Prostaglandin H synthase-2 and cytosolic phospholipase A2 in the hypoxic-ischemic brain: role in neuronal death or survival?

Brain Cytosolic Phospholipase A2: Localization, Role, and Involvement in Neurological Diseases

Cytosolic phospholipase A2 (cPLA2) hydrolyzes the arachidonoyl group from the sn-2 position of glycerophospholipids generating arachidonic acid and lysophospholipids. The products of the cPLA2-catalyzed reaction act as second messengers themselves or further metabolize to eicosanoids, platelet activating factor, and lysophosphatidic acid. cPLA2 has not been purified from brain tissue. Immunocytochemical studies have indicated that cPLA2 is expressed in neurons and astrocytes. The hindbrain and spinal cord contain dense immunoreactivity for cPLA2. Activity and immunoreactivity of cPLA2 are markedly increased in ischemia, Alzheimer’s disease, and kainic acid neurotoxicity. This increase in cPLA2 activity and immunoreactivity is accompanied by marked alterations in neural membrane phospholipid composition and the accumulation of lipid peroxides and eicosanoids. At present, it is not known whether the increased activity and immunoreactivity of cPLA2 in neural trauma (e.g., in ischemia) and neurodegenerative disease (Alzheimer’s disease) is the cause or effect of neurodegeneration. Recent studies on the role of this enzyme in brain tissue suggest that cPLA2 may be involved in synaptic plasticity, generation of second messengers, axon regeneration, and neurodegeneration.

Effect of acute administration of Pistacia lentiscus L. essential oil on rat cerebral cortex following transient bilateral common carotid artery occlusion

BACKGROUND

Ischemia/reperfusion leads to inflammation and oxidative stress which damages membrane highly polyunsaturated fatty acids (HPUFAs) and eventually induces neuronal death. This study evaluates the effect of the administration of Pistacia lentiscus L. essential oil (E.O.), a mixture of terpenes and sesquiterpenes, on modifications of fatty acid profile and endocannabinoid (eCB) congener concentrations induced by transient bilateral common carotid artery occlusion (BCCAO) in the rat frontal cortex and plasma.

METHODS

Adult Wistar rats underwent BCCAO for 20 min followed by 30 min reperfusion (BCCAO/R). 6 hours before surgery, rats, randomly assigned to four groups, were gavaged either with E.O. (200 mg/0.45 ml of sunflower oil as vehicle) or with the vehicle alone.

RESULTS

BCCAO/R triggered in frontal cortex a decrease of docosahexaenoic acid (DHA), the membrane highly polyunsaturated fatty acid most susceptible to oxidation. Pre-treatment with E.O. prevented this change and led further to decreased levels of the enzyme cyclooxygenase-2 (COX-2), as assessed by Western Blot. In plasma, only after BCCAO/R, E.O. administration increased both the ratio of DHA-to-its precursor, eicosapentaenoic acid (EPA), and levels of palmytoylethanolamide (PEA) and oleoylethanolamide (OEA).

CONCLUSIONS

Acute treatment with E.O. before BCCAO/R elicits changes both in the frontal cortex, where the BCCAO/R-induced decrease of DHA is apparently prevented and COX-2 expression decreases, and in plasma, where PEA and OEA levels and DHA biosynthesis increase. It is suggested that the increase of PEA and OEA plasma levels may induce DHA biosynthesis via peroxisome proliferator-activated receptor (PPAR) alpha activation, protecting brain tissue from ischemia/reperfusion injury.

Neuroprotective effect of apolipoprotein D against human coronavirus OC43-induced encephalitis in mice

Apolipoprotein D (apoD) is a lipocalin upregulated in the nervous system after injury or pathologies such as Alzheimer's disease, Parkinson's disease, and multiple sclerosis. We previously demonstrated that apoD protects against neuropathology by controlling the level of peroxidated lipids. Here, we further investigated the biological function of apoD in a mouse model of acute encephalitis. Our results show that apoD transcript and protein are upregulated during acute encephalitis induced by the human coronavirus OC43 (HCoV-OC43) infection. The apoD upregulation coincides with glial activation, and its expression returns to normal levels when the virus is cleared, concomitantly to a resolved glial reactivity. In addition, the overexpression of human apoD in the neurons of Thy-1/ApoD transgenic mice results in a threefold increase of the number of mice surviving to HCoV-OC43 infection. This increased survival rate is correlated with an upregulated glial activation associated with a limited innate immune response (cytokines, chemokines) and T-cell infiltration into infected brains. Moreover, the protection seems to be associated with a restricted phospholipase A2 activity. These data reveal a role for apoD in the regulation of inflammation and suggest that it protects from HCoV-OC43-induced encephalitis, most likely through the phospholipase A2 signaling pathways.

