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TNF-alpha in the brain drives sickness behaviors associated with liver disease.
Many liver disorders cause behavioral symptoms, often referred to as sickness behaviors, such as fatigue, loss of appetite, and “brain fog.” Evidence suggests that these symptoms arise from alterations in the central nervous system, but scientists don’t fully understand what drives them. Findings from a 2006 study suggest that sickness behaviors in the setting of cholestasis, a common liver disorder, are caused by the presence of tumor necrosis factor-alpha (TNF-alpha), a pro-inflammatory cytokine, in the brain.
Cholestasis is characterized by impaired bile flow and subsequent retention of bile acids, bilirubin, and other substances, including lipopolysaccharide, an endotoxin, in the liver and blood. It is a common disorder of pregnancy but can affect all demographics, including children. Most people with cholestasis report experiencing sickness behaviors, especially fatigue, which occurs in roughly 86 percent of people with the disorder.
TNF-alpha is produced by many types of immune cells. It exists in soluble and transmembrane forms, both of which mediate a variety of opposing physiological and pathological functions, depending on which of its receptors it binds to. For example, binding to TNF receptor 1 promotes apoptosis (programmed cell death) and inflammation; binding to TNF receptor 2 promotes cell survival, resolution of inflammation, immunity, and cellular repair. Elevated TNF-alpha is associated with chronic pain syndromes and anxious behaviors.
The investigators tied off the bile ducts of healthy mice to induce cholestasis. Then they isolated endothelial cells from the blood vessels in the animals' brains to see if the cells were activated and if the cells interacted with immune cells. They also measured TNF-alpha production by monocytes (white blood cells).
They found that endothelial cells were activated in the setting of cholestasis, and these activated cells readily interacted with immune cells that had been recruited to the brain. In turn, the immune cells increased their production of TNF-alpha. In light of the known effects of TNF-alpha on sickness behaviors, these findings suggest that TNF-alpha production in the brain mediates sickness behaviors in mice with liver disease.
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