(1) Cyclooxygenase (COX) enzymes represent the rate-limiting step in the metabolism of arachidonic acid (AA) to prostaglandins (PGs), a pathway activated in neuroinflammation and implicated in several neurodegenerative disorders. To gain a better insight into the specific roles of COX isoforms and characterize the interactions between upstream and downstream enzymes in brain AA cascade, we examined the expression and activity of COX-2 and phospholipase A2 enzymes (cPLA2 and sPLA2), as well as the expression of terminal prostaglandin E synthases (cPGES, mPGES-1, and mPGES- 2) in wild type and COX-1-/- mice. Brain PGE2 concentration was significantly increased, whereas thromboxane B2 (TXB2) concentration was decreased in COX-1-/- compared to wild type mice. There was a compensatory up-regulation of COX-2, accompanied by the activation of the nuclear factor (NF)-kappaB pathway, and also an increase in the upstream cPLA2 and sPLA2 enzymes. Overall, our data suggest that COX-1 and COX-2 play a distinct role in brain PG biosynthesis, with basal PGE2 production being metabolically coupled with COX-2 and TXB2 production being preferentially linked to COX-1. Additionally, COX-1 deficiency can affect the expression of reciprocal and coupled enzymes, COX-2, Ca2+ -dependent PLA2, and terminal mPGES-2.? ? (2) COX-2 is expressed under basal conditions in areas of the brain susceptible to excitotoxicity, a form of toxicity that occurs from over activation of excitatory neurotransmitter systems such as glutamate. While many studies have attempted to determine the role of COX-2 in excitotoxicity through pharmacological inhibition of the enzyme, the results are controversial. We attempted to further study the role of COX in excitotoxicity by testing the susceptibility of mice deficient in either COX-1 or COX-2 to kainic acid (KA) excitotoxicity. COX-1-/-, COX-2-/- and wild type mice were injected intraperitoneally with saline or 10 mg/kg KA, a sublethal dose which induced seizure activity, and video recorded for 2 hours after the injection. Median Racine seizure score (RSS) was significantly elevated in KA-exposed COX-2 -/- mice (RSS=4)compared to wild type (RSS=1) and COX-2+/- mice (RSS=1). COX-1 -/- mice did not differ from COX-1 +/+ mice in the magnitude of KA-induced seizures. Only COX-2 -/- mice exposed to KA exhibited neurons positive for Fluoro-Jade B (FJB), a histochemical stain that detects neuronal degeneration 24 hours after injection. FJB positive neurons were detected in the CA1 and CA3 regions of the hippocampus, amygdala and thalamus. In summary, COX-2 -/- , but not COX-1 -/-, mice exhibit an increased sensitivity to KA-induced seizure activity and neuronal damage, suggesting that COX-2 may be protective against KA-induced excitotoxicity.? ? (3) The specific role of each of COX-1 and COX-2 isoforms in neuroinflammation is still unclear. Therefore, we investigated the role of COX-1 and COX-2 in the neuroinflammatory response induced by intracerebroventricular injection of lipopolysaccharide (LPS) using COX-1-/-, COX-2-/- and wild type mice. In wild type mice LPS increased activated microglia in the cortex, corpus callosum, and cerebral ventricles and resulted in morphological changes of astrocytes in the cortex, hippocampus, and areas surrounding the cerebral ventricles. These changes were accompanied by the up-regulation of the proinflammatory mediators IL-1beta, TNF-alphaand PGE2. Reactive gliosis, expression of proinflammatory cytokines, and brain PGE2 levels, and translocation and activation of NF-kappaB and mitogen-activated protein kinases, important factors for signaling events during an inflammatory response, were significantly decreased in COX-1-/- mice. Protein oxidation, a critical factor contributing to the secondary progression of the inflammatory reaction and oxidative damage was also reduced in the COX-1-/- mice. In contrast, the mRNA expression of IL-1beta, TNF-alpha and of markers of activated microglia and astrocytes (CD11B and GFAP) was significantly increased after LPS injection in the COX-2-/- mice compared to wild-type mice. These observations suggest that COX-1 enhances whereas COX-2 attenuates LPS-induced acute neuroinflammation.? ? (4) To investigate the role of COX-2 in the cerebrovascular coupling, hemodynamic and neuronal responses to forepaw stimulation were measured in alpha-chloralose-anesthetized rats before and after intravenous injection of Meloxicam (MEL), a selective COX-2 inhibitor, and following a bolus of PGE2, a prominent vasodilatatory product of COX-2. Both MEL and PGE2 had a significant effect on the activation-elicited cerebral blood flow (CBF) and blood-oxygenation-level-dependent (BOLD) responses, quantified using continuous arterial spin labeling magnetic resonance imaging, without affecting the baseline perfusion. Meloxicam decreased brain COX enzymatic activity by 57 +/- 14% and decreased the stimulation-induced CBF response to 32 +/- 2% and BOLD to 46 +/- 1% of their respective pre-drug amplitudes. In turn, PGE2 bolus resulted in a partial recovery of functional hyperemia, with the CBF response recovering to 52 +/- 3% and the BOLD response to 56 +/- 2% of their values prior to MEL administration. These findings suggest a modulatory role of COX-2 products in the cerebrovascular coupling and provide evidence for existence of a functional metabolic buffer.

Agency
National Institute of Health (NIH)
Institute
National Institute on Aging (NIA)
Type
Intramural Research (Z01)
Project #
1Z01AG000423-03
Application #
7327031
Study Section
(BPM)
Project Start
Project End
Budget Start
Budget End
Support Year
3
Fiscal Year
2006
Total Cost
Indirect Cost
Name
Aging
Department
Type
DUNS #
City
State
Country
United States
Zip Code
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