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.

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.

Cyclooxygenases, lipoxygenases, and epoxygenases in CNS: Their role and involvement in neurological disorders

Three enzyme systems, cyclooxygenases that generate prostaglandins, lipoxygenases that form hydroxy derivatives and leukotrienes, and epoxygenases that give rise to epoxyeicosatrienoic products, metabolize arachidonic acid after its release from neural membrane phospholipids by the action of phospholipase A(2). Lysophospholipids, the other products of phospholipase A(2) reactions, are either reacylated or metabolized to platelet-activating factor. Under normal conditions, these metabolites play important roles in synaptic function, cerebral blood flow regulation, apoptosis, angiogenesis, and gene expression. Increased activities of cyclooxygenases, lipoxygenases, and epoxygenases under pathological situations such as ischemia, epilepsy, Alzheimer's disease, Parkinson disease, amyotrophic lateral sclerosis, and Creutzfeldt-Jakob disease produce neuroinflammation involving vasodilation and vasoconstriction, platelet aggregation, leukocyte chemotaxis and release of cytokines, and oxidative stress. These are closely associated with the neural cell injury which occurs in these neurological conditions. The metabolic products of docosahexaenoic acid, through these enzymes, generate a new class of lipid mediators, namely docosatrienes and resolvins. These metabolites antagonize the effect of metabolites derived from arachidonic acid. Recent studies provide insight into how these arachidonic acid metabolites interact with each other and other bioactive mediators such as platelet-activating factor, endocannabinoids, and docosatrienes under normal and pathological conditions. Here, we review present knowledge of the functions of cyclooxygenases, lipoxygenases, and epoxygenases in brain and their association with neurodegenerative diseases.

Phospholipase A2, reactive oxygen species, and lipid peroxidation in cerebral ischemia

Ischemic stroke is caused by obstruction of blood flow to the brain, resulting in energy failure that initiates a complex series of metabolic events, ultimately causing neuronal death. One such critical metabolic event is the activation of phospholipase A2 (PLA2), resulting in hydrolysis of membrane phospholipids and release of free fatty acids including arachidonic acid, a metabolic precursor for important cell-signaling eicosanoids. PLA2 enzymes have been classified as calcium-dependent cytosolic (cPLA2) and secretory (sPLA2) and calcium-independent (iPLA2) forms. Cardiolipin hydrolysis by mitochondrial sPLA2 disrupts the mitochondrial respiratory chain and increases production of reactive oxygen species (ROS). Oxidative metabolism of arachidonic acid also generates ROS. These two processes contribute to formation of lipid peroxides, which degrade to reactive aldehyde products (malondialdehyde, 4-hydroxynonenal, and acrolein) that covalently bind to proteins/nucleic acids, altering their function and causing cellular damage. Activation of PLA2 in cerebral ischemia has been shown while other studies have separately demonstrated increased lipid peroxidation. To the best of our knowledge no study has directly shown the role of PLA2 in lipid peroxidation in cerebral ischemia. To date, there are very limited data on PLA2 protein by Western blotting after cerebral ischemia, though some immunohistochemical studies (for cPLA2 and sPLA2) have been reported. Dissecting the contribution of PLA2 to lipid peroxidation in cerebral ischemia is challenging due to multiple forms of PLA2, cardiolipin hydrolysis, diverse sources of ROS arising from arachidonic acid metabolism, catecholamine autoxidation, xanthine oxidase activity, mitochondrial dysfunction, activated neutrophils coupled with NADPH oxidase activity, and lack of specific inhibitors. Although increased activity and expression of various PLA2 isoforms have been demonstrated in stroke, more studies are needed to clarify the cellular origin and localization of these isoforms in the brain, their responses in cerebral ischemic injury, and their role in oxidative stress.

Estrogen/hypothalamus-pituitary-adrenal axis interactions in the fetus: The interplay between placenta and fetal brain

OBJECTIVE

The hormonal interactions between the placenta and the fetal hypothalamus-pituitary-adrenal (HPA) axis are reviewed.

METHODS

This review addresses data obtained from the chronically catheterized fetal sheep, drawing relevant comparisons to human fetuses.

