The brain is very sensitive to high altitude hypoxia. It has been shown that HH can shorten escape latency and target quadrant residence time, and reduce the number of crossings of the target quadrant in the MWM, which suggests that spatial learning and memory ability are reduced.[1,3,17] In accordance with previous studies, we found that after 5 days of acquisition training, the ability of all of the rats to locate the platform had improved. However, compared with rats housed in a normobaric normoxia environment, rats exposed to HH for 4 weeks had a significantly longer escape latency (P < 0.05), while there was no significant between-group difference in the average swimming speed. This indicated that the model group needed more time to find the platform than the control group, which was suggestive of spatial learning dysfunction rather than physical differences.
The results of the probe test showed that the model group spent significantly less time in the original platform quadrant (P < 0.05) and had a significantly lower frequency of crossing the original platform quadrant (P < 0.05) than the control group, which indicated spatial memory dysfunction in the model group.
Together, the MWM results revealed the characteristics of spatial cognitive impairment in rats with sustained HH exposure.
Learning and memory are important cognitive abilities that involve a complex network of functional brain regions that work together to manage information. The storage and extraction of information involves many brain regions and nerve conduction pathways. In particular, the hippocampus, EC, striatum, thalamus, and basal forebrain play important roles in cognition.[17–20] Although an increasing number of pathophysiological and MRI studies involve the evaluation of brain regions other than just the hippocampus, extensive histological analysis of the whole brain is unrealistic. Therefore, it is necessary to evaluate changes of cognitive function caused by HH exposure objectively and comprehensively.
The current study is a rare example of an investigation into alterations of local spontaneous neural activity in the rat brain after sustained HH exposure. Analysis combining rs-fMRI with fALFF can measure spontaneous activity of neurons during resting state, and can effectively suppress non-specific signals and reduce interference from physiological noise. This technique is now widely regarded as an important method for measuring local functional activity of the brain. In this study, abnormal brain activity was found in multiple regions of the brain in rats from the model group compared with control group, including the hippocampus, EC, RSC, thalamus, striatum, and cerebellum.
The hippocampus is considered to be an important component of the limbic system and a crucial hub of the default mode network of the brain.[22,23] Studies have shown that hippocampal damage induced by HH tends to be aggravated by increasing altitude and exposure time. Titus et al found that the spatial learning of rats exposed to a simulated altitude of 6000 m was slightly affected after 2 days of exposure and significantly impaired after 7 days of exposure; histomorphological studies confirmed that hippocampal injury occurred simultaneously with cognitive impairment. Studies by Maiti et al[2,5,24] reported similar findings. In this study, we found that the fALFF in the bilateral hippocampus of the model group was decreased compared with that in the control group, which suggests that HH caused functional impairment in the hippocampus. This impairment may be related to the physiological and pathological changes of hippocampal pyramidal cells and the apoptosis of these cells reported in previous studies.
The EC serves as a gateway for the connection between the hippocampus and neocortex, and plays an important role in advanced cognition, such as spatial learning and memory, spatial exploration, and navigation.[25,26] The EC receives information from the neocortex, and the information is integrated into the place cells of the hippocampus for further processing to facilitate spatial cognition. It has been reported that chronic HH exposure induces a significant decrease of dendritic branching in the EC of rats. However, in the present study, the model group showed a decrease in the fALFF in the left EC, which suggested that the function of the left EC was impaired. This may be related to the impairments in locating the platform and in navigational learning observed in the model group in the MWM.
The RSC, a region that is strongly interconnected with the hippocampal formation, has been shown to play an important role in spatial navigation, learning, and memory, both in humans and in rodents.[25,27] We found that the fALFF was decreased in the RSC of rats in the model group, which is suggestive of cortical functional impairment after HH exposure. However, effects on this brain region have rarely been reported in previous studies of HH rats.
