Michael C. J. Chang

Lithium decreases turnover of arachidonate in several brain phospholipids

In vivo rates of incorporation and turnover of palmitate and arachidonate in brain phospholipids were measured in awake rats treated chronically with lithium, following intravenous infusion of radiolabeled palmitate and arachidonate, respectively. Chronic lithium, at a brain level considered to be therapeutic in humans, decreased turnover of arachidonate within brain phosphatidylinositol, phosphatidylcholine and phosphatidylethanolamine by up to 80% (P < 0.001). In contrast, lithium had a minimal effect on turnover of palmitate, causing only a 26% reduction in turnover in phosphatidylcholine (P < 0.01). These results suggest that a major therapeutic effect of lithium is to reduce turnover specifically of arachidonate, possibly by inhibiting phospholipase A2 involved in signal transduction. The effect may be secondary to the known action of lithium on the phosphoinositide cycle, by inhibiting the activity of inositol monophosphatase.

Chronic valproate treatment decreases the in vivo turnover of arachidonic acid in brain phospholipids: a possible common effect of mood stabilizers

Both (Li+) and valproic acid (VPA) are effective in treating bipolar disorder, but the pathway by which either works, and whether it is common to both drugs, is not agreed upon. We recently reported, using an in vivo fatty acid model, that Li+ reduces the turnover rate of the second messenger arachidonic acid (AA) by 80% in brain phospholipids of the awake rat, without changing turnover rates of docosahexaenoic or palmitic acid. Reduced AA turnover was accompanied by down‐regulation of gene expression and protein levels of an AA‐specific cytosolic phospholipase A2 (cPLA2). To see if VPA had the same effect on AA turnover, we used our in vivo fatty acid model in rats chronically administered VPA (200 mg/kg, i.p. for 30 days). Like Li+, VPA treatment significantly decreased AA turnover within brain phospholipids (by 28–33%), although it had no effect on cPLA2 protein levels. Thus, both mood stabilizers, Li+ and VPA have a common action in reducing AA turnover in brain phospholipids, albeit by different mechanisms.

Chronic lithium downregulates cyclooxygenase-2 activity and prostaglandin E(2) concentration in rat brain.

Rats treated with lithium chloride for 6 weeks have been reported to demonstrate reduced turnover of arachidonic acid (AA) in brain phospholipids, and decreases in mRNA and protein levels, and enzyme activity, of AA-selective cytosolic phospholipase A2(cPLA2). We now report that chronic lithium administration to rats significantly reduced the brain protein level and enzyme activity of cyclooxygenase-2 (COX-2), without affecting COX-2 mRNA. Lithium also reduced the brain concentration of prostaglandin E2 (PGE2), a bioactive product of AA formed via the COX reaction. COX-1 and the Ca2+-independent iPLA2 (type VI) were unaffected by lithium. These and prior results indicate that lithium targets a part of the AA cascade that involves cPLA2 and COX-2. This effect may contribute to lithiums therapeutic action in bipolar disorder.

85 kDa cytosolic phospholipase A2 is a target for chronic lithium in rat brain.

The mechanism by which chronic lithium exerts its therapeutic effect in brains of bipolar patients is not known. One possibility, suggested by our demonstration in the rat brain, is that chronic lithium inhibits turnover of arachidonic acid (AA) by reducing the activity of an AA-specific phospholipase A2 (PLA2). To test this further, mRNA levels of two AA-specific PLA2s, cytosolic PLA2 (cPLA2) type IV and intracellular PLA2 (iPLA2) type VIII, and protein level of cPLA2 were quantified in the brain of rats given lithium for 6 weeks. Chronic lithium markedly reduced brain mRNA and protein level of cPLA2, but had no effect on mRNA level of iPLA2. These results suggest that the final common path effect of chronic lithium administration is to reduce turnover of AA in brain by down-regulating cPLA2.

