Cerebral metabolism of plasma [14C]palmitate in awake, adult rat: subcellular localization

The Physiological and Pathological Role of Acyl-CoA Oxidation

Fatty acid metabolism, including β-oxidation (βOX), plays an important role in human physiology and pathology. βOX is an essential process in the energy metabolism of most human cells. Moreover, βOX is also the source of acetyl-CoA, the substrate for (a) ketone bodies synthesis, (b) cholesterol synthesis, (c) phase II detoxication, (d) protein acetylation, and (d) the synthesis of many other compounds, including N-acetylglutamate-an important regulator of urea synthesis. This review describes the current knowledge on the importance of the mitochondrial and peroxisomal βOX in various organs, including the liver, heart, kidney, lung, gastrointestinal tract, peripheral white blood cells, and other cells. In addition, the diseases associated with a disturbance of fatty acid oxidation (FAO) in the liver, heart, kidney, lung, alimentary tract, and other organs or cells are presented. Special attention was paid to abnormalities of FAO in cancer cells and the diseases caused by mutations in gene-encoding enzymes involved in FAO. Finally, issues related to α- and ω- fatty acid oxidation are discussed.

The Role of Acyl-CoA β-Oxidation in Brain Metabolism and Neurodegenerative Diseases

This review highlights the complex role of fatty acid β-oxidation in brain metabolism. It demonstrates the fundamental importance of fatty acid degradation as a fuel in energy balance and as an essential component in lipid homeostasis, brain aging, and neurodegenerative disorders.

Mitochondrial Metabolism in Astrocytes Regulates Brain Bioenergetics, Neurotransmission and Redox Balance

In the brain, mitochondrial metabolism has been largely associated with energy production, and its dysfunction is linked to neuronal cell loss. However, the functional role of mitochondria in glial cells has been poorly studied. Recent reports have demonstrated unequivocally that astrocytes do not require mitochondria to meet their bioenergetics demands. Then, the question remaining is, what is the functional role of mitochondria in astrocytes? In this work, we review current evidence demonstrating that mitochondrial central carbon metabolism in astrocytes regulates overall brain bioenergetics, neurotransmitter homeostasis and redox balance. Emphasis is placed in detailing carbon source utilization (glucose and fatty acids), anaplerotic inputs and cataplerotic outputs, as well as carbon shuttles to neurons, which highlight the metabolic specialization of astrocytic mitochondria and its relevance to brain function.

Determining the Bioenergetic Capacity for Fatty Acid Oxidation in the Mammalian Nervous System

The metabolic state of the brain can greatly impact neurologic function. Evidence of this includes the therapeutic benefit of a ketogenic diet in neurologic diseases, including epilepsy. However, brain lipid bioenergetics remain largely uncharacterized. The existence, capacity, and relevance of mitochondrial fatty acid β-oxidation (FAO) in the brain are highly controversial, with few genetic tools available to evaluate the question. We have provided evidence for the capacity of brain FAO using a pan-brain-specific conditional knockout (KO) mouse incapable of FAO due to the loss of carnitine palmitoyltransferase 2, the product of an obligate gene for FAO (CPT2B-/-). Loss of central nervous system (CNS) FAO did not result in gross neuroanatomical changes or systemic differences in metabolism. Loss of CPT2 in the brain did not result in robustly impaired behavior. We demonstrate by unbiased and targeted metabolomics that the mammalian brain oxidizes a substantial quantity of long-chain fatty acids in vitro and in vivo Loss of CNS FAO results in robust accumulation of long-chain acylcarnitines in the brain, suggesting that the mammalian brain mobilizes fatty acids for their oxidation, irrespective of diet or metabolic state. Together, these data demonstrate that the mammalian brain oxidizes fatty acids under normal circumstances with little influence from or on peripheral tissues.

Molecular effects of dietary fatty acids on brain insulin action and mitochondrial function

The prevalence of obesity and its co-morbidities such as insulin resistance and type 2 diabetes are tightly linked to increased ingestion of palatable fat enriched food. Thus, it seems intuitive that the brain senses elevated amounts of fatty acids (FAs) and affects adaptive metabolic response, which is connected to mitochondrial function and insulin signaling. This review will address the effect of dietary FAs on brain insulin and mitochondrial function with a special emphasis on the impact of different FAs on brain function and metabolism.

