Fos expression in the rat brain after exposure to gravito-inertial force changes

The immediate-early genes constitute useful neurobiological tools for mapping brain functional activity after sensory stimulation. We immunohistochemically investigated Fos protein expression in the brain of rats exposed to gravito-inertial force changes. Experiments were performed in hypergravity rats born and housed for 60 days in terrestrian gravity (1xg) and thereafter exposed for 90 min to 2xg or 4xg in a centrifuge, and in hypogravity rats born and housed for 60 days at 2xg and submitted for 90 min to 1xg. Data from these two experimental groups were quantified by light microscopy and compared to those from two groups of control rats born and permanently housed in either 1xg or 2xg environments that never had to adapt to novel gravito-inertial environments. Results showed a low basal Fos expression in the controls and a strong Fos staining in the experimental rats. Only the hypergravity rats displayed Fos-positive cells in vestibular-related brainstem regions (medial, inferior, and superior vestibular nuclei (VN); group y; dorsomedial cell column (DMCC) of the inferior olive (IO)). By contrast, many suprabulbar areas were strongly labeled in both the hyper- and hypogravity rats, as shown by the numerous Fos-positive cells in mesencephalic (colliculus, laterodorsal periaqueductal gray, autonomic nuclei), diencephalic (hypothalamic and thalamic nuclei), and telencephalic (parietal, temporal, entorhinal and visual cortices) structures. These spatial patterns of Fos expression suggest that an increase in gravito-inertial force activates otolith-vestibulo-olivar pathways and various suprabulbar structures underlying the corticovestibular interactions, which govern the multiple representations of vestibular information in the cortex. A decrease in gravito-inertial force has the opposite effects on the vestibulo-olivar structures as a result of otolith system disfacilitation which, in turn, modifies the activity of complex neural pathways. Exposure to both hyper- and hypogravity environments likely induces neurovegetative and/or stress effects that could account for Fos labeling in autonomic nuclei and in nervous structures involved in the hypothalamo-pituitary-adrenal axis.