Chronic lithium administration potentiates brain arachidonic acid signaling at rest and during cholinergic activation in awake rats

What is the Role of Lithium in Epilepsy?

Lithium is a well-known FDA-approved treatment for bipolar and mood disorders. Lithium has been an enigmatic drug with multifaceted actions involving various neurotransmitters and intricate cell signalling cascades. Recent studies highlight the neuroprotective and neurotrophic actions of lithium in amyotrophic lateral sclerosis, Alzheimer's disease, intracerebral hemorrhage, and epilepsy. Of note, lithium holds a significant interest in epilepsy, where the past reports expose its non-specific proconvulsant action, followed lately by numerous studies for anti-convulsant action. However, the exact mechanism of action of lithium for any of its effects is still largely unknown. The present review integrates findings from several reports and provides detailed possible mechanisms of how a single molecule exhibits marked pro-epileptogenic as well as anti-convulsant action. This review also provides clarity regarding the safety of lithium therapy in epileptic patients.

Polyunsaturated fatty acids as predictors of future suicide attempt

Polyunsaturated fatty acids (PUFAs) and cholesterol are lipids implicated in suicide risk. We prospectively studied plasma glycerophospholipid PUFAs and cholesterol as putative predictors of suicide attempts. In a multicenter cohort study, we enrolled 123 patients admitted to the emergency department (ED) for suicidal ideation or suicide attempt. Clinical assessments were performed, with follow-up telephone evaluations 6, 12, 18, and 24 months later. Blood samples were obtained in the ED and assayed for PUFAs. Using survival analysis, suicide events were not predicted by eicosapentaenoic acid (EPA, HR: -0.83, 95%CI: 0.39-1.76, p = 0.621) or docosahexaenoic acid (DHA, HR: -0.60, 95%CI: 0.19-1.86, p = 0.371). However, higher arachidonic acid (AA) was a trend for a protective factor (HR=0.30, 95%CI: 0.08-1.08, p = 0.065) in the entire trans-diagnostic sample. This protective effect was significant in all participants with a prior suicide attempt history (n = 85; HR=0.16, 95%CI: 0.04-0.67, p = 0.012), and in the subgroup of attempters with major depressive disorder (MDD; n = 55, HR=0.15, 95%CI:0.03-0.76, p = 0.002). Total LDL- and HDL-cholesterol did not predict subsequent suicide events. AA, but not DHA or EPA, positively correlated with baseline depression severity in MDD patients (r = 0.3, p = 0.006). Contrary to our hypothesis that low n-3 PUFA levels would create risk, we found that while higher AA was associated with greater depression severity at baseline, low AA unexpectedly predicted subsequent suicide attempts, the more so in higher-risk patients. Although surprising, this result agrees with a minority of reports concerning n-6 PUFAs and may represent complex interactions with sample characteristics.

Identification of a novel mutation in PLA2G6 gene in a Chinese pedigree with familial cortical myoclonic tremor with epilepsy

PURPOSE

Familial cortical myoclonic tremor with epilepsy (FCMTE) is an epileptic syndrome with autosomal dominant inheritance, of which four genetic subtypes (FCMTE1-4) have been reported. In the present study, we described the clinical and neurophysiologic features of a newly diagnosed Chinese FCMTE family, and investigated the genetic cause for this disease.

METHODS

Clinical information was obtained from affected and normal individuals of an FCMTE family comprising 41 members. Electroencephalographies were analyzed in five of six affected members (including the proband). Brain magnetic resonance imaging, somatosensory evoked potential with C-reflex analysis and magnetoencephalography was performed in the proband. Genomic DNA of three affected and two unaffected individuals was analyzed to detect the genetic mutations by using whole-exome sequencing.

RESULTS

The inheritance pattern of the pedigree was autosomal dominant. A novel missense mutation c.475C>T (p.Ala159Thr) of PLA2G6 were identified in this family. The mutated locus is highly conserved among other species. The mutation is predicted to have a functional impact, and completely co-segregated with the phenotype.

CONCLUSION

This study identifies a novel PLA2G6 mutation that is the possible genetic cause of FCMTE in this family. This mutation and associated clinical features expand the spectrum and phenotypes of PLA2G6-related disorders including neurodegenerative diseases.

Pathways of polyunsaturated fatty acid utilization: implications for brain function in neuropsychiatric health and disease

Essential polyunsaturated fatty acids (PUFAs) have profound effects on brain development and function. Abnormalities of PUFA status have been implicated in neuropsychiatric diseases such as major depression, bipolar disorder, schizophrenia, Alzheimer's disease, and attention deficit hyperactivity disorder. Pathophysiologic mechanisms could involve not only suboptimal PUFA intake, but also metabolic and genetic abnormalities, defective hepatic metabolism, and problems with diffusion and transport. This article provides an overview of physiologic factors regulating PUFA utilization, highlighting their relevance to neuropsychiatric disease.

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.

