Effects of microgravity on brain neurotransmitter receptors

Reduced Regional Cerebral Blood Flow Measured by 99mTc-Hexamethyl Propylene Amine Oxime Single-Photon Emission Computed Tomography in Microgravity Simulated by 5-Day Dry Immersion

Microgravity induces a cephalad fluid shift that is responsible for cephalic venous stasis that may increase intracranial pressure (ICP) in astronauts. However, the effects of microgravity on regional cerebral blood flow (rCBF) are not known. We therefore investigated changes in rCBF in a 5-day dry immersion (DI) model. Moreover, we tested thigh cuffs as a countermeasure to prevent potential microgravity-induced modifications in rCBF. Around 18 healthy male participants underwent 5-day DI with or without a thigh cuffs countermeasure. They were randomly allocated to a control (n=9) or cuffs (n=9) group. rCBF was measured 4days before DI and at the end of the fifth day of DI (DI5), using single-photon emission computed tomography (SPECT) with radiopharmaceutical 99mTc-hexamethyl propylene amine oxime (99mTc-HMPAO). SPECT images were processed using statistical parametric mapping (SPM12) software. At DI5, we observed a significant decrease in rCBF in 32 cortical and subcortical regions, with greater hypoperfusion in basal ganglia (right putamen peak level: z=4.71, p uncorr<0.001), bilateral occipital regions (left superior occipital peak level: z=4.51, p uncorr<0.001), bilateral insula (right insula peak level: 4.10, p uncorr<0.001), and bilateral inferior temporal (right inferior temporal peak level: 4.07, p uncorr<0.001). No significant difference was found between the control and cuffs groups on change in rCBF after 5days of DI. After a 5-day DI, we found a decrease in rCBF in cortical and subcortical regions. However, thigh cuffs countermeasure failed to prevent hypoperfusion. To date, this is the first study measuring rCBF in DI. Further investigations are needed in order to better understand the underlying mechanisms in cerebral blood flow (CBF) changes after exposure to microgravity.

Simulated Microgravity Subtlety Changes Monoamine Function across the Rat Brain

Microgravity, one of the conditions faced by astronauts during spaceflights, triggers brain adaptive responses that could have noxious consequences on behaviors. Although monoaminergic systems, which include noradrenaline (NA), dopamine (DA), and serotonin (5-HT), are widespread neuromodulatory systems involved in adaptive behaviors, the influence of microgravity on these systems is poorly documented. Using a model of simulated microgravity (SMG) during a short period in Long Evans male rats, we studied the distribution of monoamines in thirty brain regions belonging to vegetative, mood, motor, and cognitive networks. SMG modified NA and/or DA tissue contents along some brain regions belonging to the vestibular/motor systems (inferior olive, red nucleus, cerebellum, somatosensorily cortex, substantia nigra, and shell of the nucleus accumbens). DA and 5-HT contents were reduced in the prelimbic cortex, the only brain area exhibiting changes for 5-HT content. However, the number of correlations of one index of the 5-HT metabolism (ratio of metabolite and 5-HT) alone or in interaction with the DA metabolism was dramatically increased between brain regions. It is suggested that SMG, by mobilizing vestibular/motor systems, promotes in these systems early, restricted changes of NA and DA functions that are associated with a high reorganization of monoaminergic systems, notably 5-HT.

Dynamic Foot Stimulations During Short-Term Hindlimb Unloading Prevent Dysregulation of the Neurotransmission in the Hippocampus of Rats

