Reduced susceptibility to ischemic brain injury and N-methyl-D-aspartate-mediated neurotoxicity in cyclooxygenase-2-deficient mice

The distinct roles of cyclooxygenase-1 and -2 in neuroinflammation: implications for translational research

Cyclooxygenases (COX-1 and COX-2) are key enzymes in the conversion of arachidonic acid to prostaglandins and other lipid mediators. Because it can be induced by inflammatory stimuli, COX-2 has been classically considered as the most appropriate target for anti-inflammatory drugs. However, recent data indicate that COX-2 can mediate neuroprotection and that COX-1 is a major player in the neuroinflammatory process. We discuss the specific contributions of COX-1 and COX-2 in various neurodegenerative diseases and in models of neuroinflammation. We suggest that, owing to its predominant localization in microglia, COX-1 might be the major player in neuroinflammation, whereas COX-2, which is localized in neurons, might have a major role in models in which the neurons are directly challenged. Overall, the benefit of using COX-2 inhibitors should be carefully evaluated and COX-1 preferential inhibitors should be further investigated as a potential therapeutic approach in neurodegenerative diseases with an inflammatory component.

Genetic disruption of cyclooxygenase-2 does not improve histological or behavioral outcome after traumatic brain injury in mice

Increasing evidence suggests a role for cyclooxygenase-2 (COX-2) in traumatic brain injury (TBI). In the present study, the role of COX-2 in TBI was investigated using COX-2 gene-disrupted (COX-2 null) mice and wild-type (WT) controls that were subjected to the controlled cortical impact (CCI) model of TBI. There was increased expression of COX-2 in ipsilateral hippocampus in WT mice subjected to CCI. CCI resulted in a significant increase in prostaglandin E(2) concentrations in WT compared with COX-2 null hippocampi. There was a significant increase in TUNEL staining of CA1 neurons 24 hr after CCI in WT, but not in COX-2 null mice, compared with sham-operated controls, which is consistent with a protective role for COX-2 in the early phase of injury after TBI. However, there was no difference in lesion volume 21 days after CCI in COX-2 null and WT mice. COX-2 gene disruption did not alter Morris water maze performance. Taken together, these results suggest only a minor role for COX-2 activity in determining outcome after TBI in mouse.

Microglial inhibition of neuroprotection by antagonists of the EP1 prostaglandin E2 receptor

Background

The EP1 receptor for the prostanoid PGE2 is a G-protein coupled receptor that has been shown to contribute to excitotoxic neuronal death. In this study we examined the influence of non-neuronal cells on neuroprotective properties of EP1 receptor antagonists (Ono 8711 and SC 51089).

Methods

Primary neuronal cultures systems with or without non-neuronal cells were used to examine how the neuroprotective properties of EP1 antagonists were influenced by non-neuronal cells. The influence of astrocytes or microglia were individually tested in excitotoxicity assays using a co-culture system with these cells grown on permeable transwell inserts above the neuronal-enriched cultures. The influence of microglia on PGE2 synthesis and EP1 receptor expression was examined.

Results

EP1 antagonists were neuroprotective in neuronal-enriched cultures (> 90% neurons) but not in mixed cultures (30% neurons plus other non-neuronal cells). Co-cultures of microglia on permeable transwell inserts above neuronal-enriched cultures blocked neuroprotection by EP1 antagonists. Incubation of microglia with neuronal-enriched cultures for 48 hours prior to NMDA challenge was sufficient to block neuroprotection by EP1 antagonists. The loss of neuroprotection by EP1 antagonists was accompanied by a decrease of neuronal EP1 expression in the nucleus in cultures with microglia present.

Conclusion

These findings demonstrate microglial modulation of neuronal excitotoxicity through interaction with the EP1 receptor and may have important implications in vivo where microglia are associated with neuronal injury.

PGF(2alpha) FP receptor contributes to brain damage following transient focal brain ischemia

Although some of the COX-2 metabolites and prostaglandins have been implicated in stroke and excitotoxicity, the role of prostaglandin F(2alpha) (PGF(2alpha)) and its FP receptor have not been elucidated in the pathogenesis of ischemic-reperfusion (I/R) brain injury. Here we investigated the FP receptor's contribution in a unilateral middle cerebral artery (MCA) occlusion model of focal cerebral ischemia in mice. The MCA in wild type (WT) and FP knockout (FP(-/-)) C57BL/6 male mice was transiently occluded with a monofilament for 90 min. After 96 h of reperfusion, the FP(-/-) mice had 25.3% less neurological deficit (P < 0.05) and 34.4% smaller infarct volumes (P < 0.05) than those of the WT mice. In a separate cohort, physiological parameters were monitored before, during, and after ischemia, and the results revealed no differences between the groups. Because excitotoxicity is an acute mediator of stroke outcome, the effect of acute NMDA-induced neurotoxicity was also tested. Forty-eight hours after unilateral intrastriatal NMDA injection, excitotoxic brain damage was 20.8% less extensive in the FP(-/-) mice (P < 0.05) than in the WT counterparts, further supporting the toxic contribution of the FP receptor in I/R injury. Additionally, we investigated the effect of post-treatment with the FP agonist latanoprost in mice subjected to MCA occlusion; such treatment resulted in an increase in neurological deficit and infarct size in WT mice (P < 0.05), though no effects were observed in the latanoprost-treated FP(-/-) mice. Together, the results suggest that the PGF(2alpha) FP receptor significantly enhances cerebral ischemic and excitotoxic brain injury and that these results are of importance when planning for potential development of therapeutic drugs to treat stroke and its acute and/or long term consequences.

Reduced acute brain injury in PGE2 EP3 receptor-deficient mice after cerebral ischemia

Ischemic stroke is one of the leading causes of mortality and morbidity in humans. During brain ischemia and the subsequent reperfusion that occurs with stroke, the generation of the so-called "proinflammatory" prostaglandin E(2) (PGE(2)) increases significantly. Therefore, interest is growing regarding the differential functions of the individual PGE(2) receptors (EP1-4) and their relative contribution to brain damage following ischemic and inflammatory stimuli. Here, we address the contribution of the EP3 receptor in dictating early outcomes after transient cerebral ischemia. An oxygen-glucose deprivation (OGD)-induced in vitro model of brain ischemia was used in mouse hippocampal slice cultures. For transient ischemia, the right middle cerebral artery (MCA) of wildtype (WT) and EP3 knockout (EP3(-/-)) C57BL/6 male mice was occluded for 90 min and reperfused for 48 or 96 h, after which neurobehavioral scores and infarct volumes were determined. Mean arterial blood pressure, pH, blood gases (PaO(2) and PaCO(2)), cerebral blood flow, and body temperature were also determined before and during ischemia and reperfusion. OGD-induced cell death was significantly lower in brain slice cultures of EP3(-/-) mice than in those of WT mice. EP3(-/-) mice that underwent transient ischemia had significantly smaller infarct volumes than did WT mice at 48 h, but this difference was not sustained at 96 h. Neurological score deficits correlated with infarct volume, but no significant differences in the physiological parameters monitored were detected between the two genotypes. The results further support a role for EP3 receptors in contributing to acute ischemic stroke, but EP3 is not likely the sole contributor to the long-term detrimental consequences of PGE(2).