Cerebral Metabolic Changes During Sleep

PURPOSE OF REVIEW

The goal of the present paper is to review current literature supporting the occurrence of fundamental changes in brain energy metabolism during the transition from wakefulness to sleep.

RECENT FINDINGS

Latest research in the field indicates that glucose utilization and the concentrations of several brain metabolites consistently change across the sleep-wake cycle. Lactate, a product of glycolysis that is involved in synaptic plasticity, has emerged as a good biomarker of brain state. Sleep-induced changes in cerebral metabolite levels result from a shift in oxidative metabolism, which alters the reliance of brain metabolism upon carbohydrates. We found wide support for the notion that brain energetics is state dependent. In particular, fatty acids and ketone bodies partly replace glucose as cerebral energy source during sleep. This mechanism plausibly accounts for increases in biosynthetic pathways and functional alterations in neuronal activity associated with sleep. A better account of brain energy metabolism during sleep might help elucidate the long mysterious restorative effects of sleep for the whole organism.

Mouse Strain Impacts Fatty Acid Uptake and Trafficking in Liver, Heart, and Brain: A Comparison of C57BL/6 and Swiss Webster Mice

C57BL/6 and Swiss Webster mice are used to study lipid metabolism, although differences in fatty acid uptake between these strains have not been reported. Using a steady state kinetic model, [1-(14)C]16:0, [1-(14)C]20:4n-6, or [1-(14)C]22:6n-3 was infused into awake, adult male mice and uptake into liver, heart, and brain determined. The integrated area of [1-(14)C]20:4n-6 in plasma was significantly increased in C57BL/6 mice, but [1-(14)C]16:0 and [1-(14)C]22:6n-3 were not different between groups. In heart, uptake of [1-(14)C]20:4n-6 was increased 1.7-fold in C57BL/6 mice. However, trafficking of [1-(14)C]22:6n-3 into the organic fraction of heart was significantly decreased 33 % in C57BL/6 mice. Although there were limited differences in fatty acid tracer trafficking in liver or brain, [1-(14)C]16:0 incorporation into liver neutral lipids was decreased 18 % in C57BL/6 mice. In heart, the amount of [1-(14)C]16:0 and [1-(14)C]22:6n-3 incorporated into total phospholipids were decreased 45 and 49 %, respectively, in C57BL/6 mice. This was accounted for by a 53 and 37 % decrease in [1-(14)C]16:0 and 44 and 52 % decrease in [1-(14)C]22:6n-3 entering ethanolamine glycerophospholipids and choline glycerophospholipids, respectively. In contrast, there was a significant increase in [1-(14)C]20:4n-6 esterification into all heart phospholipids of C57BL/6 mice. Although changes in uptake were limited to heart, several significant differences were found in fatty acid trafficking into heart, liver, and brain phospholipids. In summary, our data demonstrates differences in tissue fatty acid uptake and trafficking between mouse strains is an important consideration when carrying out fatty acid metabolic studies.

Fat sensing and metabolic syndrome

Overconsumption of dietary fat contributes to the development of obesity and metabolic syndrome. Recent evidence suggests that high dietary fat may promote these metabolic states not only by providing calories but also by inducing impaired control of energy balance. In normal metabolic states, fat interacts with various organs or receptors to generate signals for the regulation of energy balance. Many of these interactions are impaired by high-fat diets or in obesity, contributing to the development or maintenance of obesity. These impairments may arise largely from fundamental alterations in the hypothalamus where all peripheral signals are integrated to regulate energy balance. This review focuses on various mechanisms by which fat is sensed at different stages of ingestion, circulation, storage, and utilization to regulate food intake, and how these individual mechanisms are altered by high-fat diets or in obesity.

Increased brain fatty acid uptake in metabolic syndrome

OBJECTIVE

To test whether brain fatty acid uptake is enhanced in obese subjects with metabolic syndrome (MS) and whether weight reduction modifies it.

RESEARCH DESIGN AND METHODS

We measured brain fatty acid uptake in a group of 23 patients with MS and 7 age-matched healthy control subjects during fasting conditions using positron emission tomography (PET) with [(11)C]-palmitate and [(18)F]fluoro-6-thia-heptadecanoic acid ([(18)F]-FTHA). Sixteen MS subjects were restudied after 6 weeks of very low calorie diet intervention.

