To examine the physiological sensitivity of membrane receptors to altered gravity, we examined the single channel properties of the acetylcholine receptor (AChR), in co-cultures of Xenopus myocytes and neurons, to vector-averaged gravity in the clinostat. This experimental paradigm produces an environment in which, from the cell's perspective, the gravitational vector is "nulled" by continuous averaging. In that respect, the clinostat simulates one aspect of space microgravity where the gravity force is greatly reduced. After clinorotation, the AChR channel mean open-time and conductance were statistically not different from control values but showed a rotationdependent trend that suggests a process of cellular adaptation to clinorotation. These findings therefore suggest that the AChR channel function may not be affected in the microgravity of space despite changes in the receptor's cellular organization (14).

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We carried out parallel experiments first on the slow clinostat and then in space-flight to examine the effects of altered gravity on the aggregation of the nicotinic acetylcholine receptors and the structure of the cytoskeleton in cultured Xenopus embryonic muscle cells. By examining the concordance between results from space flight and the clinostat, we tested whether the slow clinostat is a relevant simulation paradigm. Space-flown cells showed marked changes in the distribution and organization of actin filaments and had a reduced incidence of acetylcholine receptor aggregates at the site of contact with polystyrene beads. Similar effects were found after clinostat rotation. The sensitivity of synaptic receptor aggregation and cytoskeletal morphology suggests that in the microgravity of space cell behavior may be imporantly altered.

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Many environmental elements induce the stress response in organisms. In order to examine whether the space condition brings cancer causing on mammals, we propose the importance of the study about the effects of various environmental stresses on the gene expression of oncogenes and tumor-suppressor genes. Then we reviewed numerous findings about the induction of gene expression by environmental stresses. Many investigators have reported that three oncogenes of c-fos, c-jun and c-myc, so called "Early Response Genes", were induced at transcriptional level by a diverse set of stresses such as heat, UV and ionizing radiation, and that the accumulation of the product of a tumor-suppressor gene, p53 was induced at post-translational level by the same stresses.

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A noble method for the exploration of terrestrial and extraterrestrial soil microorganisms, especially targeted for Mars, has been developed. The method is based on the microscopic observation using fluorescence techniques. Microorganisms could be fluorescent by adsorption, enzymatic cleavage of extrinsic fluorescence chromophores such as acridine orange, ANS and SFDA, and also by intrinsic chromophores. The characteristic points of our fluorescence method arc shown below. 1. The present method detected all the culturable cells tested. (about 200 species from bacteria to eukaryotic cells). 2. Microorganisms in soil were much brighter than background fluorescence of soil. Cell shapes and location were clearly observed. 3. An esterase substratum, SFDA, discriminated vital (reproductive) cells from dead. On the other hand, a membrane probe, ANS, detected both vital and dead cells. 3. Pre-treatment of cells with bleaching reagents improved the detection efficiency. Especially, this pretreatment was effective in Fungi with black chromophores. 4. Some anaerobic microorganisms such as methanogenic bacteria with intrinsic chromophores can be detected without stain. 5. Application of the technique to terrestrial soil revealed that more than 100 times larger cell density was obtained compared to the value obtained by the classic plate counting technique. Vertical distribution of microorganism of soil microorganisms from Mt. Shigayama showed that, at surface, cell density was small and maximum was shown below 15 cm from surface. 6. Some pre-biotic cell (cell like aggregates composed of amino adds) could be detected by SFDA or ANS. It can be concluded that the fluorescence technique is one of the most promising method for the exploration of extraterrestrial microorganisms.

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