[Altered behaviour and expression of Fos in rats born in hypergravity and their re-adaptation to the normal gravity]

Changes in behaviour relevant to the vestibular system were studied in Long-Evans rats which were fertilized, born and housed in 2 acceleration of gravity for 4 months and thereafter exposed to 1 acceleration of gravity, and expression of Fos protein in the brain stem was examined. Data from the hypergravity rats were compared respectively with those from the rotation group and the labyrinthectomized group. Static and locomotion modes of the hypergravity rats were changed, tension of extensor was enhanced and the abilities in locomotion equalization and orientation in swimming and air-righting response were reduced. The adaptation process varied with different behaviours. The time for recovery of the ability of orientating in swimming was the longest, taking more than 1 month. The Fos protein expression provides a useful tool for mapping brain functional activities after sensory stimulation, showing a low basal level in normal and labyrinthectomized groups. The hypergravity rats, on the other hand, exhibited more Fos-positive cells in the superior colliculus, inferior colliculus, periaqueductal gray, raphe dorsal nucleus and solitary nucleus. In contrast, the inferior olivary nuclei, locus coeruleus and vestibular nuclei were not strongly labeled. These spatial patterns of Fos expression suggest that a decrease in gravity-inertial force may activate a neural pathway different from the vestibulo-olivar pathways activated by an increase in gravity-inertial force.