The effects of microgravity on the development of surface righting in rats

Gravitational change-induced alteration of the vestibular function and gene expression in the vestibular ganglion of mice

Gravity has profoundly influenced life on Earth, yet how organisms adapt to changes in gravity remains largely unknown. This study examines vestibular plasticity, specifically how the vestibular system responds to altered gravity. We subjected male C57BL/6J mice to hypergravity (2 G) followed by normal gravity (1 G) to analyze changes in vestibular function and gene expression. Mice showed significant vestibular dysfunction, assessed by righting reflex tests, which persisted for days but reversed at 1 G after exposure to 2 G. Gene expression analysis in the vestibular ganglion identified significant changes in 212 genes out of 49,585 due to gravitational changes. Specifically, 25 genes were upregulated under 2 G and recovered at 1 G after 2 G exposure, while one gene showed the opposite trend. Key neural function genes like Shisa3, Slc25a37, Ntn4, and Snca were involved. Our results reveal that hypergravity-induced vestibular dysfunction is reversible and highlight genes critical for adaptation.

Changes in metabolism and vestibular function depend on gravitational load in mice

The vestibular system is known to participate in controlling posture and metabolism. Different gravitational environments, including microgravity or hypergravity, cause plastic alteration of the vestibular system, and plasticity is important for adaptation to a novel gravitational environment. However, it is unclear whether the degree of change in vestibular-related physiological function depends on gravitational loading. To examine this, we used a hypergravity environment including 1.33 G, 1.67 G, and 2 G for 29 days. We found that a gravitational threshold induces physiological changes, including vestibular-related posture control and metabolism in mice. Body mass did not return to the preloading level in 1.67 G and 2 G mice. A significant drop in food intake, observed on the first day of hypergravity load, disappeared in all mice after longer exposure. However, a reduction in water intake was sustained in 2 G mice but not 1.33 G and 1.67 G mice. Body temperature did not return to the preloading level in 2 G mice by the final day. A decrease in the skill of the righting reflex was observed in 2 G mice but not 1.33 G and 1.67 G mice. In conclusion, this study showed that hypergravity-induced changes in metabolism and vestibular function depended on the amount of gravitational loading. The 2 G load affected vestibular-related posture control and metabolism considerably, compared with 1.33 G and 1.67 G loads.NEW & NOTEWORTHY It is unclear whether the degree of change in vestibular-related physiological function depends on gravitational loading. Present study showed that exposure to hypergravity-induced degrees of change in metabolism and vestibular function depended on the gravitational loading. The response of body mass depended on the gravitational loading size. Especially in 2 G environment, water intake, body temperature, and vestibular function were influenced. These changes could involve plastic alteration of vestibular-related autonomic and motor functions.

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.

Reproduction and the Early Development of Vertebrates in Space: Problems, Results, Opportunities

Humans and animals adapt to space flight conditions. However, the adaptive changes of fully formed organisms differ radically from the responses of vertebrate embryos, foetuses, and larvae to space flight. Development is associated with active cell proliferation and the formation of organs and systems. The instability of these processes is well known. Over 20 years has passed since the last systematic experiments on vertebrate reproduction and development in space flight. At the same time, programs are being prepared for the exploration of Mars and the Moon, which justifies further investigations into space flight's impact on vertebrate development. This review focuses on various aspects of reproduction and early development of vertebrates in space flights. The results of various experiments on fishes, amphibians, reptiles, birds and mammals are described. The experiments in which our team took part and ontogeny of the vertebrate nervous and special sensory systems are considered in more detail. Possible causes of morphological changes are also discussed. Research on evolutionarily and taxonomically different models can advance the understanding of reproduction in microgravity. Reptiles, in particular, geckos, due to their special features, can be a promising object of space developmental biology.

