Peripheral macrophage depletion prevents down regulation of central interleukin-1 receptors in mice after endotoxin administration

Immune modulation of the hypothalamic-pituitary-adrenal (HPA) axis during viral infection

Compelling data has been amassed indicating that soluble factors, or cytokines, emanating from the immune system can have profound effects on the neuroendocrine system, in particular the hypothalamic- pituitary-adrenal (HPA) axis. HPA activation by cytokines (via the release of glucocorticoids), in turn, has been found to play a critical role in restraining and shaping immune responses. Thus, cytokine-HPA interactions represent a fundamental consideration regarding the maintenance of homeostasis and the development of disease during viral infection. Although reviews exist that focus on the bi-directional communication between the immune system and the HPA axis during viral infection (188,235), others have focused on the immunomodulatory effects of glucocorticoids during viral infection (14,225). This review, however, concentrates on the other side of the bi-directional loop of neuroendocrine-immune interactions, namely, the characterization of HPA axis activity during viral infection and the mechanisms employed by cytokines to stimulate glucocorticoid release.

Cytokines in the neuroendocrine system

Cytokines are important partners in the bidirectional network interrelating the immune and the neuroendocrine systems. These substances and their specific receptors, initially thought to be exclusively present in the immune system, have recently been shown to be also expressed in the neuroendocrine system. Cytokines can modulate the responses of all endocrine axes by acting at both the central and the peripheral levels. To explain how systemic cytokines may gain access to the brain, several mechanisms have been proposed, including an active transport through the blood-brain barrier, a passage at the circumventricular organ level, as well as a neuronal pathway through the vagal nerve. The immune-neuroendocrine interactions are involved in numerous physiological and pathophysiological conditions and seem to play an important role to maintain homeostasis.

Regulation of the hypothalamic-pituitary-adrenal axis by cytokines: actions and mechanisms of action

Glucocorticoids are hormone products of the adrenal gland, which have long been recognized to have a profound impact on immunologic processes. The communication between immune and neuroendocrine systems is, however, bidirectional. The endocrine and immune systems share a common "chemical language," with both systems possessing ligands and receptors of "classical" hormones and immunoregulatory mediators. Studies in the early to mid 1980s demonstrated that monocyte-derived or recombinant interleukin-1 (IL-1) causes secretion of hormones of the hypothalamic-pituitary-adrenal (HPA) axis, establishing that immunoregulators, known as cytokines, play a pivotal role in this bidirectional communication between the immune and neuroendocrine systems. The subsequent 10-15 years have witnessed demonstrations that numerous members of several cytokine families increase the secretory activity of the HPA axis. Because this neuroendocrine action of cytokines is mediated primarily at the level of the central nervous system, studies investigating the mechanisms of HPA activation produced by cytokines take on a more broad significance, with findings relevant to the more fundamental question of how cytokines signal the brain. This article reviews published findings that have documented which cytokines have been shown to influence hormone secretion from the HPA axis, determined under what physiological/pathophysiological circumstances endogenous cytokines regulate HPA axis activity, established the possible sites of cytokine action on HPA axis hormone secretion, and identified the potential neuroanatomic and pharmacological mechanisms by which cytokines signal the neuroendocrine hypothalamus.

Activation of the hypothalamus-pituitary-adrenal axis by bacterial endotoxins: routes and intermediate signals

Peripheral administration of endotoxin induces brain-mediated responses, including activation of the hypothalamus-pituitary-adrenal (HPA) axis and changes in thermoregulation. This paper reviews the mechanisms by which endotoxin affects these responses. The effects on thermoregulation are complex and include macrophage-dependent hyperthermic and hypothermic responses. Low doses of endotoxin, given IP, activate peripheral macrophages to produce interleukin (IL)-1 beta, which enters the circulation and acts as a hormonal signal. IL-1 may pass fenestrated endothelium in the median eminence to stimulate corticotropin-releasing hormone (CRH) secretion from the CRH nerve-terminals. In addition, IL-1 may activate brain endothelial cells to produce IL-1, IL-6, prostaglandins, etc., and secrete these substances into the brain. By paracrine actions, these substances may affect neurons (e.g., CRH neurons) or act on microglial cells, which show IL-1-induced IL-1 production and therefore amplify and prolong the intracerebral IL-1 signal. In contrast, high doses of endotoxin given i.v. may directly stimulate endothelial cells to produce IL-1, IL-6, and prostaglandin-E2 (PGE2) and thereby activate the HPA axis in a macrophage-independent manner.