The Sensitivity of an Immature Vestibular System to Altered Gravity

Abstract

Stimulus deprivation or stimulus augmentation can induce long-lasting modifications to sensory and motor systems. If deprivation is effective only during a limited period of life this phase is called "critical period." A critical period was described for the development of the roll-induced vestibuloocular reflex (rVOR) of Xenopus laevis using spaceflights. Spaceflight durations and basic conditions of Xenopus' development did not make it possible to answer the question whether exposure of the immature vestibular organ to weightlessness affects rVOR development. The embryonic development of Pleurodeles waltl is slow enough to solve this problem because the rVOR cannot be induced before 15 dpf. Stage 20-21 embryos (4 dpf) were exposed to microgravity during a 10-day spaceflight, or to 3g hypergravity following the same time schedule. After termination of altered gravity, the rVOR was recorded twice in most animals. The main observations were as follows: (1) after the first rVOR appearance at stage 37 (16 dpf), both rVOR gain and amplitude increased steadily up to saturation levels of 0.22 and 20°, respectively. (2) Three days after termination of microgravity, flight and ground larvae showed no rVOR; 1 day later, the rVOR could be induced only in ground larvae. Differences disappeared after 3 weeks. (3) For 10 days after 3g exposure, rVOR development was similar to that of 1g-controls but 3 weeks later, 3g-larvae showed a larger rVOR than 1g-controls. These observations indicate that the immature vestibular system is transiently sensitive to microgravity exposure and that exposure of the immature vestibular system to hypergravity leads to a slowly growing vestibular sensitization.

Development of Vestibular Systems in Altered Gravity

“…In both species, rVOR development is retarded during early periods of life if the animals are reared for several days or permanently in complete darkness (Sebastian et al, 1996;2001). In Pleurodeles, the rVOR cannot be elicited until 15 days after fertilization (stage 37); thereafter, its amplitude and gain increase steadily and reach a saturation level of about α = 20° at stage 48 (Gabriel et al, 2012) (Fig. 16.1).…”

“…Because during the post-3g period animals were kept in regular light-dark cycles, the accelerated 1g readaptation in the low stimulation range might be caused by the normally higher light response in fish (dorsal light response) compared to Xenopus . 4) The salamander Pleurodeles differed from these normalization dynamics after hypergravity exposure because the larvae developed a delayed vestibular sensitization by 3g as shown by the augmented vestibulo-ocular reflex four weeks after termination of 3g exposure (Gabriel et al, 2012).…”

“…One week later, all larvae were responding, and the differences for the rVOR amplitude and rVOR gain were insignificant compared to 1g exposed larvae. After four weeks, 3g larvae had a significantly larger rVOR amplitude and gain than their 1g controls despite identical developmental progress (Gabriel et al, 2012).…”

“…In contrast, the mean body length L of F0g larvae was significantly reduced with respect to the G1g control until post-flight day 4. The rVOR amplitude is positively correlated with the body length; so, the absence of an rVOR on post-flight day 4 could be caused by retarded growth of the F0G salamander larvae (Gabriel et al, 2012).…”

The vestibuloocular reflex of tadpoles (Xenopus laevis) after knock-down of the isthmus related transcription factor XTcf-4

“…This curve was used to calculate the rVOR amplitude, which is the maximal peak-to-peak movement of the eyes during a 360deg lateral roll. Previous studies in Xenopus laevis, young fish (Oreochromis mossambicus) and salamander larvae (Pleurodeles waltl) have shown that the rVOR amplitude is a sensitive indicator to describe (1) modifications of the rVOR during development (Horn et al, 1986a;Sebastian and Horn, 1999;Gabriel et al, 2012), (2) the susceptibility to changes within the vestibuloocular system induced by vestibular lesions (Horn et al, 1986b), (3) the extent of vestibular plasticity after such lesions (Rayer and Horn, 1986) and (4) the impact of altered gravitational conditions on vestibular function (Horn 2006;Sebastian et al, 2001;Horn and Gabriel, 2011;Gabriel et al, 2012). The rVOR amplitude was determined by the angular difference between the mean of eye angles recorded for roll angles between 60deg to 120deg and 240deg to 300deg, respectively (cf.…”