Behaviour of adult hamsters subjected to hypergravity.

We studied vestibular function in 20 adult hamsters (3 months old) subjected to either prolonged hypergravity (n = 10) or normal gravity (n = 10) for 2 months. Locomotion and swimming of the hypergravity hamsters under light conditions were normal. Equilibrium maintenance was severely disturbed; only 6 of 10 hypergravity hamsters managed to walk on the small tube after 2 months, whereas all 10 controls were able to walk on the tube. The air-righting reflex was severely disturbed; the hypergravity hamsters made 30% correct air-righting responses, while the control hamsters made 88% correct responses. Finally, 5 of 8 hypergravity hamsters had to be saved from drowning when swimming in total darkness. Histological examination of the utricular otoconial layers afterwards, using energy dispersive X-ray element (EDAX) analysis and scanning electron microscopy, did not reveal any differences in calcium content, shape and size distribution of the otoconia between hypergravity hamsters and controls. We suggest that adult hamsters adapt to hypergravity, leading to problems in normal functioning when tested in 1 G, especially in tasks in which sensory input of the vestibular system is important for spatial orientation. These disturbances were more severe in adult hamsters than in young ones, tested in previous experiments. Therefore, we assume that age is a factor for adaptation to altered gravity conditions.