Brain Arachidonic Acid Cascade Research Articles

Lithium and the other mood stabilizers effective in bipolar disorder target the rat brain arachidonic acid cascade.

This Review evaluates the arachidonic acid (AA, 20:4n-6) cascade hypothesis for the actions of lithium and other FDA-approved mood stabilizers in bipolar disorder (BD). The hypothesis is based on evidence in unanesthetized rats that chronically administered lithium, carbamazepine, valproate, or lamotrigine each downregulated brain AA metabolism, and it is consistent with reported upregulated AA cascade markers in post-mortem BD brain. In the rats, each mood stabilizer reduced AA turnover in brain phospholipids, cyclooxygenase-2 expression, and prostaglandin E2 concentration. Lithium and carbamazepine also reduced expression of cytosolic phospholipase A2 (cPLA2) IVA, which releases AA from membrane phospholipids, whereas valproate uncompetitively inhibited in vitro acyl-CoA synthetase-4, which recycles AA into phospholipid. Topiramate and gabapentin, proven ineffective in BD, changed rat brain AA metabolism minimally. On the other hand, the atypical antipsychotics olanzapine and clozapine, which show efficacy in BD, decreased rat brain AA metabolism by reducing plasma AA availability. Each of the four approved mood stabilizers also dampened brain AA signaling during glutamatergic NMDA and dopaminergic D2 receptor activation, while lithium enhanced the signal during cholinergic muscarinic receptor activation. In BD patients, such signaling effects might normalize the neurotransmission imbalance proposed to cause disease symptoms. Additionally, the antidepressants fluoxetine and imipramine, which tend to switch BD depression to mania, each increased AA turnover and cPLA2 IVA expression in rat brain, suggesting that brain AA metabolism is higher in BD mania than depression. The AA hypothesis for mood stabilizer action is consistent with reports that low-dose aspirin reduced morbidity in patients taking lithium, and that high n-3 and/or low n-6 polyunsaturated fatty acid diets, which in rats reduce brain AA metabolism, were effective in BD and migraine patients.

Chronic clozapine reduces rat brain arachidonic acid metabolism by reducing plasma arachidonic acid availability

Chronic administration of mood stabilizers to rats down-regulates the brain arachidonic acid (AA) cascade. This down-regulation may explain their efficacy against bipolar disorder (BD), in which brain AA cascade markers are elevated. The atypical antipsychotics, olanzapine (OLZ) and clozapine (CLZ), also act against BD. When given to rats, both reduce brain cyclooxygenase activity and prostaglandin E(2) concentration; OLZ also reduces rat plasma unesterified and esterified AA concentrations, and AA incorporation and turnover in brain phospholipid. To test whether CLZ produces similar changes, we used our in vivo fatty acid method in rats given 10 mg/kg/day i.p. CLZ, or vehicle, for 30 days; or 1 day after CLZ washout. [1-(14) C]AA was infused intravenously for 5 min, arterial plasma was collected and high-energy microwaved brain was analyzed. CLZ increased incorporation coefficients ki * and decreased [corrected] rates J(in,i) of plasma unesterified AA into brain phospholipids. [corrected]. These effects disappeared after washout. Thus, CLZ and OLZ similarly down-regulated kinetics and cyclooxygenase expression of the brain AA cascade, likely by reducing plasma unesterified AA availability. Atypical antipsychotics and mood stabilizers may be therapeutic in BD by down-regulating, indirectly or directly respectively, the elevated brain AA cascade of that disease.

Gabapentin's minimal action on markers of rat brain arachidonic acid metabolism agrees with its inefficacy against bipolar disorder

In rats, FDA-approved mood stabilizers used for treating bipolar disorder (BD) selectively downregulate brain markers of the arachidonic acid (AA) cascade, which are upregulated in postmortem BD brain. Phase III clinical trials show that the anticonvulsant gabapentin (GBP) is ineffective in treating BD. We hypothesized that GBP would not alter the rat brain AA cascade. Chronic GBP (10mg/kg body weight, injected i.p. for 30 days) compared to saline vehicle did not significantly alter brain expression or activity of AA-selective cytosolic phospholipase A2 (cPLA2) IVA or secretory (s)PLA2 IIA, activity of cyclooxygenase-2, or prostaglandin E2 or thromboxane B2 concentrations. Plasma esterified and unesterified AA concentration was unaffected. These results, taken with evidence of an upregulated AA cascade in the BD brain and that approved mood stabilizers downregulate the rat brain AA cascade, support the hypothesis that effective anti-BD drugs act by targeting the brain AA cascade whereas ineffective drugs (such as GBP) do not target this pathway, and suggest that the rat model might be used for screening new anti-BD drugs.

