Swimming exercise enhances brain plasticity in fish

It is well-established that sustained exercise training can enhance brain plasticity and boost cognitive performance in mammals, but this phenomenon has not received much attention in fish. The aim of this study was to determine whether sustained swimming exercise can enhance brain plasticity in juvenile Atlantic salmon. Brain plasticity was assessed by both mapping the whole telencephalon transcriptome and conducting telencephalic region-specific microdissections on target genes. We found that 1772 transcripts were differentially expressed between the exercise and control groups. Gene ontology (GO) analysis identified 195 and 272 GO categories with a significant overrepresentation of up- or downregulated transcripts, respectively. A multitude of these GO categories was associated with neuronal excitability, neuronal signalling, cell proliferation and neurite outgrowth (i.e. cognition-related neuronal markers). Additionally, we found an increase in proliferating cell nuclear antigen (pcna) after both three and eight weeks of exercise in the equivalent to the hippocampus in fish. Furthermore, the expression of the neural plasticity markers synaptotagmin (syt) and brain-derived neurotrophic factor (bdnf) were also increased due to exercise in the equivalent to the lateral septum in fish. In conclusion, this is the first time that swimming exercise has been directly linked to increased telencephalic neurogenesis and neural plasticity in a teleost, and our results pave the way for future studies on exercise-induced neuroplasticity in fish.


Comments to the Author(s)
This is a very interesting study addressing the effects of exercise on neuronal plasticity. The results show that excersise increases the expression of genes related to neurogenesis and plasticity and at the same time reduces expression of genes related to apoptotic pathways. These are clearly novel findings, at least in teleost fish. This is also pointed out by the authors already in the abstract and throughout the manuscript. It is perhaps enough to state this novelty once. The first sentence in the abstract states that it is well established in mammals that running exercise enhances brain plasticity and cognitive performance but that this phenomenon has not been much studied in fish. First, I don't think that this effect in mammals is restricted to running but is a more general response to endurance exercise. Moreover, fish do not run. Thus I would suggest that running exercise is exchanged for endurance exercise. As I understand, only one exercise and one control tank was used in the experiment. This is a potential problem since tank effects are common and could not be controlled for. This needs to be acknowledged in the discussion.

Review form: Reviewer 2
Is the manuscript scientifically sound in its present form? Yes

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? No

Recommendation?
Accept with minor revision (please list in comments)

Comments to the Author(s)
The paper by Mes and colleagues characterizes a peculiar aspect on the potential beneficial effects of swimming exercise in juvenile specimens of Atlantic Salmon. The approach and methodology sound correct, some minor concerns need to be addressed: 1. were authors able to distinguish between male and female? 2. did they observe preference place in the tank (bottom or wall or water surface) in course of swimming exercise? 3. how many animals were used for RNA sequencing? 4. it is really interesting to see that bdnf appears upregulated in the Vv and not in the homologous of mammalian hippocampus, where it is quite abundant in other teleost species (mainly zebrafish). To better unravel the reason of this upregulation, have the authors checked also for the specific receptor TrkB? Based on these results, it could be expected to see also an upregulation of TrkB, reinforcing the hypothesis that in course of swimming exercise the homologous of lateral septum and not hippocampus is much more involved. 5. authors do not discuss the unchanged levels of NeuroD over the entire experiment and telencephalic region. Can they comment on this? Would be better to make a unique discussion on bdnf upregulation in the homologous of mammalian lateral septum. Therefore, move the paragraph from line 319 to 332 after the paragraph at the line 273. As it is sounds quite repetitive.

22-Nov-2019
Dear Dr Vindas, On behalf of the Editors, I am pleased to inform you that your Manuscript RSOS-191640 entitled "Swimming exercise enhances brain plasticity in fish" has been accepted for publication in Royal Society Open Science subject to minor revision in accordance with the referee suggestions. Please find the referees' comments at the end of this email.
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Once again, thank you for submitting your manuscript to Royal Society Open Science and I look forward to receiving your revision. If you have any questions at all, please do not hesitate to get in touch. Based on the reviewers comments I recommend the manuscript is accepted with minor changes. These changes need to be included in the revised text and explained in the cover letter with line tracking.
Special atention must be given to the Discussion section. This needs to be improved and include the recquirements from the reviewers. Please read their comments carefuly and: 1-Discuss the expression of TrkB, relating it to the hypothesis that in course of swimming exercise the homologous of lateral septum and not hippocampus is much more involved.