RESULTS

At baseline, brain global fatty acid uptake derived from [(18)F]-FTHA was 50% higher in patients with MS compared with control subjects. The mean percentage increment was 130% in the white matter, 47% in the gray matter, and uniform across brain regions. In the MS group, the nonoxidized fraction measured using [(11)C]-palmitate was 86% higher. Brain fatty acid uptake measured with [(18)F]-FTHA-PET was associated with age, fasting serum insulin, and homeostasis model assessment (HOMA) index. Both total and nonoxidized fractions of fatty acid uptake were associated with BMI. Rapid weight reduction decreased brain fatty acid uptake by 17%.

CONCLUSIONS

To our knowledge, this is the first study on humans to observe enhanced brain fatty acid uptake in patients with MS. Both fatty acid uptake and accumulation appear to be increased in MS patients and reversed by weight reduction.

A Model for Fatty Acid Transport into the Brain

A key function of fatty acid (FA) transport into the brain is to supply polyunsaturated fatty acids (PUFA) that are not synthesized in brain cells but are essential signaling molecules and components of the phospholipid membrane. In addition, common dietary FAs such as palmitic acid are also rapidly taken up by the brain and esterified to phospholipids or oxidized to provide cellular energy. Most evidence shows that FA crossing the blood brain barrier (BBB) is derived mainly from FA/albumin complexes and, to a lesser extent, from circulating lipoproteins. Our model proposes that FA diffuse across the lipid bilayer of the BBB without specific transporters to reach brain cells. They cross the luminal and transluminal leaflets of the endothelial cells and the plasma membrane of neural cells by reversible flip-flop. Acyl-CoA synthetases trap FA by forming acyl-CoA, which cannot diffuse out of the cell. Selection of FA is controlled largely by enzymes in the pathways of intracellular metabolism, beginning with the acyl-CoA synthetase.

α-Synuclein gene ablation increases docosahexaenoic acid incorporation and turnover in brain phospholipids

Previously, we demonstrated that ablation of alpha-synuclein (Snca) reduces arachidonate (20:4n-6) turnover in brain phospholipids through modulation of an endoplasmic reticulum-localized acyl-CoA synthetase (Acsl). The effect of Snca ablation on docosahexaenoic acid (22:6n-3) metabolism is unknown. In the present study, we examined the effect of Snca gene ablation on brain 22:6n-3 metabolism. We determined 22:6n-3 uptake and incorporation into brain phospholipids by infusing awake, wild-type and Snca-/- mice with [1-14C]22:6n-3 using steady-state kinetic modeling. In addition, because Snca modulates 20:4n-6-CoA formation, we assessed microsomal Acsl activity using 22:6n-3 as a substrate. Although Snca gene ablation does not affect brain 22:6n-3 uptake, brain 22:6n-3-CoA mass was elevated 1.5-fold in the absence of Snca. This is consistent with the 1.6- to 2.2-fold increase in the incorporation rate and turnover in ethanolamine glycerophospholipid, phosphatidylserine, and phosphatidylinositol pools. Increased 22:6n-3-CoA mass was not the result of altered Acsl activity, which was unaffected by the absence of Snca. While Snca bound 22:6n-3, Kd = 1.0 +/- 0.5 micromol/L, it did not bind 22:6n-3-CoA. These effects of Snca gene deletion on 22:6n-3 brain metabolism are opposite to what we reported previously for brain 20:4n-6 metabolism and are likely compensatory for the decreased 20:4n-6 metabolism in brains of Snca-/- mice.

Acyl-CoA synthetase activity links wild-type but not mutant alpha-synuclein to brain arachidonate metabolism

