Interleukin-1 (IL-1) and tumor necrosis factor alpha (TNF␣) are proinflammatory cytokines that are constitutively expressed in healthy, adult brain where they mediate normal neural functions such as sleep. They are neuromodulators expressed by and acting on neurons and glia. IL-1 and TNF␣ expression is upregulated in several important diseases/disorders. Upregulation of IL-1 and/or TNF␣ expression, elicited centrally or systemically, propagates through brain parenchyma following specific spatio-temporal patterns. We propose that cytokine signals propagate along neuronal projections and extracellular diffusion pathways by molecular cascades that need to be further elucidated. This elucidation is a prerequisite for better understanding of reciprocal interactions between nervous, endocrine and immune systems. Molecular Psychiatry (2000) 5, 604-615.

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Abstract:If cytokines are constitutively expressed by and act on neurons in normal adult brain, then we may have to modify our current view that they are predominantly inflammatory mediators. We critically reviewed the literature to determine whether we could find experimental basis for such a modification. We focused on two "proinflammatory" cytokines, interleukin (IL)-1 and tumor necrosis factor-␣ (TNF␣) because they have been most thoroughly investigated in shaping our current thinking. Evidence, although equivocal, indicates that the genes coding for these cytokines and their accessory proteins are expressed by neurons, in addition to glial cells, in normal brain. Their expression is region-and cell typespecific. Furthermore, bioactive cytokines have been extracted from various regions of normal brain. The cytokines' receptors selectively are present on all neural cell types, rendering them responsive to cytokine signaling. Blocking their action modifies multiple neural "housekeeping" functions. For example, blocking IL-1 or TNF␣ by several independent means alters regulation of sleep. This indicates that these cytokines likely modulate in the brain behavior of a normal organism. In addition, these cytokines are likely involved in synaptic plasticity, neural transmission, and Ca 2ϩ signaling. Thus, the evidence strongly suggests that these cytokines perform neural functions in normal brain. We therefore propose that they should be thought of as neuromodulators in addition to inflammatory mediators. Key Words: Interleukin-1-Tumor necrosis factor-␣-Neurons-Glia-Brain-CNS-Behavior.