Abstract
BackgroundThe 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 FindingsFirst, 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.ConclusionNocturnal 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.
Highlights
In the course of evolution, timekeeping mechanisms, such as the molecular clockwork, have evolved in all cells of the body
The autonomic innervation of the liver is essential to transmit the light information from the suprachiasmatic nucleus (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
We focused on the immediate effects of light on peripheral organs by investigating 1) the changes in clock gene expression in different organs, 2) whether the light-induced changes in clock gene expression correlate with the functional changes of an organ and 3) whether the autonomic nervous system (ANS) is responsible for the light-induced changes of hepatic gene expression
Summary
In the course of evolution, timekeeping mechanisms, such as the molecular clockwork, have evolved in all cells of the body. Since nocturnal light exposure results in a change in pineal and adrenal clock gene expression, and in changes in melatonin and corticosterone secretion [10,13,16], it has been suggested that these two hormones transmit the light signal to other tissues in order to adapt their function [17,18,19]. The influence of the SCN on peripheral clock genes is thought to be transmitted by its control over corticosterone secretion [10,18]. Studies demonstrated that light exposure affects the phase of the SCN and the functional activity of peripheral organs. 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
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