The onset of fever: new insights into its mechanism

The classical view of fever production is that it is modulated in the ventromedial preoptic area (VMPO) in response to signaling by pyrogenic cytokines elaborated in the periphery by mononuclear phagocytes and the consequent induction of cyclooxygenase (COX)-2-dependent prostaglandin (PG)E-2 in the VMPO. This mechanism has, however, been questioned, in particular because the, appearance of circulating cytokines lags the onset of the febrile response to intravenously (iv) injected bacterial endotoxic lipopolysaccharide (LPS), an exogenous pyrogen. Moreover, COX-2, in this case, is itself an inducible enzyme, the de novo synthesis of which similarly lags significantly the onset of fever. Issues also exist regarding the accessibility of the POA to blood-borne cytokines. New data adduced over the past 10 years indicate that the peripheral febrigenic message is conveyed to the VMPO via a neural rather than a humoral route, specifically by the vagus to the nucleus tractus solitarius (NST), and that the peripheral trigger is PGE(2), not cytokines; vagal afferents express PGE2 receptors (EP3). Thus, the initiation of the febrile responses to both iv and intraperitoneal (ip) LPS is temporally correlated with the appearance of LPS in the liver's Kupffer cells (Kc), its arrival immediately activating the complement (C) cascade and the consequent production of the anaphylatoxin C5a; the latter is the direct stimulus for PGE2 production, catalyzed non-differentially by constitutive COX-1 and -2. From the NST, the signal proceeds to the VMPO via the ventral noradrenergic bundle, causing the intrapreoptic release of norepinephrine (NE) which then evokes two distinct core temperature (T.) rises, viz., one alpha-adrenoceptor (AR)-mediated, rapid in onset, and PGE(2)-independent, and the other alpha(2)-AR-mediated, delayed, and COX-2/PGE(2)-dependent, i.e., the prototypic febrile pattern induced by iv LPS. The release of NE is itself modulated by nitric oxide contemporaneously released in the VMPO.