Neuromodulation of acute-phase responses to interleukin-6 in guinea pigs.

It is now generally recognized that interleukin-6 (IL6) is one of the cytokines that mediate the various nonspecific host defense responses to infectious pathogens. Among its now well-demonstrated effects on systemic administration are fever and acute-phase proteinemia. These effects are also activated by the cytokine, IL1, and it has been shown that they are modulated in the preoptic-anterior hypothalamus (POA). This study was undertaken to determine whether this brain region similarly drives the febrile and proteinemic responses to IL6. We compared, therefore, these responses of conscious guinea pigs to human recombinant (hr)IL6 administered intravenously (IV) and into the POA. hrIL6 given IV was not pyrogenic at 1 microgram/kg, caused low-grade, dose-independent fevers (0.4 +/- 0.1 degree C) at 5-20 micrograms/kg, and dose-related fevers at 50 and 100 micrograms/kg (0.6 +/- 0.0 and 0.9 +/- 0.1 degree C, respectively). However, all doses of hrIL6 induced elevations in the plasma levels of ceruloplasmin (as an indicator of acute-phase proteins), albeit not in a dose-dependent manner. Indomethacin (10 mg/kg, injected intramuscularly 20 min before hrIL6) abolished the febrile response, but did not prevent the rise in plasma ceruloplasmin levels. Fever and ceruloplasminemia were also evoked by 50 and 100 ng of hrIL6 injected into the POA (1 microliter bilaterally), but not by 25 ng. These results indicate that the inductions of fever and plasma ceruloplasmin by IL6 are, like those of IL1, modulated in the POA, albeit the effective doses are much higher than those of IL1.