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

Brain uptake and utilization of fatty acids, lipids and lipoproteins: application to neurological disorders

Transport, synthesis, and utilization of brain fatty acids and other lipids have been topics of investigation for more than a century, yet many fundamental aspects are unresolved and, indeed, subject to controversy. Understanding the mechanisms by which lipids cross the blood brain barrier and how they are utilized by neurons and glia is critical to understanding normal brain development and function, for the diagnosis and therapy of human diseases, and for the planning and delivery of optimal human nutrition throughout the world. Two particularly important fatty acids, both of which are abundant in neuronal membranes are: (a) the omega3 polyunsaturated fatty acid docosahexaenoic acid, deficiencies of which can impede brain development and compromise optimal brain function, and (b) the omega6 polyunsaturated fatty acid arachidonic acid, which yields essential, but potentially toxic, metabolic products. There is an exciting emerging evidence that modulating dietary intake of these fatty acids could have a beneficial effect on human neurological health. A workshop was held in October, 2004, in which investigators from diverse disciplines interacted to present new findings and to discuss issues relevant to lipid uptake, utilization, and metabolism in the brain. The objectives of this workshop were: (1) to assess the state-of-the-art of research in brain fatty acid/lipid uptake and utilization; (2) to discuss progress in understanding molecular mechanisms and the treatment of neurological diseases related to lipids and lipoproteins; (3) to identify areas in which current knowledge is insufficient; (4) to provide recommendations for future research; and (5) to stimulate the interest and involvement of additional neuroscientists, particularly young scientists, in these areas. The meeting was divided into four sessions: (1) mechanisms of lipid uptake and transport in the brain, (2) lipoproteins and polyunsaturated fatty acids, (3) eicosanoids in brain function, and (4) fatty acids and lipids in brain disorders. In this article, we will provide an overview of the topics discussed in these sessions.

Imaging brain phospholipase A2-mediated signal transduction in response to acute fluoxetine administration in unanesthetized rats

Fluoxetine, a selective serotonin (5-hydroxytryptamine, 5-HT) reuptake inhibitor, is used widely to treat depression and related disorders. By inhibiting presynaptic 5-HT reuptake, fluoxetine is thought to act by increasing 5-HT in the synaptic cleft, thus 5-HT binding to postsynaptic 5-HT(2A/2C) receptors. These receptors can be coupled via a G-protein to phospholipase A(2) (PLA(2)), which when activated releases the second messenger arachidonic acid from synaptic membrane phospholipids. To image this activation, fluoxetine (10 mg/kg) or saline vehicle was administered i.p. to unanesthetized rats, and regional brain incorporation coefficients k(*) of intravenously injected radiolabeled arachidonic acid were measured after 30 min. Compared with vehicle, fluoxetine significantly increased k(*) in prefrontal, motor, somatosensory, and olfactory cortex, as well as in the basal ganglia, hippocampus, and thalamus. Many of these regions demonstrate high densities of the serotonin reuptake transporter and of 5-HT(2A/2C) receptors. Brain stem, spinal cord, and cerebellum, which showed no significant response to fluoxetine, have low densities of the transporters and receptors. The results show that it is possible to image quantitatively PLA(2)-mediated signal transduction in vivo in response to fluoxetine.

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 microL x min(-1) x 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 uptake and utilization of fatty acids: applications to peroxisomal biogenesis diseases

The brain is rich in diverse fatty acids saturated, monounsaturated and polyunsaturated fatty acids with chain lengths ranging from less than 16 to more than 24 carbons that make up the complex lipids present in this organ. While some fatty acids are derived from endogenous synthesis, others must come from exogenous sources. The mechanism(s) by which fatty acids enter cells has been the subject of much debate. While some investigators argue for a protein-mediated process, others suggest that simple diffusion is sufficient. In the brain, uptake is further complicated by the presence of the blood-brain barrier. Brain fatty acid homeostasis is disturbed in many human disorders, as typified by the peroxisomal biogenesis diseases. A workshop designed to bring together researchers from varied backgrounds to discuss these issues in an open forum was held in March, 2000. In addition to assessing the current state of knowledge, areas requiring additional investigation were identified and recommendations for future research were made. A brief overview of the invited talks is presented here.

