Fos expression within vasopressin-containing neurons in the suprachiasmatic nucleus of diurnal rodents compared to nocturnal rodents

The underlying neural causes of the differences between nocturnal and diurn al animals with respect to their patterns of rhythmicity have not yet been identified. These differences could be due to differences in some subpopula tion of neurons within the suprachiasmatic nucleus (SCN) or to differences in responsiveness to signals emanating from the SCN. The experiments descri bed in this article were designed to address the former hypothesis by exami ning Fos expression within vasopressin (VP) neurons in the SCN of nocturnal and diurnal rodents. Earlier work has shown that within the SCN of the diu rnal rodent Arvicanthis niloticus, approximately 30% of VP-immunoreactive ( IR) neurons express Fos during the day, whereas Fos rarely is expressed in VP-IR neurons in the SCN of nocturnal rats. However, in earlier studies, ra ts were housed in constant darkness and pulsed with light, whereas Arvicant his were housed in a light:dark (LD) cycle. To provide data from rats that would permit comparisons with A. niloticus, the first experiment examined V P/Fos double labeling in the SCN of rats housed in a 12:12 LD cycle and per fused 4 h into the light phase or 4 h into the dark phase. Fos was signific antly elevated in the SCN of animals sacrificed during the light compared t o the dark phase, but virtually no Fos at either time was found in VP-IR ne urons, confirming that the SCN of rats and diurnal Arvicanthis are signific antly different in this regard. The authors also evaluated the relationship between this aspect of SCN function and diurnality by examining Fos-IR and VP-IR in diurnal and nocturnal forms of Arvicanthis. In this species, most individuals exhibit diurnal wheel-running rhythms, but some exhibit a dist inctly different and relatively nocturnal pattern. The authors have bred th eir laboratory colony for this trait and used animals with both patterns in this experiment. They examined Fos expression within VP-IR neurons in the SCN of both nocturnal and diurnal A. niloticus kept on a 12:12 LD cycle and perfused 4 h into the light phase or 4 h into the dark phase, and brains w ere processed for immunohistochemical identification of Fos and VP. Both th e total number of Fos-IR cells and the proportion of VP-IR neurons containi ng Fos (20%) were higher during the day than during the night. Neither of t hese parameters differed between nocturnal and diurnal animals. The implica tions of these findings are discussed.