Fos rhythms in the hypothalamus of Rattus and Arvicanthis that exhibit nocturnal and diurnal patterns of rhythmicity.

This study compared patterns of Fos expression within the suprachiasmatic nucleus (SCN), the region immediately dorsal to the SCN (the lower subparaventricular zone, LSPV), and the supraoptic nucleus (SON) of grass rats (Arvicanthis niloticus) and lab rats (Rattus norvegicus). Among grass rats we also compared individuals exhibiting nocturnal and diurnal patterns of wheel running. In the SCN of both groups of grass rats, as well as laboratory rats, Fos was elevated during the light compared to the dark portions of the day, and was expressed in 7-12% of cells containing vasoactive intestinal polypeptide (VIP). Fos was higher in the LSPV during the night compared to the day in both forms of grass rats but not in laboratory rats. In the SON, Fos rose from day to night in the diurnal grass rats and in laboratory rats, but not in nocturnal grass rats. These patterns are consistent with the hypothesis that VIP cells in the SCN function similarly in nocturnal and diurnal rodents, but that the SON and the region dorsal to the SCN are associated with intra and interspecific differences in rhythmicity, respectively.