Bacterial lipopolysaccharide induces a dose-dependent activation of neuroglia and loss of basal forebrain cholinergic cells in the rat brain

In a rat model of neuroinflammation induced with a low-dose infusion lipopolysaccharide (5.0 ng/hr, LPS), we reported that brain arachidonic acid (ARA, 20:4 n-6), but not docosahexaenoic acid (DHA, 22:6n-3), metabolism is increased compared to control rats. To further characterize the impact LPS has on the induction of injury in this model, we quantified the dose-dependent activation of neuroglia and the loss of cholinergic cells in rats subjected to increasing doses of LPS. In this study, we found that LPS produced a statistically significant and linear dose-dependent increase in the percentage of activated CD11b-positive microglia ranging from 26% to 82% following exposure to doses ranging between 0.05 and 500 ng/hr, respectively. The percentage of activated GFAP-positive astrocytes also increased linearly and significantly from 35% to 91%. Significant astroglial scaring was evident at the lateral ventricular boarder of rats treated with 50 and 500 ng/hr LPS, but not evident in control treated rats or rats treated with lower doses of LPS. A dose-dependent decrease in the numbers of ChAT-positive cells in the basal forebrain of LPS-treated rats was found at higher doses of LPS (5, 50, and 500 ng/hr) but not at lower doses. The numbers of ChAT-positive cells within individual regions of the basal forebrain (medial septum and diagonal bands) and the composite basal forebrain were similar in their response. These data demonstrate that extremely low doses of LPS are sufficient to induce significant neuroglia activation while moderate doses above 5.0 ng/hr are required to induce cholinergic cell loss.

Chronic lithium feeding reduces upregulated brain arachidonic acid metabolism in HIV-1 transgenic rat

HIV-1 transgenic (Tg) rats, a model for human HIV-1 associated neurocognitive disorder (HAND), show upregulated markers of brain arachidonic acid (AA) metabolism with neuroinflammation after 7 months of age. Since lithium decreases AA metabolism in a rat lipopolysaccharide model of neuroinflammation, and may be useful in HAND, we hypothesized that lithium would dampen upregulated brain AA metabolism in HIV-1 Tg rats. Regional brain AA incorporation coefficients k* and rates J ( in ), markers of AA signaling and metabolism, were measured in 81 brain regions using quantitative autoradiography, after intravenous [1-(14) C]AA infusion in unanesthetized 10-month-old HIV-1 Tg and age-matched wildtype rats that had been fed a control or LiCl diet for 6 weeks. k* and J ( in ) for AA were significantly higher in HIV-1 Tg than wildtype rats fed the control diet. Lithium feeding reduced plasma unesterified AA concentration in both groups and J ( in ) in wildtype rats, and blocked increments in k* (19 of 54 regions) and J ( in ) (77 of 81 regions) in HIV-1 Tg rats. These in vivo neuroimaging data indicate that lithium treatment dampened upregulated brain AA metabolism in HIV-1 Tg rats. Lithium may improve cognitive dysfunction and be neuroprotective in HIV-1 patients with HAND through a comparable effect.

Imaging brain signal transduction and metabolism via arachidonic and docosahexaenoic acid in animals and humans

The polyunsaturated fatty acids (PUFAs), arachidonic acid (AA, 20:4n-6) and docosahexaenoic acid (DHA, 22:6n-3), important second messengers in brain, are released from membrane phospholipid following receptor-mediated activation of specific phospholipase A(2) (PLA(2)) enzymes. We developed an in vivo method in rodents using quantitative autoradiography to image PUFA incorporation into brain from plasma, and showed that their incorporation rates equal their rates of metabolic consumption by brain. Thus, quantitative imaging of unesterified plasma AA or DHA incorporation into brain can be used as a biomarker of brain PUFA metabolism and neurotransmission. We have employed our method to image and quantify effects of mood stabilizers on brain AA/DHA incorporation during neurotransmission by muscarinic M(1,3,5), serotonergic 5-HT(2A/2C), dopaminergic D(2)-like (D(2), D(3), D(4)) or glutamatergic N-methyl-d-aspartic acid (NMDA) receptors, and effects of inhibition of acetylcholinesterase, of selective serotonin and dopamine reuptake transporter inhibitors, of neuroinflammation (HIV-1 and lipopolysaccharide) and excitotoxicity, and in genetically modified rodents. The method has been extended for the use with positron emission tomography (PET), and can be employed to determine how human brain AA/DHA signaling and consumption are influenced by diet, aging, disease and genetics.

Conserved valproic-acid-induced lipid droplet formation in Dictyostelium and human hepatocytes identifies structurally active compounds

Lipid droplet formation and subsequent steatosis (the abnormal retention of lipids within a cell) has been reported to contribute to hepatotoxicity and is an adverse effect of many pharmacological agents including the antiepileptic drug valproic acid (VPA). In this study, we have developed a simple model system (Dictyostelium discoideum) to investigate the effects of VPA and related compounds in lipid droplet formation. In mammalian hepatocytes, VPA increases lipid droplet accumulation over a 24-hour period, giving rise to liver cell damage, and we show a similar effect in Dictyostelium following 30 minutes of VPA treatment. Using ³H-labelled polyunsaturated (arachidonic) or saturated (palmitic) fatty acids, we shown that VPA treatment of Dictyostelium gives rise to an increased accumulation of both types of fatty acids in phosphatidylcholine, phosphatidylethanolamine and non-polar lipids in this time period, with a similar trend observed in human hepatocytes (Huh7 cells) labelled with [³H]arachidonic acid. In addition, pharmacological inhibition of β-oxidation in Dictyostelium phenocopies fatty acid accumulation, in agreement with data reported in mammalian systems. Using Dictyostelium, we then screened a range of VPA-related compounds to identify those with high and low lipid-accumulation potential, and validated these activities for effects on lipid droplet formation by using human hepatocytes. Structure-activity relationships for these VPA-related compounds suggest that lipid accumulation is independent of VPA-catalysed teratogenicity and inositol depletion. These results suggest that Dictyostelium could provide both a novel model system for the analysis of lipid droplet formation in human hepatocytes and a rapid method for identifying VPA-related compounds that show liver toxicology.