RESULTS

In the sheep, and perhaps in primate species, parturition is initiated by an increase in the activity of the HPA axis. The endogenous mechanisms underlying the increase in activity of the fetal HPA axis are incompletely understood but might involve an interplay between placenta and fetal hypothalamus and pituitary. Various hypotheses have been proposed, involving placental secretion of prostaglandins and various components of the fetal HPA axis. In the sheep, the influence of estradiol appears to be potent, and various experiments have suggested the possibility that, in late gestation, there exists a positive feedback relationship between placental estrogen secretion and pituitary adrenocorticotropin (ACTH) secretion. Estradiol circulates in concentrations known to stimulate fetal ACTH secretion. Additionally, estradiol circulates in the form of estradiol-3-sulfate, a molecular form that is taken up by the fetal brain and deconjugated by steroid sulfatase, which is expressed in the fetal brain. Recent evidence suggests that the interaction between estradiol and ACTH might involve production of paracrine or autocrine substances in the fetal brain. One candidate mediator is prostaglandin E2 (PGE2), highlighted by the action of estradiol on the expression of prostaglandin endoperoxide synthase-2 (PGHS-2 or COX-2) in brain regions known to be important for controlling HPA activity.

CONCLUSION

Estradiol, secreted by the placenta in increasing amounts in late gestation, is a potent stimulator of fetal ACTH secretion. The interactions between estradiol and the fetal HPA axis might function as a positive feedback loop that increases the concentrations of both hormones before birth.

Phospholipase A2 in the central nervous system

Phospholipase A2 (PLA2) belongs to a family of enzymes that catalyze the cleavage of fatty acids from the sn-2 position of phospholipids. There are more than 19 different isoforms of PLA2 in the mammalian system, but recent studies have focused on three major groups, namely, the group IV cytosolic PLA2, the group II secretory PLA2 (sPLA2), and the group VI Ca(2+)-independent PLA2. These PLA2s are involved in a complex network of signaling pathways that link receptor agonists, oxidative agents, and proinflammatory cytokines to the release of arachidonic acid (AA) and the synthesis of eicosanoids. PLA2s acting on membrane phospholipids have been implicated in intracellular membrane trafficking, differentiation, proliferation, and apoptotic processes. All major groups of PLA2 are present in the central nervous system (CNS). Therefore, this review is focused on PLA2 and AA release in neural cells, especially in astrocytes and neurons. In addition, because many neurodegenerative diseases are associated with increased oxidative and inflammatory responses, an attempt was made to include studies on PLA2 in cerebral ischemia, Alzheimer's disease, and neuronal injury due to excitotoxic agents. Information from these studies has provided clear evidence for the important role of PLA2 in regulating physiological and pathological functions in the CNS.

A potentially critical role of phospholipases in central nervous system ischemic, traumatic, and neurodegenerative disorders

Phospholipases are a diverse group of enzymes whose activation may be responsible for the development of injury following insult to the brain. Amongst the numerous isoforms of phospholipase proteins expressed in mammals are 19 different phospholipase A2's (PLA2s), classified functionally as either secretory, calcium dependent, or calcium independent, 11 isozymes belonging to three structural groups of PLC, and 3 PLD gene products. Many of these phospholipases have been identified in selected brain regions. Under normal conditions, these enzymes regulate the turnover of free fatty acids (FFAs) in membrane phospholipids affecting membrane stability, fluidity, and transport processes. The measurement of free fatty acids thus provides a convenient method to follow phospholipase activity and their regulation. Phospholipase activity is also responsible for the generation of an extensive list of intracellular messengers including arachidonic acid metabolites. Phospholipases are regulated by many factors including selective phosphorylation, intracellular calcium and pH. However, under abnormal conditions, excessive phospholipase activation, along with a decreased ability to resynthesize membrane phospholipids, can lead to the generation of free radicals, excitotoxicity, mitochondrial dysfunction, and apoptosis/necrosis. This review evaluates the critical contribution of the various phospholipases to brain injury following ischemia and trauma and in neurodegenerative diseases.

Phospholipase A2, hydroxyl radicals, and lipid peroxidation in transient cerebral ischemia

Phospholipid degradation is an important promoter of neuronal death after transient cerebral ischemia. Phospholipid hydrolysis by phospholipase A2 (PLA2) after transient cerebral ischemia releases arachidonic acid. Arachidonic acid metabolism results in formation of reactive oxygen species, lipid peroxides, and toxic aldehydes (malondialdehyde, 4-hydroxynonenal, and acrolein). Citicoline (cytidine-5'-diphosphocholine), an intermediate in phosphatidylcholine synthesis, has undergone 13 phase III clinical trials for stroke, and is being evaluated for treatment of Alzheimer's and Parkinson's diseases. Here we examined the effect of citicoline on PLA2 activity in relationship to attenuating hydroxyl radical (OH*) generation and lipid peroxidation after transient forebrain ischemia in gerbil. High Ca2+ dependency (millimolar range) of PLA2 activity suggests that secretory PLA2 is the predominant isoform in membrane and mitochondria. Citicoline attenuated the increase in PLA2 activity in both membrane and mitochondrial fractions. In vitro, citicoline and its components choline and cytidine had no effect on the PLA2 activity. Thus, citicoline is not a "direct PLA2 inhibitor." Citicoline also significantly attenuated loss of cardiolipin and arachidonic acid release from phosphatidylcholine and phosphatidylethanolamine. Transient cerebral ischemia resulted in significant formation of OH* and malondialdehyde, and citicoline significantly attenuated their formation. These results suggest that citicoline provides neuroprotection by attenuating the stimulation of PLA2.