Distinct pattern of microglial response, cyclooxygenase-2, and inducible nitric oxide synthase expression in the aged rat brain after excitotoxic damage

Microglial and inflammatory responses to acute damage in aging are still poorly understood, although the aged brain responds differently to injury, showing poor lesion outcome. In this study, excitotoxicity was induced by intrastriatal injection of N-methyl-D-aspartate in adult (3-4 months) and aged (22-24 months) rats. Cryostat brain sections were processed for the analysis of microglial response by lectin histochemistry and cyclooxygenase 2 (COX2) and inducible nitric oxide synthase (iNOS) expression by immunohistochemistry and confocal analysis. Aged injured animals showed more widespread area of microglial response at 12 hr postlesion (hpl) and greater microglia/macrophage density at 3 days postlesion (dpl). However, aged reactive microglia showed prevalence of ramified morphologies and fewer amoeboid/round forms. Aged injured animals presented a diminished area of COX2 expression, but a significantly larger density of COX2(+) cells, with higher numbers of COX2(+) neurons during the first 24 hpl and COX2(+) microglia/macrophages later. In contrast, the amount of COX2(+) neutrophils was diminished in the aged. iNOS was more rapidly induced in the aged injured striatum, with higher cell density at 12 hpl, when expression was mainly neuronal. From 1 dpl, both the iNOS(+) area and the density of iNOS(+) cells were reduced in the aged, with lower numbers of iNOS(+) neurons, microglia/macrophages, neutrophils, and astrocytes. In conclusion, excitotoxic damage in aging induces a distinct pattern of microglia/macrophage response and expression of inflammatory enzymes, which may account for the changes in lesion outcome in the aged, and highlight the importance of using aged animals for the study of acute age-related insults.

Post-ischemic brain damage: pathophysiology and role of inflammatory mediators

Neuroinflammatory mediators play a crucial role in the pathophysiology of brain ischemia, exerting either deleterious effects on the progression of tissue damage or beneficial roles during recovery and repair. Within hours after the ischemic insult, increased levels of cytokines and chemokines enhance the expression of adhesion molecules on cerebral endothelial cells, facilitating the adhesion and transendothelial migration of circulating neutrophils and monocytes. These cells may accumulate in the capillaries, further impairing cerebral blood flow, or extravasate into the brain parenchyma. Infiltrating leukocytes, as well as resident brain cells, including neurons and glia, may release pro-inflammatory mediators, such as cytokines, chemokines and oxygen/nitrogen free radicals that contribute to the evolution of tissue damage. Moreover, recent studies have highlighted the involvement of matrix metalloproteinases in the propagation and regulation of neuroinflammatory responses to ischemic brain injury. These enzymes cleave protein components of the extracellular matrix such as collagen, proteoglycan and laminin, but also process a number of cell-surface and soluble proteins, including receptors and cytokines such as interleukin-1beta. The present work reviewed the role of neuroinflammatory mediators in the pathophysiology of ischemic brain damage and their potential exploitation as drug targets for the treatment of cerebral ischemia.

Effects of celecoxib on volumes of hematoma and edema in patients with primary intracerebral hemorrhage

Celecoxib is a potent anti-inflammatory drug with a safety profile that has been well-demonstrated in several human diseases and is reported to have beneficial effects in a rat model of intracerebral hemorrhage (ICH). We investigated the efficacy and safety of celecoxib treatment in patients with primary ICH. We retrospectively collected clinical data of patients admitted within 48 h after the onset of primary ICH. We classified the patients administered celecoxib (400 mg/day) for > or =7 days as the celecoxib-treated group (n=17) and classified the patients who were not administered celecoxib as the control group (n=17). The volumes of hematoma and edema were analyzed with initial and follow-up (at approximately 7 days) brain CT scans. Baseline characteristics, initial severity of neurologic sequelae, and initial volumes of the lesions were not different between the groups. Celecoxib treatment significantly reduced the volumes of edema noted on follow-up brain CT scans as compared with the volumes in the control group (30.2+/-17.7 vs. 55.5+/-40.6 mL, p=0.027) and also reduced the ratio of initial hematoma and edema volumes vs. the follow-up volumes as compared with those of the control group (hematoma, 0.90+/-0.29 vs. 1.54+/-1.12, p=0.035; edema, 0.95+/-0.32 vs. 2.08+/-0.88, p<0.001). The incidences of adverse events during the admission were not different between the groups. These results suggest that celecoxib may be safe and efficacious in patients with primary ICH. To confirm this notion, a large-sized prospective study should be performed.

Glucocorticoid-regulated crosstalk between arachidonic acid and endocannabinoid biochemical pathways coordinates cognitive-, neuroimmune-, and energy homeostasis-related adaptations to stress

Arachidonic acid and its derivatives constitute the major group of signaling molecules involved in the innate immune response and its communication with all cellular and systemic aspects involved on homeostasis maintenance. Glucocorticoids spread throughout the organism their influences over key enzymatic steps of the arachidonic acid biochemical pathways, leading, in the central nervous system, to a shift favoring the synthesis of anti-inflammatory endocannabinoids over proinflammatory metabolites, such as prostaglandins. This shift modifies local immune-inflammatory response and neuronal activity to ultimately coordinate cognitive, behavioral, neuroendocrine, neuroimmune, physiological, and metabolic adjustments to basal and stress conditions. In the hypothalamus, a reciprocal feedback between glucocorticoids and arachidonate-containing molecules provides a mechanism for homeostatic control. This neurochemical switch is susceptible to fine-tuning by neuropeptides, cytokines, and hormones, such as leptin and interleukin-1beta, assuring functional integration between energy homeostasis control and the immune/stress response.

Therapeutic targets in prostaglandin E2 signaling for neurologic disease

Prostaglandins (PGs) are potent autocrine and paracrine oxygenated lipid molecules that contribute appreciably to physiologic and pathophysiologic responses in almost all organs, including brain. Emerging data indicate that the PGs, and more specifically PGE2, play a central role in brain diseases including ischemic injury and several neurodegenerative diseases. Given concerns over the potential toxicity from protracted use of cyclooxygenase inhibitors in the elderly, attention is now focused on blocking PGE2 signaling that is mediated by interactions with four distinct G protein-coupled receptors, EP1-4, which are differentially expressed on neuronal and glial cells throughout the central nervous system. EP1 activation has been shown to mediate Ca2+-dependent neurotoxicity in ischemic injury. EP2 activation has been shown to mediate microglial-induced paracrine neurotoxicity as well as suppress microglia internalization of aggregated neurotoxic peptides. Animal models support the potential efficacy of targeting specific EP receptor subtypes in Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and ischemic stroke. However promising these preclinical studies are, they have yet to be followed by clinical trials targeting any EP receptor in neurologic diseases.

Activation of liver X receptors promotes neuroprotection and reduces brain inflammation in experimental stroke

BACKGROUND

The liver X receptors (LXRs) belong to the nuclear receptor superfamily and act as transcriptional regulators of cholesterol metabolism in several tissues. Recent work also has identified LXRs as potent antiinflammatory molecules in macrophages and other immune cells. Combined changes in lipid and inflammatory profiles are likely mediating the protective role of LXRs in models of chronic injury like atherosclerosis. These beneficial actions, however, have not been illustrated in other models of acute injury such as stroke in which inflammation is an important pathophysiological feature.

METHODS AND RESULTS

We have studied LXR expression and function in the course of experimental stroke caused by permanent middle cerebral artery occlusion in rats and mice. Here, we show that administration of the synthetic LXR agonists GW3965 or TO901317 after the ischemic occlusion improves stroke outcome as shown by decreased infarct volume area and better neurological scores in rats. Neuroprotection observed with LXR agonists correlated with decreased expression of proinflammatory genes in the brain and with reduced nuclear factor-kappaB transcriptional activity. Loss of function studies using LXRalpha,beta(-/-) mice demonstrated that the effect of LXR agonists is receptor specific. Interestingly, infarcted brain area and inflammatory signaling were significantly extended in LXRalpha,beta(-/-) mice compared with control animals, indicating that endogenous LXR signaling mediates neuroprotection in this setting.

CONCLUSIONS

This work highlights the transcriptional action of LXR as a protective pathway in brain injury and the potential use of LXR agonists as therapeutic agents in stroke.

