Light exposure during daytime modulates expression of Per1 and Per2 clock genes in the suprachiasmatic nuclei of mice

Abstract

The suprachiasmatic nuclei (SCN) of the hypothalamus contain the master circadian clock in mammals. Nocturnal light pulses that reset the circadian clock also lead to rapid increases in levels of Per1 and Per2 mRNA in the SCN, suggesting that these genes are involved in the synchronization to light. During the day, when light has no phase-shifting effects in nocturnal rodents, the consequences of light exposure for Per expression have been less thoroughly studied. Therefore, the effects of light exposure during the day were assessed on Per1 and Per2 mRNA in the SCN of mice. Expression of Per1 and Per2 was generally increased by 30-min light pulses during the subjective day, with more pronounced effects in the morning. One exception was noted for a transient decrease in Per2 expression after a short light pulse applied at midday. Prolonged light exposure (up to 3 hr) starting at midday markedly increased Per2 expression but not that of Per1. Moreover, the amplitude of the daily variations of both Per and the duration of Per1 peak was increased in mice exposed to a light-dark cycle compared with those transferred to constant darkness. Finally, the amplitude of the daily variations of both Per and the basal level of Per1 were increased in mice under a light-dark cycle compared with animals synchronized to a skeleton photoperiod (i.e., with daily dawn and dusk 1-hr exposures to light). Taken together, the results indicate that prolonged light exposure during daytime positively modulates daily levels of Per1 and Per2 mRNA in the SCN of mice.

Reciprocity between phase shifts and amplitude changes in the mammalian circadian clock

“…And what is its potential adaptive significance? Daily exposure to a light/dark cycle synchronizes SCN oscillators and enhances the overall amplitude of rhythmic Per gene expression (18). Additionally, rhythmic expression of the neuropeptide VIP in phase with Per genes in the SCN reinforces the molecular oscillator (38).…”

“…Interestingly, in mammals as well as in other organisms, there is a ''critical phase'' in mid-subjective night when light reduces the amplitude of overt rhythms so severely that arrhythmicity or ''singularity'' results (13)(14)(15)(16)(17). Conversely, amplitude increase by light pulses at certain circadian phases is also well documented (18,19). Under natural seasonal changes in day-lengths, these phenomena may encode photoperiod information in the amplitudes and phases of the SCN oscillators (20,21).…”

Differential effects of constant light on circadian clock resetting by photic and nonphotic stimuli in Syrian hamsters

“…In addition, because the terminall fields of all three major projections to the SCN (the RHT, GHT, and median raphe) arje colocalized in the core, this overlap is suggestive of synaptic contacts between the three input pathways (Morin, 1994(Morin, ., 1999Morin eta/., 1992). Thus, interconnectivity provides the means for interaction, while regulation of per expression is likely the medium (Challet et a/., 2003). An example of this interaction is found in the behavioural inhibition of photic resetting, which appears to involve 5HT modulation of photic regulation (Mistlberger & Antle, 1998).…”

Hypothesis driven single cell dual oscillator mathematical model of circadian rhythms

“…In wild-type simulations (Fig 7A–7C), the rising portion of both per1 and per2 mRNA occurs during the light phase and starts to decrease during the dark phase as seen in the experiments [38, 46–48]. Under short photoperiods, per1 peaking occurs near the light offset (Fig 7A), but as photoperiod increases, peaking occurs near the middle of the light phase (Fig 7A and 7B).…”

“…Experimental results indicate that in wild-type, per1 and per2 mRNA levels in mammals rise up during the light phase and falls down during the dark phase [38, 46–48]. However, the peaking time of per1 and per2 mRNA differs significantly under different photoperiods and their phase differences increase with increase in the photo periods [38, 46].…”