Purkinje cell loss accompanies motor impairment in rats developing at altered gravity

Addressing Spaceflight Biology through the Lens of a Histologist-Embryologist

Embryogenesis and fetal development are highly delicate and error-prone processes in their core physiology, let alone if stress-associated factors and conditions are involved. Space radiation and altered gravity are factors that could radically affect fertility and pregnancy and compromise a physiological organogenesis. Unfortunately, there is a dearth of information examining the effects of cosmic exposures on reproductive and proliferating outcomes with regard to mammalian embryonic development. However, explicit attention has been given to investigations exploring discrete structures and neural networks such as the vestibular system, an entity that is viewed as the sixth sense and organically controls gravity beginning with the prenatal period. The role of the gut microbiome, a newly acknowledged field of research in the space community, is also being challenged to be added in forthcoming experimental protocols. This review discusses the data that have surfaced from simulations or actual space expeditions and addresses developmental adaptations at the histological level induced by an extraterrestrial milieu.

Neuro-consequences of the spaceflight environment

As human space exploration advances to establish a permanent presence beyond the Low Earth Orbit (LEO) with NASA's Artemis mission, researchers are striving to understand and address the health challenges of living and working in the spaceflight environment. Exposure to ionizing radiation, microgravity, isolation and other spaceflight hazards pose significant risks to astronauts. Determining neurobiological and neurobehavioral responses, understanding physiological responses under Central Nervous System (CNS) control, and identifying putative mechanisms to inform countermeasure development are critically important to ensuring brain and behavioral health of crew on long duration missions. Here we provide a detailed and comprehensive review of the effects of spaceflight and of ground-based spaceflight analogs, including simulated weightlessness, social isolation, and ionizing radiation on humans and animals. Further, we discuss dietary and non-dietary countermeasures including artificial gravity and antioxidants, among others. Significant future work is needed to ensure that neural, sensorimotor, cognitive and other physiological functions are maintained during extended deep space missions to avoid potentially catastrophic health and safety outcomes.

Cerebellar morphology and behavioural correlations of the vestibular function alterations in weightlessness

In humans and other vertebrates, the range of disturbances and behavioural changes induced by spaceflight conditions are well known. Sensory organs and the central nervous system (CNS) are forced to adapt to new environmental conditions of weightlessness. In comparison with peripheral vestibular organs and behavioural disturbances in weightlessness conditions, the CNS vestibular centres of vertebrates, including the cerebellum, have been poorly examined in orbital experiments, as well as in experimental micro- and hypergravity. However, the cerebellum serves as a critical control centre for learning and sensory system integration during space-flight. Thus, it is referred to as a principal brain structure for adaptation to gravity and the entire sensorimotor adaptation and learning during weightlessness. This paper is focused on the prolonged spaceflight effects on the vestibular cerebellum evidenced from animal models used in the Bion-M1 project. The changes in the peripheral vestibular apparatus and brainstem primary vestibular centres with appropriate behavioural disorders after altered gravity exposure are briefly reviewed. The cerebellum studies in space missions and altered gravity are discussed.

Muscle and epidermal contributions of the structural protein β-spectrin promote hypergravity-induced motor neuron axon defects in C. elegans

Biology is adapted to Earth's gravity force, and the long-term effects of varying gravity on the development of animals is unclear. Previously, we reported that high gravity, called hypergravity, increases defects in the development of motor neuron axons in the nematode Caenorhabditis elegans. Here, we show that a mutation in the unc-70 gene that encodes the cytoskeletal β-spectrin protein suppresses hypergravity-induced axon defects. UNC-70 expression is required in both muscle and epidermis to promote the axon defects in high gravity. We reveal that the location of axon defects is correlated to the size of the muscle cell that the axon traverses. We also show that mutations that compromise key proteins of hemidesmosomal structures suppress hypergravity-induced axon defects. These hemidesmosomal structures play a crucial role in coupling mechanical force between the muscle, epidermis and the external cuticle. We speculate a model in which the rigid organization of muscle, epidermal and cuticular layers under high gravity pressure compresses the narrow axon migration pathways in the extracellular matrix hindering proper axon pathfinding of motor neurons.

