Claudia Sebastian

Altered gravitational experience during early periods of life affects the static vestibulo-ocular reflex of tadpoles of the Southern Clawed Toad, Xenopus laevis Daudin

The effects of altered gravitational forces (AGF) on the development of the static vestibulo-ocular reflex (VOR) were investigated in Xenopus laevis tadpoles exposed to hypergravity (1.4g; 3g) or microgravity conditions (German spacelab mission D-2) for 9–10 days. The effects of light conditions during development were also tested by exposing tadpoles to either complete darkness (DD) or 12∶12 h light-dark conditions (LD). The static VOR was induced by lateral roll. The efficacy of the VOR circuit after termination of AGF conditions was described by the peak-to-peak amplitude of the sinusoidal VOR characteristics (i.e. amplitude). The static VOR was first observed at stage 41 for both LD and DD tadpoles. Its further development was retarded in the DD tadpoles compared with the LD tadpoles up to stage 48. Microgravity as well as hypergravity exposure caused a significant (P<0.05, at least) decrease in 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 1g ground-reared siblings at the same developmental stages. The response deficits usually disappeared but persisted for at least 2 weeks in animals whose development was retarded by hypergravity exposure. It is postulated (i) that gravity exerts an important influence on the normal development of the roll-induced static VOR; (ii) that hypergravity exposure decreases the sensitivity of the gravity-sensitive system so that recordings under 1g conditions cause a weaker static VOR; and (iii) that the vestibulo-spinal pathway possesses a higher degree of plasticity than the vestibulo-ocular pathway.

Altered gravitational forces affect the development of the static vestibuloocular reflex in fish (Oreochromis mossambicus)

Young fish (Oreochromis mossambicus) were exposed to microgravity (micro g) for 9 to 10 days during space missions STS-55 and STS-84, or to hypergravity (hg) for 9 days. Young animals (stages 11-12), which had not yet developed the roll-induced static vestibuloocular reflex (rVOR) at micro g- and hg-onset, and older ones (stages 14-16), which had already developed the rVOR, were used. For several weeks afterwards, the rVOR was recorded after termination of mug and hg. Here are the main results: (1) In the stage 11-12 fish, the rVOR gain (response angle/roll angle) measured for roll angles 15 degrees, 30 degrees, and 45 degrees was not affected by microgravity if animals were rolled from the horizontal to the inclined posture, but was increased significantly if animals were rolled in the opposite manner. The rVOR amplitude (maximal eye movement during a complete 360 degrees roll) of micro g animals increased significantly by 25% compared to 1g controls during the first postflight week, but decreased to the control level during the second postflight week. Microgravity had no effect in stage 14-16 fish on either rVOR gain or amplitude. (2) After 3g exposure, both rVOR gain and amplitude were significantly reduced for both stage 11-12 and stage 15 fish. One g readaptation was completed during the second post-3g week. Hypergravity at 2 or 2.5 g had no effect. (3) Hypergravity at all three levels tested (2g, 2.5g, and 3g) accelerated the morphological development as assessed by external morphological markers. Exposure to micro g- or 3g-periods during an early developmental period modifies the physiological properties of the neuronal network underlying the static rVOR; in susceptible developmental stages, these modifications include sensitization by microgravity and desensitization by hypergravity.

The minimum duration of microgravity experience during space flight which affects the development of the roll induced vestibulo-ocular reflex in an amphibian (Xenopus laevis)

In tadpoles of Xenopus laevis, the effects of microgravity on the development of the roll-induced vestibuloocular reflex (rVOR) was investigated. Special attention was focused on sensitive periods and the minimum duration of microgravity exposure by which the development of the rVOR is affected. The peak-to-peak excursion (rVOR amplitude) of the rVOR characteristic for a lateral 360 degrees roll was used to describe microgravity effects. Fertilization of all eggs was performed 40 h before launch. Tadpoles were exposed to microgravity either during the first (MC-group) or second half of the mission (CM-group), or throughout the 9-day mission (MM-group). Inflight, 1G-gravity was simulated by a centrifuge (CC-group). After termination of the mission, the rVOR amplitude was only reduced in the MM-group with respect to the 1 G-inflight and 1 G-ground control by approximately 20-30% while both the MC- and CM-groups were not affected by the 4-day and 5-day microG exposure, respectively. However, CM-tadpoles like MM-tadpoles showed malformation of their body characterized by a dorsal bended tail. It disappeared in both groups within 2 weeks after landing. The difference between the rVOR amplitudes of the experimental groups disappeared within 5 weeks after landing. The results demonstrate that microgravity retards the development of the rVOR if it lasted longer than 4 days but that tadpoles are susceptible even for shorter periods as shown by the malformation of the body.

Light-dependent suppression of the vestibulo-ocular reflex during development

In the fish Oreochromis mossambicus, light conditions affect the development of the roll-induced vestibuloocular reflex (rVOR). During development under continous light-dark conditions the rVOR amplitude, which is the maximum eye movement during a complete 360 degrees lateral roll, shows a secondary drop after a first peak at stage 17 by 64% (36.3 degrees at stage 17; 13.0 degrees at stage 20). This drop was shifted by 2 stages to older postembryonal stages and was 33% (29.2 degrees at stage 20; 19.5 degrees at stage 22) less pronounced in animals which were exposed to complete darkness for several days. Because the period of rVOR diminution is sensitive to light conditions, it is likely that outgrowing visual projection fibres reorganize the neuronal network underlying visual-vestibular behavior thus transiently suppressing the rVOR.

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.

A hypergravity related sensitive period during the development of the roll induced vestibuloocular reflex in an amphibian (Xenopus laevis)

In tadpoles of Xenopus laevis, the effects of an exposure to hypergravity 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 3 g period during early life. Recordings of rVOR characteristics for complete 360 degrees rolls of the tadpoles started 24 h after the end of the 3 g period. The rVOR peak-to-peak amplitudes from the 3 g samples recorded at the 2nd and 3rd day after termination of the 3 g exposure agreed with that recorded from the 1 g reared tadpoles for the youngest group, but were reduced by 30% in the older tadpoles. During further development under 1 g condition, the rVOR amplitude of tadpoles with 3 g experience did not change if the 3 g exposure started before the first appearance of the rVOR, but increased if it had started thereafter, albeit on a lower level than that of the 1 g reared siblings. The results support the existence of a sensitive period for the rVOR development, and additionally demonstrate that the period during which the rVOR appeared for the first time is an important milestone for the development of adaptive properties of the underlying neuronal network.

Readaptation of the Vestibuloocular Reflex to 1g Condition in Immature lower Vertebrates (Xenopus laevis) after Micro-or Hypergravity Exposure

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