Certain cytokines, the prototype being the highly pleiotropic TNF, have many homeostatic physiological roles, are involved in innate immunity, and cause inflammation when in excess. These cytokines have long been accepted to have central roles in the pathogenesis of systemic or local non-cerebral disease states, whether acute or chronic, and whether or not caused by infectious agents. Over the last decade they have also been appreciated to be broadly important in brain physiology. As in other organs, excessive levels in brain are harmful, and its physiological complexity leads to correspondingly complex dysfunction. This review summarizes the burgeoning literature on this topic, and how the functions of these molecules, particularly TNF, are influencing the outlook of researchers on the pathophysiology of these diseases. Basic brain physiology is thus informing knowledge of the brain dysfunction that characterizes such apparently diverse states as Alzheimer's disease, trauma (mostly, but not only, to the brain), Parkinson's disease, and severe systemic infectious states, including malaria, sepsis, viral diseases and major depression. The implication is that the anti-cytokine therapies now in use, typically directed at TNF, warrant testing in these diseases in circumstances in which the therapeutic agent enters the cerebrospinal fluid. Routinely administering such drugs to patients exhibiting the neurological changes discussed in this review would simply add another organ system to what is already a very successful strategy in the treatment of inflammatory disease at other sites, such as joints, skin and gut. Clearly, the most relevant research is focussed on Alzheimer's disease, but the principles may also apply to other encephalopathies.
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