DAILY VARIATION OF CNS GENE-EXPRESSION IN NOCTURNAL VS DIURNAL RODENTS AND IN THE DEVELOPING RAT-BRAIN

Expression of c-Sos has been shown to vary throughout the brain over t he course of the 24-h day [17]. The magnitude of these changes appear to be similar in a light: dark (LD) cycle or in constant dark (DD). To further examine whether the diurnal and circadian changes in c-fos an d other immediate-early gene (IEG) expression in brain are related to waking behaviors such as locomotor activity, we conducted three experi ments using Northern analysis. First, we compared IEG expression in no cturnal vs. diurnally active species. Second, we investigated IEG expr ession in a hibernating species during its active and inactive, phases . Third, we examined the development of IEG expression in the young po st-natal rat. As a comparison to results obtained in extra-SCN brain r egions, we also examined IEG and vasopressin expression in the SCN its elf across the circadian cycle. Animals maintained under a 12:12-h LD cycle were sacrificed in the morning (10:00-11:00 h, ZT2-ZT3) or night (22:00-23:00 h, ZT14-ZT15) or at the corresponding circadian times (C T) when kept in DD. Rats sacrificed in the morning always showed lower C-fos expression than at night in all brain areas examined while the reverse pattern was seen in squirrels under both LD and DD conditions, suggesting a direct correlation between c-fos message and activity. T he cerebellum displayed the greatest magnitude change between morning and night (often reaching 10-fold). Among other IEGs examined, the exp ression of NGFI-A and junB are similar to c-fos, but of lesser magnitu de, whereas c-jun appears to be invariant in the rat but is increased during the active phase in squirrels. During the hibernation season, s quirrels have lower levels of c-fos consistent with their low levels o f activity even during their euthermic interbout periods. c-fos expres sion in the cerebellum and rest of brain of 1-week-old rats sacrificed at ZT3 and ZT15 showed low levels at both timepoints whereas 2- and 3 -week-old animals had higher levels at night as do adults. Among other IEGs, junB and NGFI-A again were similar to c-fos while c-jun and jun D were more constant. Our observations support the idea of a diurnal r hythm of IEG expression in the CNS that is related to waking behaviors . Among IEGs, c-fos exhibits the greatest daily variation in expressio n.