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REVIEW ARTICLE
Year : 2019  |  Volume : 67  |  Issue : 8  |  Page : 221-226

Neurophysiological changes in simulated microgravity: An animal model


1 Laboratory of Medical Physics, Medical School, Aristotle University of Thessaloniki (AUTh), Thessaloniki, Greece
2 Department of Chemistry, University of Cape Town, South Africa

Date of Web Publication24-May-2019

Correspondence Address:
Dr. Christiane M Nday
Laboratory of Medical Physics, Medical School, Aristotle University of Thessaloniki, Thessaloniki - 541 24
Greece
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0028-3886.259128

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 » Abstract 


Microgravity (MG) is one of the main problems that astronauts have to cope with during space missions. Long-duration space travel can have detrimental effects on human neurophysiology. Despite scientific efforts, these effects are still insufficiently investigated. Animal earth-based analogs are used to investigate potential nervous system associated perturbations that might occur during prolonged space missions. Hindlimb unloading, Tail suspension and Pelvic suspension models are currently used in MG studies. Loss of homeostasis of certain biological pathways in the nervous system can lead to the functioning and expression of receptors/genes, and the release and functioning of neurotransmitters and neuronal membrane ion channels into specific brain regions. The potential impact of MG on molecular mechanisms linked to neurophysiology through animal earth-based analogs is reviewed. The effect of molecular signalling pathways on the decline of neuronal connectivity and cognitive and neuroplasticity function under MG simulated conditions will be studied. The role of biomarkers including neurotransmitters, genes or receptors will be highlighted in the healthy and MG-affected brain. MG-mediated neurodegenerative mechanisms linked to learning and memory impairment will be highlighted. This review depicts the current rodent models applied to simulate MG ground based approaches and investigates the MG induced changes in the nervous system. The neuropathological profile of the above animal MG ground-based models can be comparable to the effects of ageing, anxiety and other neurological disorders. The advantages and limitations of the existing approaches are discussed. MG induced neurophysiology outcomes can be extrapolated to study other clinical applications.


Keywords: Microgravity, nervous system, physiology and rodent analogs
Key Message: Long-duration space travel can have detrimental effects on human neurophysiology. Ground-based animal models, including hindlimb unloading, are used to investigate potential nervous system perturbations that may occur during prolonged space missions. Diverse molecular signalling pathways involved in neuronal connectivity and cognitive and neuroplasticity function occur under microgravity-simulated conditions.


How to cite this article:
Nday CM, Frantzidis C, Jackson G, Bamidis P, Kourtidou-Papadeli C. Neurophysiological changes in simulated microgravity: An animal model. Neurol India 2019;67, Suppl S2:221-6

How to cite this URL:
Nday CM, Frantzidis C, Jackson G, Bamidis P, Kourtidou-Papadeli C. Neurophysiological changes in simulated microgravity: An animal model. Neurol India [serial online] 2019 [cited 2019 Aug 25];67, Suppl S2:221-6. Available from: http://www.neurologyindia.com/text.asp?2019/67/8/221/259128


The environment of space is known to cause variations in many biological processes that are at a homeostatic balance under Earth's normal gravity.[1],[2] One of the conditions primarily responsible for these variations is, apart from cosmic radiation, the absence of gravity. However, the absence of gravity in space, also named zero gravity, can be simulated on Earth as microgravity (MG), or weightlessness. Among other systems, MG, as an extreme environment, can cause changes in the nervous system.[1],[2],[3]

