Neuroplastic reactivity of fish induced by altered gravity conditions: a review of recent results.

A review is being presented concerning behavioural, biochemical, histochemical and electronmicroscopical data on the influence of altered gravitational forces on the swimming performance and on the neuronal differentiation of the brain of cichlid fish larvae and adult swordtail fish that had been exposed to hyper-gravity (3g in laboratory centrifuges), hypo-gravity (>10(-2) g in a fast-rotating clinostat) and to near weightlessness (10(-4) g aboard the Spacelab D-2 mission). After long-term alterations of gravity (and parallel light deprivation), initial disturbances in the swimming behaviour followed by a stepwise regain of normal swimming modes are induced. Parallel, neuroplastic reactivities on different levels of investigation were found, such as adaptive alterations of activities of various enzymes in whole brain as well as in specific neuronal integration centers and an intraneuronal reactivity on ultrastructural level in individual brain parts and in the sensory epithelia of the inner ear. Taken together, these data reveal distinct adaptive neuroplastic reactions of fish to altered gravity conditions.     

Calcium signaling in plant cells in altered gravity.

Changes in the intracellular Ca2+ concentration in altered gravity (microgravity and clinostating) evidence that Ca2+ signaling can play a fundamental role in biological effects of microgravity. Calcium as a second messenger is known to play a crucial role in stimulus-response coupling for many plant cellular signaling pathways. Its messenger functions are realized by transient changes in the cytosolic ion concentration induced by a variety of internal and external stimuli such as light, hormones, temperature, anoxia, salinity, and gravity. Although the first data on the changes in the calcium balance in plant cells under the influence of altered gravity have appeared in 80th, a review highlighting the performed research and the possible significance of such Ca2+ changes in the structural and metabolic rearrangements of plant cells in altered gravity is still lacking. In this paper, an attempt was made to summarize the available experimental results and to consider some hypotheses in this field of research. It is proposed to distinguish between cell gravisensing and cell graviperception; the former is related to cell structure and metabolism stability in the gravitational field and their changes in microgravity (cells not specialized to gravity perception), the latter is related to active use of a gravitational stimulus by cells presumebly specialized to gravity perception for realization of normal space orientation, growth, and vital activity (gravitropism, gravitaxis) in plants. The main experimental data concerning both redistribution of free Ca2+ ions in plant cell organelles and the cell wall, and an increase in the intracellular Ca2+ concentration under the influence of altered gravity are presented. Based on the gravitational decompensation hypothesis, the consequence of events occurring in gravisensing cells not specialized to gravity perception under altered gravity are considered in the following order: changes in the cytoplasmic membrane surface tension --> alterations in the physicochemical properties of the membrane --> changes in membrane permeability, --> ion transport, membrane-bound enzyme activity, etc. --> metabolism rearrangements --> physiological responses. An analysis of data available on biological effects of altered gravity at the cellular level allows one to conclude that microgravity environment appears to affect cytoskeleton, carbohydrate and lipid metabolism, cell wall biogenesis via changes in enzyme activity and protein expression, with involvement of regulatory Ca2+ messenger system. Changes in Ca2+ influx/efflux and possible pathways of Ca2+ signaling in plant cell biochemical regulation in altered gravity are discussed.     

Effects of altered gravity on the swimming behaviour of fish.