Altered neuroinflammatory, arachidonic acid cascade and synaptic markers in postmortem Alzheimer's disease brain.

Alzheimer's disease (AD), a progressive neurodegenerative disorder, is the leading cause of dementia in the elderly. A recent positron emission tomography imaging study demonstrated upregulated brain arachidonic acid (AA) metabolism in AD patients. Further, a mouse model of AD shows an increase in AA-releasing cytosolic phospholipase A2 (cPLA2) in brain, and a reduction in cPLA2 activity ameliorated cognitive deficits. These observations led us to hypothesize that there is an upregulation of AA cascade and neuroinflammatory markers in the brain of AD patients. To test this hypothesis, we measured protein and mRNA levels of AA cascade, neuroinflammatory and synaptic markers in postmortem frontal cortex from 10 AD patients and 10 age-matched controls. Consistent with our hypothesis, AD frontal cortex showed significant increases in protein and mRNA levels of cPLA2-IVA, secretory sPLA2-IIA, cyclooxygenase-1 and -2, membrane prostaglandin (PG) synthase-1 and lipoxygenase-12 and -15. Calcium-independent iPLA2-VIA and cytosolic PGE2 synthase were decreased. In addition, interleukin-1β, tumor necrosis factor-α, glial fibrillary acidic protein and CD11b were increased. AD postmortem brain also showed signs of cellular injury, including decreased synaptophysin and drebrin, pre- and postsynaptic markers. These results indicate that increased AA cascade and inflammatory markers could contribute to AD pathology. Altered brain AA cascade enzymes could be considered therapeutic targets for future drug development.

Time-dependent changes in the brain arachidonic acid cascade during cuprizone-induced demyelination and remyelination

Phospholipases A 2 (PLA 2) are the enzymatic keys for the activation of the arachidonic acid (AA) cascade and the subsequent synthesis of pro-inflammatory prostanoids (prostaglandins and tromboxanes). Prostanoids play critical roles in the initiation and modulation of inflammation and their levels have been reported increased in several neurological and neurodegenerative disorders, including multiple sclerosis (MS). Here, we aimed to determine whether brain expression PLA 2 enzymes and the terminal prostagland in levels are changed during cuprizone-induced demyelination and in the subsequent remyelination phase. Mice were given the neurotoxicant cuprizone through the diet for six weeks to induce brain demyelination. Then, cuprizone was withdrawn and mice were returned to a normal diet for 6 weeks to allow spontaneous remyelination. We found that after 4–6 weeks of cuprizone, sPLA 2(V) and cPLA 2, but not iPLA 2(VI), gene expression was upregulated in the cortex, concomitant with an increase in the expression of astrocyte and microglia markers. Cyclooxygenase (COX)-2 gene expression was consistently upregulated during all the demyelination period, whereas COX-1 sporadically increased only at week 5 of cuprizone exposure. However, we found that at the protein level only sPLA 2(V) and COX-1 were elevated during demyelination, with COX-1 selectively expressed by activated and infiltrated microglia/macrophages and astrocytes. Levels of PGE 2, PGD 2, PGI 2 and TXB 2 were also increased during demyelination. During remyelination, none of the PLA 2 isoforms was significantly changed, whereas COX-1 and -2 were sporadically upregulated only at the gene expression level. PGE 2, PGI 2 and PGD 2 levels returned to normal, whereas TXB 2 was still upregulated after 3 weeks of cuprizone withdrawal. Our study characterizes for the first time time-dependent changes in the AA metabolic pathway during cuprizone-induced demyelination and the subsequent remyelination and suggests that sPLA 2(V) is the major isoform contributing to AA release.