In vivo fatty acid incorporation into brain phospholipids in relation to signal transduction and membrane remodeling

A method and model are described to quantify in vivo turnover rates and half-lives of fatty acids within brain phospholipids. These "kinetic" parameters can be calculated by operational equations from measured rates of incorporation of intravenously injected fatty acid radiotracers into brain phospholipids. To do this, it is necessary to determine a "dilution factor" lambda, which estimates the contribution to the brain precursor acyl-CoA pool of fatty acids released from phospholipids through the action of PLA1 or PLA2. Some calculated fatty acid half-lives are minutes to hours, consistent with active participation of phospholipids in brain function and structure. The fatty acid method can be used to identify enzyme targets of drugs acting on phospholipid metabolism. For example, a reduced brain turnover of arachidonate by chronic lithium, demonstrated in rats by the fatty acid method, suggests that this agent, which is used to treat bipolar disorder, has for its target an arachidonate-specific PLA2. In another context, when combined with in vivo imaging by quantitative autoradiography in rodents or positron emission tomography in macaques or humans, the fatty acid method can localize and quantify normal and modified PLA2-mediated signal transduction in brain.

Effects of EGb 761 on fatty acid reincorporation during reperfusion following ischemia in the brain of the awake gerbil

Transient cerebral ischemia (5 min) releases unesterified fatty acids from membrane phospholipids, increasing brain concentrations of fatty acids for up to 1 h following reperfusion. To understand the reported anti-ischemic effect of Ginkgo biloba extract (EGb 761), we monitored its effect on brain fatty acid reincorporation in a gerbil-stroke model. Both common carotid arteries in awake gerbils were occluded for 5 min, followed by 5 min of reperfusion. Animals were infused intravenously with labeled arachidonic (AA) or palmitic acid (Pam), and rates of incorporation of unlabeled fatty acid from the brian acyl-CoA pool were calculated by the model of Robinson et al. (1992), using quantitative autoradiography and biochemical analysis of brain acyl-CoA. Animals were treated for 14 d with 50 or 150 mg/kg/d EGb 761 or vehicle. Ischemia-reperfusion had no effect on the rate of unlabeled Pam incorporation into brain phospholipids from palmitoyl-CoA; this rate also was unaffected by EGb 761. In contrast, ischemia-reperfusion increased the rate of incorporation of unlabeled AA from brain arachidonoyl-CoA by a factor of 2.3-3.3 compared with the control rate; this factor was further augmented to 3.6-5.0 by pretreatment with EGb 761. There is selective reincorporation of AA compared with Pam into brain phospholipids following ischemia. EGb 761 further accelerates AA reincorporation, potentially reducing neurotoxic effects of prolonged exposure of brain to high concentrations of AA and its metabolites.

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.

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.

Sequelae of parenteral domoic acid administration in rats: comparison of effects on different metabolic markers in brain

Parenterally administered domoic acid, a structural analog of the excitatory amino acids glutamic acid and kainic acid, has specific effects on brain histology in rats, as measured using different anatomic markers. Domoic acid-induced convulsions affects limbic structures such as hippocampus and entorhinal cortex, and different anatomic markers can detect these neurotoxic effects to varying degrees. Here we report effects of domoic acid administration on quantitative indicators of brain metabolism and gliosis. Domoic acid, 2.25 mg/kg i.p., caused stereotyped behavior and convulsions in approximately 60% of rats which received it. Six to eight days after domoic acid or vehicle administration, the animals were processed to measure regional brain incorporation of the long-chain fatty acids [1-(14)C]arachidonic acid ([14C]AA) and [9,10-(3)H]palmitic acid ([3H]PA), or regional cerebral glucose utilization (rCMRglc) using 2-[1-(14)C]deoxy-D-glucose, by quantitative autoradiography. Others rats were processed to measure brain glial fibrillary acidic protein (GFAP) by enzyme-linked immunosorbent assay. Domoic acid increased GFAP in the anterior portion of cerebral cortex, the caudate putamen and thalamus compared with vehicle. However, in rats that convulsed after domoic acid GFAP was significantly increased throughout the cerebral cortex, as well as in the hippocampus, septum, caudate putamen, and thalamus. Domoic acid, in the absence of convulsions, decreased relative [14C]AA incorporation in the claustrum and pyramidal cell layer of the hippocampus compared with vehicle-injected controls. In the presence of convulsions, relative [14C]AA incorporation was decreased in hippocampus regions CA1 and CA2. Uptake of [3H]PA into brain was unaffected. Relative rCMRglc decreased in entorhinal cortex following domoic acid administration with or without convulsions. These results suggest that acute domoic acid exposure affects discrete brain circuits by inducing convulsions, and that domoic acid-induced convulsions cause chronic effects on brain function that are reflected in altered fatty acid metabolism and gliosis.

