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The static vestibuloocular reflex and its relation to fictive swimming in tadpoles (Xenopus laevis) AFter microgravity exposure. E. Horn¹, L. Gualandris-Parisot², C. Dournon³, and S. Bцser¹. ¹Gravitational Physiology, Univ of Ulm, Ulm, Germany; ²Centre de Biologie du Dйveloppement, Univ Paul-Sabatier, Toulouse cedex, France; ³EA 3442 Genetic, Signaling, Differentiation, Univ Henri Poincarй, Vandoeuvre-lиs-Nancy cedex, France.

   Sensory deprivation during early periods of life affects the development of sensory systems and causes long-lasting morphological and/or physiological modifications. Experiments with the amphibian Xenopus laevis which flew on the German D-2 mission (1993) and the Shuttle-to-Mir mission SMM-06 (1997) revealed that µg-exposure affects the development of the roll-induced vestibuloocular reflex (rVOR). During the recent Soyuz taxi flight Andromиde to ISS (October 2001), we extended these studies (1) by comparing embryos (stage 25-28) which had not yet developed their rVOR at launch with those (stage 45) which had developed it, and (2) by correlating the extent of rVOR modifications in the young group with modifications within their rhythmic activity patterns of spinal ventral roots (VR). - Results: (1) In young Xenopus tadpoles, µg induced a depression of the rVOR, in particular in animals which developed upward bended tails during the space flight; in older ones, the rVOR was depressed in tadpoles with upward-bended tails while it was augmented in tadpoles with a straight body shape. (2) Significant correlations between the extent of the rVOR and VR activity existed either only in µg-exposed animals (burst duration and rostrocaudal delay) or only in 1g-tadpoles (episode duration). In particular, the VOR-amplitude was significantly correlated with the VR-parameters burst duration, episode duration and rostrocaudal delay. In the µg-group, there was also a significant correlation between cycle length and rVOR gain during the back-down posture. - Conclusions: (1) Microgravity sensitizes or desensitizes the otolithic vestibular system to static roll-stimulation depending on pre-flight rVOR-experience; (2) the development of the rVOR network and the spinal motor system reveals features which might be based on an intrinsic overall-relation or on a dependency of these subsystems mediated by descending pathways from the brainstem to the spinal cord.

(Supported by DLR, grant 50WB0140)

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