Effects of nocturnal light on (clock) gene expression in peripheral organs: a role for the autonomic innervation of the liver

Abstract

Background: The biological clock, located in the hypothalamic suprachiasmatic nucleus (SCN), controls the daily rhythms in physiology and behavior. Early studies demonstrated that light exposure not only affects the phase of the SCN but also the functional activity of peripheral organs. More recently it was shown that the same light stimulus induces immediate changes in clock gene expression in the pineal and adrenal, suggesting a role of peripheral clocks in the organ-specific output. In the present study, we further investigated the immediate effect of nocturnal light exposure on clock genes and metabolism-related genes in different organs of the rat. In addition, we investigated the role of the autonomic nervous system as a possible output pathway of the SCN to modify the activity of the liver after light exposure.

Methodology and principal findings: First, we demonstrated that light, applied at different circadian times, affects clock gene expression in a different manner, depending on the time of day and the organ. However, the changes in clock gene expression did not correlate in a consistent manner with those of the output genes (i.e., genes involved in the functional output of an organ). Then, by selectively removing the autonomic innervation to the liver, we demonstrated that light affects liver gene expression not only via the hormonal pathway but also via the autonomic input.

Conclusion: Nocturnal light immediately affects peripheral clock gene expression but without a clear correlation with organ-specific output genes, raising the question whether the peripheral clock plays a "decisive" role in the immediate (functional) response of an organ to nocturnal light exposure. Interestingly, the autonomic innervation of the liver is essential to transmit the light information from the SCN, indicating that the autonomic nervous system is an important gateway for the SCN to cause an immediate resetting of peripheral physiology after phase-shift inducing light exposures.

Figure 1. Effects of nocturnal light exposure and liver denervation on Per1, 2 and 3 gene expression.

Effects of a 1 h light exposure at either CT14 or CT20 on mRNA levels of peripheral clock gene were studied in different organs of liver-intact and liver-denervated groups. Gray bars represent the non light-exposed control group (n = 8 at CT14 and n = 6 at CT20) and white bars represent the light-exposed group (n = 9 at CT14 and n = 6 at CT20). Hatched bars represent the liver denervated groups (non light-exposed group, n = 4, light- exposed group CD, n = 6). Each value is the mean±S.E.M. * P<0.05, ** P<0.01, *** P<0.005, compared to the respective non-light-exposed control group; ∧ P<0.05, ∧∧ P<0.01, ∧∧∧ P<0.005, compared to the control group at CT15.

Figure 2. Effects of nocturnal light exposure and liver denervation on Cry1, 2 and Dbp gene expression.

The effects of a 1 h light exposure at either CT14 or CT20 on mRNA levels of these clock genes were studied in the same groups mentioned in Figure 1. Each value is the mean±S.E.M. * P<0.05, ** P<0.01, *** P<0.005, compared to the respective non-light-exposed control group; ∧ P<0.05, ∧∧ P<0.01, ∧∧∧ P<0.005, compared to the control group at CT15.

Figure 3. Effect of nocturnal light exposure and liver denervation on the enzyme/receptor expression of peripheral tissues.

Effects of a 1 h light exposure at either CT14 or CT20 on the expression of enzymes/receptor genes in different organs of liver-intact (control) and liver-denervated (CD) animals. Gray bars represent the non-light-exposed animals (n = 8 at CT14 and n = 6 at CT20) and white bars represent the light-exposed animals (n = 9 at CT14 and n = 6 at CT20). Hatched bars represent the liver denervated groups (non light-exposed group, n = 4, light-exposed group CD, n = 6). Note that the effect of light on hepatic Per1, Per2, GLUT2 and PEPCK is absent in denervated rats. Each value is the mean±S.E.M. * P<0.05, ** P<0.01, *** P<0.005, compared to the non-light-exposed control group; ∧∧∧ P<0.005, compared to the non-denervated control group.

Figure 4. Effect of light at CT14 on hormonal levels in intact vs denervated animals.

Changes in the plasma glucose/corticosterone levels after one hour of light exposure at CT14 in sham-operated (n = 8) and CD (n = 10) animals. One-way ANOVA with repeated measures (i.e., Treatment (2 levels: light versus no-light)) and, as between-animal factor, Group (2 levels: sham-operated versus completely denervated CD) were used to assess the changes in plasma glucose/corticosterone levels after one hour of light exposure. Dark bars represent the animals in the non light-exposed condition and white bars represent the same animals after light exposure. Each value is the mean±S.E.M of the delta value (T₀–T_{60 min}).