Hypergravity induced disruption of cerebellar motor coordination

The cerebellum coordinates voluntary movements for balanced motor activity in a normal gravity condition. It remains unknown how hypergravity is associated with cerebellum-dependent motor behaviors and Purkinje cell’s activities. In order to investigate the relationship between gravity and cerebellar physiology, we measured AMPA-mediated fast currents and mGluR1-mediated slow currents of cerebellar Purkinje cells along with cerebellum-dependent behaviors such as the footprint and irregular ladder under a hypergravity condition. We found abnormal animal behaviors in the footprint and irregular ladder tests under hypergravity. They are correlated with decreased AMPA and mGluR1-mediated synaptic currents of Purkinje cells. These results indicate that gravity regulates the activity of Purkinje cells, thereby modulating cerebellum-dependent motor outputs.

Peripheral nerve injury induces adult brain neurogenesis and remodelling

Unilateral peripheral nerve chronic constriction injury (CCI) has been widely used as a research model of human neuropathic pain. Recently, CCI has been shown to induce spinal cord adult neurogenesis, which may contribute to the chronic increase in nociceptive sensitivity. Here, we show that CCI also induces rapid and profound asymmetrical anatomical rearrangements in the adult rodent cerebellum and pons. This remodelling occurs throughout the hindbrain, and in addition to regions involved in pain processing, also affects other sensory modalities. We demonstrate that these anatomical changes, partially reversible in the long term, result from adult neurogenesis. Neurogenic markers Mash1, Ngn2, doublecortin and Notch3 are widely expressed in the rodent cerebellum and pons, both under normal and injured conditions. CCI-induced hindbrain structural plasticity is absent in Notch3 knockout mice, a strain with impaired neuronal differentiation, demonstrating its dependence on adult neurogenesis. Grey matter and white matter structural changes in human brain, as a result of pain, injury or learned behaviours have been previously detected using non-invasive neuroimaging techniques. Because neurogenesis-mediated structural plasticity is thought to be restricted to the hippocampus and the subventricular zone, such anatomical rearrangements in other parts of the brain have been thought to result from neuronal plasticity or glial hypertrophy. Our findings suggest the presence of extensive neurogenesis-based structural plasticity in the adult mammalian brain, which may maintain a memory of basal sensory levels, and act as an adaptive mechanism to changes in sensory inputs.

The development of vestibular system and related functions in mammals: impact of gravity

This chapter reviews the knowledge about the adaptation to Earth gravity during the development of mammals. The impact of early exposure to altered gravity is evaluated at the level of the functions related to the vestibular system, including postural control, homeostatic regulation, and spatial memory. The hypothesis of critical periods in the adaptation to gravity is discussed. Demonstrating a critical period requires removing the gravity stimulus during delimited time windows, what is impossible to do on Earth surface. The surgical destruction of the vestibular apparatus, and the use of mice strains with defective graviceptors have provided useful information on the consequences of missing gravity perception, and the possible compensatory mechanisms, but transitory suppression of the stimulus can only be operated during spatial flight. The rare studies on rat pups housed on board of space shuttle significantly contributed to this problem, but the use of hypergravity environment, produced by means of chronic centrifugation, is the only available tool when repeated experiments must be carried out on Earth. Even though hypergravity is sometimes considered as a mirror situation to microgravity, the two situations cannot be confused because a gravitational force is still present. The theoretical considerations that validate the paradigm of hypergravity to evaluate critical periods are discussed. The question of adaption of graviceptor is questioned from an evolutionary point of view. It is possible that graviception is hardwired, because life on Earth has evolved under the constant pressure of gravity. The rapid acquisition of motor programming by precocial mammals in minutes after birth is consistent with this hypothesis, but the slow development of motor skills in altricial species and the plasticity of vestibular perception in adults suggest that gravity experience is required for the tuning of graviceptors. The possible reasons for this dichotomy are discussed.

