Macrophage inflammatory protein-1: Unique action on the hypothalamus to evoke fever
Macrophage inflammatory protein (MIP-1) administered systemically causes a fever not blocked by a prostaglandin (PGE) synthesis inhibitor. The purpose of this study was to examine the central mechanism of pyrexic action of this cytokine in the unrestrained rat. After guide cannulae for microinjection were implanted stereotaxically just above the anterior hypothalamic preoptic area (AH/POA), the body temperature of each rat was monitored by a colonic thermistor probe. Saline control vehicle or MIP-1 was microinjected into the AH/POA in one of eight concentrations ranging from 0.0028-9.0 ng per 0.5 mu 1 volume. MIP-1 induced a biphasic or monophasic fever of short latency characterized by an inverse dose-response curve. The potency of MIP-1 was in the femtomolar (10(-15)) range with the lowest dose of 0.028 ng producing a fever of over 2.0 degrees C with a latency of 15 min or less. To determine whether a PGE mediates MIP-1 fever, indomethacin was administered either intraperitoneally in a dose of 5.0 mg/kg or directly into the MIP-1 injection site in a dose of 0.5 microgram/0.5 mu 1, both injected 15 min before MIP-1. Pretreatment of the injection site in the AH/POA with indomethacin failed to prevent the febrile response evoked by MIP-1 injected at the same locus. Further, the dose of systemic indomethacin, which blocks PGE-induced fever in the rat, attenuated only partially the MIP-1 fever. The results demonstrate that MIP-1 is the most potent endopyrogen discovered thus far, and that its action is directly in the region of the hypothalamus which contains both thermosensitive and pyrogen-sensitive neurons. The local action of MIP-1 on cells of the AH/POA in evoking fever is unaffected by the PGE inhibitor which indicates, therefore, that a cellular mechanism operates in the hypothalamus to evoke fever independently of the central synthesis of a PGE.