Citicoline decreases phospholipase A2 stimulation and hydroxyl radical generation in transient cerebral ischemia

Neuroprotection by citicoline (CDP-choline) in transient cerebral ischemia has been demonstrated previously. Citicoline has undergone several Phase III clinical trials for stroke, and is being evaluated for treatment of Alzheimer's and Parkinson's diseases. Phospholipid degradation and generation of reactive oxygen species (ROS) are major factors causing neuronal injury in CNS trauma and neurodegenerative diseases. Oxidative metabolism of arachidonic acid (released by the action of phospholipases) contributes to ROS generation. We examined the effect of citicoline on phospholipase A(2) (PLA(2)) activity in relation to the attenuation of hydroxyl radical (OH.) generation after transient forebrain ischemia of gerbil. PLA(2) activity (requires mM Ca(2+)) increased significantly (P < 0.05) in both membrane (50.2 +/- 2.2 pmol/min/mg protein compared to sham 35.9 +/- 3.2) and mitochondrial fractions (77.0 +/- 1.2 pmol/min/mg protein compared to sham 33.9 +/- 1.2) after cerebral ischemia and 2 hr reperfusion in gerbil, which was significantly attenuated (P < 0.01) by citicoline (membrane, 39.9. +/- 2.2 and mitochondria, 41.9 +/- 3.2 pmol/min/mg protein). In vitro, citicoline and its components cytidine and choline had no effect on PLA(2) activity, and thus citicoline as such is not a PLA(2) inhibitor. Ischemia/reperfusion resulted in significant OH. generation (P < 0.01) and citicoline significantly (P < 0.01) attenuated their formation (expressed as 2,3-dihydroxybenzoic acid/salicylate ratio; ischemia/24 hr reperfusion, 6.30 +/- 0.23; sham, 2.56 +/- 0.27; ischemia/24 hr reperfusion + citicoline, 4.85 +/- 0.35). These results suggest that citicoline affects PLA(2) stimulation and decreases OH. generation after transient cerebral ischemia.

Pathogenesis of hypoxic-ischemic cerebral injury in the term infant: current concepts

Multiple, biochemical cascades contribute to the pathogenesis of neonatal hypoxic-ischemic brain injury. This article summarizes experimental evidence that supports the role of excitatory amino acids, calcium, free radicals, nitric oxide, proinflammatory cytokines, and bioactive lipids. Specific vulnerabilities that distinguish the response of the immature brain from that of the mature brain are highlighted. These include increased susceptibility to excitotoxicity and free radical injury, greater tendency to apoptotic death, and heightened vulnerability of developing oligodendrocytes. Available supportive evidence from human studies is also included. Implications for clinical neuroprotective strategies are discussed.

Cyclooxygenase-2 inhibition protects cultured cerebellar granule neurons from glutamate-mediated cell death

Primary insults to the brain can initiate glutamate release that may result in excitotoxicity followed by neuronal cell death. This secondary process is mediated by both N-methyl-D-aspartate (NMDA) and non-NMDA receptors in vivo and requires new gene expression. Neuronal cyclooxygenase-2 (COX2) expression is upregulated following brain insults, via glutamatergic and inflammatory mechanisms. The products of COX2 are bioactive prostanoids and reactive oxygen species that may play a role in neuronal survival. This study explores the role of neuronal COX2 in glutamate excitotoxicity using cultured cerebellar granule neurons (day 8 in vitro). Treatment with excitotoxic concentrations of glutamate or kainate transiently induced COX2 mRNA (two- and threefold at 6 h, respectively, p < 0.05, Dunnett) and prostaglandin production (five- and sixfold at 30 min, respectively, p < 0.05, Dunnett). COX2 induction peaked at toxic concentrations of these excitatory amino acids. Surprisingly, NMDA, L-quisqualate, and trans-ACPD did not induce COX2 mRNA at any concentration tested. The glutamate receptor antagonist NBQX (5 microM, AMPA/kainate receptor) completely inhibited kainate-induced COX2 mRNA and partially inhibited glutamate-induced COX2 (p < 0.05, Dunnett). Other glutamate receptor antagonists, such as MK-801 (1 microM, NMDA receptor) or MCPG (500 microM, class 1 metabotropic receptors), partially attenuated glutamate-induced COX2 mRNA. These antagonists all reduced steady-state COX2 mRNA (p < 0.05, Dunnett). To determine whether COX2 might be an effector of excitotoxic cell death, cerebellar granule cells were pretreated (24 h) with the COX2-specific enzyme inhibitor, DFU (5,5-dimethyl-3-(3-fluorophenyl)-4-(4-methylsulphonyl) phenyl-2((5)H)-furanone) prior to glutamate challenge. DFU (1 to 1000 nM) completely protected cultured neurons from glutamate-mediated neurotoxicity. Approximately 50% protection from NMDA-mediated neurotoxicity, and no protection from kainate-mediated neurotoxicity was observed. Therefore, glutamate-mediated COX2 induction contributes to excitotoxic neuronal death. These results suggest that glutamate, NMDA, and kainate neurotoxicity involve distinct excitotoxic pathways, and that the glutamate and NMDA pathways may intersect at the level of COX2.