Humans taking part in parabolic aircraft flights (PAFs) may suffer from space motion sickness-phenomena (SMS, a kinetosis). It has been argued that SMS during PAFs might not be based on microgravity alone but rather on changing accelerations from 0 g to 2 g. We test here the hypothesis that PAF-induced kinetosis is based on asymmetric statoliths (i.e., differently weighed statoliths on the right and the left side of the head), with asymmetric inputs to the brain being disclosed at microgravity. Since fish frequently reveal kinetotic behaviour during PAFs (especially so-called spinning movements and looping responses), we investigated (1) whether or not kinetotically swimming fish at microgravity would have a pronounced inner ear otolith asymmetry and (2) whether or not slow translational and continuously changing linear (vertical) acceleration on ground induced kinetosis. These latter accelerations were applied using a specially developed parabel-animal-container (PAC) to stimulate the cupular organs. The results suggest that the fish tested on ground can counter changing accelerations successfully without revealing kinetotic swimming patterns. Kinetosis could only be induced by PAFs. This finding suggests that it is indeed microgravity rather than changing accelerations, which induces kinetosis. Moreover, we demonstrate that fish swimming kinetotically during PAFs correlates with a higher otolith asymmetry in comparison to normally behaving animals in PAFs.     

Biological role of gravity: hypotheses and results of experiments on "Cosmos" biosatellites.

In order to reveal the biological significance of gravity, microgravity effects have been studied at the cellular, organism and population levels. The following questions arise. Do any gravity-dependent processes exist in a cell? Is cell adaptation to weightlessness possible; if so, what role may cytoskeleton, the genetic apparatus play in it? What are the consequences of the lack of convection in weightlessness for the performance of morphogenesis? Do the integral characteristics of living beings change in weightlessness? Is there any change in "biological capacity" of space, its resistance to expansion of life? What are the direction and intensity of microgravity action as a factor of natural selection, the driving force of evolution? These problems are discussed from a theoretical point of view, and in the light of results obtained in experiments from aboard biosatellites "Cosmos".     

Observational astrochemistry: recent results.

More than 80 molecular species have now been observed by astronomers in the dense interstellar clouds where stars and planets form or in the envelopes expelled by evolved stars. Elemental constituents of these compounds include all of the "biogenic" elements, hydrogen, carbon, nitrogen, oxygen, sulfur, and (most recently) phosphorus. In addition, silicon is found in several molecules, and a series of metal halides have recently been detected in the outflowing envelope of a nearby carbon star. Additions to the list of known interstellar molecules since the last COSPAR meeting are discussed individually. Recent measurements of the hydrogen isotopic fractionation for the cyclic molecule C3H2 are described; values up to 10,000 times the cosmic deuterium-to-hydrogen ratio are found. Knowledge of the chemical reservoirs for the major volatile elements and a comparison between observed molecular abundances and theoretical models are both discussed.     

Chromosomes and plant cell division in space: environmental conditions and experimental details.

Details of the plant cultivation system developed for the CHROMEX experiment flown aboard the Shuttle Discovery (March, 1989) in NASA's Plant Growth Unit (PGU) are presented. The physical regime as measured during Spaceflight, both within the orbiter cabin environment and within the PGU itself, is discussed. These data function as a guide to what may be representative of the environmental regime in which Space-based plant cultivation systems will be operating, at least for the near-term. Attention is also given to practical considerations involved in conducting a plant experiment in Space. Of particular importance are the differences expected to occur in moisture distribution patterns within substrates used to cultivate plants in Space vs on Earth.     

How gravity acts on Paramecium: new insights from free-fall experiments.

The swimming behaviour of Paramecium is affected by media of various specific gravities. At 1g, the negative gravitaxis of Paramecium virtually disappears in solutions the specific gravity of which is about the same as that of the organism (1.04). In solutions with a higher specific gravity (1.08), Paramecium becomes positively gravitactic. We recorded the swimming tracks of Paramecium in these media on videotape before, during and after free-falls. The records show that the density-dependent differences in the swimming behaviour disappeared immediately following the onset of the free-fall. The recorded tracks and distributions of cells in the experimental chambers were compared with computer-simulated traces and distributions based on gravitactic and gravikinetic models proposed for Paramecium. Our preliminary analysis favors a novel gravitactic mechanism involving modification of the ciliary movement The drop shaft at the Japan Microgravity Center, Hokkaido (JAMIC) was used for the free-fall experiments.     

The effect of gravity on plant germination.

