READAPTATION OF THE VESTIBULOOCULAR REFLEX TO 1G-CONDITION IN IMMATURE LOWER-VERTEBRATES (XENOPUS-LAEVIS) AFTER MICROGRAVITY OR HYPERGRAVITY EXPOSURE

The effects of altered gravitational conditions (AGC) on the developme nt of the static vestibulo-ocular reflex (VOR) and readaptation to 1g were investigated in the amphibian Xenopus laevis, Tadpoles were expos ed to microgravity (mu g) during the German Space Mission D-2 for 10 d ays, using the STATEX closed survival system, or to 3g for 9 days duri ng earth-bound experiments. At the beginning of AGC, the tadpoles had not yet developed the static VOR. The main results were: (i) Tadpoles with mu g- or 3g-experience had a lower gain of the static VOR than th e Ig-controls during the 2nd and 5th post-AGC days. (ii) Readaptation to response levels of Ig-reared controls usually occurred during the f ollowing weeks, except in slowly developing tadpoles with 3g-experienc e. Readaptation was less pronounced if, during the acute VOR test, tad poles were rolled from the inclined to the normal posture than in the opposite test situation. It is postulated that (i) gravity is necessar ily involved in the development of the static VOR, but only during a p eriod including the time before onset of the first behavioural respons e; and (ii) readaptation which is superimposed by the processes of VOR development depends on many factors including the velocity of develop ment, the actual excitation level of the vestibular systems and the ne uroplastic properties of its specific pathways.