Non-prostaglandin eicosanoids in fever and anapyrexia

Until recently, studies on the role of the metabolites of arachidonic acid (AA), eicosanoids in fever have primarily focused on prostaglandins, prostaglandin E2 (PGE2) in particular, derived from the pathway related to cyclooxygenases (COX). COX exists in two known isoforms; a constitutive COX-1, and COX-2, which is inducible upon the action of pyrogens. Data accumulated in our laboratories suggest a thermoregulatory role for two other pathways of arachidonate metabolism; 5-lipoxygenase (5-LOX) and cytochrome P-450 (epoxygenase). We have demonstrated that leukotrienes (LTs; 5-LOX-derived eicosanoids) and various isomers of epoxyeicosatrienoic acids (EETs; epoxygenase-derived eicosanoids) contribute to the process of endogenous antipyresis or cryogenesis, which limits the height of fever. In support of this are several lines of evidence based on both in vivo and in vitro experiments. 1) Intracerebroventricular (icv) injections of LTC4 at nanomolar concentrations cause a dose-dependent decrease of body temperature (Tb) in mice. 2) Lipopolysaccharide (LPS)-induced anapyrexia in mice is preceded and accompanied by elevation in hypothalamic cysteinyl-LT (CysLT) production. 3) The inhibitor of LT synthesis MK-886 suppresses both of these processes. 4) EETs as well as inducers of the epoxygenase attenuate, whereas inhibitors of epoxygenase enhance the LPS-induced fever in rats. 5) One of the isomers of EET, 11,12-EET, in in vitro studies inhibited both the generation of PGE2 and IL-6 in monocytes stimulated with LPS. These results, together with a well-established pyrogenic role of PGE2, indicate that AA cascade may be regarded as an endogenous system to regulate the temperature response upon disease. COX, 5-LOX, and epoxygenase products may act at the level of hypothalamus as proximal mediators of, respectively, fever (PGE2) or cryogenesis (CysLTs and EETs), or indirectly by influencing the other endogenous cryogens and pyrogens.