| Budget Amount *help |
¥3,600,000 (Direct Cost: ¥3,600,000)
Fiscal Year 2004: ¥1,000,000 (Direct Cost: ¥1,000,000)
Fiscal Year 2003: ¥2,600,000 (Direct Cost: ¥2,600,000)
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| Research Abstract |
Most astronauts experience spatial disorientation after exposure to weightlessness, indicating that constant gravity is utilized as a stable external reference during spatial cognition. We attempted to elucidate the role of constant gravity in spatial learning using a radial arm maze test on rats housed in a hypergravity environment (HG) produced by a centrifuge device. Male Wistar rats were kept in 2G linear acceleration for two weeks before the spatial learning task, which lasted for ten days. The control rats were placed close to the centrifuge device but not exposed to hypergravity. Spatial learning was evaluated by the accuracy and the re-entry rate, which were the rate of correct arm entries and the rate of entries into the arms that they had already visited, respectively. Locomotor activity was measured by number of entries per minute. The number of baits the animal took per minute was also measured. The results showed that accuracy was significantly inferior and the re-entry ra
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te was significantly higher in the HG rats than in the controls, suggesting that animals use a constant gravity as a stable external reference in spatial learning. However, these differences disappeared at five days later, indicating that the HG rats learned the spatial task more rapidly than the controls. Locomotor activity was higher in the HG rats and there was no difference in number of baits per minute between the HG and control animals. In conclusion, if one sensory cue necessary for spatial cognition is disturbed by gravity change, animals can subsidize with other sensory cues such as proprioceptive and motor efference copy signals through increased locomotor activities.
Effects of hypergravity load on hypothalamus-pituitary axis (HPA axis) were investigated in rats with chronically implanted canula in the cervical vein. Plasma ACTH and corticosterone levels were measured pre-, just after, 1 hour after, and 3 hours after the hypergravity load for one hour. Both hormones were increased just after and one hour after the load, however, their levels returned to the basal level 3 hours after the load. Decrease (recovery) of plasma ACTH after the load was more rapid in ACTH than corticosterone, probably because changes in corticosterone may be the downstream of ACTH changes. During the long term load (2 weeks) of hypergravity, corticosterone but not ACTH was increased at least for one week, however, both hormones returned to the control level 2 weeks after the load, suggesting that ACTH response has a short time constant and adapted to the hyoergravity load quickly. It is demonstrated that HPA axis was activated by hypergravity load and was involved in a better learning and recovery of impaired spatial navigation induced by the gravity changes. Less
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