title: Photic resetting of intrinsic rhythmicity of the rat suprachiasmatic nucleus under various photoperiods.
abstract: To date, photic entrainment of the mammalian circadian system has been studied by following phase shifts of overt rhythms in the periphery governed by a circadian pacemaker located in the suprachiasmatic nucleus (SCN). The present study follows for the first time photic resetting of intrinsic rhythmicity of the SCN itself. Rats maintained under either a shorter photoperiod, with 12 h of light and 12 h of darkness per day, or under a long, 18:6-h light-dark photoperiod were exposed to a light stimulus during the dark period and then released into darkness, and the next day the SCN rhythm in the light-stimulated c-Fos protein immunoreactivity was followed as a marker of the SCN endogenous rhythmicity. After a light stimulus in the early night, the evening rise in the photic elevation of Fos protein photoinduction as well as the morning decline were phase delayed within one cycle. After a light stimulus in the late night, only the morning decline in the photic elevation of Fos was phase advanced the next night, not the evening rise; consequently, the interval enabling high photic elevation of Fos was reduced. After a light stimulus was administered around the middle of the night, the next night the evening rise in the light-stimulated Fos was eventually phase delayed, the morning decline was phase advanced, and the rhythm amplitude was reduced significantly; under 18:6-h light-dark, a mere 5-min light exposure exhibited such effects. The data indicate that resetting of the SCN rhythmicity in the light-elevated c-Fos 1 day after a resetting stimulus administration, i.e., during transient cycles, may proceed via nonparallel phase shifts of the evening rise and of the morning decline of the light-stimulated Fos, and via amplitude lowering and suggest a complex circadian pacemaking system in the rat SCN.
authors.affiliation: Institute of Physiology, Academy of Sciences of the Czech Republic, Prague, Czech Republic.
authors.forename: A
authors.initials: A
authors.lastname: Sumová
authors.forename: H
authors.initials: H
authors.lastname: Illnerová
mesh_terms: D000818:Animals; D007150:Immunohistochemistry; D008297:Male; D017440:Photoperiod; D016760:Proto-Oncogene Proteins c-fos / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D013493:Suprachiasmatic Nucleus / Q000502:physiology*
chemical_list: D016760:Proto-Oncogene Proteins c-fos