Redundancy and multifunctionality among spinal locomotor networks

c-Fos is used to identify system-wide neural activation with cellular resolution in vivo. However, c-Fos can only capture neural activation of one event. Targeted recombination in active populations (TRAP) allows the capture of two different c-Fos activation patterns in the same animal. So far, TRAP has only been used to examine brain circuits. This study uses TRAP to investigate spinal circuit activation during resting and stepping, giving novel insights of network activation during these events. The level of colabeled (c-Fos+ and TRAP+) neurons observed after performing two bouts of stepping suggests that there is a probabilistic-like phenomenon that can recruit many combinations of neural populations (synapses) when repetitively generating many step cycles. Between two 30-min bouts of stepping, each consisting of thousands of steps, only ∼20% of the neurons activated from the first bout of stepping were also activated by the second bout. We also show colabeling of interneurons that have been active during stepping and resting. The use of the FosTRAP methodology in the spinal cord provides a new tool to compare the engagement of different populations of spinal interneurons in vivo under different motor tasks or under different conditions.NEW & NOTEWORTHY The results are consistent with there being an extensive amount of redundancy among spinal locomotor circuits. Using the newly developed FosTRAP mouse model, only ∼20% of neurons that were active (labeled by Fos-linked tdTomato expression) during a first bout of 30-min stepping were also labeled for c-Fos during a second bout of stepping. This finding suggests variability of neural networks that enables selection of many combinations of neurons (synapses) when generating each step cycle.

Emergence of Different Gaits in Infancy: Relationship Between Developing Neural Circuitries and Changing Biomechanics

How does gait-specific pattern generation evolve in early infancy? The idea that neural and biomechanical mechanisms underlying mature walking and running differ to some extent and involve distinct spinal and supraspinal neural circuits is supported by various studies. Here we consider the issue of human gaits from the developmental point of view, from neonate stepping to adult mature gaits. While differentiating features of the walk and run are clearly distinct in adults, the gradual and progressive developmental bifurcation between the different gaits suggests considerable sharing of circuitry. Gaits development and their biomechanical determinants also depend on maturation of the musculoskeletal system. This review outlines the possible overlap in the neural and biomechanical control of walking and running in infancy, supporting the idea that gaits may be built starting from common, likely phylogenetically conserved elements. Bridging connections between movement mechanics and neural control of locomotion could have profound clinical implications for technological solutions to understand better locomotor development and to diagnose early motor deficits. We also consider the neuromuscular maturation time frame of gaits resulting from active practice of locomotion, underlying plasticity of development.

Challenges to the central nervous system during human spaceflight missions to Mars

Space travel presents a number of environmental challenges to the central nervous system, including changes in gravitational acceleration that alter the terrestrial synergies between perception and action, galactic cosmic radiation that can damage sensitive neurons and structures, and multiple factors (isolation, confinement, altered atmosphere, and mission parameters, including distance from Earth) that can affect cognition and behavior. Travelers to Mars will be exposed to these environmental challenges for up to 3 years, and space-faring nations continue to direct vigorous research investments to help elucidate and mitigate the consequences of these long-duration exposures. This article reviews the findings of more than 50 years of space-related neuroscience research on humans and animals exposed to spaceflight or analogs of spaceflight environments, and projects the implications and the forward work necessary to ensure successful Mars missions. It also reviews fundamental neurophysiology responses that will help us understand and maintain human health and performance on Earth.

The heritability fallacy

The term 'heritability,' as it is used today in human behavioral genetics, is one of the most misleading in the history of science. Contrary to popular belief, the measurable heritability of a trait does not tell us how 'genetically inheritable' that trait is. Further, it does not inform us about what causes a trait, the relative influence of genes in the development of a trait, or the relative influence of the environment in the development of a trait. Because we already know that genetic factors have significant influence on the development of all human traits, measures of heritability are of little value, except in very rare cases. We, therefore, suggest that continued use of the term does enormous damage to the public understanding of how human beings develop their individual traits and identities. WIREs Cogn Sci 2017, 8:e1400. doi: 10.1002/wcs.1400 For further resources related to this article, please visit the WIREs website.

Hypergravity within a critical period impacts on the maturation of somatosensory cortical maps and their potential for use-dependent plasticity in the adult

We investigated experience-dependent plasticity of somatosensory maps in rat S1 cortex during early development. We analyzed both short- and long-term effects of exposure to 2G hypergravity (HG) during the first 3 postnatal weeks on forepaw representations. We also examined the potential of adult somatosensory maps for experience-dependent plasticity after early HG rearing. At postnatal day 22, HG was found to induce an enlargement of cortical zones driven by nail displacements and a contraction of skin sectors of the forepaw map. In these remaining zones serving the skin, neurons displayed expanded glabrous skin receptive fields (RFs). HG also induced a bias in the directional sensitivity of neuronal responses to nail displacement. HG-induced map changes were still found after 16 wk of housing in normogravity (NG). However, the glabrous skin RFs recorded in HG rats decreased to values similar to that of NG rats, as early as the end of the first week of housing in NG. Moreover, the expansion of the glabrous skin area and decrease in RF size normally induced in adults by an enriched environment (EE) did not occur in the HG rats, even after 16 wk of EE housing in NG. Our findings reveal that early postnatal experience critically and durably shapes S1 forepaw maps and limits their potential to be modified by novel experience in adulthood.