Differential effects of calcium-dependent and calcium-independent phospholipase A(2) inhibitors on kainate-induced neuronal injury in rat hippocampal slices

Brain tissue contains multiple forms of intracellular phospholipase A(2) (PLA(2)) activity that differ from each other in many ways including their response to specific inhibitors. The systemic administration of kainic acid to rats produces a marked increase in cPLA(2) activity in neurons and astrocytes. This is associated with increased lipid peroxidation as evidenced by accumulation of 4-hydroxynonenal (4-HNE) modified proteins. The present study describes the effect of specific inhibitors of Ca(2+)-dependent or Ca(2+)-independent PLA(2) on kainite-induced excitotoxic injury in rat hippocampal slices. Specific inhibitors of Ca(2+)-dependent PLA(2) prevented the decrease of a neuronal marker, GluR1, and increase in cPLA(2) and 4-HNE immunoreactivities in slices treated with kainate. This shows that cPLA(2) plays an important role in kainite-induced neurotoxicity and that cPLA(2) inhibitors can be used to protect hippocampal slices from damage induced by kainate.

Prolonged cyclooxygenase-2 induction in neurons and glia following traumatic brain injury in the rat

Cyclooxygenase-2 (COX2) is a primary inflammatory mediator that converts arachidonic acid into precursors of vasoactive prostaglandins, producing reactive oxygen species in the process. Under normal conditions COX2 is not detectable, except at low abundance in the brain. This study demonstrates a distinctive pattern of COX2 increases in the brain over time following traumatic brain injury (TBI). Quantitative lysate ribonuclease protection assays indicate acute and sustained increases in COX2 mRNA in two rat models of TBI. In the lateral fluid percussion model, COX2 mRNA is significantly elevated (>twofold, p < 0.05, Dunnett) at 1 day postinjury in the injured cortex and bilaterally in the hippocampus, compared to sham-injured controls. In the lateral cortical impact model (LCI), COX2 mRNA peaks around 6 h postinjury in the ipsilateral cerebral cortex (fivefold induction, p < 0.05, Dunnett) and in the ipsilateral and contralateral hippocampus (two- and six-fold induction, respectively, p < 0.05, Dunnett). Increases are sustained out to 3 days postinjury in the injured cortex in both models. Further analyses use the LCI model to evaluate COX2 induction. Immunoblot analyses confirm increased levels of COX2 protein in the cortex and hippocampus. Profound increases in COX2 protein are observed in the cortex at 1-3 days, that return to sham levels by 7 days postinjury (p < 0.05, Dunnett). The cellular pattern of COX2 induction following TBI has been characterized using immunohistochemistry. COX2-immunoreactivity (-ir) rises acutely (cell numbers and intensity) and remains elevated for several days following TBI. Increases in COX2-ir colocalize with neurons (MAP2-ir) and glia (GFAP-ir). Increases in COX2-ir are observed in cerebral cortex and hippocampus, ipsilateral and contralateral to injury as early as 2 h postinjury. Neurons in the ipsilateral parietal, perirhinal and piriform cortex become intensely COX2-ir from 2 h to at least 3 days postinjury. In agreement with the mRNA and immunoblot results, COX2-ir appears greatest in the contralateral hippocampus. Hippocampal COX2-ir progresses from the pyramidal cell layer of the CA1 and CA2 region at 2 h, to the CA3 pyramidal cells and dentate polymorphic and granule cell layers by 24 h postinjury. These increases are distinct from those observed following inflammatory challenge, and correspond to brain areas previously identified with the neurological and cognitive deficits associated with TBI. While COX2 induction following TBI may result in selective beneficial responses, chronic COX2 production may contribute to free radical mediated cellular damage, vascular dysfunction, and alterations in cellular metabolism. These may cause secondary injuries to the brain that promote neuropathology and worsen behavioral outcome.