Nutritional Deprivation of α‐Linolenic Acid Decreases but Does Not Abolish Turnover and Availability of Unacylated Docosahexaenoic Acid and Docosahexaenoyl‐CoA in Rat Brain

Abstract: We applied our in vivo fatty acid method to examine concentrations, incorporation, and turnover rates of docosahexaenoic acid (22:6 n‐3) in brains of rats subject to a dietary deficiency of α‐linolenic acid (18:3 n‐3) for three generations. Adult deficient and adequate rats of the F3 generation were infused intravenously with [4,5‐3H]docosahexaenoic acid over 5 min, after which brain uptake and distribution of tracer were measured. Before infusion, the plasma 22:6 n‐3 level was 0.2 nmol ml‐1 in 18:3 n‐3‐deficient compared with 10.6 nmol ml‐1 in control rats. Brain unesterified 22:6 n‐3 was not detectable, whereas docosahexaenoyl‐CoA content was reduced by 95%, and 22:6 n‐3 content in different phospholipid classes was reduced by 83‐88% in deficient rats. Neither plasma or brain arachidonic acid (20:4 n‐6) level was significantly changed with diet. Docosapentaenoic acid (22:5 n‐6) reciprocally replaced 22:6 n‐3 in brain phospholipids. Calculations using operational equations from our model indicated that 22:6 n‐3 incorporation from plasma into brain was reduced 40‐fold by 18:3 n‐3 deficiency. Recycling of 22:6 n‐3 due to deacylation‐reacylation within phospholipids was reduced by 30‐70% with the deficient diet, but animals nevertheless continued to produce 22:6 n‐3 and docosahexaenoyl‐CoA for brain function. We propose that functional brain effects of n‐3 deficiency reflect altered ratios of n‐6 to n‐3 fatty acids.

Dynamics of Docosahexaenoic Acid Metabolism in the Central Nervous System: Lack of Effect of Chronic Lithium Treatment

Using a method and model developed in our laboratory to quantitatively study brain phospholipid metabolism, in vivo rates of incorporation and turnover of docosahexaenoic acid in brain phospholipids were measured in awake rats. The results suggest that docosahexaenoate incorporation and turnover in brain phospholipids are more rapid than previously assumed and that this rapid turnover dilutes tracer specific activity in brain docoshexaenoyl-CoA pool due to release and recycling of unlabeled fatty acid from phospholipid metabolism. Fractional turnover rates for docosahexaenoate within phosphatidylinositol, choline glycerophospholipids, ethanolamine glycerophospholipids and phosphatidylserine were 17.7, 3.1, 1.2, and 0.2 %.h−1, respectively. Chronic lithium treatment, at a brain level considered to be therapeutic in humans (0.6 μmol.g−1), had no effect on turnover of docosahexaenoic acid in individual brain phospholipids. Consistent with previous studies from our laboratory that chronic lithium decreased the turnover of arachidonic acid within brain phospholipids by up to 80% and attenuated brain phospholipase A2 activity, the lack of effect of lithium on docosahexaenoate recycling and turnover suggests that a target for lithiums action is an arachidonic acid-selective phospholipase A2.

Chronic lithium treatment decreases brain phospholipase A2 activity.

Chronic lithium administration decreases the turnover of arachidonic acid (AA) in several brain phospholipids. This suggests that lithium may attenuate phospholipase A2 (PLA2) activity in brain. We now report effects of chronic lithium treatment on PLA2 activity in postnuclear supernatant from rat brain: Enzyme activity was determined by two assay methods, radiometric and fluorometric, and measured the release of the fatty acid on the second acyl position (sn2) from choline and ethanolamine phospholipids. PLA2 activity in brain postnuclear supernatant from rats chronically treated with lithium in the diet was significantly decreased (20–50%) when compared with controls. In vehicle or lithium-treated rats, PLA2 activity was not significantly augmented or attenuated by the addition of calcium chelators, divalent cations or LiCl supplementation (1.0 mM) to postnuclear supernatant. These results suggest that a major therapeutic effect of lithium is to attenuate brain PLA2 activity involved in signal transduction.