Hindlimb unloading: rodent analog for microgravity

The rodent hindlimb unloading (HU) model was developed in the 1980s to make it possible to study mechanisms, responses, and treatments for the adverse consequences of spaceflight. Decades before development of the HU model, weightlessness was predicted to yield deficits in the principal tissues responsible for structure and movement on Earth, primarily muscle and bone. Indeed, results from early spaceflight and HU experiments confirmed the expected sensitivity of the musculoskeletal system to gravity loading. Results from human and animal spaceflight and HU experiments show that nearly all organ systems and tissues studied display some measurable changes, albeit sometimes minor and of uncertain relevance to astronaut health. The focus of this review is to examine key HU results for various organ systems including those related to stress; the immune, cardiovascular, and nervous systems; vision changes; and wound healing. Analysis of the validity of the HU model is important given its potential value for both hypothesis testing and countermeasure development.

Feasibility of a Short-Arm Centrifuge for Mouse Hypergravity Experiments

To elucidate the pure impact of microgravity on small mammals despite uncontrolled factors that exist in the International Space Station, it is necessary to construct a 1 g environment in space. The Japan Aerospace Exploration Agency has developed a novel mouse habitat cage unit that can be installed in the Cell Biology Experiment Facility in the Kibo module of the International Space Station. The Cell Biology Experiment Facility has a short-arm centrifuge to produce artificial 1 g gravity in space for mouse experiments. However, the gravitational gradient formed inside the rearing cage is larger when the radius of gyration is shorter; this may have some impact on mice. Accordingly, biological responses to hypergravity induced by a short-arm centrifuge were examined and compared with those induced by a long-arm centrifuge. Hypergravity induced a significant Fos expression in the central nervous system, a suppression of body mass growth, an acute and transient reduction in food intake, and impaired vestibulomotor coordination. There was no difference in these responses between mice raised in a short-arm centrifuge and those in a long-arm centrifuge. These results demonstrate the feasibility of using a short-arm centrifuge for mouse experiments.

Neonatal intrahippocampal HIV-1 protein Tat(1-86) injection: neurobehavioral alterations in the absence of increased inflammatory cytokine activation

Pediatric AIDS caused by human immunodeficiency virus type 1 (HIV-1) remains one of the leading worldwide causes of childhood morbidity and mortality. HIV-1 proteins, such as Tat and gp120, are believed to play a crucial role in the neurotoxicity of pediatric HIV-1 infection. Detrimental effects on development, behavior, and neuroanatomy follow neonatal exposure to the HIV-1 viral toxins Tat1-72 and gp120. The present study investigated the neurobehavioral effects induced by the HIV-1 neurotoxic protein Tat1-86, which encodes the first and second exons of the Tat protein. In addition, the potential effects of HIV-1 toxic proteins Tat1-86 and gp120 on inflammatory pathways were examined in neonatal brains. Vehicle, 25 μg Tat1-86 or 100 ng gp120 was injected into the hippocampus of male Sprague-Dawley pups on postnatal day 1 (PD1). Tat1-86 induced developmental neurotoxic effects, as witnessed by delays in eye opening, delays in early reflex development and alterations in prepulse inhibition (PPI) and between-session habituation of locomotor activity. Overall, the neurotoxic profile of Tat1-86 appeared more profound in the developing nervous system in vivo relative to that seen with the first exon encoded Tat1-72 (Fitting et al., 2008b), as noted on measures of eye opening, righting reflex, and PPI. Neither the direct PD1 CNS injection of the viral HIV-1 protein variant Tat1-86, nor the HIV-1 envelope protein gp120, at doses sufficient to induce neurotoxicity, necessarily induced significant expression of the inflammatory cytokine IL-1β or inflammatory factors NF-κβ and I-κβ. The findings agree well with clinical observations that indicate delays in developmental milestones of pediatric HIV-1 patients, and suggest that activation of inflammatory pathways is not an obligatory response to viral protein-induced neurotoxicity that is detectable with behavioral assessments. Moreover, the amino acids encoded by the second tat exon may have unique actions on the developing hippocampus.

