The purpose of the present study was to investigate a possible lipopolysaccharide (LPS)-induced activation of brain cells that is mediated by the pleiotropic cytokine interleukin-6 (IL-6) and its transcription factor STAT3 during systemic or localized inflammation. In guinea pigs, intra-arterial (i.a., 10 microg x kg(-1)) or intraperitoneal (i.p., 30 microg x kg(-1)) injections of bacterial LPS cause a systemic inflammatory response which is accompanied by a robust fever. A febrile response can also be induced by administration of LPS into artificial subcutaneously implanted Teflon chambers (s.c. 100 or 10 microg x kg(-1)), which reflects an experimental model that mimics local tissue inflammation. Baseline plasma levels of bioactive IL-6 determined 60 min prior to injections of LPS or vehicle amounted to 35-80 international units (i.u.) ml(-1). Within 90 min of LPS injection, plasma IL-6 rose about 1000-fold in the groups injected i.a. or i.p., about 50-fold in the group injected s.c. with 100 microg x kg(-1) LPS, and only 5-fold in guinea pigs injected with the lower dose of LPS (10 microg x kg(-1)). At this time point, a distinct nuclear translocation pattern of the transcription factor STAT3 became evident in several brain structures. Amongst those, the sensory circumventricular organs known to lack a tight blood-brain barrier such as the area postrema, the vascular organ of the lamina terminalis and the subfornical organ, as well as the hypothalamic supraoptic nucleus showed intense nuclear STAT3 signals in the i.a. or i.p. injected groups. In contrast a moderate (s.c. group, 100 microg x kg(-1)), or even no (s.c. group, 10 microg x kg(-1)), nuclear STAT3 translocation occurred in response to s.c. injections of LPS. These results suggest that STAT3-mediated genomic activation of target gene transcription in brain cells occurred only in those cases in which sufficiently high concentrations of circulating IL-6 were formed during systemic (i.a. and i.p. groups) or localized (s.c. group, 100 microg x kg(-1)) inflammation.