Acetate supplementation attenuates lipopolysaccharide-induced neuroinflammation

Glyceryl triacetate (GTA), a compound effective at increasing circulating and tissue levels of acetate was used to treat rats subjected to a continual 28 day intra-ventricular infusion of bacterial lipopolysaccharide (LPS). This model produces a neuroinflammatory injury characterized by global neuroglial activation and a decrease in choline acetyltransferase immunoreactivity in the basal forebrain. During the LPS infusion, rats were given a daily treatment of either water or GTA at a dose of 6 g/kg by oral gavage. In parallel experiments, free-CoA and acetyl-CoA levels were measured in microwave fixed brains and flash frozen heart, liver, kidney and muscle following a single oral dose of GTA. We found that a single oral dose of GTA significantly increased plasma acetate levels by 15 min and remained elevated for up to 4 h. At 30 min the acetyl-CoA levels in microwave-fixed brain and flash frozen heart and liver were increased at least 2.2-fold. The concentrations of brain acetyl-CoA was significantly increased between 30 and 45 min following treatment and remained elevated for up to 4 h. The concentration of free-CoA in brain was significantly decreased compared to controls at 240 min. Immunohistochemical and morphological analysis demonstrated that a daily treatment with GTA significantly reduced the percentage of reactive glial fibrillary acidic protein-positive astrocytes and activated CD11b-positive microglia by 40-50% in rats subjected to LPS-induced neuroinflammation. Further, in rats subjected to neuroinflammation, GTA significantly increased the number of choline acetyltransferase (ChAT)-positive cells by 40% in the basal forebrain compared to untreated controls. These data suggest that acetate supplementation increases intermediary short chain acetyl-CoA metabolism and that treatment is potentially anti-inflammatory and neuroprotective with regards to attenuating neuroglial activation and increasing ChAT immunoreactivity in this model.

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

PURPOSE OF REVIEW

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.

RECENT FINDINGS

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).

SUMMARY

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.

Assessing serotonin receptor mRNA editing frequency by a novel ultra high-throughput sequencing method

RNA editing is a post-transcriptional modification of pre-mRNA that results in increased diversity in transcriptomes and proteomes. It occurs in a wide variety of eukaryotic organisms and in some viruses. One of the most common forms of pre-mRNA editing is A-to-I editing, in which adenosine is deaminated to inosine, which is read as guanosine during translation. This phenomenon has been observed in numerous transcripts, including the mammalian 5-HT_2C receptor, which can be edited at five distinct sites. Methods used to date to quantify 5-HT_2C receptor editing are labor-intensive, expensive and provide limited information regarding the relative abundance of 5-HT_2C receptor editing variants. Here, we present a novel, ultra high-throughput method to quantify 5-HT_2C receptor editing, compare it to a more conventional method, and use it to assess the effect of a range of genetic and pharmacologic manipulations on 5-HT_2C editing. We conclude that this new method is powerful and economical, and we provide evidence that alterations in 5-HT_2C editing appear to be a result of regional changes in brain activity, rather than a mechanism to normalize 5-HT_2C signaling.

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 A2 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 E2. 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.

Decreased GRK3 but not GRK2 expression in frontal cortex from bipolar disorder patients

Overactivation of G-protein-mediated functions and altered G-protein regulation have been reported in bipolar disorder (BD) brain. Further, drugs effective in treating BD are reported to up-regulate expression of G-protein receptor kinase (GRK) 3 in rat frontal cortex. We therefore hypothesized that some G-protein subunits and GRK levels would be reduced in the brain of BD patients. We determined protein and mRNA levels of G-protein beta and gamma subunits, GRK2, and GRK3 in post-mortem frontal cortex from 10 BD patients and 10 age-matched controls by using immunoblots and real-time RT-PCR. There were statistically significant decreases in protein and mRNA levels of G-protein subunits beta and gamma and of GRK3 in BD brain but not a significant difference in the GRK2 level. Decreased expression of G-protein subunits and of GRK3 may alter neurotransmission, leading to disturbed cognition and behaviour in BD.

Bipolar disorder and mechanisms of action of mood stabilizers

Bipolar disorder (BD) is a major medical and social burden, whose cause, pathophysiology and treatment are not agreed on. It is characterized by recurrent periods of mania and depression (Bipolar I) or of hypomania and depression (Bipolar II). Its inheritance is polygenic, with evidence of a neurotransmission imbalance and disease progression. Patients often take multiple agents concurrently, with incomplete therapeutic success, particularly with regard to depression. Suicide is common. Of the hypotheses regarding the action of mood stabilizers in BD, the "arachidonic acid (AA) cascade" hypothesis is presented in detail in this review. It is based on evidence that chronic administration of lithium, carbamazepine, sodium valproate, or lamotrigine to rats downregulated AA turnover in brain phospholipids, formation of prostaglandin E(2), and/or expression of AA cascade enzymes, including cytosolic phospholipase A(2), cyclooxygenase-2 and/or acyl-CoA synthetase. The changes were selective for AA, since brain docosahexaenoic or palmitic acid metabolism, when measured, was unaffected, and topiramate, ineffective in BD, did not modify the rat brain AA cascade. Downregulation of the cascade by the mood stabilizers corresponded to inhibition of AA neurotransmission via dopaminergic D(2)-like and glutamatergic NMDA receptors. Unlike the mood stabilizers, antidepressants that increase switching of bipolar depression to mania upregulated the rat brain AA cascade. These observations suggest that the brain AA cascade is a common target of mood stabilizers, and that bipolar symptoms, particularly mania, are associated with an upregulated cascade and excess AA signaling via D(2)-like and NMDA receptors. This review presents ways to test these suggestions.

