Background
Orexins are neuromediators that help regulate many different physiological processes; however, their role in regulating immune system functioning is unclear. This study sought to investigate the distribution of orexin in hypothalamic orexin-containing neurons during the first 6 hours after an intravenous injection of a lipopolysaccharide.
Material and Method
Orexin-containing neurons were detected by immunohistochemistry (avidin-biotin) on frontal sections from the brains of intact rats and rats after injections of either saline or lipopolysaccharide.
Results
Analysis of immunohistochemically stained sections (levels 26-32 according to Swanson's atlas) revealed that the main pool of orexin-containing neurons was located at brain levels 28 to 30. No changes were shown in the number of orexin-containing neurons in the hypothalamus of intact animals at different times of a day. Visualization of orexin-containing neurons in the lateral hypothalamus decreased only in 6 hours after an injection of lipopolysaccharide (500 microg/kg body weight) at levels 28, 29, and 30 of the rat brain; this might have been the result of changes in the balance of orexin synthesis and utilization.
Conclusions
These data suggest that orexin-containing neurons play a role in complex reactions in the brain to lipopolysaccharide application.

Keywords: Neuropeptides - immunology, Rats, Wistar, Neurons - immunology, Lipopolysaccharides - pharmacology, Intracellular Signaling Peptides and Proteins - immunology, Injections, Intravenous, Hypothalamic Area, Lateral - drug effects