Abstract
Increased exposure to light pollution perturbs physiological processes through misalignment of daily rhythms at the cellular and tissue levels. Effects of artificial light-at-night (ALAN) on diel properties of immunity are currently unknown. We therefore tested the effects of ALAN on diel patterns of cytokine gene expression, as well as key hormones involved with the regulation of immunity, in zebra finches (Taeniopygia guttata). Circulating melatonin and corticosterone, and mRNA expression levels of pro- (IL-1β, IL-6) and anti-inflammatory (IL-10) cytokines were measured at six time points across 24-h day in brain (nidopallium, hippocampus, and hypothalamus) and peripheral tissues (liver, spleen, and fat) of zebra finches exposed to 12 h light:12 h darkness (LD), dim light-at-night (DLAN) or constant bright light (LLbright). Melatonin and corticosterone concentrations were significantly rhythmic under LD, but not under LLbright and DLAN. Genes coding for cytokines showed tissue-specific diurnal rhythms under LD and were lost with exposure to LLbright, except IL-6 in hypothalamus and liver. In comparison to LLbright, effects of DLAN were less adverse with persistence of some diurnal rhythms, albeit with significant waveform alterations. These results underscore the circadian regulation of biosynthesis of immune effectors and imply the susceptibility of daily immune and endocrine patterns to ALAN.
Highlights
Most vertebrate species on earth synchronize their daily and annual activities based on information from the sun[1]
Partially-abolished diurnal rhythms in IL-6 and IL-18 mRNA expression under LL were restored by melatonin supplementation in chickens[31]
We hypothesize that cytokine gene expression will peak mostly during the day in various tissues to coincide with increased activity and higher disease risk, some cytokines in brain could peak at night to modulate sleep
Summary
Most vertebrate species on earth synchronize their daily and annual activities based on information from the sun[1]. Other studies have shown effects of circadian disruption upon innate immune function, namely inflammatory markers, as measured by pro-inflammatory cytokine gene expression in brain and peripheral tissues. While many mammalian studies report diurnal rhythms in immune responses, including expression profile of cytokines in brain and peripheral tissues[37,38], less is known in birds. The aims of this study are to (a) elucidate the diurnal variation of peripheral and neural expression of cytokine genes over the 24-hour cycle, and (b) to test whether 10-days of exposure to constant light or DLAN affects rhythmicity of cytokine gene expression, and/or the profiles of key endocrine regulators of immunity. If constant light and DLAN treatments suppress melatonin rhythms and/or alter the amplitude of cort rhythms on a chronic level, we would predict disruption of constitutive rhythms of cytokine gene expression. We predicted that DLAN would disrupt some (but not all) of the diel rhythms in immune function as opposed to complete abolishment of rhythms from constant light
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