Acute but not chronic donepezil increases muscarinic receptor-mediated signaling via arachidonic acid in unanesthetized rats

Donepezil, an acetylcholinesterase (AChE) inhibitor used for treating Alzheimer's disease patients, is thought to act by increasing brain extracellular acetylcholine (ACh), and ACh binding to cholinergic receptors. Muscarinic receptors are coupled to cytosolic phospholipase A2 (cPLA2) activation and arachidonic acid (AA) release from synaptic membrane phospholipid. This activation can be imaged in rodents as an AA incorporation coefficient k*, using quantitative autoradiography. Acute and chronic effects of donepezil on the AA signal, k* for AA, were measured in 81 brain regions of unanesthetized rats. Twenty min after a single oral dose (3.0 mg/kg) of donepezil, k* was increased significantly in 37 brain regions, whereas k* did not differ from control 7 h afterwards or following chronic (21 days) of donepezil. Pretreatment with atropine prevented the 20-min increments in k* following donepezil. Donepezil also increased the brain ACh concentration and reduced brain AChE activity, but did not change cPLA2 activity, regardless of administration regimen. These results show that donepezil acutely increases the brain AA signal that is mediated by ACh acting at muscarinic receptors, but that this signal is rapidly desensitized despite continued elevated brain ACh concentration. In contrast, the AA signal in response to arecoline was not altered following donepezil.

Chronic administration of valproic acid reduces brain NMDA signaling via arachidonic acid in unanesthetized rats

Evidence that brain glutamatergic activity is pathologically elevated in bipolar disorder suggests that mood stabilizers are therapeutic in the disease in part by downregulating glutamatergic activity. Such activity can involve the second messenger, arachidonic acid (AA, 20:4n - 6). We tested this hypothesis with regard to valproic acid (VPA), when stimulating glutamatergic N-methyl-D: -aspartate (NMDA) receptors in rat brain and measuring AA and related responses. An acute subconvulsant dose of NMDA (25 mg/kg i.p.) or saline was administered to unanesthetized rats that had been treated i.p. daily with VPA (200 mg/kg) or vehicle for 30 days. Quantitative autoradiography following intravenous [1-(14)C]AA infusion was used to image regional brain AA incorporation coefficients k*, markers of AA signaling. In chronic vehicle-pretreated rats, NMDA compared with saline significantly increased k* in 41 of 82 examined brain regions, many of which have high NMDA receptor densities, and also increased brain concentrations of the AA metabolites, prostaglandin E(2) (PGE(2)) and thromboxane B(2) (TXB(2)). VPA pretreatment reduced baseline concentrations of PGE(2) and TXB(2), and blocked the NMDA induced increases in k* and in eicosanoid concentrations. These results, taken with evidence that carbamazepine and lithium also block k* responses to NMDA in rat brain, suggest that mood stabilizers act in bipolar disorder in part by downregulating glutamatergic signaling involving AA.

Mode of action of mood stabilizers: is the arachidonic acid cascade a common target?

Bipolar disorder is a major medical, social and economic burden worldwide. However, the mechanisms of action of effective antibipolar disorder drugs remain elusive. In this paper, we review studies using a neuropharmacological approach in unanesthetized rats, combined with kinetic, biochemical and molecular biology techniques, showing that chronic administration of three Food and Drug Administration-approved mood stabilizers (lithium, valproate and carbamazepine) at therapeutically relevant doses, selectively target the brain arachidonic acid (AA) cascade. Whereas chronic lithium and carbamazepine decrease the binding activity of activator protein-2 and in turn the transcription, translation and activity of its AA-selective calcium-dependent phospholipase A(2) gene product, valproate appears to be a non-competitive inhibitor of long-chain acyl-CoA synthetase. The net overlapping effects of the three drugs are decreased turnover of AA but not of docosahexaenoic acid in rat brain phospholipids, and decreased brain cyclooxygenase-2 and prostaglandin E(2). Although these observations support the hypothesis proposed by Rapoport and colleagues that the AA cascade is a common target of mood stabilizers, this hypothesis is not necessarily exclusive of other targets. Targeting the AA cascade with drugs or diet may be a useful therapeutic approach in bipolar disorder, and examining the AA cascade in patients might help in better understanding the disease.

Flurbiprofen, A Cyclooxygenase Inhibitor, Reduces the Brain Arachidonic Acid Signal in Response to the Cholinergic Muscarinic Agonist, Arecoline, in Awake Rats

Cholinergic muscarinic receptors, when stimulated by arecoline, can activate cytosolic phospholipase A(2) (cPLA(2)) to release arachidonic acid (AA) from membrane phospholipid. This signal can be imaged in the brain in vivo using quantitative autoradiography following the intravenous injection of radiolabeled AA, as an increment in a regional brain AA incorporation coefficient k*. Arecoline increases k* significantly in brain regions having muscarinic M(1,3,5) receptors in wild-type but not in cyclooxygenase (COX)-2 knockout mice. To further clarify the roles of COX enzymes in the AA signal, in this paper we imaged k* following arecoline (5 mg/kg i.p.) or saline in each of 81 brain regions of unanesthetized rats pretreated 6 h earlier with the non-selective COX inhibitor flurbiprofen (FB, 60 mg/kg s.c.) or with vehicle. Baseline values of k* were unaffected by FB treatment, which however reduced by 80% baseline brain concentrations of prostaglandin E(2) (PGE(2)) and thromboxane B(2) (TXB(2)), eicosanoids preferentially derived from AA via COX-2 and COX-1, respectively. In vehicle-pretreated rats, arecoline increased the brain PGE(2) but not TXB(2) concentration, as well as values for k* in 77 of the 81 brain regions. FB-pretreatment prevented these arecoline-provoked changes. These results and those reported in COX-2 knockout mice suggest that the AA released in brain following muscarinic receptor-mediated activation is lost via COX-2 to PGE(2) but not via COX-1 to TXB(2), and that increments in k* following arecoline largely represent replacement by unesterified plasma AA of this loss.