NMDA-induced seizure intensity is enhanced in COX-2 deficient mice

Pharmacological inhibition or genetic deletion of cyclooxygenase (COX)-2, but not COX-1, has been shown to increase susceptibility to kainic acid (KA)-induced excitotoxicity. However, it is unclear if susceptibility to excitotoxins that act through other neurotransmitter receptors is altered by COX-2 inhibition. To further understand the involvement of COX-2 in regulating susceptibility to excitotoxicity, we investigated the effect of COX-2 deletion on excitotoxicity induced by peripheral injection of N-methyl-d-aspartate (NMDA, a specific agonist of the NMDA receptors) or lindane (a GABA(A) receptor antagonist). COX-2(-/-) mice injected intraperitoneally with NMDA (50-100mg/kg) exhibited significantly increased median seizure intensity when compared to COX-2(+/+) mice. Further, COX-2(-/-) mice exposed to NMDA showed neuronal damage, detected by Fluoro Jade B (FJB) staining, in the CA3 region of the hippocampus. There was no FJB staining nor any significant difference in median or maximal seizure intensity in COX-2(+/+) and COX-2(-/-) mice exposed to lindane. LC-MS/MS analysis of brain prostaglandin profile in COX-2(-/-) mice demonstrated a significant increase in PGF(2alpha), TXB(2), PGE(2) and PGD(2) expression 1h after administration of an excitotoxic dose of KA, but not of NMDA. Our findings demonstrate that COX-2 regulates susceptibility to KA and NMDA excitotoxicity, which directly activate glutamatergic neurotransmission, but not to lindane, which indirectly alters glutamatergic neurotransmission. Furthermore, increased levels of prostaglandins after seizures are associated with consistent manifestation of neuronal damage.

Endocannabinoid 2-arachidonoylglycerol protects neurons by limiting COX-2 elevation

Endocannabinoids are involved in synaptic signaling and neuronal protection; however, our understanding of the mechanisms by which endocannabinoids protect neurons from harmful insults remains elusive. 2-Arachidonoylglycerol (2-AG), the most abundant endogenous cannabinoid and a full agonist for cannabinoid receptors (CB1 and CB2), is a substrate for cyclooxygenase-2 (COX-2) and can be metabolized by COX-2. Here we show, however, that 2-AG is also capable of suppressing elevation of hippocampal COX-2 expression in response to proinflammatory and excitotoxic stimuli. 2-AG prevents neurodegeneration from toxic assaults that elevate COX-2 expression and inhibits the COX-2 elevation-enhanced excitatory glutamatergic synaptic transmission. The action of 2-AG on suppression of COX-2 appeared to be mediated via the pertussis toxin-sensitive G protein-coupled CB1 receptor and MAPK/NF-kappaB signaling pathways. Our results reveal that 2-AG functions as an endogenous COX-2 inhibitor protecting neurons from harmful insults by preventing excessive expression of COX-2, which provides a mechanistic basis for opening up new therapeutic approaches for protecting neurons from inflammation- and excitotoxicity-induced neurodegeneration.

S-nitrosylation/activation of COX-2 mediates NMDA neurotoxicity

Glutamate/N-methyl-d-aspartate (NMDA) receptor-mediated neurotoxicity involves cyclooxygenase (COX)-2. We demonstrate that this neurotoxicity reflects activation of COX-2 by S-nitrosylation after selective binding of neuronal nitric oxide synthase (nNOS) to COX-2. nNOS, via its PDZ domain, binds COX-2 with the generated NO S-nitrosylating and activating the enzyme. Selective disruption of nNOS-COX-2 binding prevents NMDA neurotoxicity.

Effects of Cyclooxygenase Inhibitors on Apoptotic Neuroretinal Cells

Glaucoma is characterized by a loss of retinal ganglion cells (RGC) which is associated with a decrease of visual function. Neuroprotective agents as a new therapeutic strategy could prevent the remaining neurons from apoptotic cell death. Previous studies have shown the involvement of the Cyclooxygenase (COX)-2 signalling in the apoptotic death of neurons. Herein we investigated the neuroprotective effect of COX-1/COX-2- and selective COX-2- inhibitors on apoptotic. R28, a neuroretinal cell line and determined the PGE(2) levels by ELISA. Furthermore we investigated differences in protein expression in the cells after exposure to elevated pressure compared to untreated cells by ProteinChip analysis.In addition, a protein profiling study of the cells after exposure to elevated pressure was performed. The protein expression profiles were measured by SELDI-TOF (Surface Enhanced Laser Desorption/Ionization-time of flight) Protein Chips. The protein identification was performed by mass spectrometry (MS).It could be shown that COX-2 inhibition significantly prevented the cells from apoptosis and reduced the PGE(2) concentrations. Selective COX-2 inhibitors were significant more potent than non-selective inhibitors or COX-1 inhibitors. We found differently expressed protein patterns in neuroretinal cells cultured at atmospheric pressure compared to those cells exposed to elevated pressure with or without celecoxib respectively. We identified three biomarkers, ubiquitin, HSP10 and NDKB, which were differently expressed in the groups. However, our data indicates a distinct neuroprotective effect of COX-2 inhibition. The local treatment with selective COX-2 inhibitors might provide an innovative strategy of therapeutic intervention for glaucoma.

Lysosomal release of cathepsins causes ischemic damage in the rat hippocampal slice and depends on NMDA-mediated calcium influx, arachidonic acid metabolism, and free radical production

NMDA-mediated calcium entry and reactive oxygen species (ROS) production are well-recognized perpetrators of ischemic neuronal damage. The current studies show that these events lead to the release of the protein hydrolase, cathepsin B, from lysosomes 2 h following 5-min oxygen-glucose deprivation in the rat hippocampal slice. This release reflects a lysosomal membrane permeabilization (LMP) and was measured as the appearance of diffuse immunolabeled cathepsin B in the cytosol of CA1 pyramidal neurons. Necrotic neuronal damage begins after the release of cathepsins and is prevented by inhibitors of either cathepsin B or D indicating that the release of cathepsins is an important mediator of severe damage. There was an increase in superoxide levels, measured by dihydroethidium fluorescence, at the same time as LMP and reducing ROS levels with antioxidants, Trolox or N-tert-butyl-alpha-phenyl nitrone, blocked LMP. Both LMP and ROS production were blocked by an NMDA channel blocker (MK-801) and by inhibitors of mitogen-activated protein kinase kinase (U0126), calcium-dependent/independent phospholipases A2 (methyl arachidonyl fluorophosphonate) but not calcium-independent phospholipases A2 (bromoenol lactone) and cyclooxygenase-2 (NS398). A cell-permeant specific inhibitor of calpain (PD150606) prevented LMP, but not ROS production. It is concluded that LMP results in part from calcium-initiated and extracellular signal-regulated kinase-initiated arachidonic acid metabolism, which produces free radicals; it also requires the action of calpain.

Cytosolic phospholipase A2 alpha inhibition prevents neuronal NMDA receptor-stimulated arachidonic acid mobilization and prostaglandin production but not subsequent cell death

Phospholipase A(2) (PLA(2)) enzymes encompass a superfamily of at least 13 extracellular and intracellular esterases that hydrolyze the sn-2 fatty acyl bonds of phospholipids to yield fatty acids and lysophospholipids. The purpose of this study was to characterize which phospholipase paralog regulates NMDA receptor-mediated arachidonic acid (AA) release. Using mixed cortical cell cultures containing both neurons and astrocytes, we found that [(3)H]-AA released into the extracellular medium following NMDA receptor stimulation (100 microM) increased with time and was completely prevented by the addition of the NMDA receptor antagonist MK-801 (10 microM) or by removal of extracellular Ca(2+). Neither diacylglycerol lipase inhibition (RHC-80267; 10 microM) nor selective inhibition of Ca(2+)-independent PLA(2) [bromoenol lactone (BEL); 10 microM] alone had an effect on NMDA receptor-stimulated release of [(3)H]-AA. Release was prevented by methyl arachidonyl fluorophosphonate (MAFP) (5 microM) and AACOCF(3) (1 microM), inhibitors of both cytosolic PLA(2) (cPLA(2)) and Ca(2+)-independent PLA(2) isozymes. This inhibition effectively translated to block of NMDA-induced prostaglandin (PG) production. An inhibitor of p38MAPK, SB 203580 (7.5 microM), also significantly reduced NMDA-induced PG production providing suggestive evidence for the role of cPLA(2)alpha. Its involvement in release was confirmed using cultures derived from mice deficient in cPLA(2)alpha, which failed to produce PGs in response to NMDA receptor stimulation. Interestingly, neither MAFP, AACOCF(3) nor cultures derived from cPLA(2)alpha null mutant animals showed any protection against NMDA-mediated neurotoxicity, indicating that inhibition of this enzyme may not be a viable protective strategy in disorders of the cortex involving over-activation of the NMDA receptor.