Brain arachidonic acid uptake and turnover: implications for signaling and bipolar disorder

Arachidonic acid was first detected in the brain in 1922. Although earlier work examined the role of arachidonic acid in growth and development, more recent advancements have elucidated roles for arachidonic acid in brain health and disease. In this review, we summarize evidence demonstrating that unesterified arachidonic acid in the plasma pool, which is supplied in part from adipose, is readily taken up and incorporated into brain phospholipids. By labeling plasma unesterified arachidonic acid, it is possible to trace the subsequent release of arachidonic acid from brain phospholipids upon neuroreceptor-mediated release by phospholipase A2 in response to drugs and neuroinflammation in rodents. With the synthesis of 11C labeled fatty acids, brain arachidonic acid signaling can now be measured in humans with position emission tomography. Arachidonic acid signals are known to regulate important biological functions, including neuroinflammation and excitotoxicity, and we focus on how the brain arachidonic acid cascade is a common target of drugs used to treat bipolar disorder (e.g. lithium, carbamazepine and valproate). A better understanding of the regulation of arachidonic acid uptake into the brain and the brain arachidonic acid cascade could lead to new imaging techniques and the identification of novel therapeutic targets in excitotoxicity, neuroinflammation and bipolar disorder.

Altered arachidonic acid cascade enzymes in postmortem brain from bipolar disorder patients

Mood stabilizers that are approved for treating bipolar disorder (BD), when given chronically to rats, decrease expression of markers of the brain arachidonic metabolic cascade, and reduce excitotoxicity and neuroinflammation-induced upregulation of these markers. These observations, plus evidence for neuroinflammation and excitotoxicity in BD, suggest that arachidonic acid (AA) cascade markers are upregulated in the BD brain. To test this hypothesis, these markers were measured in postmortem frontal cortex from 10 BD patients and 10 age-matched controls. Mean protein and mRNA levels of AA-selective cytosolic phospholipase A(2) (cPLA(2)) IVA, secretory sPLA(2) IIA, cyclooxygenase (COX)-2 and membrane prostaglandin E synthase (mPGES) were significantly elevated in the BD cortex. Levels of COX-1 and cytosolic PGES (cPGES) were significantly reduced relative to controls, whereas Ca(2+)-independent iPLA(2)VIA, 5-, 12-, and 15-lipoxygenase, thromboxane synthase and cytochrome p450 epoxygenase protein and mRNA levels were not significantly different. These results confirm that the brain AA cascade is disturbed in BD, and that certain enzymes associated with AA release from membrane phospholipid and with its downstream metabolism are upregulated. As mood stabilizers downregulate many of these brain enzymes in animal models, their clinical efficacy may depend on suppressing a pathologically upregulated cascade in BD. An upregulated cascade should be considered as a target for drug development and for neuroimaging in BD.

Brain Arachidonic Acid Cascade Enzymes are Upregulated in a Rat Model of Unilateral Parkinson Disease

Arachidonic acid (AA) signaling is upregulated in the caudate-putamen and frontal cortex of unilaterally 6-hydroxydopamine (6-OHDA) lesioned rats, a model for asymmetrical Parkinson disease. AA signaling can be coupled to D(2)-like receptor initiated AA hydrolysis from phospholipids by cytosolic phospholipase A(2) (cPLA(2)) and subsequent metabolism by cyclooxygenase (COX)-2. In unilaterally 6-OHDA- and sham-lesioned rats, we measured brain expression of cPLA(2), other PLA(2) enzymes, and COX-2. Activity and protein levels of cPLA(2) were significantly higher as was COX-2-protein in caudate-putamen, frontal cortex and remaining brain on the lesioned compared to intact side of the 6-OHDA lesioned rats, and compared to sham brain. Secretory sPLA(2) and Ca(2+)-independent iPLA(2) expression did not differ between sides or groups. Thus, the tonically increased ipsilateral AA signal in the lesioned rat corresponds to upregulated cPLA(2) and COX-2 expression within the AA metabolic cascade, which may contribute to symptoms and pathology in Parkinson disease.

Is the brain arachidonic acid cascade a common target of drugs used to manage bipolar disorder?