Chronic lithium administration attenuates up-regulated brain arachidonic acid metabolism in a rat model of neuroinflammation

Neuroinflammation, caused by a 6-day intracerebroventricular infusion of lipopolysaccharide (LPS) in rats, is associated with the up-regulation of brain arachidonic acid (AA) metabolism markers. Because chronic LiCl down-regulates markers of brain AA metabolism, we hypothesized that it would attenuate increments of these markers in LPS-infused rats. Incorporation coefficients k* of AA from plasma into brain, and other brain AA metabolic markers, were measured in rats that had been fed a LiCl or control diet for 6 weeks, and subjected in the last 6 days on the diet to intracerebroventricular infusion of artificial CSF or of LPS. In rats on the control diet, LPS compared with CSF infusion increased k* significantly in 28 regions, whereas the LiCl diet prevented k* increments in 18 of these regions. LiCl in CSF infused rats increased k* in 14 regions, largely belonging to auditory and visual systems. Brain cytoplasmic phospholipase A(2) activity, and prostaglandin E(2) and thromboxane B(2) concentrations, were increased significantly by LPS infusion in rats fed the control but not the LiCl diet. Chronic LiCl administration attenuates LPS-induced up-regulation of a number of brain AA metabolism markers. To the extent that this up-regulation has neuropathological consequences, lithium might be considered for treating human brain diseases accompanied by neuroinflammation.

Chronic treatment with mood stabilizers increases membrane GRK3 in rat frontal cortex

BACKGROUND

G-protein receptor kinases (GRKs) are a family of serine/threonine kinases involved in the homologous desensitization of agonist activated G-protein coupled receptors (GPCRs). G-protein coupled receptor supersensitivity, possibly as a result of decreased GRK, has been suggested in affective disorders.

METHODS

We used immunobloting to determine if chronic, therapeutically relevant doses of lithium (Li+), carbamazepine (CBZ), and valproate (VPA), would increase GRK2/3 protein levels in rat frontal cortex.

RESULTS

Chronic Li+ (24%) and CBZ (44%) significantly increased GRK3 in the membrane but not cytosol fractions. Chronic VPA had no effect on GRK3. G-protein receptor kinase 2 protein levels were unchanged by all treatments. The GRK3 membrane to cytosol ratio was increased significantly in Li+ and CBZ treated rats.

CONCLUSIONS

These results show that chronically administered Li+ and CBZ, but not VPA, increase the translocation of GRK3 from cytosol to membrane, possibly correcting supersensitivity of GPCRs in bipolar disorder.

Chronic fluoxetine increases cytosolic phospholipase A(2) activity and arachidonic acid turnover in brain phospholipids of the unanesthetized rat

RATIONALE

Fluoxetine is used to treat unipolar depression and is thought to act by increasing the concentration of serotonin (5-HT) in the synaptic cleft, leading to increased serotonin signaling. The 5-HT(2A/2C) receptor subtypes are coupled to a phospholipase A(2) (PLA(2)). We hypothesized that chronic fluoxetine would increase the brain activity of PLA(2) and the turnover rate of arachidonic acid (AA) in phospholipids of the unanesthetized rat.

MATERIALS AND METHODS

To test this hypothesis, rats were administered fluoxetine (10 mg/kg) or vehicle intraperitoneally daily for 21 days. In the unanesthetized rat, [1-(14)C]AA was infused intravenously and arterial blood plasma was sampled until the animal was killed at 5 min and its brain was subjected to chemical, radiotracer, or enzyme analysis.

RESULTS

Using equations from our fatty acid model, we found that chronic fluoxetine compared with vehicle increased the turnover rate of AA within several brain phospholipids by 75-86%. The activity and protein levels of brain cytosolic PLA(2) (cPLA(2)) but not of secretory or calcium-independent PLA(2) were increased in rats administered fluoxetine. In a separate group of animals that received chronic fluoxetine followed by a 3-day saline washout, the turnover of AA and activity and protein levels of cPLA(2) were not significantly different from controls. The protein levels of cyclooxygenases 1 and 2 as well as the concentration of prostaglandin E(2) in rats chronically administered fluoxetine did not differ significantly from controls.

CONCLUSION

The results support the hypothesis that fluoxetine increases the cPLA(2)-mediated turnover of AA within brain phospholipids.

Inhibitors of brain phospholipase A2 activity: their neuropharmacological effects and therapeutic importance for the treatment of neurologic disorders

The phospholipase A(2) family includes secretory phospholipase A(2), cytosolic phospholipase A(2), plasmalogen-selective phospholipase A(2), and calcium-independent phospholipase A(2). It is generally thought that the release of arachidonic acid by cytosolic phospholipase A(2) is the rate-limiting step in the generation of eicosanoids and platelet activating factor. These lipid mediators play critical roles in the initiation and modulation of inflammation and oxidative stress. Neurological disorders, such as ischemia, spinal cord injury, Alzheimer's disease, multiple sclerosis, prion diseases, and epilepsy are characterized by inflammatory reactions, oxidative stress, altered phospholipid metabolism, accumulation of lipid peroxides, and increased phospholipase A(2) activity. Increased activities of phospholipases A(2) and generation of lipid mediators may be involved in oxidative stress and neuroinflammation associated with the above neurological disorders. Several phospholipase A(2) inhibitors have been recently discovered and used for the treatment of ischemia and other neurological diseases in cell culture and animal models. At this time very little is known about in vivo neurochemical effects, mechanism of action, or toxicity of phospholipase A(2) inhibitors in human or animal models of neurological disorders. In kainic acid-mediated neurotoxicity, the activities of phospholipase A(2) isoforms and their immunoreactivities are markedly increased and phospholipase A(2) inhibitors, quinacrine and chloroquine, arachidonyl trifluoromethyl ketone, bromoenol lactone, cytidine 5-diphosphoamines, and vitamin E, not only inhibit phospholipase A(2) activity and immunoreactivity but also prevent neurodegeneration, suggesting that phospholipase A(2) is involved in the neurodegenerative process. This also suggests that phospholipase A(2) inhibitors can be used as neuroprotectants and anti-inflammatory agents against neurodegenerative processes in neurodegenerative diseases.