The physiological function of these nervous system processes relies on the differential expression of diverse biomarkers. These biomarkers include receptors, genes and neuronal membrane ion channels. These play an important role in cognitive mechanisms and plasticity associated with learning and memory. The neurophysiological changes that might occur under extreme conditions of MG are the loss of homeostasis in the nervous system. This leads to a cascade of biological mechanisms linked to the function and expression of specific receptors, genes, neurotransmitters and neuronal membrane ion channels in different brain locations. For instance, members of the neurotrophin family of growth factors including brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) play a role on neuronal survival, in neuronal differentiation and in the establishment of synapses.[4] They are located in the basal forebrain, hippocampus and cortex, which are connected to learning, memory and thinking. The serotonin and dopaminergic systems including 5-HT3 receptor genes though reduced (5-HT2A), monoamine oxidase A and B (MAOA and MAOB), tyrosine hydroxylase (Th), dopamine 1-class receptors (D1r), and catechol-O-methyltransferase (COMT)) are deeply involved in the regulation of emotion and behaviour. Their location in the brain determines their function. 5-HT2A is more abundant in the cortex.[5],[6],[7] B-cell lymphoma-extra large (Bcl-xL), which is encoded by the BCL2-like 1 gene, with its homologous BCL2 associated X, apoptosis regulator (BAX) genes are related with apoptosis, the programmed cell death mechanism in all organs of the body. BAX is a pro-apoptotic member of the Bcl-2 protein family.[8] MAOA and MAOB are found in most cell types of the body while tyrosine hydroxylase (Th) is detected in the central nervous system (CNS), peripheral sympathetic neurons and the adrenal medulla. Dopamine 1-class receptors (D1r), DRD1 expression in the central nervous system is highest in the dorsal striatum (caudate and putamen) and ventral striatum (nucleus accumbens and olfactory tubercle).[8] Lower levels of DRD1 mRNA expression occur in the basolateral amygdala, cerebral cortex, septum, thalamus, and hypothalamus. Catechol-O-methyltransferase (COMT) is an enzyme playing a role in dopamine degradation, mainly in the prefrontal cortex. Insulin-like growth factor 1 (IGF-1) is a hormone similar to insulin, primarily produced in the liver but found in every cell and has been implicated in biosynthesis. It is part of the metabolic pathways making bigger molecules from smaller ones in the body.[9] N-methyl-D-aspartate receptor is a glutamate receptor. Ion channel protein and glutamate neurotransmitters are located in nerve cells and play a role in the majority of synaptic connections.[10],[11] Hypothalamic-pituitary-adrenal (HPA) axis influences feedback interactions among the hypothalamus, the pituitary gland and the adrenal glands.[12] The receptors connected with this axis, such as catecholamine, corticosteroids, growth hormone, epinephrine, norepinephrine and others, are indicators for the stress responses of the organism. The antidiuretic hormone (ADH) synthesis and transformation into arginine vasopressin (AVP) take place in the hypothalamus. By being released directly into the brain from the hypothalamus, AVP may also play a role in social behaviour, mate bonding and maternal responses to stress.[13] Gamma amino butyric acid (GABA) receptors are the main inhibitory neurotransmitters present in 30%-40% of synapses in the brain, located in the substantia nigra, hippocampus, globus pallidus nuclei of the basal ganglia, periaqueductal grey matter and hypothalamus.[14] They are important in neuronal communication, through synaptic transmission. Up or down-expression of the above biomarkers might have clinical effects on the dysregulation of neurotransmission, memory and learning, leading to diverse pathological conditions such as Alzheimer's disease.[15],[16]

Ground-based animal models are employed to investigate biomarkers in MG-simulated conditions as there are practical and economic obstacles in conducting human or animal studies in space. Among other MG simulations ground-based animal models, hindlimb unloading (HU) ground-based animal model is used to assess the MG effects on the ground [17] due to its similarity to human physiology [Figure 1].
Figure 1: The hindlimb unloading (HU) ground-based animal model

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The HU rodent model [18] was initially introduced in mid-1970s at the National Aeronautics and Space Administration (NASA) Ames Research Center to enable the study of mechanisms, responses and treatments for adverse consequences, encountered during spaceflight. Since then, this MG ground-based animal model has been extensively used by laboratories worldwide to simulate weightlessness and study various physiological changes associated with absence of gravity. The HU rat model is the model of choice for studies on biological systems during simulated spaceflight.[19] Its design relies on the potential physiological MG-mediated responses that the model can simulate, to assess the data obtained, that is comparable with spaceflight animal or human studies. Though initiated and tested in 1975, the model was described in detail by Morey in 1979.[20] The simulator had a simple form, using a back harness and a cantilevered rotating beam, that allowed the head-down position of the animal model to move in 360° angle [Figure 2]. The animal model is kept in that position when MG physiological and phenotypic changes start to appear from the experimental day 7 until the allowed period duration for this kind of experiment (2 months maximum). During this time, the animal is fed normally depending on the experiment. The age of the rodent used varies from hours (postnatal) to 6 months.[21],[22],[23] Besides behavioural and brain structure-related studies that can be implemented using HU ground-based animal models, post mortem investigations are also conducted. The latter focus on biochemical, electrophysiological or histological outcomes where differential expression of biomarkers is investigated in diverse organs of interest, under MG-simulated conditions.[11],[12],[13],[14],[15],[16],[17],[18],[19],[20],[24] Usually, the initial outcome demonstrated weight and bone changes similar to changes found in astronauts of Cosmos 782 and 936 spaceflight. Since then, the model has been modified though the concept remains the same.[25],[26],[27]
Figure 2: A simple illustration of the experimental flow for creation of the simulated conditions and the HU-ground animal model. (A) Employment of normal rodent used in animal studies. (B) Normal rodent kept in a cage with its hind limbs not touching the cage surface, over a time period (2 months maximum), thus creating MG conditions simulating situations where astronauts lose their ability to contact the floor surface using their lower limbs. This is called the HU ground-based animal model. Technical details of the HU ground-based animal models have been described by Morey.[28] (C) During this period, various changes occurring in the nervous system can be studied