Because alpha-synuclein (Snca) has a role in brain lipid metabolism, we determined the impact that the loss of alpha-synuclein had on brain arachidonic acid (20:4n-6) metabolism in vivo using Snca-/- mice. We measured [1-(14)C]20:4n-6 incorporation and turnover kinetics in brain phospholipids using an established steady-state kinetic model. Liver was used as a negative control, and no changes were observed between groups. In Snca-/- brains, there was a marked reduction in 20:4n-6-CoA mass and in microsomal acyl-CoA synthetase (Acsl) activity toward 20:4n-6. Microsomal Acsl activity was completely restored after the addition of exogenous wild-type mouse or human alpha-synuclein, but not by A30P, E46K, and A53T forms of alpha-synuclein. Acsl and acyl-CoA hydrolase expression was not different between groups. The incorporation and turnover of 20:4n-6 into brain phospholipid pools were markedly reduced. The dilution coefficient lambda, which indicates 20:4n-6 recycling between the acyl-CoA pool and brain phospholipids, was increased 3.3-fold, indicating more 20:4n-6 was entering the 20:4n-6-CoA pool from the plasma relative to that being recycled from the phospholipids. This is consistent with the reduction in Acsl activity observed in the Snca-/- mice. Using titration microcalorimetry, we determined that alpha-synuclein bound free 20:4n-6 (Kd = 3.7 microM) but did not bind 20:4n-6-CoA. These data suggest alpha-synuclein is involved in substrate presentation to Acsl rather than product removal. In summary, our data demonstrate that alpha-synuclein has a major role in brain 20:4n-6 metabolism through its modulation of endoplasmic reticulum-localized acyl-CoA synthetase activity, although mutant forms of alpha-synuclein fail to restore this activity.

Alpha-synuclein gene deletion decreases brain palmitate uptake and alters the palmitate metabolism in the absence of alpha-synuclein palmitate binding

Alpha-synuclein is an abundant protein in the central nervous system that is associated with a number of neurodegenerative disorders, including Parkinson's disease. Its physiological function is poorly understood, although recently it was proposed to function as a fatty acid binding protein. To better define a role for alpha-synuclein in brain fatty acid uptake and metabolism, we infused awake, wild-type, or alpha-synuclein gene-ablated mice with [1-(14)C]palmitic acid (16:0) and assessed fatty acid uptake and turnover kinetics in brain phospholipids. Alpha-synuclein deficiency decreased brain 16:0 uptake 35% and reduced its targeting to the organic fraction. The incorporation coefficient for 16:0 entering the brain acyl-CoA pool was significantly decreased 36% in alpha-synuclein gene-ablated mice. Because incorporation coefficients alone are not predictive of fatty acid turnover in individual phospholipid classes, we calculated kinetic values for 16:0 entering brain phospholipid pools. Alpha-synuclein deficiency decreased the incorporation rate and fractional turnover of 16:0 in a number of phospholipid classes, but also increased the incorporation rate and fractional turnover of 16:0 in the choline glycerophospholipids. No differences in incorporation rate or turnover were observed in liver phospholipids, confirming that these changes in lipid metabolism were brain specific. Using titration microcalorimetry, we observed no binding of 16:0 or oleic acid to alpha-synuclein in vitro. Thus, alpha-synuclein has effects on 16:0 uptake and metabolism similar to those of an FABP, but unlike FABP, it does not directly bind 16:0; hence, the mechanism underlying these effects is different from that of a classical FABP.

Brain arachidonic acid incorporation is decreased in heart fatty acid binding protein gene-ablated mice

Heart fatty acid binding protein (H-FABP) is expressed in neurons, but its role in brain fatty acid incorporation and metabolism is poorly defined. We examined the effect of H-FABP gene ablation on brain incorporation of arachidonic ([1-(14)C]20:4n-6) or palmitic ([1-(14)C]16:0) acid in vivo. Analysis of brain mRNA confirmed gene ablation and demonstrated no compensatory changes in the levels of other FABP mRNA in the gene-ablated mice. In brains from H-FABP gene-ablated mice, the incorporation coefficient for [1-(14)C]20:4n-6 was reduced 24%, while that for [1-(14)C]16:0 was unaffected. Within the organic and aqueous fractions, significantly more [1-(14)C]20:4n-6 was distributed into the aqueous fraction, suggesting a disruption in the metabolic targeting of 20:4n-6 in these mice. There was less incorporation of [1-(14)C]20:4n-6 into total phospholipids and a marked reduction (51%) in the level of incorporation into the choline glycerophospholipids (ChoGpl). Because FABP can influence steady-state lipid mass, brain individual lipid masses were measured. The brain total phospholipid mass was reduced 17% by gene ablation, ascribed to a 27% and 32% reduction in the masses of ChoGpl and sphingomyelin, respectively. Plasmalogen subclass masses were also reduced, suggesting that H-FABP may augment brain plasmalogen synthesis. In gene-ablated mice, the phosphatidylinositol 20:4n-6 level was reduced 25%, while the proportion of total n-6 fatty acids was reduced in the major phospholipid classes. Thus, these results demonstrate for the first time that H-FABP expression influences brain 20:4n-6 uptake and trafficking as well as steady-state brain lipid levels.