The role of the angiotensin AT2 receptor on the diurnal variations of nociception and motor coordination in rats

Phasic pain demonstrates significant diurnal variation in rats. Angiotensin II modulates pain transmission and the diurnal variation in nociception in several rodent pain models. The participation of AT2 receptors in the diurnal regulation of nociception is not yet elucidated. In the present study we investigated the effects of selective peptide AT2 agonist CGP 42112A and the nonpeptide AT2 receptor antagonist PD 123319 on the nociception, motor coordination and arterial blood pressure. Male Wistar 12 weeks old rats were used. CGP 42112A was injected at single doses of 1 and 5 μg/rat intracerebroventricularly (ICV) and infused chronically ICV at a dose of 12 μg/rat/day during 14 days by osmotic minipumps. PD123319 was injected at single doses of 1 and 5 μg/rat, ICV and chronically subcutaneously at a dose of 10 mg/kg/day/14 days. Nociception was assessed by an analgesimeter, arterial blood pressure (ABP) was measured by tail cuff method, and motor coordination by Rota-rod method. Single doses of CGP 42112A (1 and 5 μg/rat) provoked a short lasting antinociception. Unlike acute injection, chronic CGP 42112A infusion increased nociception at the beginning and the end of light phase thus attenuating the diurnal variations observed in the controls. Moreover, it produced an increase of ABP and improved motor coordination. Both acute (1 μg/rat) and chronic PD 123319 treatment resulted in a decrease of pain threshold and chronic treatment attenuated its diurnal fluctuation. Our data support a role for Ang II type 2 receptors in the control of diurnal variations of nociception in rats.

Evaluation of gene, protein and neurotrophin expression in the brain of mice exposed to space environment for 91 days

Effects of 3-month exposure to microgravity environment on the expression of genes and proteins in mouse brain were studied. Moreover, responses of neurobiological parameters, nerve growth factor (NGF) and brain derived neurotrophic factor (BDNF), were also evaluated in the cerebellum, hippocampus, cortex, and adrenal glands. Spaceflight-related changes in gene and protein expression were observed. Biological processes of the up-regulated genes were related to the immune response, metabolic process, and/or inflammatory response. Changes of cellular components involving in microsome and vesicular fraction were also noted. Molecular function categories were related to various enzyme activities. The biological processes in the down-regulated genes were related to various metabolic and catabolic processes. Cellular components were related to cytoplasm and mitochondrion. The down-regulated molecular functions were related to catalytic and oxidoreductase activities. Up-regulation of 28 proteins was seen following spaceflight vs. those in ground control. These proteins were related to mitochondrial metabolism, synthesis and hydrolysis of ATP, calcium/calmodulin metabolism, nervous system, and transport of proteins and/or amino acids. Down-regulated proteins were related to mitochondrial metabolism. Expression of NGF in hippocampus, cortex, and adrenal gland of wild type animal tended to decrease following spaceflight. As for pleiotrophin transgenic mice, spaceflight-related reduction of NGF occured only in adrenal gland. Consistent trends between various portions of brain and adrenal gland were not observed in the responses of BDNF to spaceflight. Although exposure to real microgravity influenced the expression of a number of genes and proteins in the brain that have been shown to be involved in a wide spectrum of biological function, it is still unclear how the functional properties of brain were influenced by 3-month exposure to microgravity.

Absence of neurotoxicity with medicinal grade terbutaline in the rat model

To evaluate neurological effects of terbutaline, rats were injected with saline, terbutaline (Sigma or American Pharmaceutical Partners (APP™)) at 0.5 mg/kg-d or 10 mg/kg-d between postnatal days (PND) 2-5 or 11-14. Brains collected 24 h after last injection were used to determine corpus-callosum thickness, Purkinje cell and neuronal number in the cerebellum. Ambulation, distance traveled, resting time and time on rotarod were analyzed. Terbutaline (both doses/grades at PND 11-14) decreased corpus-callosum thickness. Ambulation time was significantly decreased in the 10 mg/kg-d (Sigma) and 0.5 mg/kg-d of terbutaline (APP™) (PND 2-5) juvenile-rats and 10 mg/kg-d-Sigma adult-rats, 0.5 mg/kg-d APP™ (PND 11-14) adult-rats. Resting time was increased in both doses of APP™ (PND 2-5) in juvenile-rats, 10 mg/kg-d Sigma adult-rats. 10 mg/kg-d-Sigma (PND 2-5) decreased distance traveled in adult-rats. 0.5 mg/kg-d-Sigma (PND 2-5 and PND 11-14) decreased the time spent on rotarod (30 RPM) in adult-rats. Sigma terbutaline Sigma had 2× as much free base compared to APP™. In conclusion, APP™ terbutaline did not have a deleterious effect on the developing rat brain.

