Involvement of interleukin-1 type 1 receptors in lipopolysaccharide-induced sickness responses.

Sickness responses to lipopolysaccharide (LPS) were examined in mice with deletion of the interleukin (IL)-1 type 1 receptor (IL-1R1). IL-1R1 knockout (KO) mice displayed intact anorexia and HPA-axis activation to intraperitoneally injected LPS (anorexia: 10 or 120µg/kg; HPA-axis: 120µg/kg), but showed attenuated but not extinguished fever (120µg/kg). Brain PGE2 synthesis was attenuated, but Cox-2 induction remained intact. Neither the tumor necrosis factor-α (TNFα) inhibitor etanercept nor the IL-6 receptor antibody tocilizumab abolished the LPS induced fever in IL-1R1 KO mice. Deletion of IL-1R1 specifically in brain endothelial cells attenuated the LPS induced fever, but only during the late, 3rd phase of fever, whereas deletion of IL-1R1 on neural cells or on peripheral nerves had little or no effect on the febrile response. We conclude that while IL-1 signaling is not critical for LPS induced anorexia or stress hormone release, IL-1R1, expressed on brain endothelial cells, contributes to the febrile response to LPS. However, also in the absence of IL-1R1, LPS evokes a febrile response, although this is attenuated. This remaining fever seems not to be mediated by IL-6 receptors or TNFα, but by some yet unidentified pyrogenic factor.

Immune-Induced Fever Is Dependent on Local But Not Generalized Prostaglandin E2 Synthesis in the Brain.

Fever occurs upon binding of prostaglandin E2 (PGE2) to EP3 receptors in the median preoptic nucleus of the hypothalamus, but the origin of the pyrogenic PGE2 has not been clearly determined. Here, using mice of both sexes, we examined the role of local versus generalized PGE2 production in the brain for the febrile response. In wild-type mice and in mice with genetic deletion of the prostaglandin synthesizing enzyme cyclooxygenase-2 in the brain endothelium, generated with an inducible CreERT2 under the Slco1c1 promoter, PGE2 levels in the CSF were only weakly related to the magnitude of the febrile response, whereas the PGE2 synthesizing capacity in the hypothalamus, as reflected in the levels of cyclooxygenase-2 mRNA, showed strong correlation with the immune-induced fever. Histological analysis showed that the deletion of cyclooxygenase-2 in brain endothelial cells occurred preferentially in small- and medium-sized vessels deep in the brain parenchyma, such as in the hypothalamus, whereas larger vessels, and particularly those close to the neocortical surface and in the meninges, were left unaffected, hence leaving PGE2 synthesis largely intact in major parts of the brain while significantly reducing it in the region critical for the febrile response. Furthermore, injection of a virus vector expressing microsomal prostaglandin E synthase-1 (mPGES-1) into the median preoptic nucleus of fever-refractive mPGES-1 knock-out mice, resulted in a temperature elevation in response to LPS. We conclude that the febrile response is dependent on local release of PGE2 onto its target neurons and not on the overall PGE2 production in the brain.SIGNIFICANCE STATEMENT By using mice with selective deletion of prostaglandin synthesis in brain endothelial cells, we demonstrate that local prostaglandin E2 (PGE2) production in deep brain areas, such as the hypothalamus, which is the site of thermoregulatory neurons, is critical for the febrile response to peripheral inflammation. In contrast, PGE2 production in other brain areas and the overall PGE2 level in the brain do not influence the febrile response. Furthermore, partly restoring the PGE2 synthesizing capacity in the anterior hypothalamus of mice lacking such capacity with a lentiviral vector resulted in a temperature elevation in response to LPS. These data imply that the febrile response is dependent on the local release of PGE2 onto its target neurons, possibly by a paracrine mechanism.

Differential roles of cyclooxygenase-2-related signaling in regulating hypothalamic neuronal activity under various acute stresses.