Chronic lithium chloride administration attenuates brain NMDA receptor-initiated signaling via arachidonic acid in unanesthetized rats

It has been proposed that lithium is effective in bipolar disorder (BD) by inhibiting glutamatergic neurotransmission, particularly via N-methyl-D-aspartate receptors (NMDARs). To test this hypothesis and to see if the neurotransmission could involve the NMDAR-mediated activation of phospholipase A2 (PLA2), to release arachidonic acid (AA) from membrane phospholipid, we administered subconvulsant doses of NMDA to unanesthetized rats fed a chronic control or LiCl diet. We used quantitative autoradiography following the intravenous injection of radiolabeled AA to measure regional brain incorporation coefficients k* for AA, which reflect receptor-mediated activation of PLA2. In control diet rats, NMDA (25 and 50 mg/kg i.p.) compared with i.p. saline increased k* significantly in 49 and 67 regions, respectively, of the 83 brain regions examined. The regions affected were those with reported NMDARs, including the neocortex, hippocampus, caudate-putamen, thalamus, substantia nigra, and nucleus accumbens. The increases could be blocked by pretreatment with the specific noncompetitive NMDA antagonist MK-801 ((5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate) (0.3 mg/kg i.p.), as well by a 6-week LiCl diet sufficient to produce plasma and brain lithium concentrations known to be effective in BD. MK-801 alone reduced baseline values for k* in many brain regions. The results show that it is possible to image NMDA signaling via PLA2 activation and AA release in vivo, and that chronic lithium blocks this signaling, consistent with its suggested mechanism of action in BD.

Chronic lithium chloride administration to rats elevates glucose metabolism in wide areas of brain, while potentiating negative effects on metabolism of dopamine D2-like receptor stimulation

RATIONALE AND OBJECTIVES

The regional cerebral metabolic rate for glucose (rCMRglc) can be imaged in vivo as a marker of brain functional activity. The effects of chronic lithium administration on baseline values of rCMRglc and values in response to administration of dopamine D2-like receptor agonists have not been examined in humans or rats. Knowing these effects may elucidate and localize the therapeutic action of lithium in bipolar disorder.

METHODS

In unanesthetized rats, we used the 2-deoxy-D-glucose (2-DG) technique to image the effects of a 6-week control diet or LiCl diet sufficient to produce a plasma lithium concentration therapeutically relevant to bipolar disorder, on rCMRglc at baseline and in response to the dopaminergic D2-like receptor agonist, quinpirole (1 mg/kg i.v.), or to i.v. saline.

RESULTS

Baseline rCMRglc was significantly elevated in 30 of 81 brain regions examined, in LiCl diet compared with control diet rats. Affected were visual and auditory structures, frontal cortex, amygdala, hippocampus, nucleus accumbens, caudate-putamen, interpeduncular nucleus, and substantia nigra. Acute quinpirole significantly decreased rCMRglc in four areas of the caudate-putamen in control diet rats, and in these and 19 additional brain areas in LiCl-fed rats.

CONCLUSIONS

In unanesthetized rats, chronic lithium administration widely upregulates baseline rCMRglc and potentiates the negative effects on rCMRglc of D2-like receptor stimulation. The baseline elevation may relate to lithium's reported ability to increase auditory and visual evoked responses in humans, whereas lithium's potentiation of quinpirole's negative effects on rCMRglc may be related to its therapeutic efficacy in bipolar disorder.

Regional protein levels of cytosolic phospholipase A2 and cyclooxygenase-2 in Rhesus monkey brain as a function of age

Limited evidence suggests that brain cytosolic phospholipase A(2) (cPLA(2)), which selectively releases arachidonic acid (AA) from membrane phospholipids, and cyclooxygenase-2 (COX-2), the rate-limiting enzyme for AA metabolism to prostanoids, change as a function of normal aging. In this study, we examined the protein levels of cPLA(2) and COX-2 enzymes in hippocampus, frontal pole and cerebellum from young (2-5 years old), middle-aged (8-11 years old) and old (23 years old) male and female Rhesus monkeys. In the cerebellum, cPLA(2) protein level was higher in the young brain as compared to levels seen at both middle-aged and old. Similarly, in the frontal pole, the young brain showed a higher level of COX-2 protein as compared to the levels seen at both older ages. For both, once an animal reached 8-11 years of age the levels appeared to remain relatively constant over the next decade. Immunohistochemistry of COX-2 protein within the brain demonstrated no significant change in the localization to neurons within the frontal pole. Qualitatively, a greater number of neurons were positively stained for COX-2 in the young brain than in the aged brain. Based on the previous reports of localization of cPLA(2) and COX-2 at post-synaptic sites in neurons results from the current study suggest that the elevated protein levels of the two enzymes seen in the younger brain is related to the greater potential for synaptic plasticity across multiple neurons as a function of age and that cPLA(2) and COX-2 may be considered as post-synaptic markers.