Prenatal infection decreases calbindin, decreases Purkinje cell volume and density and produces long-term motor deficits in Sprague-Dawley rats

The cerebellum is involved in the control of motor functions with Purkinje cells serving as the only output from the cerebellum. Purkinje cells are important targets for toxic substances and are vulnerable to prenatal insults. Intrauterine infection (IUI) has been shown to selectively target the developing cerebral white matter through lesioning, necrosis and inflammatory cytokine activation. Developmental and cognitive delays have been associated with animal models of IUI. The aim of this study was to determine if IUI leads to damage to Purkinje cells in the developing cerebellum and if any damage is associated with decreases in calbindin and motor behaviors in surviving pups. Pregnant rats were injected with Escherichia coli (1 × 10⁵ colony-forming units) or sterile saline at gestational day 17. Beginning at postnatal day (PND) 2, the pups were subjected to a series of developmental tests to examine developmental milestones. At PND 16, some pups were sacrificed and their brains extracted and processed for histology or protein studies. Hematoxylin and eosin (HE) staining was done to examine the general morphology of the Purkinje cells and to examine Purkinje cell density, area and volume. Calbindin expression was examined in the cerebellum via immunohistochemistry and Western blot techniques. The remaining rat pups were used to examine motor coordination and balance on a rotating rotarod at the prepubertal and adult ages. Prenatal E. coli injection did not significantly change birth weight or delivery time, but did delay surface righting and negative geotaxis in pups. Pups in the E. coli group also had a decrease in the number of Purkinje cells, as well as a decrease in Purkinje cell density and volume. HE staining demonstrated a change in Purkinje cell morphology. Calbindin expression was decreased in rats from the E. coli group as well. Locomotor tests indicated that while there were no significant changes in gross motor activity, motor coordination and balance was impaired in both prepubertal and adult rats from the E. coli group. In this model of IUI, we observed changes in Purkinje cell development which were associated with alterations in cerebellum-dependent motor behaviors. The decreases in calbindin and Purkinje cells were associated with developmental delays. These data further support the importance of IUI in brain development.

Deranged calcium signaling and neurodegeneration in spinocerebellar ataxia type 2

Spinocerebellar ataxia type 2 (SCA2) is an autosomal dominantly inherited, neurodegenerative disease caused by an expansion of polyglutamine tracts in the cytosolic protein ataxin-2 (Atx2). Cerebellar Purkinje cells (PCs) are predominantly affected in SCA2. The cause of PC degeneration in SCA2 is unknown. Here we demonstrate that mutant Atx2-58Q, but not wild-type (WT) Atx2-22Q, specifically associates with the cytosolic C-terminal region of type 1 inositol 1,4,5-trisphosphate receptor (InsP(3)R1), an intracellular calcium (Ca(2+)) release channel. Association with Atx2-58Q increased the sensitivity of InsP(3)R1 to activation by InsP(3) in planar lipid bilayer reconstitution experiments. To validate physiological significance of these findings, we performed a series of experiments with an SCA2-58Q transgenic mouse model that expresses human full-length Atx2-58Q protein under the control of a PC-specific promoter. In Ca(2+) imaging experiments, we demonstrated that stimulation with 3,5-dihydroxyphenylglycine (DHPG) resulted in higher Ca(2+) responses in 58Q PC cultures than in WT PC cultures. DHPG-induced Ca(2+) responses in 58Q PC cultures were blocked by the addition of ryanodine, an inhibitor of the ryanodine receptor (RyanR). We further demonstrated that application of glutamate induced more pronounced cell death in 58Q PC cultures than in WT PC cultures. Glutamate-induced cell death of 58Q PC cultures was attenuated by dantrolene, a clinically relevant RyanR inhibitor and Ca(2+) stabilizer. In whole animal experiments, we demonstrated that long-term feeding of SCA1-58Q mice with dantrolene alleviated age-dependent motor deficits (quantified in beam-walk and rotarod assays) and reduced PC loss observed in untreated SCA2-58Q mice by 12 months of age (quantified by stereology). Results of our studies indicate that disturbed neuronal Ca(2+) signaling may play an important role in SCA2 pathology and also suggest that the RyanR constitutes a potential therapeutic target for treatment of SCA2 patients.