Intravenously injected [1-14C]arachidonic acid targets phospholipids, and [1-14C]palmitic acid targets neutral lipids in hearts of awake rats

The differential uptake and targeting of intravenously infused [1-14C]palmitic ([1-14C]16:0) and [1-14C]arachidonic ([1-14C]20:4n-6) acids into heart lipid pools were determined in awake adult male rats. The fatty acid tracers were infused (170 microCi/kg) through the femoral vein at a constant rate of 0.4 mL/min over 5 min. At 10 min postinfusion, the rats were killed using pentobarbital. The hearts were rapidly removed, washed free of exogenous blood, and frozen in dry ice. Arterial blood was withdrawn over the course of the experiment to determine plasma radiotracer levels. Lipids were extracted from heart tissue using a two-phase system, and total radioactivity was measured in the nonvolatile aqueous and organic fractions. Both fatty acid tracers had similar plasma curves, but were differentially distributed into heart lipid compartments. The extent of [1-14C]20:4n-6 esterification into heart phospholipids, primarily choline glycerophospholipids, was elevated 3.5-fold compared to [1-14C]16:0. The unilateral incorporation coefficient, k*, which represents tissue radioactivity divided by the integrated plasma radioactivity for heart phospholipid, was sevenfold greater for [1-14C]20:4n-6 than for [1-14C]16:0. In contrast, [1-14C]16:0 was esterified mainly into heart neutral lipids, primarily triacylglycerols (TG), and was also found in the nonvolatile aqueous compartment. Thus, in rat heart, [1-14C]20:4n-6 was primarily targeted for esterification into phospholipids, while [1-14C]16:0 was targeted for esterification into TG or metabolized into nonvolatile aqueous components.

Evidence for the involvement of docosahexaenoic acid in cholinergic stimulated signal transduction at the synapse

[4,5-3H]Docosahexaenoic acid ([3H]DHA) or [9,10-3H]palmitic acid ([3H]PAM) was infused intravenously for 5 min to awake, adult male rats before and after treatment with arecoline (15 mg/kg, i.p.), a cholinergic agonist. Animals were killed 15 min post-infusion, the brains were rapidly removed and subcellular fractions were obtained after sucrose density centrifugation. In control animals, [3H]DHA and [3H]PAM were incorporated into the synaptosomal fractions, representing 50%-60% of total membrane label. Most remaining membrane label (30%-40%) was in the microsomal fraction. Both fractions contained the synaptic marker synaptophysin. The remaining 10% of radioactivity was in the myelin and mitochondrial fractions. Arecoline significantly increased [3H]DHA entry into the synaptosomal fractions by 100% and into the microsomal fraction by 50%. In these fractions 60%-65% of the [3H]DHA was in phospholipid, the rest corresponding to free fatty acid and diacylglycerol. In contrast, arecoline did not change [3H]PAM incorporation into any brain fraction. These results demonstrate that plasma [3H]DHA incorporation is selectively increased into synaptic membrane phospholipids of the rat brain in response to cholinergic activation. The increased incorporation of DHA but not of PAM into synaptic membranes in response to cholinergic stimulation indicates a primary role for DHA in phospholipid mediated signal transduction at the synapse involving activation of phospholipase A2 and/or C.

In vivo imaging of fatty acid incorporation into brain to examine signal transduction and neuroplasticity involving phospholipids

An in vivo method is presented that allows quantification and imaging of fatty acid incorporation into different brain phospholipids in relation to membrane synthesis, neuroplasticity, and signal transduction. The method can be used with positron emission tomography, and may help to evaluate brain phospholipid metabolism in humans with brain tumors, neurodegenerative disease, cerebral ischemia or trauma, or neurotoxic effects of drugs or other agents.