Spaceflight and simulated microgravity both affect learning and memory, which are mostly controlled by the hippocampus. However, data about molecular alterations in the hippocampus in real or simulated microgravity conditions are limited. Adult Wistar rats were recruited in the experiments. Here we analyzed whether short-term simulated microgravity caused by 3-day hindlimb unloading (HU) will affect the glutamatergic and GABAergic systems of the hippocampus and how dynamic foot stimulation (DFS) to the plantar surface applied during HU can contribute in the regulation of hippocampus functioning. The results demonstrated a decreased expression of vesicular glutamate transporters 1 and 2 (VGLUT1/2) in the hippocampus after 3 days of HU, while glutamate decarboxylase 67 (GAD67) expression was not affected. HU also significantly induced Akt signaling and transcriptional factor CREB that are supposed to activate the neuroprotective mechanisms. On the other hand, DFS led to normalization of VGLUT1/2 expression and activity of Akt and CREB. Analysis of exocytosis proteins revealed the inhibition of SNAP-25, VAMP-2, and syntaxin 1 expression in DFS group proposing attenuation of excitatory neurotransmission. Thus, we revealed that short-term HU causes dysregulation of glutamatergic system of the hippocampus, but, at the same time, stimulates neuroprotective Akt-dependent mechanism. In addition, most importantly, we demonstrated positive effect of DFS on the hippocampus functioning that probably depends on the regulation of neurotransmitter exocytosis.

Impairment of synaptic plasticity and novel object recognition in the hypergravity-exposed rats

The gravity is necessary for living organisms to operate various biological events including hippocampus-related functions of learning and memory. Until now, it remains inconclusive how altered gravity is associated with hippocampal functions. It is mainly due to the difficulties in generating an animal model experiencing altered gravity. Here, we demonstrate the effects of hypergravity on hippocampus-related functions using an animal behavior and electrophysiology with our hypergravity animal model. The hypergravity (4G, 4 weeks) group showed impaired synaptic efficacy and long-term potentiation in CA1 neurons of the hippocampus along with the poor performance of a novel object recognition task. Our studies suggest that altered gravity affects hippocampus-related cognitive functions, presumably through structural and functional adaptation to various conditions of gravity shift.

Long-term effects of simulated microgravity and/or chronic exposure to low-dose gamma radiation on behavior and blood-brain barrier integrity

Astronauts on lengthy voyages will be exposed to an environment of microgravity and ionizing radiation that may have adverse effects on physical abilities, mood, and cognitive functioning. However, little is known about the long-term effects of combined microgravity and low-dose radiation. We exposed mice to gamma radiation using a cobalt-57 plate (0.01 cGy/h for a total dose of 0.04 Gy), hindlimb unloading to simulate microgravity, or a combination of both for 3 weeks. Mice then underwent a behavioral test battery after 1 week, 1 month, 4 months, and 8 months to assess sensorimotor coordination/balance (rotarod), activity levels (open field), learned helplessness/depression-like behavior (tail suspension test), risk-taking (elevated zero maze), and spatial learning/memory (water maze). Aquaporin-4 (AQP4) expression was assessed in the brain after behavioral testing to determine blood–brain barrier (BBB) integrity. Mice that received unloading spent significantly more time in the exposed portions of the elevated zero maze, were hypoactive in the open field, and spent less time struggling on the tail suspension test than mice that did not receive unloading. Mice in the combination group expressed more AQP4 immunoactivity than controls. Elevated zero maze and AQP4 data were correlated. No differences were seen on the water maze or rotarod, and no radiation-only effects were observed. These results suggest that microgravity may lead to changes in exploratory/risk-taking behaviors in the absence of other sensorimotor or cognitive deficits and that combined microgravity and a chronic, low dose of gamma radiation may lead to BBB dysfunction.

Brain development, environment and sex: what can we learn from studying graviperception, gravitransduction and the gravireaction of the developing CNS to altered gravity?

As man embarks on space exploration and contemplates space habitation, there is a critical need for basic understanding of the impact of the environmental factors of space, and in particular gravity, on human survival, health, reproduction and development. This review summarizes our present knowledge on the effect of altered gravity on the developing CNS with respect to the response of the developing CNS to altered gravity (gravireaction), the physiological changes associated with altered gravity that could contribute to this effect (gravitransduction), and the possible mechanisms involved in the detection of altered gravity (graviperception). Some of these findings transcend gravitational research and are relevant to our understanding of the impact of environmental factors on CNS development on Earth.