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.

Osteopontin is not critical for otoconia formation or balance function

Unlike the structural and mechanical role of bone crystals, the inertial mass of otoconia crystals provides a shearing force to stimulate the mechanoreceptors of the utricle and saccule (the gravity receptor organ) under the stimuli of linear motion. It is not clear whether otoconia, composed primarily of CaCO3 and glycoproteins, go through similar calcification processes as bone. We have recently shown that otoconin-90 (Oc90) regulates the growth of otoconia crystals as osteopontin does bone crystals. Here, we analyzed the role of this non-collagenous bone matrix protein, osteopontin, in otoconia formation and balance function utilizing its knockout mice, whose inner ear phenotype has not been examined. Despite the presence of the protein in wild-type otoconia and vestibular hair cells, morphological, ultrastructural, and protein and calcium composition analyses of osteopontin null otoconia show that the protein is not needed for crystal formation, and no evidence of compensatory protein deposition is found. Employment of a wide spectrum of balance behavioral tests demonstrates that the protein is not critical for balance function either, which is confirmed by the normal function of the gravity receptor organ directly measured with linear vestibular-evoked potentials (VsEPs). When compared with findings on other otoconins, the data manifest a hierarchy of importance of proteins in crystallization and indicate mechanistic similarities and differences between bone and otoconia calcification.

Otoconin-90 deletion leads to imbalance but normal hearing: a comparison with other otoconia mutants

Our sense of gravitation and linear acceleration is mediated by stimulation of vestibular hair cells through displacement of otoconia in the utricle and saccule (the gravity receptor organ). We recently showed that otoconin-90 (Oc90) deletion led to formation of giant otoconia. In the present study, we determined the extent to which the giant otoconia affected balance and gravity receptor sensory input and compared the findings with other otoconia mutants. We employed a wide spectrum of balance behavioral tests, including reaching and air-righting reflexes, gait, swimming, beam-crossing, rotorod latencies, and a direct measure of gravity receptor input, vestibular evoked potentials (VsEPs). All tests on homozygous adult mutants consistently ranked the order of imbalance as (from worst to best) Nox3(het)<otopetrin 1(tlt)<Oc90 null<Oc90 wild type and C57Bl/6 mice using systematic statistical comparisons of the frequency of occurrence or the severity of abnormal functions. This order coincides with the degree of otoconia deficiencies and is consistent with VsEP measures. Notably, all mice (except Nox3(het)) showed remarkable learned adaptation to peripheral vestibular deficits by staying on the rotating rod significantly longer in each successive trial, and the rate and extent of such learned improvements ranked the same order as their initial balance ability. Despite the vestibular morbidity, Oc90 null mice had normal hearing, as measured by auditory brainstem responses (ABRs) and distortion products of otoacoustic emissions (DPOAEs). The study demonstrates that the remnant otoconia mass in Oc90 nulls does stimulate the gravity receptor organs, which was likely responsible for the improved balance performance relative to strains with absent otoconia. Furthermore, the combination of direct electrophysiological measures and a series of behavioral tests can be used to interpret the imbalance severity arising from altered inputs from the gravity receptor end organ.

Potential Role of Oxidative Stress in Mediating the Effect of Altered Gravity on the Developing Rat Cerebellum

We have previously reported that perinatal exposure to hypergravity affects cerebellar structure and motor coordination in rat neonates. In the present study, we explored the hypothesis that exposure to hypergravity results in oxidative stress that may contribute to the decrease in Purkinje cell number and the impairment of motor coordination in hypergravity-exposed rat neonates. To test this hypothesis we compared cerebellar oxidative stress marker 3-nitrotyrosine (3-NT; an index of oxidative protein modification) and 8-hydroxy-2'-deoxyguanosine (8-OH-dG; an index of oxidative DNA damage) between stationary control (SC) and rat neonates exposed to 1.65 G (HG) on a 24-ft centrifuge from gestational day (G) 8 to P21. The levels of 3-NT and 8-OH-dG were determined by specific ELISAs. We also compared the Purkinje cell number (stereorologically) and rotarod performance between the two groups. The levels of 3-NT were increased only in HG females on P6 and on P12 in the cerebellum, and only in HG females on P12 in the extracellabellar tissue. Limited cerebellar data suggests an increase in the levels of 8-OH-dG on P12 only in HG females. In extracerebellar tissue the increase in 8-OH-dG levels was observed in both HG males and HG females except on P6 when it was only observed in HG males. While preliminary, these data suggest that the effect of hypergravity on the developing brain is sex-dependent and may involve oxidative stress. Oxidative stress may, in turn, contribute to the decrease Purkinje cell number and impaired motor behavior observed in hypergravity-exposed rats.

