Regulation of the Gene Encoding Tumor Necrosis Factor Alpha (TNF-α) in the Rat Brain and Pituitary in Response to Different Models of Systemic Immune Challenge

Tumor necrosis factor (TNF)-alpha is usually referred to as a proinflammatory cytokine that plays a central role in initiating the cascade of other cytokines and factors for an appropriate immune response to infection. Like systemic phagocytes, recent studies have reported that specific cellular populations of the CNS have the ability to express and release the proinflammatory cytokine in response to peripheral administration of the bacterial endotoxin lipopolysaccharide (LPS). Whether such phenomenon represents a general mechanism of systemic immunogenic stimuli and how the severity of the challenge may influence TNF-alpha transcription in the brain has yet to be defined. Adult male rats were sacrificed 1, 3, 6, 12, 24 and 48 hours (h) after intraperitoneal (IP) injection of LPS (25-250 microg/100 g) or intramuscular (IM) injection of turpentine. Brains and pituitary glands were removed, cut, and TNF-alpha mRNA assayed by in situ hybridization using a full-length rat cRNA probe. The results show no positive signal under basal conditions or following sterile inflammation into the left hind limb. Systemic LPS caused a profound increase in the expression of the gene encoding TNF-alpha in the leptomeninges, choroid plexus (chp) and all sensorial circumventricular organs (CVOs). Interestingly, a migratory-like pattern of TNF-alpha-positive cells became apparent around the sensorial CVOs at 3 h, while a ubiquitous-like positive signal was found throughout the brain 6 h after the injection with the highest dose of LPS. The IP LPS injection also stimulated TNF-alpha transcription in the anterior pituitary lobe; the signal was maximal 1 h after the injection and returned gradually to basal levels at 12 h, whereas the mRNA encoding the cytokine was detected later in the neurohypophysis, i.e. 3 and 6 h post challenge. Dual-labeling procedure provided the evidence of an LPS-dependent induction of TNF-alpha in different phagocytic cellular populations of the brain, including parenchymal microglial cells during severe endotoxemia. The fact that these myeloid-derived cells have the ability to express the LPS receptor CD14 in the brain may well explain the transcriptional activation of the cytokine in response to the bacterial endotoxin, but not to systemic localized inflammation.