Abstract
In mammals, changing daylength (photoperiod) is the main synchronizer of seasonal functions. The photoperiodic information is transmitted through the retino-hypothalamic tract to the suprachiasmatic nuclei (SCN), site of the master circadian clock. To investigate effects of day length change on the sheep SCN, we used in-situ hybridization to assess the daily temporal organization of expression of circadian clock genes (Per1, Per2, Bmal1 and Fbxl21) and neuropeptides (Vip, Grp and Avp) in animals acclimated to a short photoperiod (SP; 8h of light) and at 3 or 15 days following transfer to a long photoperiod (LP3, LP15, respectively; 16h of light), achieved by an acute 8-h delay of lights off. We found that waveforms of SCN gene expression conformed to those previously seen in LP acclimated animals within 3 days of transfer to LP. Mean levels of expression for Per1-2 and Fbxl21 were nearly 2-fold higher in the LP15 than in the SP group. The expression of Vip was arrhythmic and unaffected by photoperiod, while, in contrast to rodents, Grp expression was not detectable within the sheep SCN. Expression of the circadian output gene Avp cycled robustly in all photoperiod groups with no detectable change in phasing. Overall these data suggest that synchronizing effects of light on SCN circadian organisation proceed similarly in ungulates and in rodents, despite differences in neuropeptide gene expression.
Highlights
Organisms living at temperate latitudes experience profound annual changes in their environment, which exert a strong selective pressure
Under short photoperiod (SP), Per1 expression was maximal during the early day (CoG = ZT1.47 ± 2.83), after which levels steadily decreased to reach a nadir during the mid-dark phase (ZT16)
The photoperiodic switch led to clear delay in phasing relative to lights on within 3 days, with maximal levels observed at ZT8 (CoG = ZT7.53 ± 2.18) and a trough during the mid-dark phase, similar to the SP condition
Summary
Organisms living at temperate latitudes experience profound annual changes in their environment (e.g. temperature, food availability), which exert a strong selective pressure. Effects of Photoperiod Extension on Clock Gene and Neuropeptide RNA Expression in the Sheep SCN. The retina is the sole photoreceptive tissue and relays photoperiodic information to the main central circadian clock located in the suprachiasmatic nuclei (SCN) of the hypothalamus. Circadian clocks revolve around a set of clock genes, which interact to cyclically activate and inhibit their own expression. A heterodimer of transcription factors composed of CLOCK and BMAL1 turns on the expression of Period 1–3 and Cryptochrome 1–2 whose proteins feed-back onto the dimer to switch off transcription. The CLOCK/BMAL1 dimer directs transcription of genes that do not directly function in the core clock mechanism but add robustness or instill rhythmicity to various intracellular pathways and higher integrative physiological functions [3,4]. Within the SCN, Arginine Vasopressin (AVP) is a well-known example of a clock-controlled gene [5]: AVP can act on distant targets through diffusion in the cerebrospinal fluid and synaptically within adjacent regions, such as the paraventricular nuclei, to which the SCN project [6]
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