Cyclooxygenase-2 in synaptic signaling

Cyclooxygenase-2 (COX-2), a rate-limiting enzyme converting arachidonic acid to prostaglandins and a key player in neuroinflammation, has been implicated in the pathogenesis of neurodegenerative diseases such as multiple sclerosis, Parkinson's and Alzheimer's diseases, and in traumatic brain injury- and ischemia-induced neuronal damage, and epileptogenesis. Accumulated information suggests that the contribution of COX-2 to neuropathology is associated with its involvement in synaptic modification. Inhibition or elevation of COX-2 has been shown to suppress or enhance excitatory glutamatergic neurotransmission and long-term potentiation (LTP). These events are mainly mediated via PGE(2), the predominant reaction product of COX-2, and the PGE(2) subtype 2 receptor (EP(2))-protein kinase A pathway. Recent evidence shows that endogenous cannabinoids are substrates for COX-2 and can be oxygenated by COX-2 to form new classes of prostaglandins (prostaglandin glycerol esters and prostaglandin ethanolamides). These COX-2 oxidative metabolites of endocannabinoids, as novel signaling mediators, modulate synaptic transmission and plasticity and cause neurodegeneration. The actions of these COX-2 metabolites are likely mediated by mitogen-activated protein kinase (MAPK) and inositol 1,4,5-trisphosphate (IP(3)) signal transduction pathways. These discoveries suggest that the contributions of COX-2 to neurotransmission and brain malfunction result not only from its conversion of arachidonic acid to classic prostaglandins but also from its oxidative metabolism of endocannabinoids to novel prostaglandins. Thus, elucidation of COX-2 in synaptic signaling may provide a mechanistic basis for designing new drugs aimed at preventing, treating or alleviating neuroinflammation-associated neurological disorders.

Neuroinflammatory response to lipopolysaccharide is exacerbated in mice genetically deficient in cyclooxygenase-2

Background

Cyclooxygenases (COX) -1 and -2 are key mediators of the inflammatory response in the central nervous system. Since COX-2 is inducible by inflammatory stimuli, it has been traditionally considered as the most appropriate target for anti-inflammatory drugs. However, the specific roles of COX-1 and COX-2 in modulating a neuroinflammatory response are unclear. Recently, we demonstrated that COX-1 deficient mice show decreased neuroinflammatory response and neuronal damage in response to lipopolysaccharide (LPS).

Methods

In this study, we investigated the role of COX-2 in the neuroinflammatory response to intracerebroventricular-injected LPS (5 μg), a model of direct activation of innate immunity, using COX-2 deficient (COX-2^-/-) and wild type (COX-2^+/+) mice, as well as COX-2^+/+ mice pretreated for 6 weeks with celecoxib, a COX-2 selective inhibitor.

Results

Twenty-four hours after LPS injection, COX-2^-/- mice showed increased neuronal damage, glial cell activation, mRNA and protein expression of markers of inflammation and oxidative stress, such as cytokines, chemokines, iNOS and NADPH oxidase. Brain protein levels of IL-1β, NADPH oxidase subunit p67^phox, and phosphorylated-signal transducer and activator of transcription 3 (STAT3) were higher in COX-2^-/- and in celecoxib-treated mice, compared to COX-2^+/+ mice. The increased neuroinflammatory response in COX-2^-/- mice was likely mediated by the upregulation of STAT3 and suppressor of cytokine signaling 3 (SOCS3).

Conclusion

These results show that inhibiting COX-2 activity can exacerbate the inflammatory response to LPS, possibly by increasing glial cells activation and upregulating the STAT3 and SOCS3 pathways in the brain.

Cyclooxygenase-2 inhibition improves amyloid-beta-mediated suppression of memory and synaptic plasticity

Non-steroidal anti-inflammatory agents (NSAIDs) are associated with a marked reduction in the risk of developing Alzheimer's disease, a form of dementia characterized by the accumulation of amyloid plaques containing the amyloid-beta protein (Abeta). Studies of the effects of NSAIDs upon the inflammatory response surrounding amyloid plaques and upon the generation of Abeta from the amyloid precursor protein (APP) have led to two proposed mechanisms by which NSAIDs may protect against Alzheimer's disease: one, the selective lowering of Abeta42 by a subset of NSAIDs; and two, the reduction of inflammation. Although Alzheimer's disease is a disorder of brain and synaptic function, the effects of NSAIDs on Abeta-mediated suppression of synaptic plasticity and memory function have never been reported. We therefore investigated how three different NSAIDs, chosen for their distinct effects on Abeta42 production and the inhibition of the cyclooxygenase (COX) isoenzymes, COX-1 and COX-2, affect memory function and synaptic plasticity. By focusing upon brain and synapse function, we made novel observations about the effects of NSAIDs on Abeta-mediated neural processes. Here we report that the selective inhibition of COX-2, but not COX-1, acutely prevented the suppression of hippocampal long-term plasticity (LTP) by Abeta. The non-selective NSAIDs, ibuprofen and naproxen, and a selective COX-2 inhibitor, MF-tricyclic, each restored memory function in Tg2576 mice over-expressing APP, and also blocked Abeta-mediated inhibition of LTP. There was no advantage of ibuprofen, a selective Abeta42-lowering agent (SALA), over the non-SALAs, naproxen and MF-tricyclic. The beneficial effects on memory did not depend upon lowered levels of Abeta42 or the inflammatory cytokines, tumour necrosis factor alpha (TNF-alpha) and interleukin 1beta (IL-1beta). Intriguingly, improved memory function was inversely related to prostaglandin E2 (PGE2) levels. Conversely, exogenous PGE2 prevented the restorative effects of COX-2 inhibitors on LTP. The data indicate that the inhibition of COX-2 blocks Abeta-mediated suppression of LTP and memory function, and that this block occurs independently of reductions in Abeta42 or decreases in inflammation. The results lead us to propose a third possible mechanism by which NSAIDs may protect against Alzheimer's disease, involving the blockade of a COX-2-mediated PGE2 response at synapses.

Misoprostol, an anti-ulcer agent and PGE2 receptor agonist, protects against cerebral ischemia

Induction of COX-2 activity in cerebral ischemia results in increased neuronal injury and infarct size. Recent studies investigating neurotoxic mechanisms of COX-2 demonstrate both toxic and paradoxically protective effects of downstream prostaglandin receptor signaling pathways. We tested whether misoprostol, a PGE(2) receptor agonist that is utilized clinically as an anti-ulcer agent and signals through the protective PGE(2) EP2, EP3, and EP4 receptors, would reduce brain injury in the murine middle cerebral artery occlusion-reperfusion (MCAO-RP) model. Administration of misoprostol, at the time of MCAO or 2h after MCAO, resulted in significant rescue of infarct volume at 24 and 72h. Immunocytochemistry demonstrated dynamic regulation of the EP2 and EP4 receptors during reperfusion in neurons and endothelial cells of cerebral cortex and striatum, with limited expression of EP3 receptor. EP3-/- mice had no significant changes in infarct volume compared to control littermates. Moreover, administration of misoprostol to EP3+/+ and EP3-/- mice showed similar levels of infarct rescue, indicating that misoprostol protection was not mediated through the EP3 receptor. Taken together, these findings suggest a novel function for misoprostol as a protective agent in cerebral ischemia acting via the PGE(2) EP2 and/or EP4 receptors.

