title: Interleukin-6 levels fluctuate with the light-dark cycle in the brain and peripheral tissues in rats.
abstract: Interleukin-6 (IL-6) has been implicated in excessive daytime sleepiness (EDS) in humans, and exogenous IL-6 also induces sleep alterations both in humans and rats. The IL-6 levels in human blood vary with the light-dark cycle with IL-6 levels being high during the dark period and low during the light period, whereas in the pituitary of rats the IL-6 levels are elevated during the light period compared to the dark period. However, it is unknown whether IL-6 in the brain is affected by the light-dark cycle. We hypothesized that IL-6 levels in the brain are regulated by the light-dark cycles and are elevated during the period that is predominantly occupied by sleep. To test this hypothesis, we measured IL-6 levels in the brain, blood, and adipose tissue of rats across light-dark cycle every 4 h. IL-6 levels were significantly higher during the light period than during the dark period in the cortex, hippocampus and hypothalamus. In the brainstem, IL-6 levels did not significantly vary across the light-dark cycles. IL-6 levels in the blood and adipose tissues were also significantly higher during the light period than during the dark period. IL-6 levels were positively correlated between the blood and adipose tissue, between hypothalamus and blood, and between the hypothalamus and hippocampus. These observations suggest that IL-6 levels vary across the light-dark cycle among different tissues and that IL-6 levels are elevated both centrally and peripherally during the period predominantly occupied by sleep but decreased during the period that primarily consists of wakefulness.
authors.affiliation: Department of Psychiatry, Pennsylvania State University College of Medicine, Hershey, PA 10733, USA.
authors.forename: Zhiwei
authors.initials: Z
authors.lastname: Guan
authors.forename: Alexandros N
authors.initials: AN
authors.lastname: Vgontzas
authors.forename: Takenori
authors.initials: T
authors.lastname: Omori
authors.forename: Xuwen
authors.initials: X
authors.lastname: Peng
authors.forename: Edward O
authors.initials: EO
authors.lastname: Bixler
authors.forename: Jidong
authors.initials: J
authors.lastname: Fang
mesh_terms: D000273:Adipose Tissue / Q000378:metabolism*; D000818:Animals; D001921:Brain / Q000378:metabolism*; D002540:Cerebral Cortex / Q000378:metabolism; D002940:Circadian Rhythm / Q000502:physiology*; D003624:Darkness; D006624:Hippocampus / Q000378:metabolism; D007031:Hypothalamus / Q000378:metabolism; D015850:Interleukin-6 / Q000097:blood / Q000378:metabolism* / Q000528:radiation effects; D008027:Light; D008297:Male; D017440:Photoperiod; D051381:Rats; D017207:Rats, Sprague-Dawley; D014018:Tissue Distribution
chemical_list: D015850:Interleukin-6