Sensorimotor training during expression of the leg extension response (LER) in 1-day-old rats

In newborn rats, the leg extension response (LER) is a coordinated hyperextension of the hindlimbs that is shown in response to anogenital stimulation. Here we examined the influence of sensorimotor training on LER expression in postnatal day 1 rats. In Experiment 1, we examined if proprioceptive feedback facilitates LER expression. We did this by repeatedly stimulating the pup's anogenital region with a vibrotactile device, to experimentally evoke the LER, thus increasing LER-relevant hindlimb proprioceptive feedback during training. In trained subjects, the LER was evoked every 4 min for 15 trials, followed by a final LER test. Results indicated that proprioceptive feedback on its own did not alter later expression of the LER. In Experiment 2, we examined the effect of both proprioceptive and cutaneous feedback on LER expression, through the use of a range of motion (ROM) restriction during training. During ROM restriction, a Plexiglas plate was placed beneath the pup at 50% of limb length. After the 15th training trial, a final LER test occurred with no ROM restriction in place. Compared to controls, pups that experienced ROM restriction exhibited a significantly shorter LER duration, and smaller hip and ankle angles during the LER test (indicating greater limb flexion). Together these findings show that concurrent proprioceptive and cutaneous feedback, but not proprioceptive feedback alone, has persistent effects on expression of this newborn action pattern.

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.

Exposure to hypergravity during the preweaning but not postweaning period reduces vestibular-related stress responses in rats

Gravitational forces, including hypergravity or microgravity, induce plasticity of vestibular-related functions. These functions are not easily reversed if exposure to the gravitational forces occurs during vestibular development. In the present study, we hypothesized that vestibular-related stress responses might be suppressed in rats exposed to hypergravity during the vestibular development period. We exposed the rats to 2 g (hypergravity) during the preweaning (BW-HG; embryonic day 14 to postnatal week 3) or postweaning (AW-HG; postnatal weeks 4-6) periods. After recovery for 4 wk at 1 g, we conducted rotarod tests and then exposed the rats to 2 g for 90 min. In BW-HG rats, vestibular-related motor coordination on the rotarod test was partially, but not fully, restored to the level of AW-HG rats or rats raised at 1 g (1-G group). Loading-induced plasma adrenocorticotropic hormone and corticosterone levels were significantly suppressed in BW-HG and in rats with a vestibular lesion compared with AW-HG and 1-G rats. Arginine vasopressin and Fos expression levels in the paraventricular hypothalamic nucleus were also significantly lower in BW-HG and vestibular lesion rats than in AW-HG and 1-G rats. By contrast, there was no difference in the electrical foot shock-induced increase in plasma corticosterone among the experimental groups, suggesting that the nonvestibular-related stress response was not suppressed by exposure to 2 g during preweaning. These results indicated that exposure to hypergravity during preweaning specifically suppressed the vestibular-related stress response, and this suppression did not recover after 4 wk at 1 g.

The molecular basis of experience-dependent motor system development

Neurons in the vertebrate nervous system acquire their mature features over an extended period in pre-natal and early post-natal life. The interaction of the organism with its environment (“experience”) has been shown to profoundly influence sensory neuron development. Over the past ~2 decades, it has become increasingly clear that motor system development is also experience-dependent. Glutamate receptors of the N-methyl-D-aspartate (NMDA) subtype have been implicated in both sensory and motor system experience-dependent development. An additional molecular mechanism involves the GluA1 subunit of the 2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl)propanoic acid (AMPA) subtype glutamate receptors. GluA1-dependent development operates in an NMDA-R independent manner and uses a distinct set of signaling molecules. The synapse associated protein of 97 kDa molecular weight (SAP97) is key. A deeper understanding of how experiences guides motor system development may lead to new ways to improve function after central nervous system insult.