The thalamus, which functions as a relay station of the nervous system, transmits information between different subcortical regions and the cortex. The mammillary region and the anterior thalamic nucleus play an important role in spatial learning and memory, and it has been shown that injury of the anterior thalamic nucleus leads to more serious spatial memory impairment. The present study found that the fALFF was decreased in the left thalamus and bilateral mammillary regions of rats in the model group, which suggested that dysfunction of these regions may play a role in spatial cognitive impairment in HH.
Previous studies have shown that the striatum is also vulnerable to HH. The apoptosis of striatum was consistent with the damaged hippocampus and cortex under such conditions, which is associated with behavioral impairment in the MWM test. As an important part of the basal forebrain, the septal region has extensive fibrous connections with the hippocampus through the septohippocampal pathway. Some studies have shown that escape latency in the MWM is prolonged and that spatial cognition is impaired in rats with septal area damage. We also found that the fALFF was decreased in the left striatum and the septal region of rats after HH, and that abnormal activation of these brain regions in the model group may be related to changes in cognitive function.
In addition, the fALFF was decreased in some areas of the cerebellum in the model group. In recent years, some studies have reported that the cerebellum is not only responsible for motor coordination, but also participates in some cognitive processes. Rondi-Reig et al summarized the current understanding of cerebellar monitoring of sensory information involved in spatial representation and concluded that the cerebellum updates spatial representations by monitoring sensory information and interacting with navigation pathways to maintain a sense of direction and location. However, the abnormal change in this brain region may be due to pathophysiological changes or because of the weakening of the afferent information from the higher cognitive network; we do not know which of these is the case because there is almost no relevant pathophysiological evidence in any relevant studies.
There are also some limitations in this research. First, since MWM training and testing were conducted on rats under normobaric normoxia conditions after 4 weeks of HH exposure in HH-simulated cabin. The MWM and MR scans were performed simultaneously to reduce the impact of normobaric normoxia exposure time on the accuracy of fMRI. Therefore, the correlation analysis between behavior test and fMRI results had no chance to be conducted. The plateau filed experimental study should be done in the future to complement the data. Second, comparing to the control group, there were extensive areas of fALFF reduction but no obvious fALFF increased area was found in the model group, which may cause by neuronal injury or decompensation of the functional brain regions. In further study, multiple model groups with different exposure time (1 day, 7 days, 14 days, and 28 days) should be compared, that continuous observation of spontaneous brain activity may help us to get better understanding of the procedure from compensatory to decompensatory of functional brain areas in model rats.
In conclusion, research of this kind has rarely used rs-fMRI combined with fALFF analysis to observe changes in spontaneous brain activity in rats after sustained HH exposure. Different from relevant previous pathophysiological studies that focused on the hippocampus and local cortex of rats, we found widespread reductions in fALFF throughout the brain. Furthermore, some brain regions with abnormal activation were located at the hub of the default mode network; this finding may provide another key insight into the potential mechanism underlying the cognitive impairment of rats following HH exposure.
This work was supported by grants from the National Natural Science Foundation of China (No. 61527807) and the Key Program of the National Natural Science Foundation of China (No. 81630003).
1. Lefferts WK, DeBlois JP, White CN, Day TA, Heffernan KS, Brutsaert TD. Changes in cognitive function and latent processes of decision-making during incremental ascent to high altitude. Physiol Behav
2019; 201:139–145. doi: 10.1016/j.physbeh.2019.01.002.
2. Maiti P, Singh SB, Mallick B, Muthuraju S, Ilavazhagan G. High altitude memory impairment is due to neuronal apoptosis in hippocampus, cortex and striatum. J Chem Neuroanat
2008; 36:227–238. doi: 10.1016/j.jchemneu.2008.07.003.
3. Martin K, McLeod E, Périard J, Rattray B, Keegan R, Pyne DB. The impact of environmental stress on cognitive performance: a systematic review. Hum Factors
2019; 19:18720819839817doi: 10.1177/0018720819839817.
