NEUROBIOLOGY OF FISH UNDER ALTERED GRAVITY CONDITIONS

In vertebrates (including man), an altered gravitational environment s uch as weightlessness can induce malfunction of the inner ear, based o n an irregular dislocation of the otoliths from the corresponding sens ory epithelia. This dislocation leads to an illusionary tilt, since th e otolithic inputs are not in register with other sensory organs. This results in an intersensory conflict. Vertebrates in orbit therefore f ace severe orientation problems. In humans, the intersensory conflict may additionally lead to a malaise, commonly referred to as space moti on sickness (SMS). During the first days in weightlessness, the orient ation problems (and SMS) disappear, since the brain develops a new com pensatory interpretation of the available sensory data. The present re view reports the neurobiological responses-particularly in fish-observ ed at altered gravitational states, concerning behaviour and neuroplas tic reactivities. Recent investigations employing microgravity (spacef light, parabolic aircraft flights, clinostat) and hyper-gravity (labor atory centrifuges as ground based research tools) yielded clues and in sights into the understanding of the respective basic phenomena. The p ossible 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 (we ight, size). (C) 1998 Elsevier Science B.V. All rights reserved.