Research reportCyclooxygenase 2 mRNA expression in rat brain after peripheral injection of lipopolysaccharide
Introduction
Prostaglandin E2 (PGE2) levels of brain interstitial fluid rise following peripheral injection of LPS [20]. Pharmacological blockade of PGE2 synthesis attenuates many peripheral LPS-induced responses, such as fever [20], brain c-fos expression, hypothalamic–pituitary axis activation [16], increased splenic sympathetic activity [14], activation of serotonergic and noradrenergic neurotransmission in hippocampus [13], and increased blood–brain barrier (BBB) permeability [7]. Increased production of PGE2 in brain, therefore, is critically involved in these central nervous system (CNS)-linked responses to peripheral LPS.
There are three enzymes that are essential for the production of PGE2: phospholipase A2 (PLA2), cyclooxygenase, and PGE2 isomerase. COX is a rate-limiting enzyme in PGE2 synthesis, which catalyzes the conversion of arachidonic acid to prostaglandin H2[11]. Two isoforms of COX exist. COX 1 is a constitutively expressed form, and its levels are relatively insensitive to inflammatory stimulation. COX 2, on the other hand, has low constitutive levels of expression, and it is strongly induced by inflammatory factors such as LPS [8]. The induced expression of COX 2 in the brain is thought to play an important role in the elevation of central PGE2 levels in response to peripheral LPS 4, 5.
Only two studies have investigated the induction of COX 2 mRNA in the brain after peripheral LPS injection. The results of these studies show that COX 2 mRNA expression is induced in cells of the blood–brain barrier (BBB) after peripheral LPS injection 3, 5. Cellular responses in the CNS to peripherally injected LPS, however, depend upon the dose and the route of LPS administration as well as the time after the LPS injection 22, 24. A detailed analysis of COX 2 mRNA expression in response to peripheral LPS injection has not been performed. Furthermore, the phenotypes of the cells producing COX 2 mRNA have not been identified. In this study, we examined the induction of COX 2 mRNA at multiple time points after administration of a wide range of doses of LPS in the rat by either intravenous (i.v.) or intraperitoneal (i.p.) injection. We also identified the phenotypes of COX 2 mRNA expressing cells by combined immunohistochemistry and in situ hybridization histochemistry.
Section snippets
Animals and LPS injection
Male Sprague–Dawley rats (175–200 g; Taconic Farms, Germantown, NY) were group housed and handled daily prior to experimentation. Initially, they were injected i.p. with 2.5 mg/kg of LPS dissolved in 0.9% saline or 0.9% sterile saline alone. Animals were then killed by decapitation at 0.5, 1, 2, 4, 8, 12 or 24 h after the injection (n=3–5 per time point). To control for the effects due to saline injection alone, control animals which received no injection were also sacrificed (n=3). Injections
Results
Representative film autoradiographs of COX 2 mRNA hybridization in coronally cut sections at the level of pituitary are shown in Fig. 1. Constitutively expressed COX 2 mRNA was labeled in cortical regions, hippocampus, and dentate gyrus (Fig. 1A), and the labeling appeared to be neuronal. The same pattern and intensity of COX 2 mRNA hybridization were found in all the saline-injected and non-injected animals (data not shown).
After LPS injection (2.5 mg/kg, i.p.), induction of COX 2 mRNA was
Discussion
The present study confirms the previous results showing that COX 2 mRNA is induced by peripheral LPS primarily in the vascular and leptomeningeal cells of the brain 4, 5whereas no consistent changes in neuronal expression of COX 2 mRNA were detected. Unique to the present study, we made the following observations: (1) The COX 2 mRNA induction always occurs in the close vicinity of blood vessels regardless of the dose and route of the peripheral LPS administration. (2) The COX 2 mRNA induction
Conclusion
Peripheral LPS injections induces rapid COX 2 mRNA expression in endothelial cells of the blood vessels of the brain, which may elevate prostaglandin levels globally to modulate the function of CNS.
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