Electrophysiological and behavioural observations have shown that changes in the sleep-waking activity occur in astronauts during the space flight. Experiments performed in ground-based experiments have previously shown that the immediate early gene (IEG) c-fos, a marker of neuronal activation, can be used as a molecular correlate of sleep and waking. However, while Fos expression peaks within 2-4 hours after the stimulus and returns to baseline within 6-8 hours, other IEGs as the FRA proteins which are also synthetized soon after their induction, persist in the cell nuclei for longer periods of time, ranging from 1-2 days to weeks. 2. Both Fos and FRA expression were evaluated in several adult albino rats sacrificed at different time points of the space flight, i.e. either at FD2 and FD14, i.e. at launch and about two weeks after launch, respectively, or at R + 1 and R + 13, i.e. at the reentry and about two weeks after landing. The changes in Fos and FRA expression were then compared with those obtained in ground controls. These experiments demonstrate activation of several brain areas which varies during the different phases of the space flight. Due to their different time of persistence, Fos and FRA immunohistochemistry can provide only correlative observations. In particular, FRA expression has been quite helpful to identify the occurrence of short-lasting events such as those related either to stress or to REM-sleep, whose episodes last in the rat only a few min and could hardly be detected by using only Fos expression. 3. Evidence was presented indicating that at FD2 and FD14 Fos-labeled cells were observed in several brain areas in which Fos had been previously identified as being induced by spontaneous or forced waking in ground-based experiments. In contrast to these findings FLT rats sacrificed at R + 1 showed low levels of Fos immunostaining in the cerebral cortex (neocortex) and several forebrain structures such as the hypothalamus and thalamus. Some Fos staining was also present in limbic cortical areas, the septum, and the hippocampus. The main area of the forebrain of FLT rats sacrificed at R + 1, showing an increased expression of Fos, was the central nucleus of the amygdala (CeA) (cf. 127), as well as the noradrenergic locus coeruleus (LC) nucleus (cf. 122). At R + 13 Fos immunostaining was variable among FLT rats. However, none of these rats showed a significant number of Fos-positive cells in CeA. 4. Most of the rats studied for Fos expression were also tested for FRA expression. In particular, a scattered amount of FRA expression occurred at FD14 in different areas of the neocortex and in limbic forebrain regions (such as the cingulate, retrosplenial and entorhinal cortex). It included also the hippocampus, the lateral septum, the caudate/putamen, as well as some hypothalamic regions. At the reentry (R + 1) it was previously shown that a prominent increase in FRA expression occurred in the LC of FLT rats (cf. 122). This finding was associated with an increase in FRA expression which affected not only the nucleus paragigantocellularis lateralis of the medulla, which sends excitatory glutamatergic afferents to the LC (cf. 31 for ref.), but also structures which are known to produce corticotropin-releasing factor (CRF), a neuropeptide which activates the noradrenergic LC neurons during stress. 5. These findings which result from acceleration stress were followed by REMS episodes, which probably occurred after a long period of sleep deprivation following exposure to microgravity. It was previously shown that an increase in Fos and FRA expression occurred at the reentry in some pontine and medullary reticular structures (cf. 128), which are likely to be involved in both the descending (postural atonia) and the ascending manifestations of PS. These findings can be integrated by results of the present experiments showing that at the reentry high levels of FRA expression occurred in the hippocampus and the limbic system, i.e. in structures which are involved in the generalized pattern of EEG desynchronization and the theta activity, typical of REMS (cf. 83, 84). A prominent increase in FRA expression also affected at the reentry some components of the amygdaloid complex, particularly the CeA. as well as some related structures, such as the lateral parabrachial nucleus (cf. 122) and the nucleus of the tractus solitarius (cf. 127). These structures are known to contribute to the PGO waves, which drive the oculomotor system either directly or through the medial vestibular nuclei (128, cf. also 126). Unfortunately due to our brainstem transections we were unable to evaluate the changes in gene expression which could affect the dorsolateral pontine structures during the occurrence of REMS episodes. Further experiments are thus required to investigate the role that these pontine structures exert in determining adaptive changes following exposure to microgravity after launch as well as readaptation to the terrestrial environment after landing.
