Phase response curve and light-induced Fos expression in the suprachiasmatic nucleus and adjacent hypothalamus of Arvicanthis niloticus

This article describes the phase response curve (PRC), the effect of light on Fos immunoreactivity (Fos-IR) in the suprachiasmatic nucleus (SCN), and the effect of SCN lesions on circadian rhythms in the murid rodent, Arvican this niloticus. In this species, all individuals are diurnal when housed wi thout a running wheel, but running in a wheel induces a nocturnal pattern i n some individuals. First, the authors characterized the PRC in animals wit h either the nocturnal or diurnal pattern. Both groups of animals were less affected by light during the middle of the subjective day than during the night and were phase delayed and phase advanced by pulses in the early and late subjective night, respectively. Second, the authors characterized the Fos response to light at circadian times 5, 14, or 22. Light induced an inc rease in Fos-IR within the SCN during the subjective night but not subjecti ve day; this effect was especially pronounced in the ventral SCN, where ret inal inputs are most concentrated, but was also evident in other regions. B oth light and time influenced Fos-IR within the lower subparaventricular ar ea. Third, SCN lesions caused animals to become arrhythmic when housed in a light-dark cycle as well as constant darkness. In summary, Arvicanthis app ear to be very similar to nocturnal rodents with respect to their PRC, temp oral patterns of light-induced Fos expression in the SCN, and the effects o f SCN lesions on activity rhythms.