Interleukin-1 reduces food intake and body weight in rat by acting in the arcuate hypothalamus.

A reduction in food intake is commonly observed after bacterial infection, a phenomenon that can be reproduced by peripheral administration of Gram-negative bacterial lipopolysaccharide (LPS) or interleukin-1beta (IL-1ß), a pro-inflammatory cytokine released by LPS-activated macrophages. The arcuate nucleus of the hypothalamus (ARH) plays a major role in food intake regulation and expresses IL-1 type 1 receptor (IL-1R1) mRNA. In the present work, we tested the hypothesis that IL-1R1 expressing cells in the ARH mediate IL-1ß and/or LPS-induced hypophagia in the rat. To do so, we developed an IL-1ß-saporin conjugate, which eliminated IL-R1-expressing neurons in the hippocampus, and micro-injected it into the ARH prior to systemic IL-1ß and LPS administration. ARH IL-1ß-saporin injection resulted in loss of neuropeptide Y-containing cells and attenuated hypophagia and weight loss after intraperitoneal IL-1ß, but not LPS, administration. In conclusion, the present study shows that ARH NPY-containing neurons express functional IL-1R1s that mediate peripheral IL-1ß-, but not LPS-, induced hypophagia. Our present and previous findings indicate that the reduction of food intake after IL-1ß and LPS are mediated by different neural pathways.

Uncoupling of interleukin-6 from its signalling pathway by dietary n-3-polyunsaturated fatty acid deprivation alters sickness behaviour in mice.

Sickness behaviour is an adaptive behavioural response to the activation of the innate immune system. It is mediated by brain cytokine production and action, especially interleukin-6 (IL-6). Polyunsaturated fatty acids (PUFA) are essential fatty acids that are highly incorporated in brain cell membranes and display immunomodulating properties. We hypothesized that a decrease in n-3 (also known as omega3) PUFA brain level by dietary means impacts on lipopolysaccharide (LPS)-induced IL-6 production and sickness behaviour. Our results show that mice exposed throughout life to a diet containing n-3 PUFA (n-3/n-6 diet) display a decrease in social interaction that does not occur in mice submitted to a diet devoid of n-3 PUFA (n-6 diet). LPS induced high IL-6 plasma levels as well as expression of IL-6 mRNA in the hippocampus and cFos mRNA in the brainstem of mice fed either diet, indicating intact immune-to-brain communication. However, STAT3 and STAT1 activation, a hallmark of the IL-6 signalling pathway, was lower in the hippocampus of LPS-treated n-6 mice than n-3/n-6 mice. In addition, LPS did not reduce social interaction in IL-6-knockout (IL-6-KO) mice and failed to induce STAT3 activation in the brain of IL-6-KO mice. Altogether, these findings point to alteration in brain STAT3 as a key mechanism for the lack of effect of LPS on social interaction in mice fed with the n-6 PUFA diet. The relative deficiency of Western diets in n-3 PUFA could impact on behavioural aspects of the host response to infection.

In vitro and in vivo evidence for a role of the P2X7 receptor in the release of IL-1 beta in the murine brain.

The P2X(7) receptor (P2X(7)R) is a purinoceptor expressed predominantly by cells of immune origin, including microglial cells. P2X(7)R has a role in the release of biologically active proinflammatory cytokines such as IL-1 beta, IL-6 and TNFalpha. Here we demonstrate that when incubated with lipopolysaccharide (LPS), glial cells cultured from brain of P2X(7)R(-/-) mice produce less IL-1 beta compared to glial cells from brains of wild-type mice. This is not the case for TNFalpha and IL-6. Our results indicate a selective effect of the P2X7R gene deletion on release of IL-1 beta release but not of IL-6 and TNFalpha. In addition, we confirm that only microglial cells produce IL-1beta, and this release is dependent on P2X(7)R and ABC1 transporter. Because IL-1 beta is a key regulator of the brain cytokine network and P2X(7)R is an absolute requirement for IL-1 beta release, we further investigated whether response of brain cytokines to LPS in vivo was altered in P2X(7)R(-/-) mice compared to wild-type mice. IL-1 beta and TNFalpha mRNAs were less elevated in the brain of P2X(7)R(-/-) than in the brain of wild-type mice in response to systemic LPS. These results show that P2X7R plays a key role in the brain cytokine response to immune stimuli, which certainly applies also to cytokine-dependent alterations in brain functions including sickness behavior.

Cytokine-induced sickness behavior.

The behavioral repertoire of humans and animals changes dramatically following infection. Sick individuals have little motivation to eat, are listless, complain of fatigue and malaise, loose interest in social activities and have significant changes in sleep patterns. They display an inability to experience pleasure, have exaggerated responses to pain and fail to concentrate. Proinflammatory cytokines acting in the brain cause sickness behaviors. These nearly universal behavioral changes are a manifestation of a central motivational state that is designed to promote recovery. Exaggerated symptoms of sickness in cancer patients, such as cachexia, can be life-threatening. However, quality of life is often drastically impaired before the cancer becomes totally debilitating. Although basic studies in psychoneuroimmunology have defined proinflammatory cytokines as the central mediators of sickness behavior, a much better understanding of how cytokine and neurotransmitter receptors communicate with each other is needed. Advances that have been made during the past decade should now be extended to clinical studies in an attempt to alleviate sickness symptoms and improve quality of life for cancer patients.