DL, Romanovsky AA. Cyclooxygenase-1 or -2-which one mediates lipopolysaccharide-induced hypothermia? Am J Physiol Regul Integr Comp Physiol 297: R485-R494, 2009. First published June 10, 2009 doi:10.1152/ajpregu.91026.2008.-Systemic inflammation is associated with either fever or hypothermia. Fever, a response to mild systemic inflammation, is mediated by cyclooxygenase (COX)-2 and not by COX-1. However, it is still disputed whether COX-2, COX-1, neither, or both mediate(s) responses to severe systemic inflammation, and, in particular, the hypothermic response. We compared the effects of SC-236 (COX-2 inhibitor) and SC-560 (COX-1 inhibitor) on the deep body temperature (Tb) of rats injected with a lower (10 g/kg iv) or higher (1,000 g/kg iv) dose of LPS at different ambient temperatures (Tas). At a neutral Ta (30°C), the rats responded to LPS with a polyphasic fever (lower dose) or a brief hypothermia followed by fever (higher dose). SC-236 (2.5 mg/kg iv) blocked the fever induced by either LPS dose, whereas SC-560 (5 mg/kg iv) altered neither the febrile response to the lower LPS dose nor the fever component of the response to the higher dose. However, SC-560 blocked the initial hypothermia caused by the higher LPS dose. At a subneutral T a (22°C), the rats responded to LPS with early (70 -90 min, nadir) dose-dependent hypothermia. The hypothermic response to either dose was enhanced by SC-236 but blocked by SC-560. The hypothermic response to the higher LPS dose was associated with a fall in arterial blood pressure. This hypotensive response was attenuated by either SC-236 or SC-560. At the onset of LPS-induced hypothermia and hypotension, the functional activity of the COX-1 pathway (COX-1-mediated PGE2 synthesis ex vivo) increased in the spleen but not liver, lung, kidney, or brain. The expression of splenic COX-1 was unaffected by LPS. We conclude that COX-1, but not COX-2, mediates LPS hypothermia, and that both COX isoforms are required for LPS hypotension. body temperature; thermoregulation; fever; inflammation SO STRONGLY IS SYSTEMIC INFLAMMATION associated with changes in deep body temperature (T b ) that every clinical definition of the systemic inflammatory response syndrome includes a change in T b (11,48). Whereas the majority (ϳ90%) of patients with systemic inflammation have an increased T b , a number of them (ϳ10%) have a lowered T b (6, 17). Thermoregulatory manifestations are also present in animal models of systemic inflammation. In a rat model of systemic inflammation induced by bacterial LPS, the pattern of T b change depends on the ambient temperature (T a ) and the LPS dose. At a neutral or supraneutral T a (warm environment), fever is the prevailing response; the fever is monophasic when the dose of LPS is low (just suprathreshold), but it turns polyphasic as the dose increases (66, 68, 69, 71, 79); a mild hypothermia may precede the polyphasic fever when the dose of LPS is high (68). At a subneutral T a (cool environment), hypothermia followed by fever is the predominant response; th...