Brain incorporation of [11C]arachidonic acid in young healthy humans measured with positron emission tomography.

Arachidonic acid (AA) is an important second messenger involved in signal transduction mediated by phospholipase A2. The goal of this study was to establish an in vivo quantitative method to examine the role of AA in this signaling process in the human brain. A simple irreversible uptake model was derived from rat studies and modified for positron emission tomography (PET) to quantify the incorporation rate K*of [11C]AA into brain. Dynamic 60-minute three-dimensional scans and arterial input functions were acquired in 8 young healthy adults studied at rest. Brain radioactivity was corrected for uptake of the metabolite [11C]CO2. K* and cerebral blood volume (Vb) were estimated pixel-by-pixel and were calculated in regions of interest. K* equaled 5.6 ± 1.2 and 2.6 ± 0.5 μL · min−1 · mL−1 in gray and white matter, respectively. K* and Vb values were found to be unchanged with data analysis periods from 20 to 60 minutes. Thus, PET can be used to obtain quantitative images of the incorporation rate K* of [11C]AA in the human brain. As brain incorporation of labeled AA has been shown in awake rats to be increased by pharmacological activation associated with phospholipase A2-signaling, PET and [11C]AA may be useful to measure signal transduction in the human brain.

Brain incorporation of [1-11C]arachidonate in normocapnic and hypercapnic monkeys, measured with positron emission tomography.

Positron emission tomography (PET) was used to determine brain incorporation coefficients k* of [1-11C]arachidonate in isoflurane-anesthetized rhesus monkeys, as well as cerebral blood flow (CBF) using [15O]water. Intravenously injected [1-11C]arachidonate disappeared from plasma with a half-life of 1.1 min, whereas brain radioactivity reached a steady-state by 10 min. Mean values of k* were the same whether calculated by a single-time point method at 20 min after injection began, or by least-squares fitting of an equation for total brain radioactivity to data at all time points. k* equalled 1.1-1.2 x 10(-4) ml x s(-1) x g(-1) in gray matter and was unaffected by a 2.6-fold increase in CBF caused by hypercapnia. These results indicate that brain incorporation of [1-11C]arachidonate can be quantified in the primate using PET, and that incorporation is flow-independent.

Chronic nutritional deprivation of n-3 α-linolenic acid does not affect n-6 arachidonic acid recycling within brain phospholipids of awake rats

Using an in vivo fatty acid model and operational equations, we reported that esterified and unesterified concentrations of docosahexaenoic acid (DHA, 22 : 6 n‐3) were markedly reduced in brains of third‐generation (F3) rats nutritionally deprived of α‐linolenic acid (18 : 3 n‐3), and that DHA turnover within phospholipids was reduced as well. The concentration of docosapentaenoic acid (DPA, 22 : 5 n‐6), an arachidonic acid (AA, 20 : 4 n‐6) elongation/desaturation product, was barely detectable in control rats but was elevated in the deprived rats. In the present study, we used the same in vivo model, involving the intravenous infusion of radiolabeled AA to demonstrate that concentrations of unesterified and esterified AA, and turnover of AA within phospholipids, were not altered in brains of awake F3‐generation n‐3‐deficient rats, compared with control concentrations. Brain DPA‐CoA could be measured in the deprived but not control rats, and AA‐CoA was elevated in the deprived animals. These results indicated that AA and DHA are recycled within brain phospholipids independently of each other, suggesting that recycling is regulated independently by AA‐ and DHA‐selective enzymes, respectively. Competition among n‐3 and n‐6 fatty acids within brain probably does not occur at the level of recycling, but at levels of elongation and desaturation (hence greater production of DPA during n‐3 deprivation), or conversion to bioactive eicosanoids and other metabolites.