Effects of simulated microgravity on the development and maturation of dissociated cortical neurons

Although a wealth of evidence supports the hypothesis that some functions of the nervous system may be altered during exposure to microgravity, the possible changes in basic neuronal physiology are not easy to assess. Indeed, few studies have examined whether microgravity affects the development of neurons in culture. In the present study, a suspension of dissociated cortical cells from rat embryos were exposed to 24 h of simulated microgravity before plating in a normal adherent culture system. Both preexposed and control cells were used after a period of 7-10 d in vitro. The vitality and the level of reactive oxygen species of cultures previously exposed did not differ from those of normal cultures. Cellular characterization by immunostaining with a specific antibody displayed normal neuronal phenotype in control cells, whereas pretreatment in simulated microgravity revealed an increase of glial fibrillary acidic protein fluorescence in the elongated stellate glial cells. Electrophysiological recording indicated that the electrical properties of neurons preexposed were comparable with those of controls. Overall, our results indicate that a short time of simulated microgravity preexposure does not affect dramatically the ability of dissociated neural cells to develop and differentiate in an adherent culture system.

Effect of culture in a rotating wall bioreactor on the physiology of differentiated neuron-like PC12 and SH-SY5Y cells

A variety of evidence suggests that nervous system function is altered during microgravity, however, assessing changes in neuronal physiology during space flight is a non-trivial task. We have used a rotating wall bioreactor with a high aspect ratio vessel (HARV), which simulates the microgravity environment, to investigate the how the viability, neurite extension, and signaling of differentiated neuron-like cells changes in different culture environments. We show that culture of differentiated PC12 and SH-SY5Y cells in the simulated microgravity HARV bioreactor resulted in high cell viability, moderate neurite extension, and cell aggregation accompanied by NO production. Neurite extension was less than that seen in static cultures, suggesting that less than optimal differentiation occurs in simulated microgravity relative to normal gravity. Cells grown in a mixed vessel under normal gravity (a spinner flask) had low viability, low neurite extension, and high glutamate release. This work demonstrates the feasibility of using a rotating wall bioreactor to explore the effects of simulated microgravity on differentiation and physiology of neuron-like cells.

Effects of microgravity and bone morphogenetic protein II on GFAP in rat brain

This study evaluated effects of bone morphogenetic protein II (BMP) on glial fibrillary acidic protein (GFAP) in the brain of female Fischer 344 rats during 14 days of spaceflight. GFAP mRNA decreased in vehicle-implanted rats flown on the space shuttle by 53 and 48% in the stratum moleculare and stratum lacunosum moleculare hippocampal subregions, respectively. GFAP mRNA was not significantly affected by BMP implantation during spaceflight. Rats returning from space exhibited a 56% increase in serum corticosterone. BMP treatment did not additively increase corticosterone elevations in microgravity but appeared to increase serum corticosterone and reduce GFAP mRNA in the stratum moleculare in control rats. These data suggest that exposure to microgravity reduces GFAP expression in hippocampal astrocytes.

Changes in the central nervous system during long-duration space flight: implications for neuro-imaging

The purpose of this paper is to review the potential functional and morphological effects of long duration space flight on the human central nervous system (CNS) and how current neuroimaging techniques may be utilized to study these effects. It must be determined if there will be any detrimental changes to the CNS from long term exposure to the space environment if human beings are to plan interplanetary missions or establish permanent space habitats. Research to date has focused primarily on the short term changes in the CNS as the result of space flight. The space environment has many factors such as weightlessness, electromagnetic fields, and radiation, that may impact upon the function and structure of the CNS. CNS changes known to occur during and after long term space flight include neurovestibular disturbances, cephalic fluid shifts, alterations in sensory perception, changes in proprioception, psychological disturbances, and cognitive changes. Animal studies have shown altered plasticity of the neural cytoarchitecture, decreased neuronal metabolism in the hypothalamus, and changes in neurotransmitter concentrations. Recent progress in the ability to study brain morphology, cerebral metabolism, and neurochemistry in vivo in the human brain would provide ample opportunity to investigate many of the changes that occur in the CNS as a result of space flight. These methods include positron emission tomography (PET), single photon emission computed tomography (SPECT), and magnetic resonance imaging (MRI).