Repeated acute exposures to hypergravity during early development subtly affect CD-1 mouse neurobehavioural profile

Exposure to altered gravitational environment, especially during critical ontogenetic phases, may induce persistent nervous system modifications and behavioural anomalies. This study evaluated the effects of hypergravity exposure on the development of the nervous system and assessed the relevance of parity in the mother's responses to altered gravitational stimuli. CD-1 mouse pups of both sexes delivered by primiparous and biparous dams were exposed to 1h of 2 G rotationally induced hypergravity from PND2 to PND9. Sensorimotor responses and somatic growth were daily measured (PND2-PND15), ultrasonic vocalisations recorded on PNDs 2, 5 and 9, and homing behaviour evaluated on PND12. In addition, spatial orientation ability was assessed in a T-maze on PND18, while mice exploratory behaviour and locomotor activity were evaluated in an open-field test (PND21). Long-term effects of hypergravity exposure on both spatial learning (Morris water-maze test) and brain levels of NGF and BDNF were also investigated at adulthood. Rotation per se induced a delay in somatic growth, sensorimotor responses and ultrasonic vocalisation profile, while hypergravity highlighted sex differences in open-field behaviour. Strategies to solve a spatial learning task, rather than learning per se, were affected by early exposure to rotation, while hypergravity selectively altered behavioural profile in the reversal phase of the test. Early exposure to rotation per se also decreased hypothalamic BDNF levels, while hypergravity reduced NGF levels in the frontal cortex. Previous maternal experience did not interact with hypergravity exposure, while differences between offspring of primiparous and biparous dams were observed in sensorimotor development and exploratory behaviour.

Exposure to Altered Gravity During Specific Developmental Periods Differentially Affects Growth, Development, the Cerebellum and Motor Functions in Male and Female Rats

We previously reported that perinatal exposure to hypergravity affects cerebellar structure and motor coordination in rat neonates. In the present study, we explored the hypothesis that neonatal cerebellar structure and motor coordination may be particularly vulnerable to the effects of hypergravity during specific developmental stages. To test this hypothesis, we compared neurodevelopment, motor behavior and cerebellar structure in rat neonates exposed to 1.65 G on a 24-ft centrifuge during discrete periods of time: the 2(nd) week of pregnancy [gestational day (G) 8 through G15; group A], the 3(rd) week of pregnancy (G15 through birth on G22/G23; group B), the 1(st) week of nursing [birth through postnatal day (P) 6; group C], the 2(nd) and 3(rd) weeks of nursing (P6 through P21; group D), the combined 2(nd) and 3(rd) weeks of pregnancy and nursing (G8 through P21; group E) and stationary control (SC) neonates (group F). Prenatal exposure to hypergravity resulted in intrauterine growth retardation as reflected by a decrease in the number of pups in a litter and lower average mass at birth. Exposure to hypergravity immediately after birth impaired the righting response on P3, while the startle response in both males and females was most affected by exposure during the 2(nd) and 3(rd) weeks after birth. Hypergravity exposure also impaired motor functions, as evidenced by poorer performance on a rotarod; while both males and females exposed to hypergravity during the 2(nd) and 3(rd) weeks after birth performed poorly on P21, male neonates were most dramatically affected by exposure to hypergravity during the second week of gestation, when the duration of their recorded stay on the rotarod was one half that of SC males. Cerebellar mass was most reduced by later postnatal exposure. Thus, for the developing rat cerebellum, the postnatal period that overlaps the brain growth spurt is the most vulnerable to hypergravity. However, male motor behavior is also affected by midpregnancy exposure to hypergravity, suggesting discrete and sexually dimorphic windows of vulnerability of the developing central nervous system to environmental perturbations.