We have previously suggested that activation of the hypothalamic-pituitary-adrenal (HPA) axis is dependent on cyclooxygenase (COX)-2-related signaling under infectious and restraint stresses, but less dependent on it under hypoglycemic stress. In the present study, we evaluated the neuronal activity in the brain to elucidate the possible mechanisms underlying a stress-specific relevance between COX-2-related signaling and activation of the HPA axis under infectious (lipopolysaccharide, LPS), hypoglycemic (2-deoxy-D-glucose, 2DG) and restraint (1 hr) stress conditions. The number of c-Fos-immunoreactive (IR) cells in several brain regions including the paraventricular nucleus (PVN) and supraoptic nucleus (SON) was increased at 120 min after application of all stress stimuli. The number of c-Fos-IR cells at 30 min was increased only by 2DG in the SON, but not in the PVN. In the PVN, a selective COX-2 inhibitor (NS-398) did not affect the number of c-Fos-IR cells at any time points. On the other hand, in the SON, NS-398 increased c-Fos-IR cells at 30 min after LPS and restraint stresses, but not after 2DG injection. These results suggest that, among the brain regions responding to acute stresses, the PVN and SON are commonly activated under three acute stresses. In addition, it is also suggested that COX-2-related signaling decreases neuronal activity in the SON under infectious and restraint, but not hypoglycemic, stresses, which may be involved in the suppression of the HPA axis.

Cyclooxygenase-2-related signaling in the hypothalamus plays differential roles in response to various acute stresses.

We previously suggested that cyclooxygenase (COX)-2 plays a role as a common mediator of stresses in the brain. In the present study, we evaluated the possible involvement of COX-2-related signaling in the activation of the hypothalamic-pituitary-adrenal (HPA) axis under three different stress conditions, namely infectious (lipopolysaccharide, LPS), hypoglycemic (2-deoxy-d-glucose, 2DG) and restraint (1h) stresses in rats. Both an unselective COX inhibitor (indomethacin) and a selective COX-2 inhibitor (NS-398) significantly attenuated the increase of serum corticosterone levels after LPS and restraint stresses, but not after 2DG injection. COX-2 and microsomal prostaglandin E synthase (mPGES)-1 mRNA levels in the hypothalamus were significantly increased after LPS injection in intact rats. In adrenalectomized (ADX) rats, the expression of both genes was significantly increased after 2DG and restraint stresses, which was blocked by treatment with corticosterone. Interleukin-1ß (IL-1ß) mRNA levels in the hypothalamus in intact rats were increased only by LPS injection, though those in ADX rats were increased by all three stress stimuli. These results suggest that the relationship between COX-2-related signaling and activation of the HPA axis is stress-specific, and that COX-2-related signaling preferably mediates infectious and restraint stresses. Furthermore, the expression of COX-2 and mPGES-1 mRNA under the infectious stress condition was not negatively regulated by endogenous glucocorticoids, likely due to an increase in IL-1ß levels.

Taurine counteracts the suppressive effect of lipopolysaccharide on neurogenesis in the hippocampus of rats.

Neurogenesis has been generally accepted to happen in the subventricular zone lining the lateral ventricular and subgranular zone (SGZ) in the hippocampus of adult mammalian brain. Recent studies have reported that inflammatory stimuli, such as injection of lipopolysaccharide (LPS), impair neurogenesis in the SGZ. Taurine, a sulfur-containing ß-amino acid, is a major free intracellular amino acid in many tissues of mammals and having various supplementary effects on the mammalian body functions including the brain. Recently, it has been also reported that taurine levels in the brain significantly increase under stressful conditions. The present study was aimed to evaluate the possible beneficial effects of taurine on the neurogenesis in the SGZ under the condition of acute inflammatory stimuli by LPS. Adult male rats were intraperitoneally injected with taurine once a day for 39 days. Twenty-four hours before the animals were sacrificed on the last day of taurine treatment, LPS was injected simultaneously with bromodeoxyuridine (BrdU). Immunohistochemistry for BrdU, Ki67, and Iba-1 in the brain was performed, and serum levels of TNF-α and IL-1ß 2 h after LPS injection were determined. The results showed that LPS significantly decreased the number of immunoreactive cells for both BrdU and Ki67 in the SGZ, while increased that for Iba-1, all of which were restored by taurine administration. Meanwhile, the serum concentrations of TNF-α and IL-1ß were significantly increased, which were significantly attenuated by taurine administration. These results suggest that taurine effectively maintains neurogenesis in the SGZ under the acute infectious condition by attenuating the increase of microgliosis in the hippocampus as well as proinflammatory cytokines in the peripheral circulation.