2-Discuss the unchanged levels of NeuroD.
Reviewer comments to Author:

Reviewer: 1 Comments to the Author(s)
This is a very interesting study addressing the effects of exercise on neuronal plasticity. The results show that excersise increases the expression of genes related to neurogenesis and plasticity and at the same time reduces expression of genes related to apoptotic pathways. These are clearly novel findings, at least in teleost fish. This is also pointed out by the authors already in the abstract and throughout the manuscript. It is perhaps enough to state this novelty once. The first sentence in the abstract states that it is well established in mammals that running exercise enhances brain plasticity and cognitive performance but that this phenomenon has not been much studied in fish. First, I don't think that this effect in mammals is restricted to running but is a more general response to endurance exercise. Moreover, fish do not run. Thus I would suggest that running exercise is exchanged for endurance exercise. As I understand, only one exercise and one control tank was used in the experiment. This is a potential problem since tank effects are common and could not be controlled for. This needs to be acknowledged in the discussion.
Reviewer: 2 Comments to the Author(s) The paper by Mes and colleagues characterizes a peculiar aspect on the potential beneficial effects of swimming exercise in juvenile specimens of Atlantic Salmon. The approach and methodology sound correct, some minor concerns need to be addressed: 1. were authors able to distinguish between male and female? 2. did they observe preference place in the tank (bottom or wall or water surface) in course of swimming exercise? 3. how many animals were used for RNA sequencing? 4. it is really interesting to see that bdnf appears upregulated in the Vv and not in the homologous of mammalian hippocampus, where it is quite abundant in other teleost species (mainly zebrafish). To better unravel the reason of this upregulation, have the authors checked also for the specific receptor TrkB? Based on these results, it could be expected to see also an upregulation of TrkB, reinforcing the hypothesis that in course of swimming exercise the homologous of lateral septum and not hippocampus is much more involved. 5. authors do not discuss the unchanged levels of NeuroD over the entire experiment and telencephalic region. Can they comment on this? It is a pleasure to accept your manuscript entitled "Swimming exercise enhances brain plasticity in fish" in its current form for publication in Royal Society Open Science. The comments of the reviewer(s) who reviewed your manuscript are included at the foot of this letter.

RESULTS
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Thank you for your fine contribution. On behalf of the Editors of Royal Society Open Science, we look forward to your continued contributions to the Journal. Please find enclosed our revised manuscript "Swimming exercise enhances brain plasticity in fish" (RSOS-191640).
We are grateful for your, and the reviewers, evaluation of our work and would like to thank for instructive and helpful comments. We have given close attention to the issues raised and addressed these in our revised manuscript. Please find a detailed response to each comment below. Sincerely,

Marco A. Vindas
Associate Editor Comments to Author (Dr Susana Lopes): Based on the reviewers comments I recommend the manuscript is accepted with minor changes. These changes need to be included in the revised text and explained in the cover letter with line tracking.
Special atention must be given to the Discussion section. This needs to be improved and include the recquirements from the reviewers. Please read their comments carefuly and: 1. Discuss the expression of TrkB, relating it to the hypothesis that in course of swimming exercise the homologous of lateral septum and not hippocampus is much more involved.
We have now included the role of TrkB in BDNF signaling in Ln 283-287. We do not currently have gene expression levels for this receptor since developing the molecular tools necessary for this endeavor represents a project in itself and goes beyond the context of the current paper. Encouraged by the results obtained in this experiment we are in the process of developing several research tools that will be used in future projects for the further study into the mechanisms at play in swimming-enhanced neuroplasticity in fish 2. Discuss the unchanged levels of NeuroD.
A more extensive discussion of these results is now found in Ln 352-366 Reviewer 1. Comments to the Author(s) 1. This is a very interesting study addressing the effects of exercise on neuronal plasticity. The results show that excersise increases the expression of genes related to neurogenesis and plasticity and at the same time reduces expression of genes related to apoptotic pathways. These are clearly novel findings, at least in teleost fish. This is also pointed out by the authors already in the abstract and throughout the manuscript. It is perhaps enough to state this novelty once.