An age-dependent sensitivity of the roll-induced vestibuloocular reflex to hypergravity exposure of several days in an amphibian (Xenopus laevis).

In tadpoles of the Southern Clawed Toad (Xenopus laevis), the effects of an exposure to hypergravity of several days duration on the development of the roll-induced static vestibuloocular reflex (rVOR) were investigated. Special attention was given to the onset of the 9 or 12 days lasting 3G-period during early life. First recordings of the rVOR characteristics for complete 360 degrees rolls of the tadpoles were performed 24 hrs after the end of the 3G-period. The rVOR peak-to-peak amplitudes as well as the VOR-gain for a roll angle of 15 degrees from 3G-and 1G-samples recorded at the 2nd and 3rd day after 3G-termination agreed for the youngest group, but were reduced by approx. 30% in the older tadpoles. Long-term observations lasting up to 8 weeks after termination of the 3G-period, demonstrated (i) an early retardation of the development, and (ii) a developmental acceleration in all groups so that after 2 weeks in the stage 6/9- and 33/36-samples and after 8 weeks in the stage 45-tadpoles, the rVOR-amplitude as well as the rVOR-gain for a 15 degrees roll were at the same level in both the 3G- and the 1G-samples. The results support the existence of a sensitive period for the rVOR development, and additionally demonstrate the importance of the period of the first appearance of the rVOR for the development of adaptive properties of the underlying neuronal network. They also demonstrate the dominant efficiency of genetic programs in the functional development of the vestibular system. Methodological approaches are discussed which will be useful in the further description of the critical period. They include studies on the neuronogenesis and synaptic maturation within the vestibular pathways as well as on the fundamentals of buoyancy control during swimming. A modular but closed mini-system for experimental use is described which allows survival periods lasting many weeks and multiple types of treatments of developing aquatic animals in orbit, controlled automatically.