Cyclooxygenase-2 deficiency enhances Th2 immune responses and impairs neutrophil recruitment in hepatic ischemia/reperfusion injury

Cyclooxygenase-2 (COX-2) is a prostanoid-synthesizing enzyme that is critically implicated in a variety of pathophysiological processes. Using a COX-2-deficient mouse model, we present data that suggest that COX-2 has an active role in liver ischemia/reperfusion (I/R) injury. We demonstrate that COX-2-deficient mice had a significant reduction in liver damage after I/R insult. The inability of COX-2(-/-) to elaborate COX-2 products favored a Th2-type response in these mice. COX-2(-/-) livers after I/R injury showed significantly decreased levels of IL-2, as well as IL-12, a cytokine known to have a central role in Th1 effector cell differentiation. Moreover, such livers expressed enhanced levels of the anti-inflammatory cytokine IL-10, shifting the balance in favor of a Th2 response in COX-2-deficient mice. The lack of COX-2 expression resulted in decreased levels of CXCL2, a neutrophil-activating chemokine, reduced infiltration of MMP-9-positive neutrophils, and impaired late macrophage activation in livers after I/R injury. Additionally, Bcl-2 and Bcl-x(L) were normally expressed in COX-2(-/-) livers after injury, whereas respective wild-type controls were almost depleted of these two inhibitors of cell death. In contrast, caspase-3 activation and TUNEL-positive cells were depressed in COX-2(-/-) livers. Therefore, our data support the concept that COX-2 is involved in the pathogenic events occurring in liver I/R injury. The data also suggest that potential valuable therapeutic approaches in liver I/R injury may result from further studies aimed at identifying specific COX-2-derived prostanoid pathways.

Antiinflammatory and neuroprotective actions of COX2 inhibitors in the injured brain

Overexpression of COX2 appears to be both a marker and an effector of neural damage after a variety of acquired brain injuries, and in natural or pathological aging of the brain. COX2 inhibitors may be neuroprotective in the brain by reducing prostanoid and free radical synthesis, or by directing arachidonic acid down alternate metabolic pathways. The arachidonic acid shunting hypothesis proposes that COX2 inhibitors' neuroprotective effects may be mediated by increased formation of potentially beneficial eicosanoids. Under conditions where COX2 activity is inhibited, arachidonic acid accumulates or is converted to eicosanoids via lipoxygenases and cytochrome P450 (CYP) epoxygenases. Several P450 eicosanoids have been demonstrated to have beneficial effects in the brain and/or periphery. We suspect that arachidonic acid shunting may be as important to functional recovery after brain injuries as altered prostanoid formation per se. Thus, COX2 inhibition and arachidonic acid shunting have therapeutic implications beyond the suppression of prostaglandin synthesis and free radical formation.

Nuclear factor-kappaB activation and postischemic inflammation are suppressed in CD36-null mice after middle cerebral artery occlusion

CD36, a class-B scavenger receptor involved in multiple functions, including inflammatory signaling, may also contribute to ischemic brain injury through yet unidentified mechanisms. We investigated whether CD36 participates in the molecular events underlying the inflammatory reaction that accompanies cerebral ischemia and may contribute to the tissue damage. We found that activation of nuclear factor-kappaB, a transcription factor that coordinates postischemic gene expression, is attenuated in CD36-null mice subjected to middle cerebral artery occlusion. The infiltration of neutrophils and the glial reaction induced by cerebral ischemia were suppressed. Treatment with an inhibitor of inducible nitric oxide synthase, an enzyme that contributes to the tissue damage, reduced ischemic brain injury in wild-type mice, but not in CD36 nulls. In contrast to cerebral ischemia, the molecular and cellular inflammatory changes induced by intracerebroventricular injection of interleukin-1beta were not attenuated in CD36-null mice. The findings unveil a novel role of CD36 in early molecular events leading to nuclear factor-kappaB activation and postischemic inflammation. Inhibition of CD36 signaling may be a valuable therapeutic approach to counteract the deleterious effects of postischemic inflammation.

Expression of enzymes involved in the prostanoid metabolism by cortical astrocytes after LPS-induced inflammation

Neuroinflammatory processes are a common epiphenomenon for a number of neurological and neurodegenerative diseases. Besides microglia, astrocytes are implicated in brain inflammation in response to harmful stimuli and pathological processes. Bacterial endotoxins can induce the synthesis and release of proinflammatory mediators, i.e., cytokines and chemokines, by astroglia. In this study, we have investigated the effect of lipopolysaccharide (LPS) treatment on the expression of enzymes of prostanoid synthesis and degradation in cultured mouse cortical astrocytes using an Affymetrix Gene Chip array, quantitative reverse transcriptase polymerase chain reaction (RT-PCR), and an enzyme-immunosorbent assay. LPS treatment induced an upregulation of enzymes responsible for prostaglandin E2 synthesis, a downregulation of enzymes that catalyzes prostaglandin E2 (PGE2) degradation and production of proinflammatory leukotrienes. Changes in enzyme expression were accompanied by a highly significant increase in extracellular PGE2. Our data demonstrate that astrocytes are directly involved in the complex regulation of proinflammatory prostanoids in the CNS under pathological processes, thus being of potential interest as targets for therapeutical interventions. Further studies are required to unravel the different roles and interactions between astroglia and other cells of the brain-intrinsic innate immune system during inflammation.

Seizure-induced up-regulation of P-glycoprotein at the blood-brain barrier through glutamate and cyclooxygenase-2 signaling

Increased expression of drug efflux transporters at the blood-brain barrier accompanies epileptic seizures and complicates therapy with antiepileptic drugs. This study is concerned with identifying mechanistic links that connect seizure activity to increased P-glycoprotein expression at the blood-brain barrier. In this regard, we tested the hypothesis that seizures increase brain extracellular glutamate, which signals through an N-methyl-d-aspartate (NMDA) receptor and cyclooxygenase-2 (COX-2) in brain capillaries to increase blood-brain barrier P-glycoprotein expression. Consistent with this hypothesis, exposing isolated rat or mouse brain capillaries to glutamate for 15 to 30 min increased P-glycoprotein expression and transport activity hours later. These increases were blocked by 5H-dibenzo[a,d]cyclohepten-5,10-imine (dizocilpine maleate) (MK-801), an NMDA receptor antagonist, and by celecoxib, a selective COX-2 inhibitor; no such glutamate-induced increases were seen in brain capillaries from COX-2-null mice. In rats, intracerebral microinjection of glutamate caused locally increased P-glycoprotein expression in brain capillaries. Moreover, using a pilocarpine status epilepticus rat model, we observed seizure-induced increases in capillary P-glycoprotein expression that were attenuated by administration of indomethacin, a COX inhibitor. Our findings suggest that brain uptake of some antiepileptic drugs can be enhanced through COX-2 inhibition. Moreover, they provide insight into one mechanism that underlies drug resistance in epilepsy and possibly other central nervous system disorders.

