Tomio Naitoh

Motion sickness in amphibians.

We explored the question of whether amphibians get motion sickness by exposing anurans (frogs) and urodeles (salamanders) to the provocative stimulus of parabolic aircraft flight. Animals were fed before flight, and the presence of vomitus in their containers after flight was used to indicate motion-induced emesis. None of the species that we studied vomited during the 8 to 10 parabolas of each flight. However, at least one specimen from each of the anuran species Rana rugosa, Rana nigromaculata, Hyla japonica, and Rhacophorus schlegelii vomited in a period of 0.5 to 42 h after flight. Some specimens of R. nigromaculata, H. japonica, and R. schlegelii were also observed retching without emesis either during or shortly after exposure to parabolic flight. We were unable to induce either emesis or retching behavior in the aquatic from Xenopus laevis. Among the urodeles studied we saw no signs of motion sickness in either adult or larval Cynops pyrrhogaster, but at least one larval Hynobius nebulosus vomited shortly after parabolic flight. The amphibian species that exhibited the most motion sickness were the same ones that showed the greatest amount of tumbling during the microgravity phases of their parabolic flights. The most distinctive difference between motion sickness in amphibians and mammals that vomit, including man, is the long delay between a provocative stimulus and emesis proper in the amphibians. The retching behavior we induced in the frogs was identical to that described previously for frogs treated with emetic drugs. H. japonica, exposed to extended periods of microgravity on the MIR Space Station, flattened their bellies against the substrate and dorsiflexed their heads in a manner reminiscent of drug-induced nausea. In light of our current observations of retching behavior in motion sick H. japonica, we suggest that the previously observed behavior of three frogs on the MIR Space Station was a manifestation of motion sickness.

The Physiology, Morphology, and Ontogeny of Emetic Behavior in Anuran Amphibians

We examined the responses of Rana catesbeiana, Rana clamitans, and Xenopus laevis to a variety of emetic drug treatments. No treatment caused regurgitation or any emetic behavior (i. e., retching) in any premetamorphic tadpoles. The ability to vomit first appeared during metamorphosis, precisely when the transforming froglets were first able to swallow a large bolus of food. Postmetamorphic frogs exhibited intergeneric differences in their sensitivity to different emetics. The emetic behavior of Rana did not change when it was transferred from a terrestrial to an aquatic environment. Mechanical stimulus, that is, distention of the foregut, alone can induce regurgitation. Movements of the torso, along with contraction of the abdominal wall, appear to be essential for raising abdominal pressure during emesis in anurans. Rana and Xenopus differ in the body movements they exhibit during emesis: Rana dorsiflexes its presacral vertebral column on its sacroiliac joint; Xenopus instead telescopes its torso by sliding the ilia rostrocaudally on the sacral vertebra. These same sacroiliac movements have been previously related to differences in the locomotion of these two frogs. Our data suggest that the sacroiliac musculoskeletal complex of frogs is as important to the elevation of abdominal pressure during actions such as emesis, oviposition, micturition, etc., as it is to adult locomotion. Anurans do not develop their elongated ilia and their sacroiliac joints until metamorphosis. The inability to deform the torso may be one mechanistic reason why tadpoles neither vomit nor display any of the other behaviors requiring abrupt elevation of abdominal pressure.

Startle Response and Turning Bias in Microhyla Tadpoles

Abstract Rana tadpoles are known to have a left-handed turning bias whereas Xenopus larvae lack such a preference in turning direction. Since Rana tadpoles have a single, external, left-handed spiracle, we previously suggested that a turning bias in tadpoles may be obligatorily linked to that external asymmetry. We have tested this idea by examining turning bias in the startle response of Microhyla ornata tadpoles. Microhyla tadpoles are, like Xenopus, externally symmetrical, but phylogenetically they are more closely related to Rana. Individual Microhyla tadpoles were startled by a solenoid-driven plunger that sent a shock wave up through the bottom of a container holding each tadpole. High speed videography (250 frames per second) was used to witness the tadpoles response. Microhyla tadpoles show no turning bias during the first few days post-hatching, when they are very small. However, they develop a left-handed turning bias while still in the earliest free-swimming stage (Gosner stage 25) and that bias persists through stage 41. At stage 42, after forelimbs emerge, the laterality in startle responses fades away. Since Microhyla larvae are externally symmetrical, yet preferentially turn to the left, we can reject the hypothesis that a turning bias in tadpoles is obligated by external morphological asymmetry. An alternative working hypothesis, given the limited taxa that have been examined to date, is that handedness in tadpoles is phylogenetically conserved and independent of spiracle position in tadpoles.

Behavior of Japanese tree frogs under microgravity on MIR and in parabolic flight.

