Immediate early gene expression in the vestibular nuclei and related vegetative areas in rats during space flight

Changes in neuronal activity resulting in somatic and vegetative deficits o ccur during different space flight conditions. Immediate early genes (IEGs: c-fos and Fos-related antigen [FRA]) are useful indicators of changes in n euronal activity and plasticity. They are induced within minutes of several extracellular stimulations, while the corresponding proteins persist for h ours (Fos) or days (FRAs). Changes in IEG expression are likely to contribu te to adaptation to microgravity and readaptation to the terrestrial enviro nment. During the NASA Neurolab Mission (STS-90), changes in IEG expression were studied in adult male albino rats (Fisher 344) sacrificed at flight d ay (FD) 2 (24 h after launch), FD14 and at similar time points after re-ent ry (R + 1, 24 h after re-entry, and R + 13). These time points were chosen to maximize the probability of detecting changes in IEG expression related to changes in gravitational fields occurring during the mission, e.g. (i) i ncrease in gravitational force from I to 3g during the launch, before reach ing about Og at FD2; (ii) adaptation to 0g at FD14; (iii) increase in gravi ty from 0 to approximate to 1.5-1.8g before reaching Ig at R + 1; and (iv) readaptation to Ig at R + 13. Fos- and FRA-positive cells were identified i n the brainstem of flight rats and ground-based controls using immunocytoch emistry. With respect to control rats, the number of labeled cells increase d in flight animals in the medial and spinal vestibular nuclei (but not in the lateral vestibular nucleus) at FD2, decreased at FD14, greatly increase d at R + 1 and returned to baseline levels at R + 13. Similar changes in IE G expression were also observed in the nucleus of the solitary tract, the a rea postrema and the central nucleus of the amygdala. In particular, in the se vegetative areas the number of Fos-positive cells decreased in flight ra ts with respect to controls at FD14, i.e. after exposure to Og, but signifi cantly increased at R + 1, i.e. after return to 1g. Thus, altered gravitati onal fields produced molecular changes in vestibular nuclei controlling som atic functions, as well as in related medullary and basal forebrain structu res regulating vegetative functions.