Involvement of endogenous prostaglandin F2alpha on kainic acid-induced seizure activity through FP receptor: the mechanism of proconvulsant effects of COX-2 inhibitors

COX-2 and prostaglandins (PGs) might play important roles in epilepsy. In kainic acid-induced seizures, the brain largely increases PGD(2), first from COX-1 and later COX-2-induced PGF(2alpha). Pre-treatment with COX-2 inhibitors such as indomethacin, nimesulide, and celecoxib is known to aggravate kainic acid (KA)-induced seizure activity. However it is not known whether the proconvulsant effect of those non-steroidal anti-inflammatory drugs (NSAIDs) is due to changes in endogenous prostaglandins (PGs), or what types of PGs are involved. The purpose of this study was to determine the effect of intracisternally administered PGs on KA-induced seizures aggravated by pre- or post-treatment with COX-2 inhibitors. Systemic KA injection (10 mg/kg i.p.) in mice evoked mild seizure activity within 15 min. PGs were administrated intracisternally 20 min prior to KA administration. COX inhibitors (indomethacin, nimesulide, and ketoprofen, 10 mg/kg i.p.) were injected 1 h before or 15 min after KA. An additional COX-2 inhibitor, celecoxib, was administered orally. Intracisternally administered PGF(2alpha) (700 ng), but not PGD(2) (700 ng) or PGE(2) (700 ng) completely alleviated KA-induced seizures potentiated by COX-2 inhibitors, and also reduced KA-induced hippocampal neuronal death aggravated by indomethacin. PGF(2alpha) alone did not affect KA-induced seizures. However, an FP receptor antagonist, AL 8810 (10 or 50 ng) which is an 11beta-fluoro analogue of PGF(2alpha) potentiated KA-induced seizure activity dose-dependently. In summary, pre- or post-treatment with COX-2 inhibitors aggravates KA-induced seizures, which suggests to change the endogenous PGF(2alpha). Seizure-induced PGF(2alpha) might act as an endogenous anticonvulsant through FP receptors.

Neuroprotection by PGE2 receptor EP1 inhibition involves the PTEN/AKT pathway

The prostanoid synthesizing enzyme cyclooxygenase-2 (COX-2) is involved in the mechanisms of cerebral ischemia, an effect mediated by prostaglandin E2 through activation of EP1 receptors. Thus, inhibition of EP1 receptors is neuroprotective in models of ischemic stroke, but the molecular mechanisms of the effect have not been fully elucidated. We used oxygen glucose deprivation (OGD) in hippocampal slices as an injury model to investigate whether the neuroprotection afforded by EP1 receptor inhibition involves the PI3K/AKT survival pathway. EP1 receptor inhibition with SC51089 or SC51322 reduced the hippocampal damage produced by ODG by 28+/-2% and 32+/-3%, respectively (p<0.05). OGD induced a transient reduction of AKT activity that was partly counteracted by SC51089. LY294002 blocked the increase in phospho-AKT evoked by SC51089 and abolished the associated protective effect. The AKT activation induced by SC51089 was associated with phosphorylation of PTEN, the phosphatase that negatively regulates AKT. Furthermore, SC51089 attenuated the mitochondrial translocation of the proapoptotic protein BAD. These data indicate that EP1 receptor inhibition improves the survival of hippocampal slices by preventing the attenuation in AKT activity induced by OGD, and by reducing the mitochondrial translocation of BAD. The findings provide evidence for a link between EP1 receptors and the PI3K/AKT survival pathway and shed light on the molecular mechanisms of the prosurvival effect of EP1 receptor inhibition.

HIV-1 envelope glycoprotein 120 induces cyclooxygenase-2 expression in astrocytoma cells through a nuclear factor-kappaB-dependent mechanism

Human immunodeficiency virus-1 gp120 alters astroglial function, which compromises the function of the nearby of neuronal cells contributing to the cognitive impairment in human immunodeficiency virus-1 infection. Cyclooxygenase (COX)-2 has been involved in this process, although the intracellular pathways and second messengers involved are yet unknown. We have investigated the role of gp120-induced COX-2 in the astrocytoma human cell line U-87, and the different pathways involved in this activation. COX-2 mRNA and protein expression were detected in gp120-stimulated cells. Moreover, gp120 induces COX-2 promoter transcription. The effect of gp120 was abrogated by a neutralizing antibody against the chemokine receptor CXCR4 neutralizing antibody. Analysis of the promoter show that deletion or mutation of a proximal nuclear factor (NF)-kappaB site completely abrogated gp120-dependent transcription. NF-kappaB but neither Activating protein-1 nor nuclear factor of activated T-cells-dependent transcription was induced by gp120, as shown by reporter and electrophoretic mobility shift assays. In addition, transfection assays with the NF-kappaB inhibitor, IkappaBalpha, prevented gp120-mediated COX-2 induction. In contrast, there was no inhibition of COX-2 promoter transcription by expressing a dominant negative c-Jun, or nuclear factor of activated T-cells constructs. The antioxidant pyrrolidine dithiocarbamate inhibited COX-2 protein expression and COX-2 transcriptional activity induced by gp120. Thus, our results indicate that gp120 induced COX-2 transcription through NF-kappaB activation in astrocytoma cells.

Altered GABAergic neurotransmission is associated with increased kainate-induced seizure in prostaglandin-endoperoxide synthase-2 deficient mice

Excitotoxicity involves over activation of brain excitatory glutamate receptors and has been implicated in neurological, neurodegenerative and neuropsychiatric diseases. Metabolism of arachidonic acid (AA) through the phospholipase A(2) (PLA(2))/prostaglandin-endoperoxide synthase (PTGS) pathway is increased after excitotoxic stimulation. However, the individual roles of the PTGS isoforms in this process are not well established. We assessed the role of the PTGS isoforms in the process of excitotoxicity by exposing mice deficient in either PTGS-1 (PTGS-1(-/-)) or PTGS-2 (PTGS-2(-/-)) to the prototypic excitotoxin, kainic acid (KA). Seizure intensity and neuronal damage were significantly elevated in KA-exposed PTGS-2(-/-), but not in PTGS-1(-/-), mice. The increased susceptibility was not associated with an alteration in KA receptor binding activity or mediated through the CB1 endocannabinoid receptor. The frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) was decreased in the CA1 pyramidal neurons of PTGS-2(-/-) mice, suggesting an alteration of GABAergic function. In wild-type mice, six weeks treatment with the PTGS-2 selective inhibitor celecoxib recapitulated the increased susceptibility to KA-induced excitotoxicity observed in PTGS-2(-/-) mice, further supporting the role of PTGS-2 in the excitotoxic process. The increased susceptibility to KA was also associated with decreased brain levels of PGE(2), a biomarker of PTGS-2 activity. Our results suggest that PTGS-2 activity and its specific products may modulate neuronal excitability by affecting GABAergic neurotransmission. Further, inhibition of PTGS-2, but not PTGS-1, may increase the susceptibility to seizures.

Chronic carbamazepine administration reduces N-methyl-D-aspartate receptor-initiated signaling via arachidonic acid in rat brain

BACKGROUND

Lithium and carbamazepine (CBZ) are used to treat mania in bipolar disorder. When given chronically to rats, both agents reduce arachidonic acid (AA) turnover in brain phospholipids and downstream AA metabolism. Lithium in rats also attenuates brain N-methyl-D-aspartic acid receptor (NMDAR) signaling via AA. We hypothesized that, like chronic lithium, chronic CBZ administration to rats would reduce NMDAR-mediated signaling via AA.

METHODS

We used our fatty acid method with quantitative autoradiography to image the regional brain incorporation coefficient k* of AA, a marker of AA signaling, in unanesthetized rats that had been given 25 mg/kg/day I.P. CBZ or vehicle for 30 days, then injected with NMDA (25 mg/kg I.P.) or saline. We also measured brain concentrations of two AA metabolites, prostaglandin E(2) (PGE(2)) and thromboxane B(2) (TXB(2)).

RESULTS

In chronic vehicle-treated rats, NMDA compared with saline increased k* significantly in 69 of 82 brain regions examined, but did not change k* significantly in any region in CBZ-treated rats. In vehicle- but not CBZ-treated rats, NMDA also increased brain concentrations of PGE(2) and TXB(2).

CONCLUSIONS

Chronic CBZ administration to rats blocks increments in the AA signal k*, and in PGE(2) and TXB(2) concentrations that are produced by NMDA in vehicle-treated rats. The clinical action of antimanic drugs might involve inhibition of brain NMDAR-mediated signaling involving AA and its metabolites.