Japanese tree frogs (Hyla japonica) were flown to the space station MIR and spent eight days in orbit during December, 1990. Under microgravity, their postures and behaviors were observed and recorded. On the MIR, floating frogs stretched four legs out, bent their bodies backward and expanded their abdomens. Frogs on a surface often bent their neck backward and walked backwards. This behavior was observed on parabolic flights and resembles the retching behavior of sick frogs on land--a possible indicator of motion sickness. Observations on MIR were carried out twice to investigate the frogs adaptation to space. The frequency of failure in landing after a jump decreased in the second observation period. After the frogs returned to earth, readaptation processes were observed. The frogs behaved normally as early as 2.5 hours after landing.

Emesis and space motion sickness in amphibians.

Amphibians possess the ability to vomit in response to a variety of stimuli that provoke emesis in mammals. Pharmacological studies have establish that the ejection of gastric contents and the basic mechanism for vomiting have been phylogenetically conserved among these tetrapods. As part of on-going comparative studies on emesis in vertebrates, we previously documented that some postmetamorphic anurans and salamander larvae experience motion-induced emesis when exposed to the provocative stimulus of parabolic aircraft flight. However, more recent experiments suggest that there are strict conditions for inducing emesis in amphibians exposed to parabolic flight and that amphibians are not as sensitive to this stimulus as mammals. Further studies on emesis in lower vertebrates may help us understand the processes that cause emesis in abnormal gravitational regimes.

Allometry in vestibular responses of anurans.

Frogs and toads turn either their heads or bodies opposite to angular accelerations applied around the yaw axis. Thresholds exist for the minimum angular acceleration that induces this vestibulomotor response in individual frogs. These thresholds were recorded for several anuran species that cover a broad range of sizes and life styles. Interspecific variation in the magnitude of the thresholds, which correlated with the ecology and behavior of the species, was documented. Also an allometric relationship was observed between this threshold and body size; the larger the frog, the lower the threshold. In many species, the threshold value for reflexive vestibulomotor responses to angular acceleration was proportional to the -0.4 (+/-0.2) power of body mass. Physical dimensions of the semicircular canals determine, in part, vestibular sensitivity to angular acceleration. Hence changes with growth in the semicircular canals are believed to contribute to the slope of -0.4. The biological significance of this allometry in vestibular responses is discussed and compared to trends in vestibular sensitivity and semicircular canal morphology of other vertebrate classes.

Movements of the large intestine in the anuran larvae, Xenopus laevis

1. The contractile behavior of the large intestine of Xenopus laevis tadpoles was studied. 2. The large intestine is divided into a colon and rectum, and shows three types of movements: rhythmic ascending (antiperistaltic) waves of contraction originating at the anal end of the large bowel, rhythmic longitudinal contractions in the rectum and colon, and irregular contractions. The first two patterns occur in the large bowel in situ and thus appear mature. The last one occurred only in older preparations, and thus appeared pathological. 3. Antiperistaltic waves of contractions and longitudinal contractions are generated independent of each other, suggesting that circular muscles and longitudinal muscles contract separately. 4. Acetylcholine, adrenaline and noradrenaline augment motility. 5. The premetamorphic motility of the large bowel is similar to that seen in adult frogs. Comparable motility was not observed elsewhere in the larval alimentary tract. The large intestine appears to be the first portion of the anuran alimentary tract to acquire the adult physiological and morphological profile.

Inhibitor and Temperature Effect on Catalase in the Liver of Adult Diploid and Haploid Rana rugosa

The authors succeeded in raising a single mature haploid Rana rugosa female to the age of 2 years from an egg artificially fertilized with ultraviolet-irradiated sperm. In order to discover why this particular haploid individual should survive so long, hydrogen peroxide detoxifying catalase in the liver of this individual and age-matched diploids was examined and compared for total activity, temperature stability, and chemical inhibition. Total activity was found to be significantly higher in the haploid frog than in the diploids, suggesting that this particular haploid had a unique system for hydrogen peroxide detoxification which protected the liver against cell death, preventing hepatic failure, and leading to a prolonged survival. Liver catalase from the haploid proved to be more labile to aminotriazole and urea, losing 60-70% of its original activity after 30 min treatment, whereas diploid catalase lost only 40% under the same conditions. Haploid and diploid catalase responded similarly to heat, however. It seems likely that inhibitor-binding sites differ considerably between the catalase of normal diploids and the catalase of this particular haploid, while overall structure is generally similar.

Emesis in larval salamanders, Hynobius nebulosus (Hynobiidae)

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Studying the visceral physiology of tadpoles through their naturally transparent abdominal walls.

We propose using anuran tadpoles with naturally transparent abdominal skin to study the visceral physiology of amphibian larvae under microgravity. The transparency of the abdominal wall in certain tadpoles enables one to evaluate the basal physiological state and temporal changes in viscera from their movements without any invasive treatment. In order to validate our experimental design, the intestinal motility and heart rate of Rhacophorus tadpoles were examined as indices of physiological responses to stepwise changes in temperature.