Gene targets related to phospholipid and fatty acid metabolism in schizophrenia and other psychiatric disorders: an update

Phospholipids make up about 60% of the brain's dry weight and play key roles in many brain signal tranduction mechanisms. A recent review(1)identified the increasing evidence that abnormal phospholipid and related fatty acid metabolism may contribute to illnesses such as schizophrenia, bipolar disorder, depression and attention deficit hyperactivity disorder. This current paper reviews the main pathways of phospholipid metabolism, emphasizing the role of phospholipases of the A2 in signal tranduction processes. It also updates the chromosomal locations of regions likely to be involved in these disorders, and relates these to the known locations of genes directly or indirectly involved in phospholipid and fatty acid metabolism.

Central nervous system regulation of reflex responses to hypotension during fetal life

The ability of the fetus to survive, grow, and successfully complete the transition from fetal to neonatal life is critically dependent on the appropriate regulation of fetal blood pressure, blood volume, and fluid dynamics. This is a short review of the physiological mechanisms controlling the fetal cardiovascular system, focusing mainly on the neural and endocrine elements in the schema of cardiovascular function and control. The fetal cardiovascular system is arranged anatomically to provide for perfusion of the umbilical-placental circulation, the organ of gas exchange of the fetus, and to largely bypass the lungs. Fetal blood volume and pressure, maintained at levels that are appropriate for this function, are influenced by neural and endocrine control mechanisms, which are similar to, but quantitatively different from, the adult animal. Baroreceptors and chemoreceptors located in the carotid sinuses and aortic arch sense changes in blood pressure and blood gases and comprise the afferent limb of the major reflexes that maintain normal fetal blood pressure and volume. Fetal hypotension stimulates reflex decreases in fetal heart rate, which are apparently mediated by chemoreceptor input. Arginine vasopressin responses to hypotension are most likely mediated by baroreceptor input. Recent evidence suggests that the reflex responses to hypotension in the fetus are modulated by paracrine or endocrine factors. For example, baroreceptor or chemoreceptor reflex pathways are modulated by the endogenous production of prostanoids and by the preparturient changes in fetal plasma estrogen concentration.

Immunohistochemical nitrotyrosine distribution in neonatal rat cerebrocortical slices during and after hypoxia

The peroxynitrite contributions to hypoxic damage in brain slices that arise from N-methyl-D-aspartate (NMDA) receptor activation were studied by following the temporal-spatial course of nitrotyrosine (NT) formation during six conditions: hypoxia (pO(2)<5 mmHg) with or without 10 microM MK-801 treatment; with exposure to 10, 100 and 1000 microM NMDA; and no treatment (control). In each experiment, twenty 350-micrometer thick cerebrocortical slices, obtained from the parietal lobes of ten 7-day-old Sprague-Dawley rats, were metabolically recovered and allowed to respire in a well-oxygenated perfusion system. Thirty minutes exposures to hypoxia or NMDA were followed by 2 h of oxygenated reperfusion. MK-801 administration began 15 min prior to hypoxia and was discontinued during reperfusion. Anti-NT serum immunohistochemistry stains in 20-micrometer frozen sections of slices taken during oxygenated reperfusion, after hypoxia or NMDA exposure, were positive in both neurons and endothelial cells. NT-positive neurons were detected sooner after hypoxia than after NMDA exposure, suggesting that mechanisms of superoxide generation were different in both groups. After hypoxia and even more so after NMDA exposure, more intense NT-positive staining was observed in endothelial cells than in neurons. Cell damage after hypoxia was attenuated by MK-801. MK-801 decreased post-hypoxia counts of NT-stained endothelial cells by 78.5% (p<0. 001) and NT-stained neurons by 54.1% (p<0.05). Our findings suggest that NMDA receptor activation in hypoxic brain slices is associated with increased post-hypoxic peroxynitrite production that contributes to acute neuronal death and endothelial cell injury. Peroxynitrite injury to endothelial cells, caused either by increased peroxynitrite from within or from increased vulnerability to peroxynitrite from without, might play an important role in hypoxic-ischemic brain injury and NMDA-induced brain injury.