Resveratrol potently reduces prostaglandin E2 production and free radical formation in lipopolysaccharide-activated primary rat microglia

Background

Neuroinflammatory responses are triggered by diverse ethiologies and can provide either beneficial or harmful results. Microglial cells are the major cell type involved in neuroinflammation, releasing several mediators, which contribute to the neuronal demise in several diseases including cerebral ischemia and neurodegenerative disorders. Attenuation of microglial activation has been shown to confer protection against different types of brain injury. Recent evidence suggests that resveratrol has anti-inflammatory and potent antioxidant properties. It has been also shown that resveratrol is a potent inhibitor of cyclooxygenase (COX)-1 activity. Previous findings have demonstrated that this compound is able to reduce neuronal injury in different models, both in vitro and in vivo. The aim of this study was to examine whether resveratrol is able to reduce prostaglandin E₂ (PGE₂) and 8-iso-prostaglandin F_2α (8-iso-PGF_2α) production by lipopolysaccharide (LPS)-activated primary rat microglia.

Methods

Primary microglial cell cultures were prepared from cerebral cortices of neonatal rats. Microglial cells were stimulated with 10 ng/ml of LPS in the presence or absence of different concentrations of resveratrol (1–50 μM). After 24 h incubation, culture media were collected to measure the production of PGE₂ and 8-iso-PGF_2α using enzyme immunoassays. Protein levels of COX-1, COX-2 and microsomal prostaglandin E synthase-1 (mPGES-1) were studied by Western blotting after 24 h of incubation with LPS. Expression of mPGES-1 at the mRNA level was investigated using reverse transcription-polymerase chain reaction (RT-PCR) analysis.

Results

Our results indicate that resveratrol potently reduced LPS-induced PGE₂ synthesis and the formation of 8-iso-PGF_2α, a measure of free radical production. Interestingly, resveratrol dose-dependently reduced the expression (mRNA and protein) of mPGES-1, which is a key enzyme responsible for the synthesis of PGE₂ by activated microglia, whereas resveratrol did not affect the expression of COX-2. Resveratrol is therefore the first known inhibitor which specifically prevents mPGES-1 expression without affecting COX-2 levels. Another important observation of the present study is that other COX-1 selective inhibitors (SC-560 and Valeroyl Salicylate) potently reduced PGE₂ and 8-iso-PGF_2α production by LPS-activated microglia.

Conclusion

These findings suggest that the naturally occurring polyphenol resveratrol is able to reduce microglial activation, an effect that might help to explain its neuroprotective effects in several in vivo models of brain injury.

Cannabinoid activation of PPAR alpha; a novel neuroprotective mechanism

BACKGROUND AND PURPOSE

Although CB(1) receptor activation evokes neuroprotection in response to cannabinoids, some cannabinoids have been reported to be peroxisome proliferator activated receptor (PPAR) ligands, offering an alternative protective mechanism. We have, therefore, investigated the ability of a range of cannabinoids to activate PPAR alpha and for N-oleoylethanolamine (OEA), an endogenous cannabinoid-like compound (ECL), to evoke neuroprotection.

EXPERIMENTAL APPROACH

Assays of PPAR alpha occupancy and gene transactivation potential were conducted in cell-free and transfected HeLa cell preparations, respectively. In vivo estimates of PPAR alpha activation through fat mobilization and gene transcription were conducted in mice. Neuroprotection in vivo was investigated in wild-type and PPAR alpha gene-disrupted mice.

KEY RESULTS

The ECLs OEA, anandamide, noladin ether and virodhamine were found to bind to the purified PPAR alpha ligand binding domain and to increase PPAR alpha-driven transcriptional activity. The high affinity synthetic CB(1/2) cannabinoid agonist WIN 55212-2 bound to PPAR alpha equipotently with the PPARalpha agonist fenofibrate, and stimulated PPARalpha-mediated gene transcription. The phytocannabinoid delta 9 tetrahydrocannabinol was without effect. OEA and WIN 55212-2 induced lipolysis in vivo, while OEA pre-treatment reduced infarct volume from middle cerebral artery occlusion in wild-type, but not in PPAR alpha-null mice. OEA treatment also led to increased expression of the NFkappa B-inhibitory protein, Ikappa B, in mouse cerebral cortex, while expression of the NFkappa B-regulated protein COX-2 was inhibited.

CONCLUSIONS AND IMPLICATIONS

These data demonstrate the potential for a range of cannabinoid compounds, of diverse structures, to activate PPAR alpha and suggest that at least some of the neuroprotective properties of these agents could be mediated by nuclear receptor activation.

Neuronal Cyclooxygenase-2 Activity and Prostaglandins PGE2, PGD2, and PGF2α Exacerbate Hypoxic Neuronal Injury in Neuron-enriched Primary Culture

Cyclooxygenase-2 (COX-2) activity has been implicated in the pathogenesis of cerebral ischemia. To determine whether COX-2 activity within the neuron itself exacerbates hypoxic neuronal injury, neuron-enriched cultures were subjected to anoxia. Treatment with COX-2 selective antagonists decreased cell death. Neurons cultured from homozygous COX-2 gene disrupted mice were resistant to hypoxia compared to those of heterozygotes. Infection of primary neurons with AAV expressing COX-2 exacerbated cell death compared to neurons infected with enhanced green fluorescent protein (EGFP) control vector. Addition of PGE2, PGD2 or PGF2 alpha to the medium exacerbated injury, suggesting that the deleterious effects of COX-2 overexpression in hypoxia could be mediated by direct receptor mediated effects of prostaglandins. Overexpression of COX-2 did not increase expression of cyclin D1 or phosphoretinoblastoma protein (pRb), or cleavage of caspase 3 suggesting that this cell cycle mechanism does not mediate COX-2 toxicity in this model.

CEREBROVASCULAR EFFECTS OF OESTROGEN: MULTIPLICITY OF ACTION

1. Cerebral vessels express oestrogen receptors (ER) in both the smooth muscle and endothelial cell layers of cerebral blood vessels. Levels of ERalpha are higher in female rats chronically exposed to oestrogen, either endogenous or exogenous. 2. Chronic exposure to oestrogen, either endogenous (normally cycling females) or exogenous (ovariectomized with oestrogen replacement), results in cerebral arteries that are more dilated than arteries from ovariectomized counterparts when studied in vitro. This effect is primarily mediated by an increase in the production of vasodilator factors, including nitric oxide (NO) and prostacylin. In contrast, oestrogen appears to suppress the production of endothelial-derived hyperpolarizing factor. Oestrogen treatment increases cerebrovascular levels of endothelial nitric oxide synthase (eNOS), cyclo-oxygenase (COX)-1 and prostacyclin synthase. In addition, via activation of the phosphatidylinositol 3-kinase/Akt pathway, both acute and chronic oestrogen exposure increases eNOS phosphorylation, increasing NO production. 3. Oestrogen receptors have also been localized to cerebrovascular mitochondria and exposure to oestrogen increases the efficiency of energy production while simultaneously reducing mitochondrial production of reactive oxygen species. Oestrogen increases the production of mitochondrial proteins encoded by both mitochondrial and nuclear DNA, including cytochrome c, subunits I and IV of complex IV and Mn-superoxide dismutase. Oestrogen treatment increases the activity of citrate synthase and complex IV and decreases mitochondrial production of H(2)O(2). 4. Oestrogen also has potent anti-inflammatory effects in the cerebral circulation that may have important implications for the incidence and severity of cerebrovascular disease. Administration of lipopolysaccharide or interleukin-1beta to ovariectomized female rats induces cerebrovascular COX-2 and inducible nitric oxide synthase (iNOS) protein expression and increases prostaglandin E(2) expression. Levels of COX-2 and iNOS expression vary with the stage of the oestrous cycle, and the cerebrovascular inflammatory response is suppressed in ovariectomized animals treated with oestrogen. Interleukin-1beta induction of COX-2 protein is prevented by treatment with a nuclear factor (NF)-kappaB inhibitor, and oestrogen treatment reduces cerebrovascular NF-kappaB activity. 5. Cerebrovascular dysfunction and pathology contribute to the pathogenesis of stroke, brain trauma, oedema and dementias, such as Alzheimer's disease. A better understanding of the action of oestrogen on cerebrovascular function holds promise for the development of new therapeutic entities that could be useful in preventing or treating a wide variety of cerebrovascular diseases.

