Elsevier

Neuroscience

Volume 170, Issue 4, 10 November 2010, Pages 1172-1178
Neuroscience

Cognitive, Behavioral, and Systems Neuroscience
Research Paper
Nighttime dim light exposure alters the responses of the circadian system

https://doi.org/10.1016/j.neuroscience.2010.08.009Get rights and content

Abstract

The daily light dark cycle is the most salient entraining factor for the circadian system. However, in modern society, darkness at night is vanishing as light pollution steadily increases. The impact of brighter nights on wild life ecology and human physiology is just now being recognized. In the present study, we tested the possible detrimental effects of dim light exposure on the regulation of circadian rhythms, using CD1 mice housed in light/dim light (LdimL, 300 lux:20 lux) or light/dark (LD, 300 lux:1 lux) conditions. We first examined the expression of clock genes in the suprachiasmatic nucleus (SCN), the locus of the principal brain clock, in the animals of the LD and LdimL groups. Under the entrained condition, there was no difference in PER1 peak expression between the two groups, but at the trough of the PER 1 rhythm, there was an increase in PER1 in the LdimL group, indicating a decrease in the amplitude of the PER1 rhythm. After a brief light exposure (30 min, 300 lux) at night, the light-induced expression of mPer1 and mPer2 genes was attenuated in the SCN of LdimL group. Next, we examined the behavioral rhythms by monitoring wheel-running activity to determine whether the altered responses in the SCN of LdimL group have behavioral consequence. Compared to the LD controls, the LdimL group showed increased daytime activity. After being released into constant darkness, the LdimL group displayed shorter free-running periods. Furthermore, following the light exposure, the phase shifting responses were smaller in the LdimL group. The results indicate that nighttime dim light exposure can cause functional changes of the circadian system, and suggest that altered circadian function could be one of the mechanisms underlying the adverse effects of light pollution on wild life ecology and human physiology.

Section snippets

Animals

Male CD1 mice (28 days old) were purchased from Charles River and were randomly assigned to one of two groups. The control group was housed in a 12:12 h light: dark (LD, 300 lux: 1 lux) cycle. Light phase illumination was produced by cool white fluorescent bulbs. A red safety light, with a maximum light intensity of 1 lux, was on during the dark phase to allow for animal care and maintenance. The experimental group was housed in a 12:12 h light: dim light (LdimL 300 lux: 20 lux) condition, in

Effect of nighttime dim light on the daily oscillation of the SCN

PER1 expression was high at ZT12 and low at ZT23 under both lighting condition (Fig. 1). At ZT12 (Fig. 1A, left column), densely packed PER1-ir nuclei were seen throughout the SCN in both LD and LdimL. At ZT23, scattered PER1-ir nuclei were seen at the center to dorsal but absent in the ventral region of the SCN in LD, showing the distribution characteristic to this time as previously reported (King et al., 2003). However, for the animals housed in LdimL, PER1-ir nuclei were distributed in both

Discussion

The results from the present study revealed that both the time-keeping and entrainment functions of the SCN were altered by a brighter night and that these changes had consequences downstream, in the form of changes in the overt behavioral rhythms of the animals. One of the main findings of the present study was an elevated baseline expression of clock genes in the SCN. At ZT23, which is the trough time for PER1 expression, the number of PER1-ir cells in the LdimL group was significantly higher

Acknowledgments

We would like to thank Drs. Antonio Nunez and Laura Smale for critical reading of previous drafts of this manuscript. We also thank Ashley Tomczak, Madison Operacz, Mona Shah and Kathleen Thomas for assisting the experiment and data analysis. We would also like to thank Drs. Matthew Blanchard, Taosheng Liu, Jie Huang at MSU and Dr. Arnold Wilkins at University of Essex for their help in characterizing and interpreting the light conditions. This work was supported by Psychology Department

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