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 » Hindlimb Unloading Ground-Based Animal Model and Its Neurophysiological Linked Changes Top


  1. Several alterations in the physiology of the nervous system have been demonstrated employing the HU ground-based animal model. Recently, Kulikova et al.,[29] examined the compliance of HU with the effect of actual spaceflight on brain neuroplasticity assessing brain neurotrophic factors (GDNF, cerebral dopamine derived neurotropic factor [CDNF]), apoptotic factors (Bcl-xL, BAX), serotonin and dopaminergic systems (5-HT2A, MAOA, MAOB, Th, D1r, COMT). The expression of dopanimergic genes was found to be highly abundant in HU in the striatum region of the brain, while MAOA and 5-HT2A receptor genes were not altered. Expression of Bcl-xL in the hippocampus under simulated conditions, using HU ground-based animal model was similar and comparable, unlike its gene expression under actual space environment conditions.[30] HU linked MG neurophysiology, projects damaged cortical motor map organization and corticospinal irritability.[31] It has been hypothesized that cortical reshaping may affect the motor function of HU rodents.[32] Consequently, sensorimotor restriction can influence the motor cortex organization by causing deficits in motor performance of HU. Insulin-like growth factor 1 (IGF-1) of HU was demonstrated to contribute to protection of the cortex from degeneration. The reduction of IGF-1 function results in age-related variations in individuals. In addition it inhibited the deterioration of potential acute functions that had an influence over ground locomotion [33]
  2. HU ground-based animal models have been reported to develop anxiety-like behavior. This was demonstrated by the decreased level of NR2A/2B subunits of the N-methyl-D-aspartate receptor and glutamate levels.[34] The latter also play a crucial role in synaptic plasticity as well as memory function. An important issue is to determine to what extent the variations in organ systems are a result of stress responses and activation of the HPA axis. The levels of adrenal corticosteroids, growth hormone, epinephrine, norepinephrine, body mass and growth, lymphoid organ atrophy and adrenal gland mass are parameters used for assessment of the stress responses. An increase in catecholamine production and corticosteroid levels have been reported in HU models.


The MG effects on HU rats have been associated with certain biomarkers, in a time-dependent manner. Alterations in antidiuretic hormone (ADH) biomarker have been observed. Based on the MG time-dependent effects in the HU model, ADH is activated significantly on the 7th day, suggesting that the ADH system may be involved in anti-diuretic phenomenon in early spaceflight period. It is presumed that the ADH system may require 14 days for adaptation to MG. The plasma ADH as well as angiotensin II level peak on the 7th day of HU, and return to their initial values on the 14th day. Electrolytes such as sodium, potassium and chloride do not demonstrate any changes during the HU period. In immune histochemical studies, the ADH and c-Fos immune reactivities (IR) were maximum on the 7th day of the HU process, in the paraventricular and supraoptic nucleus. The aquaporin 2 (AQP2) IR also increased in water re-absorption on the same day, showing a similar pattern as seen with ADH.

Under MG conditions, the expression of GABAA receptors was noticed in the rostral ventrolateral medulla (VLM) in the HU ground animal model while no GABAA alteration was found in the caudal VLM.[35] This was in contradiction to the over expression noticed in other receptors like sodium channel, nonvoltage-gated 1 beta, glutamate receptor, voltage-dependent anion channel 1 as well as calcium channel beta 3 subunit.[36] These receptors are related to learning, memory, ion channels and cell junctions.


