Anorexia is part of the body’s acute-phase response to illness. Whereas anorexia is a common behavioral response to infectious diseases, the reasons for and mechanisms behind this observation are still unknown. The anorexia of infection is part of the host’s acute phase response (APR). Despite being beneficial in the beginning, long lasting anorexia delays recovery and is ultimately deleterious.
When it is considered on an evolutionary basis, the organism must have net benefits from anorexia. The first response to infection is the development of acute phase response (APR). The APR is triggered by microbial products and characterized by production of several cytokines known to induce anorexia. Microbial products such as bacterial cell wall compounds (e.g., lipopolysaccharides and peptidoglycans), microbial nucleic acids (e. g., bacterial DNA and viral double-stranded RNA), and viral glycoproteins trigger the APR and presumably also the anorexia during infections. Microbial products stimulate the production of proinflammatory cytokines (e.g., interleukins [ILs], tumor necrosis factor-alpha, interferons), which serve as endogenous mediators. Several microbial products and cytokines reduce food intake after parenteral administration, suggesting a role of these substances in the anorexia during infection. Microbial products are mainly released and cytokines are produced in the periphery during most infections; they might inhibit feeding through neural and humoral pathways activated by their peripheral actions.
Several microbial products and cytokines reduce food intake after parenteral administration, suggesting a role of these substances in the anorexia during infection. Locally released cytokines may inhibit feeding by activating peripheral sensory fibers directly or indirectly, and without a concomitant increase in circulating cytokines. However, the final center for appetite or eating is the central nervous system (CNS). Thus, these peripheral signals must reach and interact with brain regions that control appetite. In addition, a direct action of cytokines and microbial products on the CNS is presumably involved in the anorexia during infection.
Activation of peripheral afferents by locally produced cytokines is involved in several cytokine effects, but is not crucial for the anorectic effect of microbial products and IL-1beta. Cytokines increase leptin expression in the adipose tissue, and leptin may contribute to, but is also not essential for, the anorectic effects of microbial products and cytokines. In addition, a direct action of cytokines and microbial products on the central nervous system (CNS) is presumably involved in the anorexia during infection. Cytokines can reach CNS receptors through circumventricular organs and through active or passive transport mechanisms or they can act through receptors on endothelial cells of the brain vasculature and stimulate the release of subsequent mediators such as eicosanoids. De novo CNS cytokine synthesis occurs in response to peripheral infections, but its role in the accompanying anorexia is still open to discussion. Central mediators of the anorexia during infection appear to be neurochemicals involved in the normal control of feeding, such as serotonin, dopamine, histamine, corticotropin releasing factor, neuropeptide Y, and alpha-melanocyte-stimulating hormone. Reciprocal, synergistic, and antagonistic interactions between various pleiotropic cytokines, and between cytokines and neurochemicals, form a complex network that mediates the anorexia during infection. Current knowledge on the mechanisms involved suggests some therapeutic options for treatment. Substances that block common key steps in cytokine synthesis or cytokine action, or inhibitors of eicosanoid synthesis, may hold more promise than attempts to antagonize specific cytokines. To target the neurochemical mediation of the anorexia during infection may be even more efficient. Future research should address these neurochemical mechanisms and the cytokine actions at the blood-brain barrier. Further unanswered questions concern the modulation of the anorexia during infection by gender and nutritional state.
Lipopolysaccharides and Cytokines
Microbial products such as lipopolysaccharides (LPS), which are also commonly used to model acute illness, trigger the acute-phase response and cause anorexia mainly through pro-inflammatory cytokines. LPS stimulate cytokine production through the cell-surface structural molecule CD14 and toll-like receptor-4. Cytokines ultimately change neural activity in brain areas controlling food intake and energy balance. The blood-brain barrier endothelial cells (BBB EC) are an important site of cytokine action in this context. BBB EC and perivascular cells (microglia and macrophages) form a complex regulatory interface that modulates neuronal activity by the release of messengers (e.g. PG, NO) in response to peripheral challenges. Serotonergic neurons originating in the raphe nuclei and glucagon-like peptide-1-expressing neurons in the hindbrain may be among the targets of these messengers, because serotonin (5-HT), acting through the 5-HT2C receptor, and glucagon-like peptide-1 have recently emerged as neurochemical mediators of LPS anorexia. The central melanocortin system, which is a downstream target of serotonergic neurons, also appears to be involved in mediation of LPS anorexia. Interestingly, LPS also reduce orexin expression and the activity of orexin neurons in the lateral hypothalamic area of fasted mice. As the eating-stimulatory properties of orexin are apparently related to arousal, the inhibitory effect of LPS on orexin neurons might be involved in LPS-induced inactivity and anorexia. In summary, the immune signalling pathways of LPS-induced, and presumably acute illness-induced, anorexia converge on central neural signalling systems that control food intake and energy balance in healthy individuals.
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2. Langhans W. Anorexia of infection: current prospects. Nutrition. 2000 Oct;16(10):996-1005.
3. Langhans W. Signals generating anorexia during acute illness. Proc Nutr Soc. 2007 Aug;66(3):321-30.
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