4. Bailey DM, Brugniaux JV, Filipponi T, Marley CJ, Stacey B, Soria R, et al. Exaggerated systemic oxidative-inflammatory-nitrosative stress in chronic mountain sickness is associated with cognitive decline and depression. J Physiol
2019; 597:611–629. doi: 10.1113/JP276898.
5. Maiti P, Singh SB, Muthuraju S, Veleri S, Ilavazhagan G. Hypobaric hypoxia
damages the hippocampal pyramidal neurons in the rat
brain. Brain Res
2007; 1175:1–9. doi: 10.1016/j.brainres.2007.06.106.
6. Zhang J, Chen J, Fan C, Li J, Lin J, Yang T, et al. Alteration of spontaneous brain activity after hypoxia-reoxygenation: a resting-state fMRI study. High Alt Med Biol
2017; 1:20–26. doi: 10.1089/ham.2016.0083.
7. Chen J, Fan C, Li J, Han Q, Lin J, Yang T, et al. Increased intraregional synchronized neural activity in adult brain after prolonged adaptation to high-altitude hypoxia: a resting-state fMRI study. High Alt Med Biol
2016; 17:16–24. doi: 10.1089/ham.2015.0104.
8. Chen X, Zhang Q, Wang J, Liu J, Zhang W, Qi S, et al. Cognitive and neuroimaging changes in healthy immigrants upon relocation to a high altitude: a panel study. Hum Brain Mapp
2017; 38:3865–3877. doi: 10.1002/hbm.23635.
9. Cramer NP, Korotcov A, Bosomtwi A, Xu X, Holman DR, Whiting K, et al. Neuronal and vascular deficits following chronic adaptation to high altitude. Exp Neurol
2019; 311:293–304. doi: 10.1016/j.expneurol.2018.10.007.
10. Wei W, Wang X, Gong Q, Fan M, Zhang J. Cortical thickness of native tibetans in the Qinghai-tibetan plateau. AJNR Am J Neuroradiol
2017; 38:553–560. doi: 10.3174/ajnr.A5050.
11. Hackett PH, Yarnell PR, Weiland DA, Reynard KB. Acute and evolving MRI of high-altitude cerebral edema: microbleeds, edema, and pathophysiology. AJNR Am J Neuroradiol
2019; 40:464–469. doi: 10.3174/ajnr.A5897.
12. Chen L, Cai C, Yang T, Lin J, Cai S. Changes in brain iron concentration after exposure to high-altitude hypoxia measured by quantitative susceptibility mapping. Neuroimage
2017; 147:488–499. doi: 10.1016/j.neuroimage.2016.12.033.
13. Chen J, Li J, Han Q, Lin J, Yang T, Chen Z, et al. Long-term acclimatization to high-altitude hypoxia modifies interhemispheric functional
and structural connectivity in the adult brain. Brain Behav
2016; 6:e512doi: 10.1002/brb3.512.
14. Vorhees CV, Williams MT. Morris water maze: procedures for assessing spatial and related forms of learning and memory. Nat Protoc
2006; 1:848–858. doi: 10.1038/nprot.2006.116.
15. Nie B, Chen K, Zhao S, Liu J, Gu X, Yao Q, et al. A rat
brain MRI template with digital stereotaxic atlas of fine anatomical delineations in paxinos space and its automated application in voxel-wise analysis. Hum Brain Mapp
2013; 34:1306–1318. doi: 10.1002/hbm.21511.
16. Nie B, Hui J, Wang L, Chai P, Gao J, Liu S, et al. Automatic method for tracing regions of interest in rat
brain magnetic resonance imaging
studies. J Magn Reson Imaging
2010; 32:830–835. doi: 10.1002/jmri.22283.
17. Qaid E, Zakaria R, Sulaiman SF, Yusof NM, Shafin N, Othman Z, et al. Insight into potential mechanisms of hypobaric hypoxia
–induced learning and memory deficit-lessons from rat
studies. Hum Experiment Toxicol
2017; 36:1315–1325. doi: 10.1177/0960327116689714.