An axis clinostat was constructed to create micro and negative gravity also a rotated flat disk was constructed with different rotation rates to give increased gravity, by centrifugal force up to 48 g. Rice seeds were grown on agar in tubes at the constant air temperature of 20 degrees C under an average light condition of 110 micromol/m2/sec(PPF). Humidity was not controlled but was maintained above 90%. Since the tube containers were not large enough for long cultivation, shoot and root growth were observed every 12 hours until the sixth day from seeding. The lengths of shoots and roots for each individual plant were measured on the last day. The stem lengths were increased by microgravity but the root lengths were not. Under the negative gravity, negative orthogeotropism and under microgravity, diageotropism was observed. No significant effect of increased gravity was observed on shoot and root growth.     

Spaceflight hardware for conducting plant growth experiments in space: the early years 1960-2000.

The best strategy for supporting long-duration space missions is believed to be bioregenerative life support systems (BLSS). An integral part of a BLSS is a chamber supporting the growth of higher plants that would provide food, water, and atmosphere regeneration for the human crew. Such a chamber will have to be a complete plant growth system, capable of providing lighting, water, and nutrients to plants in microgravity. Other capabilities include temperature, humidity, and atmospheric gas composition controls. Many spaceflight experiments to date have utilized incomplete growth systems (typically having a hydration system but lacking lighting) to study tropic and metabolic changes in germinating seedlings and young plants. American, European, and Russian scientists have also developed a number of small complete plant growth systems for use in spaceflight research. Currently we are entering a new era of experimentation and hardware development as a result of long-term spaceflight opportunities available on the International Space Station. This is already impacting development of plant growth hardware. To take full advantage of these new opportunities and construct innovative systems, we must understand the results of past spaceflight experiments and the basic capabilities of the diverse plant growth systems that were used to conduct these experiments. The objective of this paper is to describe the most influential pieces of plant growth hardware that have been used for the purpose of conducting scientific experiments during the first 40 years of research.     

Effects of gravity and cosmic rays on cell proliferation kinetics in Paramecium tetraurelia.

Space flights resulted in a stimulating effect on kinetics of proliferation in Paramecium tetraurelia. Additional experiments were performed in order to determine the origin of this phenomena. Paramecia were cultivated in balloon flights or in a slow clinostat, or were exposed to different levels of hypergravity. The results suggest that changes in cell proliferation rate are related to cosmic rays and to a direct effect of microgravity.     

Short-term responses of gravitaxis to altered gravity in Paramecium.

Negative gravitaxis of Paramecium almost disappeared in solutions having specific gravity about the same as that of the organisms (1.04). The taxis turned to positive in solutions of specific gravity 1.08. Using a drop shaft at the Japan Microgravity Center, Hokkaido (JAMIC) we examined how swimming behaviour in these media was modified by changing gravitational conditions before, during and after free-fall. Tracks of swimming cells recorded on videotape indicate that the swimming cells continued upward and downward shift depending on the specific gravity of the external medium under 1-g conditions and these vertical displacements disappeared immediately after the moment of launch. The effectiveness of changing gravity to induce displacement of the cells seems to depend on the orientation of the cells to gravity. These results suggest a corelation between vertical displacement of the cell through the medium and a gravitactic mechanism in Paramecium.     

Cell biology of plant gravity sensing.

The debate about whether gravity sensing relies upon statoliths (amyloplasts that sediment) has intensified with recent findings of gravitropism in starchless mutants and of claims of hydrostatic gravity sensing. Starch and significant plastid sedimentation are not necessary for reduced sensing in mutant roots, but plastids might function here if there were a specialized receptor for plastid mass e.g. in the ER. Alternatively, components in addition to amyloplasts might provide mass for sensing. The nucleus is dense and its position is regulated, but no direct data exist for its role in sensing. If the weight of the protoplast functioned in sensing, why would there be specific cytological specializations favoring sedimentation rather than cell mass? Gravity has multiple effects on plants in addition to gravitropism. There may be more than one mechanism of gravity sensing.     