Cyclooxygenase-2 selective inhibitors aggravate kainic acid induced seizure and neuronal cell death in the hippocampus

Cyclooxygenase-2 (COX-2) in the brain is expressed constitutively and also increased in pathological conditions such as seizure, cerebral ischemia, and inflammation. This study examined the role of COX-2 in kainic acid-induced seizure and in the following neuronal death by using selective inhibitors. Systemic kainate injection (50 mg/kg; i.p.) in mice evoked seizure within 15 min and led to 29% mortality within 2 h. TUNEL-positive neuronal death peaked at 3 days after injection and was prominent in CA(3a) regions of the hippocampus. NS-398 or celecoxib (10 mg/kg, COX-2 selective inhibitor) and indomethacin (5 mg/kg, nonselective inhibitor) exaggerated kainic acid-induced seizure activity and mortality. COX-2 selective inhibitors induced the seizure at earlier onset and more severe mortality within the first hour than indomethacin and aspirin. NS-398 also aggravated kainic acid-induced TUNEL positive neuronal death and decreased Cresyl violet stained viable neurons, and extended lesions to CA(1) and CA(3b). Kainic acid increased the levels of PGD(2), PGF(2a) and PG E(2) in the hippocampus immediately after injection. Indomethacin attenuated the production of basal and kainic acid-induced prostaglandins. In contrast, NS-398 failed to reduce until the first 30 min after kainic acid injection, during which the animals were severely seizured. It has been challenged the endogenous PGs might have anticonvulsant properties. Thus, COX-2 selective inhibitor, including nonselective inhibitor such as indomethacin, aggravated kainic acid-induced seizure activity and the following hippocampal neuronal death even with variable prostaglandin levels.

Ischemia-reperfusion rapidly increases COX-2 expression in piglet cerebral arteries

In the newborn, cyclooxygenase (COX)-derived products play an important role in the cerebrovascular dysfunction after ischemia-reperfusion (I/R). We examined effects of I/R on expression of COX-1 and COX-2 isoforms in large cerebral arteries of anesthetized piglets. The circle of Willis, the basilar, and the middle cerebral arteries were collected from piglets at 0.5-12 h after global ischemia (2.5-10 min, n = 50), hypoxia (n = 3), or hypercapnia (n = 2) and from time-control (n = 19) or untreated animals (n = 7). Tissues were analyzed for COX-1 and COX-2 mRNA and protein using RNase protection assay and immunoblot analysis, respectively. Ischemia increased COX-2 mRNA by 30 min, and maximal levels were reached at 2 h. Hypoxia or hypercapnia had minimal effects on COX-2 mRNA. COX-2 protein levels were also consistently elevated by 8 h after I/R. Increases in COX-2 mRNA or protein were not influenced by pretreatment with either indomethacin (5 mg/kg iv, n = 5) or nitro-L-arginine methyl ester (15 mg/kg iv, n = 7). COX-1 mRNA levels were low in time controls, and ischemic stress had no significant effect on COX-1 expression. Thus ischemic stress leads to relatively rapid, selective induction of COX-2 in cerebral arteries.

Localization of cytosolic phospholipase A2 messenger RNA mainly in neurons in the rat brain

Ca2(+)-sensitive 85,000 mol. wt cytosolic phospholipase A2 plays an essential role in the selective and stimulus-dependent release of arachidonic acid from membrane phospholipids. Cytosolic phospholipase A2-catalysed lipid mediators including arachidonic acid and its metabolites have been suggested to be involved in a variety of neuronal functions in the CNS. Since the cellular localization of cytosolic phospholipase A2 is still controversial and obscure, we tried an improved method of rapid processing of each specimens and succeeded in obtaining intense signals of cytosolic phospholipase A2 messenger RNA in the normal rat brain by northern blot analysis and in situ hybridization. Northern blot analysis showed the abundant distribution of cytosolic phospholipase A2 messenger RNA in most regions of the brain, with intense signals observed in the pineal gland and pons. Macroautoradiographs prepared after in situ hybridization with three different antisense riboprobes gave essentially similar patterns of localization; significant signals were widely detected in the gray matter of various regions, i.e. the olfactory bulb, cerebral cortex, hippocampus, amygdala, several thalamic and hypothalamic nuclei and cerebellum. Microautoradiographs showed that most of the intense signals were predominant in neurons, and that faint signals were from glial cells and other non-neuronal cells in the choroid plexus, inner surface cells of veins and the leptomeninges. In addition, the cycloheximide treatment increased the cytosolic phospholipase A2 messenger RNA level in the same cell populations originally possessing messenger RNA signals. Predominant expression of cytosolic phospholipase A2 messenger RNA in neurons may provide the basis for the contribution of cytosolic phospholipase A2-catalysed lipid mediators to a variety of neurotransmission and synaptic functions in the CNS.