Ontogeny of mouse vestibulo-ocular reflex following genetic or environmental alteration of gravity sensing

The vestibular organs consist of complementary sensors: the semicircular canals detect rotations while the otoliths detect linear accelerations, including the constant pull of gravity. Several fundamental questions remain on how the vestibular system would develop and/or adapt to prolonged changes in gravity such as during long-term space journey. How do vestibular reflexes develop if the appropriate assembly of otoliths and semi-circular canals is perturbed? The aim of present work was to evaluate the role of gravity sensing during ontogeny of the vestibular system. In otoconia-deficient mice (ied), gravity cannot be sensed and therefore maculo-ocular reflexes (MOR) were absent. While canals-related reflexes were present, the ied deficit also led to the abnormal spatial tuning of the horizontal angular canal-related VOR. To identify putative otolith-related critical periods, normal C57Bl/6J mice were subjected to 2G hypergravity by chronic centrifugation during different periods of development or adulthood (Adult-HG) and compared to non-centrifuged (control) C57Bl/6J mice. Mice exposed to hypergravity during development had completely normal vestibulo-ocular reflexes 6 months after end of centrifugation. Adult-HG mice all displayed major abnormalities in maculo-ocular reflexe one month after return to normal gravity. During the next 5 months, adaptation to normal gravity occurred in half of the individuals. In summary, genetic suppression of gravity sensing indicated that otolith-related signals might be necessary to ensure proper functioning of canal-related vestibular reflexes. On the other hand, exposure to hypergravity during development was not sufficient to modify durably motor behaviour. Hence, 2G centrifugation during development revealed no otolith-specific critical period.

Exposure to hypergravity during specific developmental periods differentially affects metabolism and vestibular reactions in adult C57BL /6j mice

The development of the posturo-motor control of movement is conditioned by Earth's gravity. Missing or altered gravity during the critical periods of development delays development and induces durable changes in the vestibular, cerebellar, or muscular structures, but these are not consistently mirrored at a functional level. The differences in the time schedule of vestibular and motor development could contribute to this inconstancy. To investigate the influence of gravity on the development of vestibular and locomotor functions, we analysed the performance of adult mice subjected to hypergravity during the time covering either the vestibular or locomotor development. The mice were centrifuged at 2 g from embryonic day (E) 0 to postnatal day (P) 10 (PRE), from P10 to P30 (POST), from E0 to P30 (FULL), and from E7 to P21. Their muscular force, anxiety level, vestibular reactions, and aerobic capacity during treadmill training were then evaluated at the age of 2 and 6 months. The performance of young adults varied in relation to the period of exposure to hypergravity. The mice that acquired locomotion in hypergravity (POST and FULL) showed a lower forelimb force and delayed vestibular reactions. The mice centrifuged from conception to P10 (PRE) showed a higher aerobic capacity during treadmill training. The differences in muscular force and vestibular reactions regressed with age, but the metabolic changes persisted. These results confirmed that early exposure to hypergravity induces qualitative changes depending on the period of exposure. They validated, at a functional level, the existence of several critical periods for adaptation to gravity.

GluR1 controls dendrite growth through its binding partner, SAP97

Activity-dependent dendrite elaboration influences the pattern of interneuronal connectivity and network function. In the present study, we examined the mechanism by which the GluR1 subunit of AMPA receptors controls dendrite morphogenesis. GluR1 binds to SAP97, a scaffolding protein that is a component of the postsynaptic density, via its C-terminal 7 aa. We find that elimination of this interaction in vitro or in vivo (by deleting the C-terminal 7 aa of GluR1, GluR1Delta7) does not influence trafficking, processing, or cell surface GluR1 expression but does prevent translocation of SAP97 from the cytosol to membranes. GluR1 and SAP97 together at the plasma membrane promotes dendrite branching in an activity-dependent manner, although this does not require physical association. Our findings suggest that the C-terminal 7 aa of GluR1 are essential for bringing SAP97 to the plasma membrane, where it acts to translate the activity of AMPA receptors into dendrite growth.