Although lithium has been used therapeutically to treat patients with bipolar disorder for over 50 years, its mechanism of action, as well as that of other drugs used to treat bipolar disorder, is not agreed upon. In the present paper, I review studies in unanaesthetized rats using a neuropharmacological approach, combined with kinetic, biochemical and molecular biology techniques, demonstrating that chronic administration of three commonly used mood stabilizers (lithium, valproic acid and carbamazepine), at therapeutically relevant doses, selectively target the brain arachidonic acid cascade. Upon chronic administration, lithium and carbamazepine decrease the binding activity of activator protein-2 and, in turn, the transcription, translation and activity of its arachidonic acid-selective calcium-dependent phospholipase A(2) gene product, whereas chronic valproic acid non-competitively inhibits long-chain acyl-CoA synthetase. The net overlapping effects of the three mood stabilizers are decreased turnover of arachidonic acid, but not of docosahexaenoic acid, in rat brain phospholipids, as well as decreased brain cyclo-oxygenase-2 and prostaglandin E(2). As an extension of this theory, drugs that are thought to induce switching to mania, especially when administered during bipolar depression (fluoxetine and imipramine), up-regulate enzymes of the arachidonic acid cascade and turnover of arachidonic acid in rat brain phospholipids. Future basic and clinical studies on the arachidonic acid hypothesis of bipolar disorder are warranted.

Chronic Administration of Lamotrigine Downregulates COX-2 mRNA and Protein in Rat Frontal Cortex

Chronic administration to rats of mood-stabilizers that are effective against mania in bipolar disorder, is reported to downregulate markers of the brain arachidonic acid cascade. We hypothesized that chronic administration of lamotrigine, which is used to treat depression and rapid cycling in bipolar disorder, might do so as well. Male CDF rats were administered a therapeutically relevant dose of lamotrigine (10 mg/kg) or vehicle intragastrically once daily for 42 days. Protein levels of isoforms of phospholipase A(2) (PLA(2)) and of cyclooxygenase (COX), and the mRNA level of COX-2, were quantified in the frontal cortex using immunoblotting and RT-PCR, respectively. Compared to vehicle-treated rats, chronic lamotrigine significantly decreased frontal cortex protein and mRNA levels of COX-2 without altering protein levels of the PLA(2) isoforms. Consistent with the hypothesis, lamotrigine and other mood-stabilizers have a common downregulatory action on COX-2 expression in rat brain, which may account in part for their efficacy in bipolar disorder.

Antimanic therapies target brain arachidonic acid signaling: Lessons learned about the regulation of brain fatty acid metabolism

Bipolar disorder is a major medical, social and economic burden worldwide. However, the biochemical basis of the disorder and the mechanisms of action of effective antibipolar disorder drugs remain elusive. In this paper, we review how combining a kinetic approach to studying the turnover of fatty acids within brain phospholipids of unanesthetized rats along with chronic administration of antimanic drugs (lithium, valproate and carbamazepine) at therapeutically relevant doses, shows that the brain arachidonic acid cascade is a common target of these drugs. The overlapping effects of the three drugs are decreased turnover of arachidonic acid but not of docosahexaenoic acid in rat brain phospholipids, and decreased brain cyclooxygenase-2 and prostaglandin E 2. Whereas lithium and carbamazepine target the transcription of the arachidonic acid-selective calcium-dependent cytosolic phospholipase A 2, valproate is a non-competitive inhibitor of an arachidonic acid-selective acyl-CoA synthetase. Two potential models of bipolar disorder, chronic N-methyl- d-aspartate and n-3 polyunsaturated fatty acid deprivation, opposite to the antimanic drugs, increase the turnover and markers of the arachidonic acid cascade in rat brain. These observations support the hypothesis proposed by Rapoport and colleagues that the arachidonic acid cascade is a common target of mood stabilizers and that by targeting substrate-specific enzymes the turnover of individual fatty acids can be regulated within the brain.

