Pergamon www.elsevier.nl/locatelasr zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Adv. Space Res. Vol. 25. No. 10. pp. 1985-1995.2000 0 2000 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177/00 $20.00 + 0.00 PII: SO273-1177(99)01007-8 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONML GRAVITATIONAL NEUROBIOLOGY OF FISH H. Rahmann, and R. H. Anken Zoological Institute, University of Stuttgart-Hohenheim, Garbenstr. 30, D-70593 Stuttgart, Germany ABSTRACT In vertebrates (including man), altered gravitational environments such as weightlessness can induce malfunctions of the inner ears, based on irregular movements of the semicircular cristae or on dislocations of the inner ear otoliths from the-corresponding sensory epithelia. This will lead to illusionary tilts, since the vestibular inputs are not confirmed by the other sensory organs, which results in an intersensory conflict. Vertebrates in orbit therefore face severe orientation problems. In humans, the intersensory conflict may additionally lead to a malaise, commonly referred to as space motion sickness (SMS), a kinetosis. During the first days at weightlessness, the orientation problems (and SMS) disappear, since the brain develops a new compensatory interpretation of the available sensory data. The present review reports on the neurobiological responses - particularly of fish - observed at altered gravitational states, concerning behaviour and neuroplastic reactivities. Recent investigations employing microgravity (spaceflight, parabolic aircraft flights, clinostat) and hyper-gravity (laboratory centrifuges as ground based research tools) yielded clues and insights into the understanding of the respective basic phenomena. 0 2000 COSPAR. Published by Elsevier zyxwvutsrqponmlkjihgfe Science Ltd. INTRODUCTION Since the dawn of life on Earth some four billions of years ago, gravity has been a more or less stable environmental factor. Therefore, gravity became most suitable with regard to spatial orientation. Animals use statoliths to bend sensory cilia for the transformation of acceleration into a computable signal. In vertebrates, the bending of cilia mechanically affects ion channels, which alter the electrical current of the respective sensory cell in the inner ear. On the level of the sensory cells, the transformation to computable action potentials takes place. Signal transduction at the level of the central nervous system (CNS) finally causes a motor output (signal response), i.e. a behavioural reaction. For spatial orientation and postural control in the environment, the vertebrate CNS integrates information from the inner ear vestibular endorgans together with tactile, proprioceptive and visual cues (e.g., ClCment and Berthoz, 1994). At unexpected gravitational sensations, especially when diminishing the g-force, those parts of the vestibular apparatus which are sensitive to changes in position (i.e., the statolithic or otolithic organs, which are used to measure linear acceleration and the non-otolithic cupular organs, which sense angular acceleration) may transmit information to the brain, which do not necessarily completely agree with, e.g., the visual cues needed for a correct postural control. This can result in orientation problems, often accompanied by motion sickness (kinetosis) and vomiting in human subjects. In the following, we focus our report mainly on research results using fish as vertebrate model systems (Figure 1). They are easy to rear, they are able to float around freely in the water, which makes them highly sensitive to altered gravitational conditions, and - last not least - their peripheral and central vestibular system (inner ear and vestibular brain nuclei, respectively) is largely comparable to that of higher vertebrates including man. 1985