Fos-related antigens are involved in the transcriptional responses of locus coeruleus neurons to altered gravitational fields in rats

Locus coerulcus (LC) neurons, which have widespread projections to the whol e brain, respond to natural stimulation of macular receptors. Using immunoc ytochemistry we investigated whether rats exposed to altered gravitational fields showed changes in Fos and Fos-related antigen (FRA) protein levels i n the LC. Fos protein is induced very rapidly and returns to basal levels w ithin hours after stimulation, while FRAs persist for days or weeks after i nduction. Adult male albino rats (Fisher 344) were sacrificed at different time points during a space flight (NASA Neurolab Mission, STS-90) and the n umbers of Fos- and FRA-positive cells in the LC were counted and compared t o those in ground-based control rats. No significant changes in Fos protein expression were detected in the LC under different space flight conditions . In contrast, the number of FRA-positive cells increased on average to 167 % of that of the controls at FD2, i.e. when gravity increased from 1 to 3g during the launch before reaching about Og. FRA-labeled neurons then decrea sed to 46% of control values at FD14, i.e. after adaptation to Og, but incr eased again to 317% of control values at R + 1, when the animals were expos ed to an increase in gravitational force from 0 to 1.5-1.8g before reaching Ig during landing. The number of labeled cells was 193% of the control val ues at R + 13, i.e. after readaptation to 1g. Thus gravitational force appe ars to be very effective in inducing a long-term increase in FRA protein ex pression in the LC. Because activity in the noradrenergic LC neurons may in crease Fos expression in several target structures, we postulate that the l ong-lasting induction of FRAs in the LC at FD2, and more prominently at R 1, may contribute to the long-term molecular changes which probably occur in the brain during adaptation to Og and readaptation to 1g.