Roles of brain phosphatidylinositol-specific phospholipase C and diacylglycerol lipase in centrally administered histamine-induced adrenomedullary outflow in rats

Recently, we reported that intracerebroventricularly (i.c.v.) administered histamine evokes the secretion of noradrenaline and adrenaline from adrenal medulla by brain cyclooxygenase-1- and thromboxane A2-mediated mechanisms in rats. These results suggest the involvement of brain arachidonic acid cascade in the histamine-induced activation of the central adrenomedullary outflow. Arachidonic acid is released mainly by phospholipase A2 (PLA2)-dependent pathway or phospholipase C (PLC)/diacylglycerol lipase-dependent pathway. In the present study, histamine (27 nmol/animal, i.c.v.) -induced elevation of plasma noradrenaline and adrenaline was dose-dependently reduced by U-73122 (PLC inhibitor) (10 and 100 nmol/animal, i.c.v.), ET-18-OCH3 (phosphatidylinositol-specific PLC inhibitor) (10 and 30 nmol/animal, i.c.v.) and RHC-80267 (diacylglycerol lipase inhibitor) (1.3 and 2.6 micromol/animal, i.c.v.). However, mepacrine (PLA2 inhibitor) (1.1 and 2.2 micromol/animal, i.c.v.) and D609 (phosphatidylcholine-specific PLC inhibitor) (30, 100 and 300 nmol/animal, i.c.v.) had no effect. These results suggest the involvement of brain phosphatidylinositol-specific PLC and diacylglycerol lipase in the centrally administered histamine-induced activation of the adrenomedullary outflow in rats.

Cyclooxygenase‐1 and ‐2 enzymes differentially regulate the brain upstream NF‐κB pathway and downstream enzymes involved in prostaglandin biosynthesis

We have recently reported that cyclooxygenase (COX)-2-deficiency affects brain upstream and downstream enzymes in the arachidonic acid (AA) metabolic pathway to prostaglandin E2 (PGE2), as well as enzyme activity, protein and mRNA levels of the reciprocal isozyme, COX-1. To gain a better insight into the specific roles of COX isoforms and characterize the interactions between upstream and downstream enzymes in brain AA cascade, we examined the expression and activity of COX-2 and phospholipase A2 enzymes (cPLA2 and sPLA2), as well as the expression of terminal prostaglandin E synthases (cPGES, mPGES-1, and - 2) in wild type and COX-1(-/-) mice. We found that brain PGE2 concentration was significantly increased, whereas thromboxane B2 (TXB2) concentration was decreased in COX-1(-/-) mice. There was a compensatory up-regulation of COX-2, accompanied by the activation of the NF-kappaB pathway, and also an increase in the upstream cPLA2 and sPLA2 enzymes. The mechanism of NF-kappaB activation in the COX-1(-/-) mice involved the up-regulation of protein expression of the p50 and p65 subunits of NF-kappaB, as well as the increased protein levels of phosphorylated IkappaBalpha and of phosphorylated IKKalpha/beta. Overall, our data suggest that COX-1 and COX-2 play a distinct role in brain PG biosynthesis, with basal PGE2 production being metabolically coupled with COX-2 and TXB2 production being preferentially linked to COX-1. Additionally, COX-1 deficiency can affect the expression of reciprocal and coupled enzymes, COX-2, Ca2+ -dependent PLA2, and terminal mPGES-2, to overcome defects in brain AA cascade.

Valproic acid selectively inhibits conversion of arachidonic acid to arachidonoyl–CoA by brain microsomal long-chain fatty acyl–CoA synthetases: relevance to bipolar disorder

