Motion sickness in lower vertebrates: Studies in weightlessness and under normal conditions

This review presents both literature data and results of our own studies aimed at finding out if lower vertebrates are susceptible to motion sickness. In our experiments, fish and amphibians were submitted to motion for 2 h and longer on a centrifuge (f = 0.25 Hz, acentrifugal = 0.144 g) and on a parallel swing (f = 0.2 Hz, ahorizontal = 0.059 g). The performed studies did not revealed in 4 fish species and in toads any single feature characteristic of motion sickness (sopite syndrome, pre-vomiting behavior or emesis). At the same time, in toads, the characteristic stress reaction appeared (escape reaction, increase of urination frequency, worsening of appetite) as well as some other responses not associated with motion sickness (synchronized head swinging, eye retraction). In trout fry, the used stimulation promoted division of individuals into groups differing by locomotor reaction to stress as well as individuals with a well-pronounced compensatory response that we called the otolithotropic reaction. Our conclusions are confirmed by analysis of results obtained by other authors. Therefore, the lower vertebrates, unlike mammals, are not susceptible to motion sickness either under terrestrial conditions or under conditions of weightlessness. Based on available experimental data and theoretical concepts of mechanism of development of the motion sickness, which have been formulated in several hypotheses (the hypothesis of discoordination, Treisman’s hypothesis, resonance hypotheses), there is put forward the synthetic hypothesis of sopite that is of conceptual values. According to this hypothesis, the unusual stimulation producing sensory-motor or sensory-sensory discoordinations or action of vestibular and visual stimuli of frequency of about 0.2 Hz is perceived by the CNS as poisoning and causes the corresponding reactions. The sopiting is, in fact, a side effect of technological evolution. It is suggested that in the lower vertebrates, unlike mammals, there is absent a hypothetic vomiting center, therefore, they are not submitted to motion sickness.