Effects of gravity on early development.

The development of embryonic and larval stages of the South African Toad Xenopus laevis D, was investigated in hyper-g up to 5 g (centrifuge), in simulated 0 g (fast-rotating clinostat), in alternating low g, hyper-g (parabolic flights) and in microgravity (Spacelab missions D1, D-2). The selected developmental stages are assumed to be very sensitive to environmental stimuli. The results showed that the developmental reaction processes run normal also in environments different to 1 g and that aberrations in behavior and morphology normalize after return to 1 g. Development, differentiation, and morphology of the gravity perceiving parts of the vestibular system (macula-organs) had not been affected by exposure to different g-levels.     

The FLUKA code for space applications: recent developments.

The FLUKA Monte Carlo transport code is widely used for fundamental research, radioprotection and dosimetry, hybrid nuclear energy system and cosmic ray calculations. The validity of its physical models has been benchmarked against a variety of experimental data over a wide range of energies, ranging from accelerator data to cosmic ray showers in the earth atmosphere. The code is presently undergoing several developments in order to better fit the needs of space applications. The generation of particle spectra according to up-to-date cosmic ray data as well as the effect of the solar and geomagnetic modulation have been implemented and already successfully applied to a variety of problems. The implementation of suitable models for heavy ion nuclear interactions has reached an operational stage. At medium/high energy FLUKA is using the DPMJET model. The major task of incorporating heavy ion interactions from a few GeV/n down to the threshold for inelastic collisions is also progressing and promising results have been obtained using a modified version of the RQMD-2.4 code. This interim solution is now fully operational, while waiting for the development of new models based on the FLUKA hadron-nucleus interaction code, a newly developed QMD code, and the implementation of the Boltzmann master equation theory for low energy ion interactions.     

Influence of long-term altered gravity on the swimming performance of developing cichlid fish: including results from the 2nd German Spacelab Mission D-2.

This study presents qualitative and quantitative data concerning gravity-dependent changes in the swimming behaviour of developing cichlid fish larvae (Oreochromis mossambicus) after a 9 resp. 10 days exposure to increased acceleration (centrifuge experiments), to reduced gravity (fast-rotating clinostat), changed accelerations (parabolic aircraft flights) and to near weightlessness (2nd German Spacelab Mission D-2). Changes of gravity initially cause disturbances of the swimming performance of the fish larvae. With prolonged stay in orbit a step by step normalisation of the swimming behaviour took place in the fish. After return to 1g earth conditions no somersaulting or looping could be detected concerning the fish, but still slow and disorientated movements as compared to controls occurred. The fish larvae adapted to earth gravity within 3-5 days. Fish seem to be in a distinct early developmental stages extreme sensitive and adaptable to altered gravity; However, elder fish either do not react or show compensatory behaviour e.g. escape reactions.     

Histochemical investigations on the influence of long-term altered gravity on the CNS of developing cichlid fish: results from the 2nd German Spacelab Mission D-2.

The effect of long-term (10 days) altered gravitational conditions upon succinate dehydrogenase (SDH) reactivity in total brains as well as in individual brain nuclei of developing cichlid fish larvae had been investigated by means of semiquantitative histochemical methods (densitometric grey value analysis). Increasing accelerations from near weightlessness (spaceflight) via 1g controls to 3g hyper gravity (centrifuge) resulted in slightly increasing "all over the brain" (total brain) SDH reactivity. When focusing on distinct neuronal integration centers within the same brains in order to find the anatomical substratum of the gross histochemical data, significant effects of altered gravity only within vestibulum related brain parts were obtained.     

Exposure to altered gravity affects all stages of endochondral cartilage differentiation.

