Systemic injection of the bacterial endotoxin lipopolysaccharide (LPS) provides a very good mean for increasing the release of proinflammatory cytokines by circulating monocytes and tissue macrophages. There is now considerable evidence that LPS exerts its action on mononuclear phagocytes via the cell surface receptor CD14. The aim of the present study was to verify the hypothesis that the brain has also the ability to express the gene encoding the LPS receptor, which may allow a direct action of the endotoxin onto specific cellular populations during blood sepsis. Adult male Sprague-Dawley rats were sacrificed 1, 3, 6 and 24 h after systemic (i.v. or i.p.) injection of LPS or the vehicle solution. Brains were cut from the olfactory bulb to the medulla in 30-microm coronal sections and mRNA encoding rat CD14 was assayed by in situ hybridization histochemistry using a specific 35S-labeled riboprobe. The results show low levels of CD14 mRNA in the leptomeninges, choroid plexus and along blood vessels of the brain microvasculature under basal conditions. Systemic injection of the bacterial endotoxin caused a profound increase in the expression of the gene encoding CD14 within these same structures as well as in the circumventricular organs (CVOs) the organum vasculosum of the lamina terminalis, subfornical organ, median eminence and area postrema. In most of these structures, the signal for CD14 mRNA was first detected at 1 h, reached a peak at 3 h post-injection, declined at 6 h, and return to basal levels 24 h after LPS treatment. Quite interestingly, a migratory-like pattern of CD14 positive cells was observed from all sensorial CVOs to deeper parenchymal brain 3 and 6 h after LPS injection. At 6 h post-challenge, small positive cells were found throughout the entire parenchymal brain and dual-labeling procedure indicated that different cells of myeloid origin have the ability to express CD14 in response to systemic LPS. These included CVO microglia, choroid plexus and leptomeninge macrophages, parenchymal and perivascular-associated microglial cells, although specific nonmyeloid cells were also positive for the LPS receptor. These results provide the very first evidence of a direct role of LPS on specific cell populations of the central nervous system, which is likely to be responsible for the transcription of proinflammatory cytokines; first within accessible structures from the blood and thereafter through scattered parenchymal cells during severe sepsis.