 » Advantages Top


The HU studies have advantages and limitations. Understanding the response of rodents to a simulated MG environment at a molecular level is important in understanding the astronauts' health. It also contributes to the understanding of age-related disorders and musculoskeletal system changes on Earth. The faster development of HU and the shorter lifespan provides an immediate and effective data. This could lead to scientific conclusions about neurophysiological changes that spaceflight induces in living organisms. HU ground-based experiments can be scheduled without concern for the real MG environment. Cost effective mid-course experimental modifications can be done. The precautions required are more limited than in spaceflight experiments. Parameters can be measured at multiple time points within a single experiment. Additionally, experiments involving the HU for MG investigations can be repeated and extended.[26]


 » Limitations Top


Optimized animal models supporting and providing strong evidence of overall effects of MG on the body physiology are limited. For instance, the fact that part of the animal body remains suspended and the rest of the body is still touching the ground surface of the cage [Figure 1], might have an influence on the outcome. In real MG conditions, the entire body is floating in the air, weightlessly due to loss of gravity. The time of HU use is limited to 2 months. Hence, long-term MG effects cannot be investigated. Robust terrestrial technological infrastructure for investigating physiological changes and validating countermeasures such as physical activity and nutrition are still to be developed. This will help to counterbalance the MG effects on astronauts and space travellers, leading to longer stays. The HU data cannot ipso facto be extrapolated to humans as the nervous system anatomy and physiology are different. Rat and human embryos brain at the same stage of development (18 and 63 days of rat and human embryo, respectively) differ. Rat thalamus, including the neuroepithelium of the cerebral cortex, is not equivalent to that in the human embryo. This could have an impact on the final development of MG-related outcome. Neuroepithelium is the source of most neurons that will populate the highly folded neocortex in the mature human brain. Inconsistencies in the stress responses to the HU depending on the experiment, continued the gravitational loading of the forequarters; and, lack of clarity about the influence of HU on the spine, are also concerns.


 » Perspectives Top


For future investigations, the following need to be considered:

  1. Nervous system is the part of an animal that coordinates its actions by transmitting signals to and from different parts of the body. Investigations on neurophysiology should include all interconnected organs. This would contribute to a fuller understanding of how brain-related function influences rest of the body
  2. More efforts are needed to advance the HU approach to offer more and comparable results to real space conditions. The creation of a more innovative MG-ground animal model as an analog of human (neuro) physiology is essential. This would provide results closer to human neurophysiological variations. More scientific studies have been done in implementing the human ground-based analog of spaceflight where male or female participants are immobilized for certain periods of time (usually up to 60 days) with 6° head-down tilt bed rest. This analog is the most widely accepted human Earth study [37]
  3. Exposing experimental animals (rodents) to real spaceflight environment to investigate MG-related neurophysiological changes would be helpful. Few spaceflight animal experiments have taken place so far [Table 1].[38], [39], [40] These offer more tangible results on the outcome regarding the real MG effects on the nervous system, projecting the importance of rodents in research while reducing the costs and complying with the ethical issues of conducting space human research.
Table 1: Representative investigated biomarkers and mechanism of implication of specific brain regions under real MG conditions

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 » Conclusions Top


Investigation of the MG effects on neurophysiological functions has been simulated in the HU ground-based models. Not many studies have been implemented detailing the loss of homeostasis of certain biological pathways in the nervous system, particularly those responsible for function of receptors/genes, neurotransmitters, neuronal membrane ion channels into specific brain regions. More work is still needed specifically on molecular signalling pathways dealing with neuronal connectivity and cognitive and neuroplasticity functions under MG-simulated conditions. Creation of novel ground-based animal models could provide new insights on space-related research in order to find solutions to biological as well as biomedical space-related questions.

Financial support and sponsorship

The bed rest study was funded by the European Space Agency, ESA (4000113871-15-NL) and the current work is based on the research supported in part by the National Research Foundation of South Africa (Grant Numbers 93450 and 85466 to GEJ) and the University of Cape Town Research Committee.

Conflicts of interest

There are no conflicts of interest.



 
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