bakoyiannis et al., investigates the role of specific ventral hippocampal circuits (specifically to nucleus accumbens and mPFC) in high-fat diet-induced memory deficits. The authors had previously shown that increases in activity in the ventral hippocampus accompany high-fat diet-induced memory deficits, and that inhibition of activity thereby normalizes those memory deficits. In this manuscript, the authors extend these findings to specific projections, showing that they normalize different types of memories by inhibiting the two different pathways.
The strengths of the paper include the pathway-specific manipulations that reveal a difference between the two types of memory. The results are a modest step forward for the field of feeding and learning and memory and would be of interest to that subgroup of neuroscientists. However, the paper also has a number of weaknesses which I detail below.
1. First, the authors show an effect of cfos from both pathways in Figure 2 on object learning. However, the inactivation studies show a pathway-specific effect on object recognition and object location, with no experiments to delineate how this divergence occurs. The authors do not specify whether they compared cfos in the control group between NAcc and mPFC projections (presumably they did some controls with each injection), which might reveal differences.
2. Related to this, it is unclear how the pathways end up diverging for memory if they do not show any differences in cfos during training. Perhaps there are pathway-specific differences in cfos following the ORM and OLM tests? It is difficult to support the claim that there are pathway differences in memory following inactivation if we do not see any pathway-specific change in activity.
3. Figure 2 and Figure 3 are also hard to interpret because of the usage of a 1-way ANOVA which is not the appropriate statistical test when there are two independent variables (HFD and DREADD manipulation). Indeed, noticing the statistical test also reveals that a critical control missing: HFD -, hM4di+CNO +. It is possible that inactivation simply brings down cfos levels regardless of diet. While this might benefit memory in the case of HFD, it is critical to know whether the manipulation is specific to the overactivation caused by HFD or just provides a general decrease in activity.
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