Circadian rhythms and different photoresponses of Clock gene transcription in the rat suprachiasmatic nucleus and pineal gland.

Aug 15, 2006Sheng li xue bao : [Acta physiologica Sinica]

Daily rhythms and light responses of Clock gene activity in the rat’s body clock and pineal gland

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Abstract

The mRNA levels of the Clock gene in the suprachiasmatic nucleus (SCN) of rats exhibited circadian oscillation under constant darkness conditions (P<0.05).

Clock gene transcription in the pineal gland (PG) showed similar rhythmic profiles to that in the SCN throughout the day, with peaks during subjective night and troughs during subjective day (P>0.05).
Under a light-dark cycle, Clock gene transcription in the SCN also displayed circadian oscillation, but the rhythmic profile was anti-phasic compared to the constant darkness condition (P<0.05).
The amplitude and mRNA levels of Clock gene transcription in the SCN were significantly higher under the light-dark cycle compared to constant darkness (P<0.05), while the PG showed significantly decreased rhythmic parameters under light-dark conditions (P<0.05).
Circadian profiles of Clock gene transcription in the PG under the light-dark cycle were distinctly different from those in the SCN, displaying anti-phasic rhythms with peaks and troughs occurring at different times (P<0.05).
The findings indicate a synchronous endogenous nature of Clock gene circadian transcriptions in the SCN and PG, with differing responses to light regimes.

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The aim of this study was to observe and compare the endogenous circadian rhythm and photoresponse of Clock gene transcription in the suprachiasmatic nucleus (SCN) and pineal gland (PG) of rats. With free access to food and water in special darkrooms, Sprague-Dawley rats were housed under the light regime of constant darkness (DD) for 8 weeks (n=36) or 12 hour-light: 12 hour-dark cycle (LD) for 4 weeks (n=36), respectively. Then, their SCN and PG were dissected out every 4 h in a circadian day, 6 rats at each time (n=6). All animal treatments and sampling during the dark phases were conducted under red dim light (<0.1 lux). The total RNA was extracted from each sample and the semi-quantitative RT-PCR was used to determine the temporal mRNA changes of Clock gene in the SCN and PG at different circadian times (CT) or zeitgeber times (ZT). The grayness ratio of Clock/H3.3 bands was served as the relative estimation of Clock gene expression. The experimental data were analyzed by the Cosine method and the Clock Lab software to fit original results measured at 6 time points and to simulate a circadian rhythmic curve which was then examined for statistical difference by the amplitude F test. The main results are as follows: (1) The mRNA levels of Clock gene in the SCN under DD regime displayed the circadian oscillation (P<0.05). The endogenous rhythmic profiles of Clock gene transcription in the PG were similar to those in the SCN (P>0.05) throughout the day with the peak at the subjective night (CT15 in the SCN or CT18 in the PG) and the trough during the subjective day (CT3 in the SCN or CT6 in the PG). (2) Clock gene transcription in the SCN under LD cycle also showed the circadian oscillation (P<0.05), and the rhythmic profile was anti-phasic to that under DD condition (P<0.05). The amplitude and the mRNA level at the peak of Clock gene transcription in the SCN under LD were significantly increased compared with that under DD (P<0.05), while the value of corresponding rhythmic parameters in the PG under LD were remarkably decreased (P<0.05). (3) Under LD cycle, the circadian profiles of Clock gene transcription induced by light in the PG were quite different from those in the SCN (P<0.05). Their Clock transcription rhythms were anti-phasic, i.e., showing peaks at the light phase ZT10 in the SCN or at the dark time ZT17 in the PG and troughs during the dark time ZT22 in the SCN or during the light phase ZT5 in the PG. The findings of the present study indicate a synchronous endogenous nature of the Clock gene circadian transcriptions in the SCN and PG, and different roles of light regime in modulating the circadian transcriptions of Clock gene in these two central nuclei.

Circadian rhythms and different photoresponses of Clock gene transcription in the rat suprachiasmatic nucleus and pineal gland.

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