18. Titus AD, Shankaranarayana RB, Harsha HN, Ramkumar K, Srikumar BN, Singh SB, et al. Hypobaric hypoxia
-induced dendritic atrophy of hippocampal neurons is associated with cognitive impairment in adult rats. Neuroscience
2007; 14:265–278. doi: 10.1016/j.neuroscience.2006.11.037.
19. Zhu M, Xu M, Zhang K, Li J, Ma H, Xia G, et al. Effect of acute exposure to hypobaric hypoxia
on learning and memory in adult Sprague-Dawley rats. Behav Brain Res
2019; 367:82–90. doi: 10.1016/j.bbr.2019.03.047.
20. Ochi G, Yamada Y, Hyodo K, Suwabe K, Fukuie T, Byun K, et al. Neural basis for reduced executive performance with hypoxic exercise. NeuroImage
2018; 171:75–83. doi: 10.1016/j.neuroimage.2017.12.091.
21. Zou QH, Zhu CZ, Yang Y, Zuo XN, Long XY, Cao QJ, et al. An improved approach to detection of amplitude of low-frequency fluctuation (ALFF) for resting-state fMRI: fractional ALFF. J Neurosci Methods
2008; 172:137–141. doi: 10.1016/j.jneumeth.2008.04.012.
22. Lisman J, Buzsáki G, Eichenbaum H, Nadel L, Ranganath C, Redish AD. Viewpoints: how the hippocampus contributes to memory, navigation and cognition
. Nat Neurosci
2017; 20:1434–1447. doi: 10.1038/nn.4661.
23. Eichenbaum H. The role of the hippocampus in navigation is memory. J Neurophysiol
2017; 117:1785–1796. doi: 10.1152/jn.00005.2017.
24. Maiti P, Muthuraju S, Ilavazhagan G, Singh SB. Hypobaric hypoxia
induces dendritic plasticity in cortical and hippocampal pyramidal neurons in rat
brain. Behav Brain Res
2008; 189:233–243. doi: 10.1016/j.bbr.2008.01.007.
25. Epstein RA, Patai EZ, Julian JB, Spiers HJ. The cognitive map in humans: spatial navigation and beyond. Nat Neurosci
2017; 20:1504–1513. doi: 10.1038/nn.4656.
26. Grieves RM, Duvelle É, Wood ER, Dudchenko PA. Field repetition and local mapping in the hippocampus and the medial entorhinal cortex. J Neurophysiol
2017; 118:2378–2388. doi: 10.1152/jn.00933.2016.
27. Ash JA, Lu H, Taxier LR, Long JM, Yang Y, Stein EA, et al. Functional
connectivity with the retrosplenial cortex predicts cognitive aging in rats. Proc Nat Acad Sci U S A
2016; 113:12286–12291. doi: 10.1073/pnas.1525309113.
28. Hwang K, Bertolero MA, Liu WB, D’Esposito M. The human thalamus is an integrative hub for functional
brain networks. J Neurosci
2017; 37:5594–5607. doi: 10.1523/JNEUROSCI.0067-17.2017.
29. Wu M, Shanabrough M, Leranth C, Alreja M. Cholinergic excitation of septohippocampal GABA but not cholinergic neurons: implications for learning and memory. J Neurosci
2000; 20:3900–3908. doi: 10.1523/JNEUROSCI.20-10-03900.2000.
30. Ang ST, Ariffin MZ, Khanna S. The forebrain medial septal region and nociception. Neurobiol Learn Mem
2017; 138:238–251. doi: 10.1016/j.nlm.2016.07.017.
31. Rondi-Reig L, Paradis A, Lefort JM, Babayan BM, Tobin C. How the cerebellum may monitor sensory information for spatial representation. Front Sys Neurosci
2014; 8:205doi: 10.3389/fnsys.2014.00205.
32. Schmahmann JD. The cerebellum and cognition
. Neurosci Lett
2019; 688:62–75. doi: 10.1016/j.neulet.2018.07.005.