Chronic carbamazepine decreases the incorporation rate and turnover of arachidonic acid but not docosahexaenoic acid in brain phospholipids of the unanesthetized rat: relevance to bipolar disorder

BACKGROUND

The basis for carbamazepine's efficacy in treating bipolar disorder is not agreed on. One hypothesis is that, similar to lithium and valproate (antibipolar drugs), carbamazepine might selectively decrease the kinetics of arachidonic acid (AA) in brain phospholipids.

METHODS

To assess whether it targets brain AA kinetics, we administered carbamazepine (25 mg/kg/day, IP) to rats for 30 days and then determined its effect compared with that of vehicle on incorporation and turnover rates of AA and docosahexaenoic acid (DHA) in brain phospholipids. In unanesthetized rats that had received carbamazepine or vehicle, [1-14C]AA or [1-14C]DHA was infused intravenously, and arterial blood plasma was sampled until the animal was killed at 5 min and its brain, after being microwaved, was used for acyl-coenzyme A (acyl-CoA) and phospholipid fatty acid analysis.

RESULTS

Chronic carbamazepine, compared with vehicle, decreased the rate of incorporation of AA-CoA (27%-29%) and turnover of AA (25%-27%) but not of DHA-CoA or DHA in brain phospholipids.

CONCLUSIONS

The results, which are comparable to published findings after chronic administration of lithium and valproic acid to rats, support the hypothesis that drugs effective against mania in bipolar disorder act by selectively downregulating the incorporation rate of AA-CoA and turnover of AA in brain phospholipids.

Resting and arecoline-stimulated brain metabolism and signaling involving arachidonic acid are altered in the cyclooxygenase-2 knockout mouse

Abstract Studies were performed to determine if cyclooxygenase (COX)-2 regulates muscarinic receptor-initiated signaling involving brain phospholipase A2 (PLA2) activation and arachidonic acid (AA; 20 : 4n-6) release. AA incorporation coefficients, k* (brain [1-14C]AA radioactivity/integrated plasma radioactivity), representing this signaling, were measured following the intravenous injection of [1-14C]AA using quantitative autoradiography, in each of 81 brain regions in unanesthetized COX-2 knockout (COX-2(-/-)) and wild-type (COX-2(+/+)) mice. Mice were administered arecoline (30 mg/kg i.p.), a non-specific muscarinic receptor agonist, or saline i.p. (baseline control). At baseline, COX-2(-/-) compared with COX-2(+/+) mice had widespread and significant elevations of k*. Arecoline increased k* significantly in COX-2(+/+) mice compared with saline controls in 72 of 81 brain regions, but had no significant effect on k* in any region in COX-2(-/-) mice. These findings, when related to net incorporation rates of AA from brain into plasma, demonstrate enhanced baseline brain metabolic loss of AA in COX-2(-/-) compared with COX-2(+/+) mice, and an absence of a normal k* response to muscarinic receptor activation. This response likely reflects selective COX-2-mediated conversion of PLA2-released AA to prostanoids.

Topiramate does not alter expression in rat brain of enzymes of arachidonic acid metabolism

RATIONALE

When administered chronically to rats, drugs that are effective in bipolar disorder-lithium and the anticonvulsants, valproic acid and carbamazepine-have been shown to downregulate the expression of certain enzymes involved in brain arachidonic acid (AA) release and cyclooxygenase (COX)-mediated metabolism. Phase II clinical trials with the anticonvulsant topiramate [2,3:4,5-bis-O-(1-methylethylidene)-beta-D-fructopyranose sulfamate] suggest that this drug may also be effective for bipolar disorder.

OBJECTIVES

To see if topiramate has effects similar to those of the other three drugs, we administered topiramate to rats for 14 days at 20 mg/kg, p.o. twice daily.

RESULTS

Compared with p.o. vehicle, topiramate treatment did not significantly affect the brain activity or protein level of cytosolic phospholipase A2, secretory PLA2, or Ca2+-independent iPLA2. Additionally, brain protein levels of COX-1, COX-2, 5-lipoxygenase, and cytochrome P450 epoxygenase were unchanged.

CONCLUSIONS

These results suggest that topiramate does not modify expression of the enzymes involved in brain AA metabolism that have been shown to be targeted by lithium, valproic acid, or carbamazepine. If topiramate proves effective in bipolar disorder, it may not act by modulating brain AA metabolism. In view of the proven anticonvulsant effect of topiramate, our results also suggest that the AA cascade is not involved in the anti-seizure properties of the drug.

Chronic lithium chloride administration to unanesthetized rats attenuates brain dopamine D2-like receptor-initiated signaling via arachidonic acid

We studied the effect of lithium chloride on dopaminergic neurotransmission via D2-like receptors coupled to phospholipase A2 (PLA2). In unanesthetized rats injected i.v. with radiolabeled arachidonic acid (AA, 20:4 n-6), regional PLA2 activation was imaged by measuring regional incorporation coefficients k* of AA (brain radioactivity divided by integrated plasma radioactivity) using quantitative autoradiography, following administration of the D2-like receptor agonist, quinpirole. In rats fed a control diet, quinpirole at 1 mg/kg i.v. increased k* for AA significantly in 17 regions with high densities of D2-like receptors, of 61 regions examined. Increases in k* were found in the prefrontal cortex, frontal cortex, accumbens nucleus, caudate-putamen, substantia nigra, and ventral tegmental area. Quinpirole, 0.25 mg/kg i.v. enhanced k* significantly only in the caudate-putamen. In rats fed LiCl for 6 weeks to produce a therapeutically relevant brain lithium concentration, neither 0.25 mg/kg nor 1 mg/kg quinpirole increased k* significantly in any region. Orofacial movements following quinpirole were modified but not abolished by LiCl feeding. The results suggest that downregulation by lithium of D2-like receptor signaling involving PLA2 and AA may contribute to lithium's therapeutic efficacy in bipolar disorder.

