Abstract
The suprachiasmatic nuclei of the hypothalamus (SCN) are the master circadian clock in mammals. Transcriptional activity in this master clock has a marker in the immediate‐early gene c‐Fos. Within the SCN, distinct differences in c‐Fos in the ventrolateral and the dorsomedial SCN have been reported for rodent species such as rats, mice, and hamsters. We studied C‐FOS expression in the common vole (Microtus arvalis) SCN under LD 12∶12 h and under constant dim light conditions. In the vole dorsomedial SCN, rhythmic C‐FOS expression was seen in LD with a clear peak in the middle of the light period. Under constant dim light, we report constitutive, non‐rhythmic expression of C‐FOS in the dorsomedial SCN. This pattern is consistent with the circadian organization of behavioral activity, which is weak in voles and may be lost under constant dim-light conditions. In the ventrolateral SCN, we observed a rise in C‐FOS expression under LD conditions prior to lights‐on, followed by peak expression at lights‐on. Another peak was seen at lights‐off. In an additional experiment, we subjected animals to LD 16∶8 to test the hypothesis that the dawn and dusk peaks in ventrolateral C‐FOS expression change phase along with the photoperiod. The peak in C‐FOS expression did not shift with the time of lights on, but remained at the same external time 6. The results are consistent with the interpretation that in the vole, c‐Fos expression reports transcriptional activity associated more likely with an internal, gating process than with an external effect of light.