Neonatal intrahippocampal injection of the HIV-1 proteins gp120 and Tat: differential effects on behavior and the relationship to stereological hippocampal measures

HIV-1 proteins, such as Tat and gp120, are believed to play a crucial role in the central nervous system (CNS) pathology of acquired immune deficiency syndrome (AIDS). The present study sought to determine the potential role of Tat and/or gp120 on behavioral development and the relationship to the long-term effects of the HIV-1 proteins on the rat hippocampus. Male pups of 13 Sprague-Dawley litters were bilaterally injected on postnatal day (P)1. Every litter contributed an animal to each of four treatment condition: VEH (0.5 microl sterile buffer), gp120 (100 ng), Tat (25 microg) or combined gp120+Tat (100 ng+25 microg). Body weight was not affected by either protein treatment. Tat revealed a transient effect on many of the behavioral assessments early in development as well as on preattentive processes and spatial memory in adulthood. Gp120 had more selective effects on negative geotaxis (P8-P10) and on locomotor activity (P94-P96). Combined gp120+Tat effects were noted for eye opening with potential interactive effects of gp120 and Tat on negative geotaxis. Anatomical assessment at approximately 7 1/2 months of age was conducted by using design-based stereology to quantify the total cell number in five hippocampal subregions [granule layer (GL), hilus of the dentate gyrus (DGH), cornu ammonis fields (CA)2/3, CA1, and subiculum (SUB)] [Fitting, S., Booze, R.M., Hasselrot, U., Mactutus, C.F., 2007a. Differential long-term neurotoxicity of HIV-1 proteins in the rat hippocampal formation: a design-based stereological study. Hippocampus 18(2), 135-147]. A relationship between early reflex development and estimated cell number in the adult hippocampus was indicated by simple regression analyses. In addition, estimated number of neurons and astrocytes in the DGH explained 81% of the variance of the distribution of searching behavior in the probe test. Collectively, these data indicate that the DGH may participate in the spatial memory alterations observed in adulthood consequent to neonatal exposure to HIV-1 proteins.

Impairment of vestibular-mediated cardiovascular response and motor coordination in rats born and reared under hypergravity

It is well known that environmental stimulation is important for the proper development of sensory function. The vestibular system senses gravitational acceleration and then alters cardiovascular and motor functions through reflex pathways. The development of vestibular-mediated cardiovascular and motor functions may depend on the gravitational environment present at birth and during subsequent growth. To examine this hypothesis, arterial pressure (AP) and renal sympathetic nerve activity (RSNA) were monitored during horizontal linear acceleration and performance in a motor coordination task in rats born and reared in 1-G or 2-G environments. Linear acceleration of +/-1 G increased AP and RSNA. These responses were attenuated in rats with a vestibular lesion, suggesting that the vestibular system mediated AP and RSNA responses. These responses were also attenuated in rats born in a 2-G environment. AP and RSNA responses were partially restored in these rats when the hypergravity load was removed, and the rats were maintained in a 1-G environment for 1 wk. The AP response to compressed air, which is mediated independently of the vestibular system, did not change in the 2-G environment. Motor coordination was also impaired in the 2-G environment and remained impaired even after 1 wk of unloading. These results indicate that hypergravity impaired both the vestibulo-cardiovascular reflex and motor coordination. The vestibulo-cardiovascular reflex was only impaired temporarily and partially recovered following 1 wk of unloading. In contrast, motor coordination did not return to normal in response to unloading.

Embryonic motor neuron dendrite growth is stunted by inhibition of nitric oxide-dependent activation of soluble guanylyl cyclase and protein kinase G

We have examined the participation of a neuronal nitric oxide synthase (nNOS) signaling pathway in the elaboration of motor neuron dendrites during embryonic life. During chick embryogenesis, nNOS is expressed by interneurons that surround the motor neuron pools in the ventral horn. Pseudorabies virus tracing suggests that these cells, while juxtaposed to motor neurons are not synaptically connected to them. The downstream effectors, soluble guanylyl cyclase (sGC) and protein kinase G (PKG), are found in motor neurons as well as several other populations of spinal cord cells. To determine the functional significance of the nNOS/sGC/PKG signaling pathway, pharmacological inhibitors were applied to chick embryos and the effects on motor neuron dendrites monitored. Inhibition of nNOS activity led to a lasting reduction in the overall size and degree of branching of the dendritic tree. These alterations in dendritic architecture were also seen when the activity of sGC or PKG was blocked. Our results suggest that normal motor neuron dendrite elaboration depends, in part, on the activity-dependent generation of NO by ventral horn interneurons, which then activates sGC and PKG in motor neurons.