A quantitative method for measuring regional in vivo fatty-acid incorporation into and turnover within brain phospholipids: review and critical analysis

An experimental method and its associated mathematical model are described to quantitate in vivo incorporation rates into and turnovers of fatty acids (FAs) within stable brain metabolic compartments, particularly phospholipids. A radiolabeled FA is injected i.v. in a rat, and arterial plasma unacylated FA radioactivities and unlabeled concentrations are sampled until the animal is killed after 15 min, when the brain is analyzed biochemically or with quantitative autoradiography. Unbound unacylated label in blood easily crosses the blood-brain barrier; rapidly equilibrates in the unacylated FA, acyl-CoA and phosphatidate-diacylglycerol brain pools; then is incorporated into phospholipids and other stable metabolic compartments. Uptake and incorporation of labeled FAs are independent of cerebral blood flow at constant brain blood volume. Different labeled FAs enter specific sn positions of different brain phospholipids, suggesting that a combination of probes can be used to investigate metabolism of these phospholipids. Thus, [9,10-3-H]palmitate preferentially labels the sn1 position of phosphatidylcholine; [1-14C]arachidonate the sn2 positions of phosphatidylinositol and phosphatidylcholine; and [1-14C]docosahexaenoate the sn2 positions of phosphatidylethanolamine and phosphatidylcholine. The FA model provides an operational equation for rates of incorporation of FAs into brain phospholipids, taking into account intracerebral recycling and de novo synthesis of the FA, as well as entry into brain of FA from acylated blood sources. The equation is essentially independent of specific details of the proposed model, and can be used to calculate turnovers and half-lives of FAs within different phospholipid classes. For the model to be most applicable, experiments should satisfy conditions for pulse-labeling of the phospholipids, with brain sampling times short enough to minimize exchange of label between stable metabolic compartments. A 15-20 min sampling time satisfies these criteria. The FA method has been used to elucidate the dynamics of brain phospholipids metabolism in relation to brain development, brain tumor, chronically reduced auditory input, transient ischemic insult, axotomy with and without nerve regeneration, and cholinergic stimulation in animals with or without a chronic unilateral lesion of the nucleus basalis magnocellularis.

In vivo brain incorporation of [1-14C]arachidonate in awake rats, with or without cholinergic stimulation, following unilateral lesioning of nucleus basalis magnocellularis

Regional brain incorporation of a radiolabeled unsaturated fatty acid, [1-14C]arachidonic acid (14C-AA), was measured in awake rats following unilateral lesioning of the nucleus basalis magnocellularis (NBM). Right-sided lesions were produced in 3-month-old, male rats by stereotaxic injection of 10 micrograms ibotenic acid. Two weeks after lesioning, rats were subjected to one of two protocols: (1) 5 min intravenous infusion of 14C-AA (170 microCi/kg); or (2) i.p. injection of arecoline (5 mg/kg), a cholinergic agonist, followed by 5 min intravenous infusion of 14C-AA. All animals were killed 15 min postinfusion. Brains were frozen and sectioned for quantitative autoradiography or were stained for acetylcholinesterase (AChE). Animals with unilateral NBM lesions displayed reduced AChE staining in prefrontal, frontal and parietal cortices of the lesioned side, but there was no right-left difference in incorporation of 14C-AA without cholinergic stimulation. Arecoline administration increased 14C-AA incorporation into the prefrontal and frontal cortices ipsilateral to the NBM lesion as compared to the contralateral side and the increase was most prominent in deeper cortical layers such as layers IV and V. Right-left differences in incorporation were not apparent in parietal, temporal, or occipital cortices, where reduction of AChE activity was minimal or absent, nor in subcortical structures. The results suggest that the intravenous 14C-AA technique combined with cholinergic stimulation can be used to detect compensatory regulation of phospholipid-coupled signal transduction caused by a deficit in cholinergic input into the cerebral cortex.

In vivo incorporation of [9,10(-3)H]-palmitate into a rat metastatic brain-tumor model