Evidence for membrane remodeling in ipsilateral thalamus and amygdala following left amygdala-kindled seizures in awake rats

We examined regional cerebral metabolic rates for glucose (rCMRglc) and brain incorporation coefficients (k*) of each of three intravenously infused fatty acid radiotracers, [9,10-(3H)]palmitate ([3H]PAM), [1-(14C)]arachidonate ([14C]AA) and [1-(14C)]docosahe-xaenoate ([14C]DHA), in awake rats fully kindled by once-daily electrical stimulation of the left amygdala. Compared with sham-stimulated animals, rCMRglc was increased bilaterally during a seizure, particularly in midbrain-brain stem regions, thalamus and basolateral nucleus of the amygdala. At 24 h and 2 weeks after a seizure, there was no significant change in k* for either [14C]AA or [14C]DHA in any brain region, whereas k* for [3H]PAM at 24 h was increased significantly (by 32-53%) ipsilateral to stimulation in regions of the amygdala and thalamus. Contralateral regions showed no significant change. Two weeks after a seizure, k* for [3H]PAM was increased in the ipsilateral lateral dorsal nucleus of the thalamus. These results argue for membrane remodeling involving phosphatidylcholine in the ipsilateral amygdala and thalamus at the completed phase of amygdala kindling. Remodeling may continue for up to 2 weeks after a seizure during the completed phase.

Effects of chronic beta-amyloid treatment on fatty acid incorporation into rat brain

The present study evaluated the effects of chronic A beta administration on radio-labeled plasma fatty acid incorporation in rat brain. A beta was chronically infused intraventricularly via an osmotic minipump, for 1 week, at a concentration of 460 microM. After the infusion, fatty acid incorporation was quantified using an in vivo method developed in this laboratory. Three radiolabeled fatty acids were separately infused IV in awake animals. Biochemical analyses of fatty acid incorporation and histology for A beta showed no differences between control (vehicle infusion only) and experimental groups. However, in vitro tests on the cytotoxicity of A beta showed that it caused significant cell death relative to controls (PC-12 cells). The lack of effect of infused A beta on radiolabeled fatty acid incorporation is discussed.

In vivo imaging of brain incorporation of fatty acids and of 2-deoxy-D-glucose demonstrates functional and structural neuroplastic effects of chronic unilateral visual deprivation in rats

Regional cerebral 'incorporation coefficients' k* of each of 3 labeled long-chain fatty acids -[9,10-3H]palmitate ([3H]PA), [1-14C]arachidonate ([14C]AA) and [1-14C]docosahexaenoate ([14C]DHA)-were measured using quantitative autoradiography in 11 bilateral brain visual areas of 3.5-month-old awake, hooded, Long-Evans rats, and were compared with regional cerebral metabolic rates for glucose (rCMRglc). The rats, which had undergone unilateral orbital enucleation at 15 days of age, were studied either in the dark with eyelids of the intact eye sutured, or when stimulated in a light box with the intact eye open. rCMRglc did not differ between homologous contralateral and ipsilateral visual areas in the dark or during stimulation, but was elevated bilaterally by 25% or more in many visual areas during stimulation compared with dark. Contralateral compared with ipsilateral k* was lower for each fatty acid tracer in superficial gray of the superior colliculus (in dark and during stimulation) and dorsal nucleus of lateral geniculate body (during stimulation). In the dark, k* for [3H]PA was correlated significantly with rCMRglc for the 22 visual areas studied, whereas during stimulation k* for [14C]AA was correlated with rCMRglc. These results suggest that central neuroplastic changes following chronic unilateral enucleation are accompanied by reduced incorporation of [3H]PA, [14C]AA and [14C]DHA into contralateral brain ares that normally receive crossed retinofugal fibers, and by symmetry of rCMRglc in the dark but increased bilateral symmetrical responsiveness of rCMRglc to visual stimulation of the intact eye.