Neonatal intrahippocampal gp120 injection: an examination early in development

The presence of human immunodeficiency virus type 1 (HIV-1) in the brain is believed to be responsible for mediating the pathogenesis of neurological abnormalities through the viral toxins gp120 and Tat. Numerous studies indicate neurotoxic effects of the HIV-1-protein Tat, with demonstrated neurobehavioral and cognitive alterations. However, less clear is the neurotoxic effect of gp120 on neurobehavior. This study was designed to characterize the potential deficits in sensory-motor and preattentive functions, following intrahippocampal administration of gp120. Using a randomized-block design, male and female pups of eight Sprague-Dawley litters were injected bilaterally with either vehicle (VEH) (1 microl volume) or one of the three gp120 doses (1.29, 12.9, or 129 ng/microl) at postnatal day (P)1. Sensory-motor functions were assessed at P3, as measured by the righting reflex and at P8, as measured by negative geotaxis. At P24 animals were tested on preattentive processes, as indexed by sensorimotor gating. Sensorimotor gating was measured by prepulse inhibition (PPI) of the auditory startle response (ASR) (ISIs of 0, 8, 40, 80, 120, and 4000 ms, six trial blocks, Latin-square design). Results indicated gp120-induced neurotoxicity on the righting reflex but not negative geotaxis. For sensorimotor gating, the PPI test demonstrated a reduced inhibition response on peak ASR latency as the dose of gp120 increased. No effect was noted for response inhibition on peak ASR amplitude. These data suggest that intrahippocampal injection of gp120 (0, 1.29, 12.9, or 129 ng/microl) had transient neurotoxic effects on sensory-motor function and limited effects on preattentive processes early in development.

Expression of serum- and glucocorticoid-inducible kinase is regulated in an experience-dependent manner and can cause dendrite growth

The interaction of an animal with its environment during a critical period in early postnatal life has lifelong effects on the structure and function of sensory and motor systems. To gain insight into the molecular mechanisms of experience-dependent development, we challenged young rats to adapt to a new environment that engenders novel motor behavior. Rats born in the gravitational field (1G) of the earth subsequently were reared for 2 weeks either in the absence of gravity (microgravity) or at 1G. A comparison of gene expression using microarrays led to the identification of a panel of differentially regulated transcripts. We report here that the abundance of serum- and glucocorticoid-inducible kinase (SGK) is increased in spinal cord tissue from animals reared in microgravity in comparison with 1G-reared controls. The induction of SGK expression also can be achieved by administration of glucocorticoids to animals at 1G or neurons in vitro. Expression of constitutively active SGK in neurons leads to the elaboration of neuronal dendrites and their branching. Glucocorticoids also lead to dendrite elaboration, and this effect can be abrogated by inhibiting SGK activity. Changes in the level of expression of SGK could be part of the mechanism for experience-dependent acquisition of mature neuronal properties.

Long-term effects of microgravity on the swimming behaviour of young rats

The postnatal development of sensory systems has been shown in studies over the last four decades to be influenced by experience during critical periods of development. We report here that similar experience-dependent development can be observed in the swimming behaviour of young rats reared from postnatal day 14 (P14) to P30 in the reduced gravitational field of low earth orbit. Animals flown in space when placed in the water on the day of landing maintained their head and forelimbs in a balanced posture. However, until the animals began to swim, their hindquarters showed little lateral postural control resulting in rotation about the longitudinal axis (60 degrees+/-4 deg). Such results suggest an 'unlinking' of postural control of the forequarters from the hindquarters in the early hours after landing. Similar instability seen in animals age-matched to the day of launch (97+/-7 deg) and in ground control animals (9+/-3 deg) was corrected within one or two rotations, even in the absence of swimming. Animals flown in space began to swim sooner after being placed in the water, and the duration of swimming strokes was shorter than in control animals. Motion analysis revealed a difference in the swimming style on landing day. In flight animals, the knee joint was more flexed throughout the stroke, there was a narrower range of movement, and the linear velocity of the tip of the foot was faster throughout most of the stroke than in age-matched control animals. Thus, posture in the water as well as swimming speed and style were altered in the animals flown in space. Some of these characteristics persisted for as long as the animals were followed (30 days). These included the short pre-swimming interval and short stroke duration in flight animals. These findings clearly show that an altered gravitational field influences the postnatal development of motor function. The nature of the differences between animals reared in space for 16 days and those remaining on the ground reflects an adaptation of the flight animals to the microgravity environment. The data suggest that the most fundamental of these adaptations is a resetting of the basic motor rhythm to a higher frequency.