Decoding photoperiodic time through Per1 and ICER gene amplitude.

The mammalian Per1 gene is expressed in the suprachiasmatic nucleus of the hypothalamus, where it is thought to play a critical role in the generation of circadian rhythms. Per1 mRNA also is expressed in other tissues. Its expression in the pars tuberalis (PT) of the pituitary is noteworthy because, like the suprachiasmatic nucleus, it is a known site of action of melatonin. The duration of the nocturnal melatonin signal encodes photoperiodic time, and many species use this to coordinate physiological adaptations with the yearly climatic cycle. This study reveals how the duration of photoperiodic time, conveyed through melatonin, is decoded as amplitude of Per1 and ICER (inducible cAMP early repressor) gene expression in the PT. Syrian hamsters display a robust and transient peak of Per1 and ICER gene expression 3 h after lights-on (Zeitgeber time 3) in the PT, under both long (16 h light/8 h dark) and short (8 h light/16 h dark) photoperiods. However, the amplitude of these peaks is greatly attenuated under a short photoperiod. The data show how amplitude of these genes may be important to the long-term measurement of photoperiodic time intervals.