Abstract
The cellular mechanisms of neuroplastic changes in the structure of motoneurons and neuropils of the oculomotor (III) nuclei in mice after a 30-day space flight and 7 days after landing were studied. The results showed that microgravity caused degenerative phenomena in neurons: a decrease in the number of terminal dendritic branches was found both after flight and after readaptation to Earth's gravity. In mice after the flight, the number of axodendritic synapses was less than in the control, and their number was not restored after the readaptation. The number of mitochondria in the motoneurons of animals after the flight also decreased and after the readaptation reached only the control value. In addition, a significant number of dark motorneurons were found in mice after readaptation, which indicates that degeneration was caused not only by microgravity, but also by a reaction to the landing of the biosatellite. On the contrary, in the trochlear nucleus, as we showed earlier (Mikheeva et al. in Brain Res 15(1795):148077. https://doi.org/10.1016/j.brainres.2022.148077, 2022), after readaptation, the dendrites and synaptic contacts were restored, and mitogenesis is significantly enhanced. It has been suggested that morphological changes in the oculomotor nucleus may be the main cause of microgravity-induced nystagmus.
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Differences in the response of these nuclei to microgravity may be due to their anatomical and functional differences. The number of motor neurons in the oculomotor nucleus is much higher than in the trochlear nucleus, and they are somewhat smaller in size. In addition, neurons in the oculomotor nucleus innervate four muscles of the eyeball, in contrast to the trochlear nucleus, which innervates only one muscle. It can be assumed that the stability threshold of the motoneurons of the oculomotor nucleus was exceeded under the conditions of the space experiment, and therefore degenerative phenomena began in them.
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Acknowledgements
The Bion-M1 project was organized by the State Space Corporation ROSCOSMOS (previously Russian Federal Space Agency), the Russian Academy of Sciences, the Joint Stock Company Space Rocket Centre (JSC SRC) Progress (previously Russian Space Center “Progress”) with the participation of the State Scientific Center of Russian Federation Institute of Biomedical Problems of the Russian Academy of Sciences (SSC RF–IBMP RAS).
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The work was partially supported by the State Assignment No. 075-01025-23-00.
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Methodology, investigation, visualization: MIB; VGM, ZNS, SRSh; conceptualization, validation, data curation, writing—original draft: AVI and PLL.
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Mikheeva, I., Zhujkova, N., Mikhailova, G. et al. Morphological changes in motoneurons of the oculomotor nucleus of mice after a 30-day space flight and through a 7-day period of readaptation to earth gravity. Brain Struct Funct 228, 2041–2049 (2023). https://doi.org/10.1007/s00429-023-02704-0
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DOI: https://doi.org/10.1007/s00429-023-02704-0