Neuritogenesis: a model for space radiation effects on the central nervous system

Pivotal to the astronauts' functional integrity and survival during long space flights are the strategies to deal with space radiations. The majority of the cellular studies in this area emphasize simple endpoints such as growth related events which, although useful to understand the nature of primary cell injury, have poor predictive value for extrapolation to more complex tissues such as the central nervous system (CNS). In order to assess the radiation damage on neural cell populations, we developed an in vitro model in which neuronal differentiation, neurite extension, and synaptogenesis occur under controlled conditions. The model exploits chick embryo neural explants to study the effects of radiations on neuritogenesis. In addition, neurobiological problems associated with long-term space flights are discussed.

Immunocytochemical localization of atrial natriuretic factor (ANF)-like peptides in the brain and heart of the treefrog Hyla japonica: effect of weightlessness on the distribution of immunoreactive neurons and cardiocytes

The localization of atrial-natriuretic factor (ANF)-like immunoreactivity was investigated in the brain and heart of the treefrog Hyla japonica by the indirect immunofluorescence technique. Concurrently, the effect of weightlessness on the distribution of ANF-containing neurons and cardiocytes was studied in frogs that were sent into space for 9 days on the space station "MIR." In control animals, the amygdala contained the most prominent group of ANF-immunoreactive cells and fibers. ANF-positive neurons and nerve processes were also detected in other areas of the telencephalon such as the nucleus olfactorius, the pallium mediale, and the striatum. In "space frogs," the intensity of labeling of the amygdala and nucleus olfactorius was similar to that seen in control animals. In contrast, the pallium and the striatum of "space frogs" were totally devoid of positive cell bodies. In the diencephalon, of all animals, numerous ANF-immunoreactive perikarya and fibers were seen in the hypothalamus, the anterior thalamus, the infundibulum, and the median eminence. ANF-positive cell bodies were also noted in the lateral forebrain bundle of control frogs but were absent in "space frogs." The major difference between control and "space frogs" was observed in the posterior nuclei of the thalamus. In "space frogs," the nucleus posterocentralis thalami and the nucleus posterolateralis thalami exhibited large ANF-immunoreactive perikarya, while, in control frogs, these nuclei only contained scarce positive nerve fibers. In the mesencephalon, ANF-positive cell bodies and nerve processes were seen in the nucleus tegmenti mesencephali, the interpeduncular nucleus, and the nucleus cerebelli of all animals. However, stained perikarya were only observed in the nucleus reticularis isthmi of control frogs. In the heart, atrial cardiocytes exhibited intense ANF-like immunoreactivity. ANF-positive myocytes were also detected in the subpericardial region of the ventricle. The density and distribution of the staining were identical in the heart of control and "space frogs." These data support the concept that prolonged exposure to microgravity affects biosynthesis and/or release of ANF-related peptides in discrete regions of the amphibian brain.

Vector-free gravity disrupts synapse formation in cell culture

Terrestrial organisms evolved under and are subjected to the constancy of gravity. The organisms having adapted to this environmental factor, it is possible that embryonic development may be modified by exposure to altered gravity. To test the effects of gravity on embryonic development, we monitored the formation of nerve-associated acetylcholine receptor patches (NARPs) as an index of synaptogenesis. Embryonic spinal neuron and myotomal myocyte cocultures were placed in a horizontally rotating clinostat. From the cell's perspective, this results in the cancellation of the gravitational vector because of continuous averaging, thus mimicking the reduced gravitational force encountered in space. NARPs from cultures in which nerve-muscle contact was established before the onset of rotation were unaffected. In contrast, cultures in which nerve contact took place during rotation showed a marked inhibition of NARPs. Moreover, in the myocytes which did exhibit NARPs, the area of the patch was significantly reduced compared with control sister cultures. Several paradigms were used to ascertain that these findings did not result simply from loss of contact between neurites and myocytes, accelerated diffusion of a putative aggregating factor secreted by neurites, or from turbulence in the medium. Our data suggest that the process of synapse formation is sensitive to the gravitational vector. Embryonic development of the nervous system, in space, may therefore be markedly different from that normally occurring on earth.