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Ventral root recordings during fictive swimming in tadpoles (Xenopus laevis) with microgravity or hypergravity experience. S. Bцser¹, C. Dournon², L. Gualandris-Parisot³, and E. Horn¹. ¹Gravitational Physiology, Univ of Ulm, Ulm, Germany; ²EA 3442 Genetic, Signaling, Differentiation, Univ Henri Poincarй, Vandoeuvre-lиs-Nancy cedex, France; ³Centre de Biologie du Dйveloppement, Univ Paul-Sabatier, Toulouse cedex, France.

   Tadpole swimming, a stereotyped rhythmical activity, is a model to examine effects of altered gravity on the motor system since the movement simplicity makes it easy to detect basic changes. High speed camera recordings have shown that tadpoles (Xenopus laevis) raised in microgravity exhibited a lower tailbeat frequency than 1g-controls (Fejtek et al. 1998, J Exp Biol 201:1917-26). Swimming pattern is generated by a central oscillator. It is possible to observe the rhythmical, burstlike activity of motoneurons by extracellular recordings from ventral roots (VR) of the spinal cord in paralysed animals (fictive swimming). Typical for the swimming pattern is the rostrocaudal delay of activity in ipsilateral myotomes of different segments. - Experiments were performed with Xenopus tadpoles exposed either to 10- to 11-day 3g-hypergravity on ground or to 10-day microgravity during the Andromиde mission to ISS in October 2001. At onset of  altered gravity, they were younger than stage 28. VR-recordings started 4 hrs after the end of 3g- and 48 hrs after the end of  µg-exposure. Fictive swimming was induced by a mechanical stimulus. We determined burst duration, rostrocaudal delay, episode duration and cycle length of VR-activity. – Results: The 3g-exposure increased significantly the VR burst duration. After µg-exposure, the duration of episodes of fictive swimming was increased and the rostrocaudal delay was decreased compared to ground-reared 1g-controls; in addition, the burst duration of fictive swimming was slightly decreased. Readaptation of VR-acitivity was completed within 8 days for both 3g- and µg-animals. – Conclusions: (1) There are evidences that 3g- or µg-induced changes in fictive swimming have a vestibular origin. (2) The µg-induced reduction of macular activity may affect the development of descending reticulospinal and raphespinal projections to the spinal cord in a reversible manner. (Supported by DLR, grant 50WB0140 to Horn)

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