The effect of methyl palmoxirate on incorporation of [U-14C]palmitate into rat brain

We examined the dose response, time course and reversibility of the effect of methyl 2-tetradecylglycidate (McN-3716, methyl palmoxirate or MEP), an inhibitor of beta-oxidation of fatty acids, on incorporation of radiolabeled palmitic acid ([U-14C]PA) from plasma into brain lipids of awake rats. MEP (0.1, 1 and 10 mg/kg) or vehicle was administered intravenously from 10 min to 72 hr prior to infusion of [U-14C]PA. Two hr pretreatment with MEP (0.1 to 10 mg/kg) increased brain organic radioactivity 1.2 to 1.8 fold and decreased brain aqueous radioactivity by 1.2 to 3.0 fold when compared to control values. At 10 mg/kg, MEP significantly increased brain organic fraction from 40% in controls to 85%, 30 min to 6 hr pretreatment, and resulted in a redistribution of the radiolabeled fatty acid toward triacylglycerol. MEP changed the lipid/aqueous brain ratio of incorporated [U-14C]PA from 0.67 to 5.7. The incorporation rate coefficient, k*, was significantly increased by MEP (10 mg/kg) at 2 hr (31%), 4 hr (59%) and 6 hr (34%). All effects were reversed by 72 hr, consistent with a half-life of approximately 2 days for carnitine palmitoyl transferase I. These results indicate that intravenous MEP may be used with [1-11C]palmitic acid for studying brain lipid metabolism in vivo by positron emission tomography, as it significantly reduces the large unincorporated aqueous fraction that would result in high background radioactivity.

Effect of inhibition of beta-oxidation on incorporation of [U-14C]palmitate and [1-14C]arachidonate into brain lipids

The purpose of the present study was to determine the effect of inhibiting the mitochondrial beta-oxidation of free fatty acids on the incorporation of radiolabeled free fatty acids into brain lipids. To this end, methyl 2-tetradecylglycidate (MEP), an irreversible inhibitor of carnitine palmitoyltransferase I, was given orally to male rats 2, 4, and 6 h prior to an intravenous infusion of the saturated fatty acid [U-14C]palmitic acid (PA) or the polyunsaturated fatty acid [1-14C]arachidonate (AA). With [U-14C]PA, MEP (10-25 mg/kg) increased brain organic radioactivity 2-fold and decreased brain aqueous radioactivity 3- to 5-fold relative to control values at all pretreatment times. The effect was due to prolongation of the plasma integral of [U-14C]PA due to peripheral inhibition of beta-oxidation, and to direct inhibition of beta-oxidation of the tracer within the brain. MEP had no effect on brain organic radioactivity after infusion of [1-14C]AA. Increasing the interval between MEP administration and [U-14C]PA infusion from 2 to 6 h resulted in a dramatic redistribution of [U-14C]PA within brain lipids. The percentage of radioactivity in phospholipids decreased from 65 to 33%, whereas that in the free fatty acid fraction increased from 10 to 47% and that in triglycerides was elevated 2-3 fold over control levels. These results indicate that MEP may facilitate the use of radiolabeled PA as an in vivo probe of brain lipid metabolism using quantitative autoradiography or positron emission tomography.