5-HT2A/2C receptor signaling via phospholipase A2 and arachidonic acid is attenuated in mice lacking the serotonin reuptake transporter

SUBJECTS

The serotonin reuptake transporter (SERT) helps to regulate brain serotonergic transmission and is the target of some antidepressants. To further understand SERT function, we measured a marker of regional brain phospholipase A2 (PLA2) activation in SERT knockout mice (SERT-/-) and their littermate controls (SERT+/+).

METHODS

Following administration of 1.5 mg/kg s.c. (+/-)-2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI), a 5-HT(2A/2C) receptor agonist, to unanesthetized mice injected intravenously with radiolabeled arachidonic acid (AA), PLA2 activation, represented as the regional incorporation coefficient k* of AA, was determined with quantitative autoradiography in each of 71 brain regions.

RESULTS

In SERT+/+ mice, DOI significantly increased k* in 27 regions known to have 5-HT(2A/2C) receptors, including the frontal, motor, somatosensory, pyriform and cingulate cortex, white matter, nucleus accumbens, caudate putamen, septum, CA1 of hippocampus, thalamus, and hypothalamus. In contrast, DOI did not increase k* significantly in any brain region of SERT-/- mice. Head twitches following DOI, which also were measured, were robust in SERT+/+ mice but were markedly attenuated in SERT-/- mice.

CONCLUSIONS

These results show that a lifelong elevation of the synaptic 5-HT concentration in SERT-/- mice leads to downregulation of 5-HT(2A/2C) receptor-mediated PLA2 signaling via AA and of head twitches, in response to DOI.

Increased arachidonic acid concentration in the brain of Flinders Sensitive Line rats, an animal model of depression

Depression may be associated with impaired membrane PUFA composition, especially decreased n-3 PUFA. This assumption has not been tested at the level of brain tissue. Moreover, most studies were confounded by dietary variability. We examined the FA composition of selected brain areas in an animal model of depression, the Flinders Sensitive Line (FSL) rat, and compared the findings with those in controls fed identical diets. In all brain regions studied, the concentration of arachidonic acid (AA) was significantly higher in the FSL rats: in the hypothalamus by 21%, in the nucleus accumbens by 24%, in the prefrontal cortex by 31%, and in the striatum by 23%. No significant differences were observed for n-3 PUFA or for the saturated and monounsaturated FAs. Our results confirm the existence of altered brain PUFA composition in an animal model of depression. The finding of increased AA, an n-6 PUFA, rather than decreased n-3 PUFA, emphasizes the importance of both PUFA families in the pathophysiological processes underlying depression. The FSL rat is a useful tool for further elucidation of the FA disturbances in depression.

Chronic lithium administration to rats selectively modifies 5-HT2A/2C receptor-mediated brain signaling via arachidonic acid

The effects of chronic lithium administration on regional brain incorporation coefficients k* of arachidonic acid (AA), a marker of phospholipase A2 (PLA2) activation, were determined in unanesthetized rats administered i.p. saline or 1 mg/kg i.p. (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI), a 5-HT2A/2C receptor agonist. After injecting [1-(14)C]AA intravenously, k* (brain radioactivity/integrated plasma radioactivity) was measured in each of 94 brain regions by quantitative autoradiography. Studies were performed in rats fed a LiCl or a control diet for 6 weeks. In the control diet rats, DOI significantly increased k* in widespread brain areas containing 5-HT2A/2C receptors. In the LiCl-fed rats, the significant positive k* response to DOI did not differ from that in control diet rats in most brain regions, except in auditory and visual areas, where the response was absent. LiCl did not change the head turning response to DOI seen in control rats. In summary, LiCl feeding blocked PLA2-mediated signal involving AA in response to DOI in visual and auditory regions, but not generally elsewhere. These selective effects may be related to lithium's therapeutic efficacy in patients with bipolar disorder, particularly its ability to ameliorate hallucinations in that disease.

Synaptic lipid signaling: significance of polyunsaturated fatty acids and platelet-activating factor

Neuronal cellular and intracellular membranes are rich in specialized phospholipids that are reservoirs of lipid messengers released by specific phospholipases and stimulated by neurotransmitters, neurotrophic factors, cytokines, membrane depolarization, ion channel activation, etc. Secretory phospholipases A2 may be both intercellular messengers and generators of lipid messengers. The highly networked nervous system includes cells (e.g., astrocytes, oligodendrocytes, microglial cells, endothelial microvascular cells) that extensively interact with neurons; several lipid messengers participate in these interactions. This review highlights modulation of postsynaptic membrane excitability and long-term synaptic plasticity by cyclooxygenase-2-generated prostaglandin E2, arachidonoyldiacylcylglycerol, and arachidonic acid-containing endocannabinoids. The peroxidation of docosahexaenoic acid (DHA), a critical component of excitable membranes in brain and retina, is promoted by oxidative stress. DHA is also the precursor of enzyme-derived, neuroprotective docosanoids. The phospholipid platelet-activating factor is a retrograde messenger of long-term potentiation, a modulator of glutamate release, and an upregulator of memory formation. Lipid messengers modulate signaling cascades and contribute to cellular differentiation, function, protection, and repair in the nervous system. Lipidomic neurobiology will advance our knowledge of the brain, spinal cord, retina, and peripheral nerve function and diseases that affect them, and new discoveries on networks of signaling in health and disease will likely lead to novel therapeutic interventions.