Lipid metabolism of an intracerebrally implanted brain tumor and normal brain was investigated in awake Fischer 344 rats using intravenously injected [9,10(-3)H]-palmitate as a probe. A suspension of Walker 256 carcinosarcoma cells (250 cells in 5 microliters medium), with or without 1% low-melting-point agar, was implanted into the caudate nucleus of rats 8 to 9 weeks old. Control animals received an intracerebral injection without tumor cells. Seven days after implantation, awake rats were infused intravenously for 5 minutes with [9,10(-3)H]-palmitate (6.4 mCi/kg). The rats were killed 20 minutes after initiation of the infusion and coronal brain slices were obtained for quantitative autoradiography and light histological study. Tumor cell masses were histologically well demarcated from the surrounding brain tissue. Tumor tissue incorporation of [9,10(-3)H]-palmitate was heterogeneous, ranging on average from 3.1- to 6.1-fold greater than in the corresponding contralateral brain. In addition, incorporation corresponded to regional tumor cell density. The incorporation rate constant of [9,10(-3)H]-palmitate in tumor was significantly increased compared to control brain and was independent of tumor size. Necrotic areas within tumors showed no incorporation of radiolabeled palmitate. Brain surrounding the tumors and control injection sites showed reactive gliosis, and possessed 30% greater incorporation of [9,10(-3)H]-palmitate than contralateral normal brain. These results suggest that [9,10(-3)H]-palmitate can be used to image brain tumors in vivo, measuring turnover and/or synthesis of tumor and brain lipid.

Fatty acid oxidation and ketogenesis by astrocytes in primary culture

The oxidation of the fatty acids octanoate and palmitate to CO2 and the ketone bodies acetoacetate and D-(-)-3-hydroxybutyrate was examined in astrocytes that were prepared from cortex of 2-day-old rat brain and grown in primary culture to confluence. Accumulation of acetoacetate (by mass) in the culture medium of astrocytes incubated with octanoate (0.3-0.5 mM) was 50-90 nmol C2 units h-1 mg of protein-1. A similar rate was obtained using radiolabeled tracer methodology with [1-14C]octanoate as labeled substrate. The results from the radiolabeled tracer studies using [1-14C]- and [7-14C]octanoate and [1-14C]-, [13-14C]-, and [15-14C]palmitate indicated that a substantial proportion of the omega-terminal four-carbon unit of these fatty acids bypassed the beta-ketothiolase step of the beta-oxidation pathway and the 3-hydroxy-3-methylglutaryl (HMG)-CoA cycle of the classic ketogenic pathway. The [14C]acetoacetate formed from the 1-14C-labeled fatty acids, obligated to pass through the acetyl-CoA pool, contained 50% of the label at carbon 3 and 50% at carbon 1. By contrast, the [14C]acetoacetate formed from (omega-1)-labeled fatty acids contained 90% of the label at carbon 3 and 10% at carbon 1, whereas that formed from the (omega-3)-labeled fatty acid contained 20% of the label at carbon 3 and 80% at carbon 1. These results indicate that acetoacetate is primarily formed either by the action of 3-oxo-acid-CoA transferase (EC 2.8.3.5) or acetoacetyl-CoA deacylase (EC 3.1.2.11) or both on acetoacetyl-CoA and not by the action of the mitochondrial HMG-CoA cycle involving HMG-CoA lyase (EC 4.1.3.4), which was readily detected, and HMG-CoA synthase (EC 4.1.3.5), which was barely measurable.

Quantitative brain autoradiography of [9,10-3H]palmitic acid incorporation into brain lipids

The distribution of radioactivity within brain metabolic compartments was examined following the intravenous injection of [9,10-3H]palmitate into awake rats. Brain radioactivity reached a maximum value by 15 min after [9,10-3H]palmitate injection and remained unchanged for at least 4 hr. Regional differences in radioactivity could be determined with high resolution by quantitative autoradiography, at the level of cell layers within the hippocampus and cerebral cortex, and between striosomes of the caudate nucleus. Regional brain radioactivities were converted to normalized regional radioactivities (k) by dividing them by the integrated plasma fatty acid radioactivity (integrated over the time course of the experiment). These values reflected incorporation mainly into brain phospholipids; radioactivity due to nonlipid components was minimal. Indeed, about 85% of brain radioactivity was within lipids between 5 min and 4 hr postinjection, the remainder being equally divided between protein-associated pellet and aqueous-soluble metabolites. The major lipids labeled were phospholipids, particularly phosphatidylcholine, which contained about 75% of phospholipid radioactivity. The results show that [9,10-3H]palmitate can be used to examine incorporation of plasma palmitate into individual brain regions via quantitative autoradiography. Furthermore, the tracer is a rather selective marker for phosphatidylcholine and can be used to examine turnover and synthesis of this phospholipid. [9,10-3H]palmitate has advantages over [U-14C]palmitate for autoradiographic studies of incorporation; following the 14C-tracer, significant label even at 4 hr after injection is in nonlipid compartments (glutamate and aspartate), and the long path length of 14C limits resolution at the cell layer level.