Differences in rates of incorporation of intravenously injected radiolabeled fatty acids into phospholipids of intracerebrally implanted tumor and brain in awake rats

This study investigates the incorporation of three intravenously administered radiolabeled fatty acids, [9,10-3H]palmitate (3H-PAM), [1-14C]arachidonate (14C-ACH) and [1-14C]docosahexaenoate (14C-DHA), into lipids of intracerebrally implanted tumor and contralateral brain cortex in awake rats. A suspension of Walker 256 carcinosarcoma cells (1 x 10(6) cells) was implanted into the right cerebral hemisphere of an 8- to 9-week-old Fischer-344 rat. Seven days later, the awake rat was infused intravenously for 5 min with 3H-PAM (6.4 mCi/kg), 14C-ACH (170 microCi/kg) or 14C-DHA (100 microCi/kg). Twenty min after the start of infusion, the rat was killed and intracranial tumor mass and brain cortex were removed for lipids analysis. Each radiolabel was incorporated more into tumor than into brain cortex. Ratios of net incorporation rate coefficients (k*) into tumor as compared with brain were 4.5, 3.4 and 1.7 for 3H-PAM, 14C-ACH and 14C-DHA, respectively. Lipid radioactivity comprised more than 80% of total tumor or brain radioactivity for each probe. Phospholipids contained 58%, 89% and 68% of tumor lipid radioactivity, and 58%, 82% and 74% of brain lipid radioactivity, for 3H-PAM, 14C-ACH and 14C-DHA, respectively. Incorporation coefficients (k*i) for a phospholipid class (i)--choline phosphoglycerides (PC), inositol monophosphoglycerides (PI), ethanolamine phosphoglycerides (PE), serine phosphoglycerides (PS), and sphingomyelin (SM)--were greater in tumor than in brain for each fatty acid probe, except that values for k*PE and k*PS using 14C-DHA were equivalent. Differences in k*i between tumor and brain were largest for SM and PC and the change in k*PC accounted for 65-90% of the increase in the net phospholipid incorporation rate for each probe. Differences in k*PI, k*PE and k*PS were smaller than those in were smaller than those in k*PC and k*SM, and varied with the probe. Differences in k*i were related to differences in tumor and brain phospholipid composition and metabolism. The results indicate that suitably radiolabeled fatty acids may be used to image and characterize metabolism of lipid compartments of a brain tumor in vivo using positron emission tomography.

In vivo cerebral incorporation of radiolabeled fatty acids after acute unilateral orbital enucleation in adult hooded Long-Evans rats

We examined effects of acute unilateral enucleation on incorporation from blood of intravenously injected unsaturated [1-14C]arachidonic acid ([14C]AA) and [1-14C]docosahexaenoic acid ([14C]DHA), and of saturated [9,10-3H]palmitic acid ([3H]PA), into visual and nonvisual brain areas of awake adult Long-Evans hooded rats. Regional cerebral metabolic rate for glucose (rCMRglc) values also were assessed with 2-deoxy-D-[1-14C]glucose ([14C]DG). One day after unilateral enucleation, an awake rat was placed in a brightly lit visual stimulation box with black and white striped walls, and a radiolabeled fatty acid was infused for 5 min or [14C]DG was injected as a bolus. [14C]DG also was injected in a group of rats kept in the dark for 4 h. Fifteen minutes after starting an infusion of a radiolabeled fatty acid, or 45 min after injecting [14C]DG, the rat was killed and the brain was prepared for quantitative autoradiography. Incorporation coefficients k* of fatty acids, or rCMRglc values, were calculated in homologous brain regions contralateral and ipsilateral to enucleation. As compared with ipsilateral regions, rCMRglc was reduced significantly (by as much as -39%) in contralateral visual areas, including the superior colliculus, lateral geniculate body, and layers I, IV, and V of the primary (striate) and secondary (association, extrastriate) visual cortices. Enucleation did not affect incorporation of [3H]PA into contralateral visual regions, but reduced incorporation of [14C]AA and of [14C]DHA by -18.5 to -2.1%. Percent reductions were correlated with percent reductions in rCMRglc in most but not all regions. No effects were noted at any of nine non-visual structures that were examined. These results indicate that enucleation acutely reduces neuronal activity in contralateral visual areas of the awake rat and that the reductions are coupled to reduced incorporation of unsaturated fatty acids into sn-2 regions of phosphatidylcholine, phosphatidylinositol, and phosphatidylethanolamine. Reduced fatty acid incorporation likely reflects reduced activity of phospholipases A2 and/or phospholipase C.