Chondrogenesis has a number of well-defined steps: (1) condensation, which involves cell aggregation, adhesion and communication; (2) activation of cartilage genes, which is accompanied by rounding up of the cells and intracellular differentiation; and (3) production and secretion of cartilage specific matrix molecules. Our studies show that each of these steps is affected by exposure to gravitational changes. Clinorotation and centrifugation affected initial aggregation and condensation. In the CELLS experiment, where cells were exposed to microgravity after some condensation occurred preflight, intracellular differentiation and matrix production were delayed relative to controls. Once cartilage has developed, in rats, further differentiation (hypertrophy, matrix production) was also affected by spaceflight and hind limb suspension. For the process of chondrogenesis to proceed as we know it, loading and other factors present at 1g are required at each step of the process. This requirement means that not only will skeletal development and bone healing, processes involving chondrogenesis, be altered by long term exposure to microgravity, but that continuous intervention will be necessary to correct any defects produced by altered gravity environments.     

Chairman's introduction: mechanisms, models and experiments in space radiation research.

Radiation risk estimate in space is a moral obligation and a scientific challenge requiring the combined efforts of physicists and biologists. This introductory paper presents some thoughts about problems to be solved and the possible directions of research. It stresses the necessity of cooperation across disciplines and the combination of space and ground based investigations.     

Cell inactivation, repair and mutation induction in bacteria after heavy ion exposure: results from experiments at accelerators and in space.

To understand the mechanisms of accelerated heavy ions on biological matter, the responses of spores of B. subtilis to this structured high LET radiation was investigated applying two different approaches. 1) By the use of the Biostack concept, the inactivation probability as a function of radial distance to single particles' trajectory (i.e. impact parameter) was determined in space experiments as well as at accelerators using low fluences of heavy ions. It was found that spores can survive even a central hit and that the effective range of inactivation extends far beyond impact parameters where inactivation by delta-ray dose would be effective. Concerning the space experiment, the inactivation cross section exceeds those from comparable accelerator experiments by roughly a factor of 20. 2) From fluence effect curves, cross sections for inactivation and mutation induction, and the efficiency of repair processes were determined. They are influenced by the ions characteristics in a complex manner. According to dependence on LET, at least 3 LET ranges can be differentiated: A low LET range (app. < 200 keV/micrometers), where cross sections for inactivation and mutation induction follow a common curve for different ions and where repair processes are effective; an intermediate LET range of the so-called saturation cross section with negligible mutagenic and repair efficiency; and a high LET range (>1000 keV/micrometers) where the biological endpoints are majorly dependent on atomic mass and energy of the ion under consideration.     

Adaptations of the vestibular system to short and long-term exposures to altered gravity.

Long-term space flight creates unique environmental conditions to which the vestibular system must adapt for optimal survival of a given organism. The development and maintenance of vestibular connections are controlled by environmental gravitational stimulation as well as genetically controlled molecular interactions. This paper describes the effects of hypergravity on axonal growth and dendritic morphology, respectively. Two aspects of this vestibular adaptation are examined: (1) How does long-term exposure to hypergravity affect the development of vestibular axons? (2) How does short-term exposure to extremely rapid changes in gravity, such as those that occur during shuttle launch and landing, affect dendrites of the vestibulocerebellar system? To study the effects of longterm exposures to altered gravity, embryonic rats that developed in hypergravity were compared to microgravity-exposed and control rats. Examination of the vestibular projections from epithelia devoted to linear and angular acceleration revealed that the terminal fields segregate differently in rat embryos that gestated in each of the gravitational environments.To study the effects of short-term exposures to altered gravity, mice were exposed briefly to strong vestibular stimuli and the vestibulocerebellum was examined for any resulting morphological changes. My data show that these stimuli cause intense vestibular excitation of cerebellar Purkinje cells, which induce up-regulation of clathrin-mediated endocytosis and other morphological changes that are comparable to those seen in long-term depression. This system provides a basis for studying how the vestibular environment can modify cerebellar function, allowing animals to adapt to new environments.