VESTIBULAR INDUCED
BEHAVIOUR
OF RATS BORN AND RAISED UNDER HYPERGRAVITY CONDITIONS
R. J. Wubbels and
H. A. A. de Jong
Vestibular Department ENT, Academic Medical Centre, PO Box 22660,
1100 DD Amsterdam, The Netherlands, e-mail: r.j.wubbels@amc.uva.nl
Introduction
Rats were bred in a centrifuge, simulating a hypergravity (HG) environment
of 2.5 times normal gravity (NG), to investigate how the vestibular system is
affected by long-term HG conditions. There is no acute distress among centrifuge-bred
rats when the centrifuge is stopped, because all animals immediately become
very active.
The airrighting reflex (turning from a supine to prone position during fall) and the reappearance at the surface after falling into a water basin were observed (IR illumination). Both types of behaviour depend on the capability to orientate relative to the direction of gravity. The question is: Is this behaviour affected by the ontogenetic development of the vestibular system under HG conditions?
Airrichtning Reflex
The figure above shows the percentage of correct airrighting reflexes (left)
and their mean duration (right), for rats bred under NG or HG conditions. Individual
animals (14 HG and 15 NG rats) were dropped 3 times each experimental session.
For both groups, the score is more or less the same and doesn’t change very
much with age. These results are in sharp contrast with those obtained for hamsters,
for which it was shown that airrighting of HG-bred animals is less successful
(Sondag et al., Brain Res. Bull. 43, 289-294, 1997). With respect to
the duration of the airrighting reflex, the results do not show remarkable differences
between HG and NG rats, although young NG animals appear to be somewhat faster.
Finding the Water
Surface
The percentage of successful reappearances, within 3 seconds, at
the water surface (left) is always 100% for the NG group. For young HG animals
finding the surface appears more difficult (just 36% at age 6 weeks). Video
recordings show animals floating at the surface, but with their heads submerged.
Surfacings of young NG rats occur considerably faster than those of HG animals
of the same age (right).
Correlations
In
our experimental design three consecutive airrighting reflexes and surfacings
are measured. which may lead to correlations within the data set. For instance,
a successful airrighting reflex is usually followed by a faster surfacing.
The histogram above shows the distribution of duration differences between the first and second surfacing (mean± SD: 172± 449 ms), and between the first and third surfacing (313± 453 ms) for HG rats. Normal distributions around the respective means (dotted and dashed curve) and around 0 (solid curve) are also drawn. The latter distribution is expected when the duration of surfacings is independent of preceding trials. Thus, the second surfacing takes significantly longer than the first (t-test, p<<0.005), and the third surfacing again takes longer than the second (p<<0.005). For NG animals, the duration difference between second and first surfacing (36± 341 ms) does not significantly deviate from 0. Also, the duration difference between third and first surfacing (72± 308 ms) is small (but deviating from 0, p@ 0.01).
Conclusions
* In contrast with findings for hamsters, the number of correct airrighting reflexes of HG rats is not less than for the NG group.
* Young NG rats perform the airrighting reflex faster than HG of the same age. Older animals of both groups need about the same amount of time to perform an airrighting reflex.
* Young HG rats have some trouble with surfacing in the first few weeks after weaning.
* NG rats and older HG rats have no problem in finding the water surface.
* The duration of a surfacing of a NG rat does not depend (or hardly depends) on a preceding trial while a surfacing of a HG rat tends to take longer than the preceding surfacing.
* The results of our study show that vestibular induced behaviour of HG rats is not permanently deteriorated by their ontogenetic development.
This study is financially supported by the Space Research Organization of the Netherlands (SRON; project mg-044).