Intravenously injected radiolabelled fatty acids image brain tumour phospholipids in vivo: differential uptakes of palmitate, arachidonate and docosahexaenoate

This paper investigates the incorporation of intravenously (i.v.) administered radiolabelled fatty acids--[9,10(3)-H]palmitate (3H-PA), [1-14C]arachidonate (14C-AA) and [1-14C]docosahexaenoate (14C-DHA)--into intracerebrally implanted tumours in awake Fischer-344 rats. A suspension of Walker 256 carcinosarcoma tumour cells (1 x 10(6) cells) was implanted into the right cerebral hemisphere of 8- to 9-week-old rats. Seven days after implantation, the awake rat was infused i.v. for 5 min with 3H-PA (6.4 mCi/kg), 14C-AA (170 microCi/kg) or 14C-DHA (100 microCi/kg). Twenty minutes after the start of infusion, the rat was killed and coronal brain sections were obtained for quantitative autoradiography and histology. Each fatty acid showed well-demarcated incorporation into tumour tissue. Areas of necrosis or haemorrhage showed no or small levels of incorporation. The ratios of incorporation into the tumour to incorporation into contralateral brain regions were 2.8-5.5 for 3H-PA, 2.1-3.3 for 14C-AA and 1.5-2.2 for 14C-DHA. The mean ratios differed significantly between the fatty acids (P < 0.01). 3H-PA was not incorporated into necrotic tumours despite the presence of an open blood-tumour barrier, indicated by extravasated horseradish peroxidase. The incorporation rate constant of 3H-PA was similar for small intracerebral and large extracerebral tumours. The results show that 3H-PA, 14C-AA and 14C-DHA are incorporated more readily into tumour tissue than into brain, and that the increase is primarily due to increased utilization of fatty acids by tumour cells and not due to a high blood-tumour permeability. The relative increases in rates of incorporation for the different fatty acids may be related to lipid composition of the tumour and to the requirement of and specific role of these fatty acids in tumour cell growth and division.

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.

Arecoline-stimulated brain incorporation of intravenously administered fatty acids in unanesthetized rats

Brain incorporation of [1-14C]arachidonate ([14C]AA; 170 microCi/kg), [1-14C]docosahexaenoate ([14C]DA; 100 microCi/kg), or [9,10-3H]palmitate ([3H]PA; 6.4 mCi/kg) infused intravenously for 5 min was examined in the awake rat following systemic administration of the cholinomimetic arecoline (15 mg/kg i.p.). The rat was killed 15 min after infusion, and the brain was removed, frozen, and prepared for biochemical analysis and autoradiography. Brain radioactivity, normalized for plasma exposure, was increased by 41 and 45% in arecoline-treated rats given [14C]AA and [14C]DA, respectively. Pretreatment with atropine prevented the increase in fatty acid incorporation. Arecoline treatment had no effect on brain incorporation of [3H]PA. Quantitative autoradiography indicated regionally selective increases in brain [14C]AA and [14C]DA incorporation in response to arecoline. The results suggest that intravenously administered radiolabeled fatty acids can be used to study neurotransmitter-stimulated brain lipid metabolism in vivo.

Brain tumor imaging in rats using the positron emitting fatty acid dl-erythro-9,10-[18F]difluoropalmitate

Positron emitting dl-erythro-9,10[18F]difluoropalmitate, [18F]DFPA, was synthesized for the in vivo imaging of brain tumors in rats. Male Fischer 344 rats were intracerebrally implanted with Walker 256 carcinosarcoma tumor cells (1 x 10(6) in 5 microliters tissue culture media) and 7 days later were infused with [18F]DFPA (500-1000 mCi/mmol) i.v. for 5 min. Rats were killed after 20 min. Brains were removed and either prepared for autoradiography, or brain and tumor were separated and their radioactivity quantified by gamma spectroscopy. Brain tumors were well demarcated from surrounding and normal brain in autoradiographs, and closely paralleled tumor growth in histological sections. The mean optical density of tumor was significantly greater, by 318 +/- 68 per cent (P less than 0.025, n = 3), than normal brain in autoradiographs, and that of edematous brain surrounding a large tumor was intermediately increased. [18F]DFPA proved of value to image and circumscribe intracerebral tumors in awake rats, and studies are continuing to facilitate its clinical application in brain tumor patients.