ALTERED GRAVITATIONAL EXPERIENCE DURING EARLY PERIODS OF LIFE AFFECTSTHE STATIC VESTIBULOOCULAR REFLEX OF TADPOLES OF THE SOUTHERN CLAWED TOAD, XENOPUS-LAEVIS DAUDIN

The effects of altered gravitational forces (AGF) on the development o f the static vestibule-ocular reflex (VOR) were investigated in Xenopu s laevis tadpoles exposed to hypergravity (1.4g; 3g) or microgravity c onditions (German spacelab mission D-2) for 9-10 days. The effects of light conditions during development were also tested by exposing tadpo les to either complete darkness (DD) or 12:12h light-dark conditions ( LD). The static VOR was induced by lateral roll. The efficacy of the V OR circuit after termination of AGF conditions was described by the pe ak-to-peak amplitude of the sinusoidal VOR characteristics (i.e. ampli tude). The static VOR was first observed at stage 41 for both LD and D D tadpoles. Its further development was retarded in the DD tadpoles co mpared with the LD tadpoles up to stage 48. Microgravity as well as hy pergravity exposure caused a significant (P<0.05, at least) decrease i n the static VOR amplitude during the first week after termination of the AGF period. The decreases were 39.4% in the microgravity group, 16 .2% in the 1.4g group and 24.9-42.9% in the 3g group compared with the Ig ground-reared siblings at the same developmental stages. The respo nse deficits usually disappeared but persisted for at least 2 weeks in animals whose development was retarded by hypergravity exposure. It i s postulated (i) that gravity exerts an important influence on the nor mal development of the roll-induced static VOR; (ii) that hypergravity exposure decreases the sensitivity of the gravity-sensitive system so that recordings under Ig conditions cause a weaker static VOR; and (i ii) that the vestibule-spinal pathway possesses a higher degree of pla sticity than the vestibule-ocular pathway.