Neurobiology of fish under altered gravity conditions

In vertebrates (including man), an altered gravitational environment such as weightlessness can induce malfunction of the inner ear, based on an irregular dislocation of the otoliths from the corresponding sensory epithelia. This dislocation leads to an illusionary tilt, since the otolithic inputs are not in register with other sensory organs. This 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). During the first days in weightlessness, the orientation problems (and SMS) disappear, since the brain develops a new compensatory interpretation of the available sensory data. The present review reports the neurobiological responses-particularly in 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. The possible sources of human space sickness (a kinetosis) and of the space adaptation syndrome (when a sensory reinterpretation of gravitational and visual cues takes place) are particularly highlighted with regard to the functional significance of bilaterally asymmetric otoliths (weight, size).