A method for examining turnover and synthesis of palmitate-containing brain lipids in vivo

1. A theoretical three compartment model is presented which gives the rate of incorporation of plasma palmitate into brain, Jpalm, in terms of turnover and synthesis of palmitate-containing lipids, de novo synthesis of palmitate from acetate, and recycling of palmitate within lipids. 2. Jpalm equals 4 h brain radioactivity following intravenous injection of [U-14C]-palmitate (determined with quantitative autoradiography), divided by integrated plasma specific activity of palmitate. Jpalm follows the time course of brain lipid synthesis during development of the rat, but is age-invariant in the adult. 3. At 1-7 days after 5 min of bilateral carotid occlusion in the awake gerbil, intravascular [14C]-palmitate incorporation is reduced in the CA1 pyramidal layer of the hippocampus, consistent with delayed neuronal death, but is elevated in the CA3 and CA4 pyramidal layers and dentate gyrus, suggesting synthesis of new membrane during recovery from the ischaemic insult. 4. Several weeks after unilateral destruction of the cochlea in 11 day old rats, incorporation of [14C]-palmitate from plasma into appropriate central auditory regions is reduced, corresponding to reduced cell size and altered morphology. 5. [14C]-palmitate incorporation into the left hypoglossal nucleus is increased during and following axonal regeneration (up to 23% compared with control side) following transection of the left hypoglossal nerve in Fischer-344 rats, whereas incorporation is decreased 6-7% when regeneration is prevented. Time courses of incorporation in both cases correspond to histological changes. 6. The results show that the palmitate method can be used to examine regional turnover and synthesis of brain lipids following injury, sensory deprivation, development, regeneration and ageing.

Incorporation of plasma [14C]palmitate into the hypoglossal nucleus following unilateral axotomy of the hypoglossal nerve in adult rat, with and without regeneration

Incorporation of plasma [14C]palmitate into the hypoglossal nucleus of the rat following transection, with and without regeneration of the hypoglossal nerve, was studied using quantitative autoradiography. The left hypoglossal nerve of 3-month-old, male Fischer-344 rats was transected using either: (1) an R-operation, which allowed nerve regeneration; or (2) a D-operation, in which regeneration was prevented. One to 84 days after axotomy, [14C]palmitate was injected intravenously and its rates of incorporation into stable structures of the left and right hypoglossal nuclei were measured at 4 h after injection. Following the R-operation, incorporation into the left hypoglossal nucleus was increased during and following axonal regeneration (up to 23% compared to control side), whereas incorporation was decreased 6-7% in the absence of regeneration, using the D-operation. The time courses of incorporation in both cases corresponded to histological changes, especially cell membrane changes following axotomy and suggest that [14C]palmitate incorporation reflects regenerative and degenerative neuronal changes associated with changes in lipid synthesis.

Regional cerebral palmitate incorporation after unilateral auditory deprivation in immature and adult Fischer-344 rats

Regional cerebral incorporation of intravenously injected [U-14C]palmitate was measured from 1 day to 13 weeks after left cochlear destruction in 11-day- and 3 month-old, awake Fischer-344 rats. In 11-day-old animals, statistically significant left-right differences in incorporation were absent 1 day after cochlear destruction and were found only in parts of the cochlear nucleus and inferior colliculus after 1 week. After 6 to 13 weeks, consistent with functional neuroanatomy of central auditory regions, incorporation was reduced by 6 to 9% in the left cochlear nucleus and left lateral superior olivary nucleus, compared with corresponding right-side regions. The right medial superior olivary nucleus, medial nucleus of the trapezoid body, lateral lemniscus nucleus, inferior colliculus, medial geniculate body, and auditory cortex had 5 to 9% less incorporation than did corresponding left-side regions. Fewer significant differences after chronic auditory deprivation occurred in 3-month-old rats than in 11-day-old rats following cochlear destruction. Reduced incorporation corresponded to reported changes in cell morphology, which also were greater in immature than mature rats following auditory deprivation. The results suggest that the palmitate method can be used to identify long-term regional changes in the turnover of brain lipids after sensory deprivation.