Several drugs used to treat bipolar disorder (lithium and carbamazepine), when administered chronically to rats, reduce the turnover of arachidonic acid, but not docosahexaenoic acid, in brain phospholipids by decreasing the activity of an arachidonic acid-selective phospholipase A(2). Although chronic valproic acid produces similar effects on brain arachidonic acid and docosahexaenoic acid turnover, it does not alter phospholipase A(2) activity, suggesting that it targets a different enzyme in the turnover pathway. By isolating rat brain microsomal long-chain fatty acyl-CoA synthetases (Acsl), we show in vitro that valproic acid is a non-competitive inhibitor of Acsl, as it reduces the maximal velocity of the reaction without changing the affinity of the substrate for the enzyme. While valproic acid inhibited the synthesis of arachidonoyl-CoA, palmitoyl-CoA, and docosahexaenoyl-CoA, the K (i )for inhibition of arachidonoyl-CoA synthesis (14.1 mM) was approximately one fifth the K (i) for inhibiting palmitoyl-CoA (85.4 mM) and docosahexaenoyl-CoA (78.2 mM) synthesis. As chronic administration of valproic acid in bipolar disorder achieves whole-brain levels of 1.0 to 1.5 mM, inhibition of arachidonoyl-CoA formation can occur at brain concentrations that are therapeutically relevant to this disease. Furthermore, brain microsomal Acsl did not produce valproyl-CoA. This study shows that valproic acid acts as a non-competitive inhibitor of brain microsomal Acsl, and that inhibition is substrate-selective. The study supports the hypothesis that valproic acid acts in bipolar disorder by reducing the brain arachidonic acid cascade, by inhibiting arachidonoyl-CoA formation.

Do lithium and anticonvulsants target the brain arachidonic acid cascade in bipolar disorder?

Lithium and certain anticonvulsants, including carbamazepine and valproic acid, are effective antimanic drugs for treating bipolar disorder, but their mechanisms of action remain uncertain. Feeding rats lithium chloride for 6 weeks, to produce a brain lithium concentration of 0.7mM, reduced arachidonic acid turnover within brain phospholipids by 75%. The effect was highly specific, as turnover rates of docosahexaenoic acid and palmitic acid were unaffected. Arachidonate turnover in rat brain also was reduced by long-term valproic acid administration. Lithium's reduction of arachidonate turnover corresponded to its down-regulating gene expression and enzyme activity of cytosolic phospholipase A(2), an enzyme that selectively liberates arachidonic but not docosahexaenoic acid from phospholipids. Lithium also reduced the brain protein level and activity of cyclooxygenase 2, as well as the brain concentration of prostaglandin E(2), an arachidonate metabolite produced via cyclooxygenase 2. These results give rise to the hypothesis that lithium and antimanic anticonvulsants act by targeting parts of the "arachidonic acid cascade," which may be functionally hyperactive in mania. Thus, drugs that target enzymes in the cascade, such as cyclooxygenase 2 inhibitors, might be candidate treatments for mania. Also, in view of competition between arachidonic and docosahexaenoic acids in a number of functional processes, docosahexaenoic acid or its precursors would be expected to be therapeutic. Neither of these predictions is evident from other current hypotheses for the antimanic action of lithium and anticonvulsant drugs.

Role of brain arachidonic acid cascade on central CRF 1 receptor-mediated activation of sympatho-adrenomedullary outflow in rats

The present experiments were designed to characterize the mechanisms involved in the corticotropin releasing factor (CRF)-induced activation of central sympatho-adrenomedullary outflow in rats. Intracerebroventricularly (i.c.v.) administered CRF and urocortin (0.5, 1.5 and 3.0 nmol/animal) effectively and dose-dependently elevated plasma levels of adrenaline and noradrenaline, and the effect of urocortin was almost the same as that of CRF. The elevation of catecholamines induced by CRF and urocortin (1.5 nmol/animal) was reduced by CP-154,526(butyl-ethyl-{2,5-dimethyl-7-(2,4,6trimethylphenyl)-7 H-pyrrolo [2,3- d] pyrimidin-4-yl]amine), a selective CRF 1 receptor antagonist, in a dose dependent manner (1.2 and/or 2.4 μmol/animal, i.c.v.), and abolished by indomethacin (1.2 μmol/animal, i.c.v.), an inhibitor of cyclooxygenase. Furegrelate (1.8 μmol/animal, i.c.v.), an inhibitor of thromboxane A 2 synthase, abolished the CRF-induced elevation of adrenaline, but had no effect on the evoked release of noradrenaline. These results suggest that activation of brain CRF 1 receptor facilitates the central sympathetic and adrenomedullary outflow in distinct central pathways in rats: brain thromboxane A 2 is involved in the central adrenomedullary outflow; an active metabolite of arachidonic acid other than thromboxane A 2 (probably prostaglandin E 2) may be involved in the central sympathetic outflow.