Upregulation of cytosolic phospholipase A2 correlates with apoptosis in mouse superior cervical and dorsal root ganglia neurons

The involvement of cytosolic phospholipase A2 (cPLA2) in apoptosis of adult mouse superior cervical and dorsal root ganglia neurons has been investigated by the use of immunohistochemistry for cPLA2 and DNA nick-end labeling for apoptotic cells, respectively. cPLA2 immunoreactivity was strongly upregulated in neurons of both preparations during in vitro culturing. By double labeling it was unequivocally demonstrated that cPLA2 was present and upregulated only in neurons undergoing apoptosis. A similar picture emerged when cPLA2 immunoreactivity was compared with staining with Fluoro-Jade, a novel fluorochrome marker for neuronal degeneration. The preferential presence of cPLA2 in apoptotic and degenerating cells suggests that the enzyme is important for some mechanism involved in or intimately coupled to neuronal cell death.

Neuronal death and survival in two models of hypoxic-ischemic brain damage

Two unilateral hypoxic-ischemia (HI) models (moderate and severe) in immature rat brain have been used to investigate the role of various transcription factors and related proteins in delayed neuronal death and survival. The moderate HI model results in an apoptotic-like neuronal death in selectively vulnerable regions of the brain while the more severe HI injury consistently produces widespread necrosis resulting in infarction, with some necrosis resistant cell populations showing evidence of an apoptotic type death. In susceptible regions undergoing an apoptotic-like death there was not only a prolonged induction of the immediate early genes, c-jun, c-fos and nur77, but also of possible target genes amyloid precursor protein (APP751) and CPP32. In contrast, increased levels of BDNF, phosphorylated CREB and PGHS-2 were found in cells resistant to the moderate HI insult suggesting that these proteins either alone or in combination may be of importance in the process of neuroprotection. An additional feature of both the moderate and severe brain insults was the rapid activation and/or proliferation of glial cells (microglia and astrocytes) in and around the site of damage. The glial response following HI was associated with an upregulation of both the CCAAT-enhancer binding protein alpha (microglia only) and NFkappaB transcription factors.

Activity and injury-dependent expression of inducible transcription factors, growth factors and apoptosis-related genes within the central nervous system

This review primarily discusses work that has been performed in our laboratories and that of our direct collaborators and therefore does not represent an exhaustive review of the current literature. Our aim is to further discuss the role that gene expression plays in neuronal plasticity and pathology. In the first part of this review we examine activity-dependent changes in the expression of inducible transcription factors (ITFs) and neurotrophins with long-term potentiation (LTP) and kindling. This work has identified particular ITFs (Krox-20 and Krox-24) and neurotrophin systems (particularly the brain-derived neurotrophic factor (BDNF)/tyrosine receptor kinase-B, Trk-B system) that may be involved in stabilizing long-lasting LTP (i.e. LTP3). We also show that changes in the expression of other ITFs (Fos, Jun-D and Krox-20) and the BDNF/trkB neurotrophin system may play a central role in the development of hippocampal kindling, an animal model of human temporal lobe epilepsy. In the next part of this review we examine changes in gene expression after neuronal injuries (ischemia, prolonged seizure activity and focal brain injury) and after nerve transection (axotomy). We identify apoptosis-related genes (p53, c-Jun, Bax) whose delayed expression selectively increases in degenerating neurons, further suggesting that some forms of neuronal death may involve apoptosis. Moreover, since overexpression of the tumour-suppressor gene p53 induces apoptosis in a wide variety of dividing cell types we speculate that it may perform the same function in post-mitotic neurons following brain injuries. Additionally, we show that neuronal injury is associated with rapid, transient, activity-dependent expression of neurotrophins (BDNF and activinA) in neurons, contrasting with a delayed and more persistent injury-induced expression of certain growth factors (IGF-1 and TGFbeta) in glia. In this section we also describe results linking ITFs and neurotrophic factor expression. Firstly, we show that while BDNF and trkB are induced as immediate-early genes following injury, the injury-induced expression of activinA and trkC may be regulated by ITFs. We also discuss whether loss of retrograde transport of neurotrophic factors such as nerve growth factor following nerve transection triggers the selective and prolonged expression of c-Jun in axotomized neurons and whether c-Jun is responsible for regeneration or degeneration of these axotomized neurons. In the last section we further examine the role that gene expression may play in memory formation, epileptogenesis and neuronal degeneration, lastly speculating whether the expression of various growth factors after brain injury represents an endogenous neuroprotective response of the brain to injury. Here we discuss our results which show that pharmacological enhancement of this response with exogenous application of IGF-1 or TGF-beta reduces neuronal loss after brain injury.