iNOS-derived NO and nox2-derived superoxide confer tolerance to excitotoxic brain injury through peroxynitrite

Sublethal injurious stimuli induce tolerance to subsequent lethal insults, a phenomenon termed preconditioning. Inducible nitric oxide synthase (iNOS) is essential for the preconditioning induced by transient bilateral common carotid artery occlusion (BCCAO) or by systemic administration of the endotoxin lipopolysaccharide (LPS). We used a model of brain injury produced by neocortical injection of N-methyl-D-aspartate (NMDA) to investigate the mechanisms by which iNOS-derived nitric oxide (NO) contributes to tolerance induced by LPS or BCCAO. We found that the tolerance is blocked by the iNOS inhibitor aminoguanidine, is not observed in iNOS-null mice, and is rescued by the NO donor DTPA NONOate. Lipopolysaccharide failed to induce preconditioning in mice lacking the nox2 subunit of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, suggesting that superoxide derived from NADPH oxidase is needed for the induction of the tolerance. Because superoxide reacts with NO to form peroxynitrite, we investigated the role of peroxynitrite. We found that LPS induces the peroxynitrite marker 3-nitrotyrosine in cortical neurons and that the peroxynitrite decomposition catalyst FeTPPS abolishes LPS-induced preconditioning. These results suggest that the protective effect of iNOS-derived NO is mediated by peroxynitrite formed by the reaction of NO with NADPH oxidase-derived superoxide. Thus, peroxynitrite, in addition to its well-established deleterious role in ischemic brain injury and neurodegeneration, can also be beneficial by inducing tolerance to excitotoxicity.

Statins: multiple mechanisms of action in the ischemic brain

Although substantial epidemiological studies have failed to find a correlation between cholesterol levels and stroke, clinical trials have shown that HMG-CoA reductase inhibitors (or statins, the most potent hypocholesterolemic drugs available) greatly reduce the incidence of stroke. These clinical observations have opened the way to a number of studies of the non-cholesterol-dependent (or pleiotropic) effects in animal models of stroke, indicating that the neuroprotection is attributable to multiple activities. One of the main protective mechanisms elicited by statin administration is the increase in nitric oxide bioavailability that regulates cerebral perfusion and improves endothelial function, but others include antioxidant properties, the inhibition of inflammatory responses, immunomodulatory actions, the regulation of progenitor cells, and the stabilization of atherosclerotic plaques. Many of these effects are due to the inhibited synthesis of isoprenoid intermediates, which serve as lipid attachments for a variety of intracellular signaling molecules. This article describes the mechanisms involved in the neuroprotective effects of statins.

Roles of prostaglandin synthesis in excitotoxic brain diseases

Cyclooxygenase (COX) is a rate-limiting enzyme in prostaglandin synthesis. COX consists of two isoforms, constitutive COX-1 and inducible COX-2. We have first found that COX-2 expression in the brain is tightly regulated by neuronal activity under physiological conditions, and electroconvulsive seizure robustly induces COX-2 mRNA in the brain. Our recent in-depth studies reveal COX-2 expression is divided into two phases, early in neurons and late in non-neuronal cells, such as endothelial cells or astrocytes. In this review, we present that early synthesized COX-2 facilitates the recurrence of hippocampal seizures in rapid kindling model, and late induced COX-2 stimulates hippocampal neuron loss after kainic acid treatment. Hence, we consider the potential role of COX-2 inhibitors as a new therapeutic drug for a neuronal loss after seizure or focal cerebral ischemia. The short-term and sub-acute medication of selective COX-2 inhibitors that suppresses an elevation of prostaglandin E(2) (PGE(2)) may be an effective treatment to prevent neuronal loss after onset of neuronal excitatory diseases. This review also discusses a novel role of vascular endothelial cells in brain diseases. We found that these cells produce PGE(2) by synthesizing COX-2 and microsomal prostaglandin E synthase-1 (mPGES-1) in response to excitotoxicity and neuroinflammation. We also show a possible mechanisms of neuronal damage associated with seizure via astrocytes and endothelial cells. Further analysis of the interaction among neurons, astrocytes and endothelial cells may provide a better understanding of the processes of neuropathological disorders, as well as facilitating the development of new treatments.

PGD(2) DP1 receptor protects brain from ischemia-reperfusion injury

Prostaglandin D(2) is the most abundant prostaglandin in the brain. It has long been described as a modulator of the neuroinflammatory process, but little is known regarding the role of its Galpha(s)-coupled receptor, DP1. Therefore, in this study, the effect of the DP1 receptor on the outcome of cerebral ischemia in wildtype (WT) and DP1 knockout (DP1(-/-)) C57Bl/6 mice was investigated. Ischemia-reperfusion injury was produced by a 90-min occlusion of the right middle cerebral artery followed by a 4-day reperfusion. Infarct size was 49.0 +/- 11.0% larger in DP1(-/-) mice (n = 11; P < 0.01) than in WT mice (n = 9 per group). However, no differences were detected in the relative cerebral blood flow (CBF) or any of the physiological parameters measured (n = 5 per group) or in the large blood vessel anatomy (n = 3 per group). To further address whether the DP1 protective role in the brain could be extended to neurons, mouse primary corticostriatal neuronal cultures were exposed to the DP1-selective agonist, BW245C, which provided dose-dependent protection against excitotoxicity induced by glutamate. Protection was significant at a dose as low as 0.05 microm. The results indicate that the DP1 receptor is neuroprotective in both in vivo and in vitro paradigms. Development of drugs to stimulate the DP1 receptor in brain could provide a new therapeutic strategy against cerebral ischemia and potentially other neurological conditions.

COX-2 oxidative metabolite of endocannabinoid 2-AG enhances excitatory glutamatergic synaptic transmission and induces neurotoxicity

Neuroinflammation has been implicated in the pathogenesis of neurodegenerative diseases. Cyclooxygenase-2 (COX-2), an inducible enzyme converting arachidonic acid (AA) to prostaglandins, is the key player in neuroinflammation. It has been long thought that the COX-2-mediated neuronal injury/degeneration is attributed to the increased production of AA-derived prostaglandins. Recent studies show that endogenous cannabinoid 2-arachidonoylglycerol (2-AG) is a natural substrate for COX-2, and it can be oxygenated by COX-2 to form prostaglandin glyceryl esters. In this study, we demonstrate that prostaglandin E(2) glyceryl ester (PGE(2)-G), a major COX-2 oxidative metabolite of 2-arachidonoylglycerol, enhanced hippocampal glutamatergic synaptic transmission indicated by the increased frequency of miniature excitatory post-synaptic currents, and induced neuronal injury/death revealed by the terminal transferase dUTP nick end labeling staining and caspase 3 activation. The actions of PGE(2)-G are not mediated via a cannabinoid receptor 1, but mediated through ERK, p38 mitogen-activated protein kinase, IP(3), and NF-kappaB signal transduction pathways. In addition, the PGE(2)-G-induced neurotoxicity is attenuated by blockade of the NMDA receptors. Our results suggest that the COX-2 oxidative metabolism of endocannabinoids is an important mechanism contributing to the inflammation-induced neurodegeneration.

Neuroprotection and neuroplasticity – A holistic approach and future perspectives

Modern medicine is facing an increasing number of treatments available for vascular and neurodegenerative brain diseases, but no causal or neuroprotective treatment has yet been established. Almost all neurological conditions are characterized by progressive neuronal disfunction, which, regardless of the pathogenetic mechanism, finally leads to neuronal death. Many agents that proved neuroprotective in experimental studies failed in achieving this goal within clinical studies. This paper briefly reviews the latest etiopathogenetic theories regarding nervous system disorders and the most important endeavors in neuroprotection.