Seasonal changes of daylength (photoperiod) affect the expression of hormonal and behavioral circadian rhythms in a variety of organisms. In mammals, such effects might reflect photoperiodic changes in the circadian pace-making system [located in the suprachiasmatic nucleus (SCN) of the hypothalamus] that governs these rhythms, but to date no functionally relevant, intrinsic property of the SCN has been shown to be photoperiod dependent. We have analyzed the temporal regulation of light-induced c-fos gene expression in the SCN of rats maintained in long or short photoperiods. Both in situ hybridization and immunohistochemical assays show that the endogenous circadian rhythm of light responsiveness in the SCN is altered by photoperiod, with the duration of the photosensitive subjective night under the short photoperiod 5-6 h longer than under the long photoperiod. Our results provide evidence that a functional property of the SCN is altered by photoperiod and suggest that the nucleus is involved in photoperiodic time measurement.

• MeSH
• Time Factors MeSH
• Circadian Rhythm * MeSH
• Activity Cycles MeSH
• Gene Expression MeSH
• Genes, fos MeSH
• In Situ Hybridization MeSH
• Rats MeSH
• RNA, Messenger analysis   biosynthesis   MeSH
• Suprachiasmatic Nucleus physiology   MeSH
• Rats, Wistar MeSH
• Proto-Oncogene Proteins c-fos biosynthesis   MeSH
• Light MeSH
• Darkness MeSH
• Animals MeSH
• Check Tag
• Rats MeSH
• Male MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Research Support, U.S. Gov't, P.H.S. MeSH
• Names of Substances
• RNA, Messenger MeSH
• Proto-Oncogene Proteins c-fos MeSH