The Light/Dark cycle disruption affects hepatic function both in metabolic parameters and tissue structure in a...

Biological rhythms, such as Light/Dark (LD) cycles, are an integral component of virtually all aspects of life. These rhythms are controlled in large part by circadian clocks, allowing the organism to adapt its internal rhythmic metabolism to changes in the external environment created by daily fluctuations in the LD cycle. Therefore, changes in the daily duration of the lighting could lead to adverse health consequences. The aim of the study was to investigate, in a nocturnal desert rodent, Gerbillus tarabuli, the effects of the LD cycle disruption on the structure of the hepatic tissue and the content of carbohydrate and lipid parameters as indicators of metabolic state.

The present study was conducted on two gerbil groups: control group was exposed to a standard lighting cycle (LD: 12:12), and the shifted group was subjected to a chronic disrupted LD cycle, alternating a standard cycle (LD: 12:12) with a modified cycle (LD: 20:4), i.e., the light phase of 24-h cycle was prolonged by 8 h on every second day during a period of 12 weeks. We used: (i) routine histology and histochemical staining for tissue analysis; (ii) immunohistochemistry (IHC) for MPO detection; (iii) biochemical methods for hepatic glycogen and lipids extraction and quantification. Blood metabolic parameters were assessed by enzymatic methods.

Our structural results indicate in the shifted group an alteration of tissue architecture, showing widely scattered inflammatory foci with many dilated sinusoids and prominent leukocyte infiltration with connective fibrotic extension. IHC revealed also increased hepatic myeloperoxidase (MPO) expression confirming neutrophils' presence. In parallel, the histochemical study revealed a strong depletion of hepatocytic glycogen and lipid inclusions; these observations were also supported by the measurements of glycogen and total lipids in extracted tissue indicating a reduction in liver content. These results were accompanied by a decrease in body weight relative to the reduction of food intake, as well as hyperglycemia and some alterations in serum lipid parameters (triglycerides and cholesterol) suggesting a metabolic disturbance.

We conclude that a phase difference between the endogenous activity rhythm of the species and the daily cycle of illumination has a strong impact on the liver morphology as well as on the metabolic activity of liver cells.