Docosahexaenoic acid (cervonic acid) incorporation into different brain regions in the awake rat

Omega-3 fatty acids and health of auditory and vestibular systems: a comprehensive review

PURPOSE

The purpose of this study was to comprehensively review animal and human studies that explore the role of omega-3 PUFAs in maintaining the health of the auditory organ across all life stages.

METHODS

This narrative review involved searching Scopus, PubMed, Google Scholar, and Cochrane Library databases for relevant articles from December 1980 to July 2023.

RESULTS

some animal and human studies suggest that both deficiency and excessive intake of long-chain omega-3 PUFAs, particularly docosahexaenoic acid (DHA), can lead to auditory neural conduction impairment and reduced hearing acuity from fetal development to old age (presbycusis). These effects are likely to be dependent on the dosage. Some research indicates that an excessive intake of omega-3, rather than a deficiency, can result in nutritional toxicity and hearing impairments. Animal studies highlight the positive impact of omega-3 supplements with high DHA content in addressing hearing damage, but human research on this subject is limited. Furthermore, certain studies propose that omega-3 PUFAs may prevent or delay age-related hearing loss, with high plasma omega-3 concentration, particularly long-chain omega-3 PUFA, linked to reduced hearing loss. Additionally, consuming fish more than twice a week may be associated with a lower risk of hearing loss in adulthood, with these effects potentially influenced by age and gender. However, the majority of studies have been conducted on animals, and clinical trials are scarce. Research on the influence of omega-3 PUFAs on the peripheral and central vestibular systems remains limited.

CONCLUSION

This article delves into the impact of omega-3 on the auditory-vestibular system, exploring its influence on neurodevelopment, protection, and treatment. It not only highlights specific research gaps but also offers valuable insights for potential future studies.

Dietary LC-PUFA in iron-deficient anaemic pregnant and lactating guinea pigs induce minor defects in the offsprings' auditory brainstem responses

OBJECTIVES

We previously demonstrated that a mild pre-natal/early post-natal iron-deficient anaemic (IDA) diet devoid of long-chain polyunsaturated fatty acids (LC-PUFA) affected development, neurophysiology, and cerebral lipid biochemistry of the guinea pigs' progeny. Impacts of dietary LC-PUFA on altered cerebral development resulting from pre-natal IDA are unknown. To address this health issue, impacts of mild gestational IDA in the presence of dietary LC-PUFA on the offsprings' neural maturation were studied in guinea pigs using auditory brainstem responses (ABRs) and assessments of brain fatty acids (FAs).

METHODS

Female guinea pigs (n = 10/group) were fed an iron sufficient (IS) or IDA diet (146 and 12.7 mg iron/kg, respectively) with physiological amounts of LC-PUFA, during the gestation and lactation periods. From post-natal day (PNd) 9 onwards, the IS + PUFA diet was given to both groups of weaned offspring. Cerebral tissue and offsprings' ABR were collected on PNd24.

RESULTS

There was no difference in peripheral and brainstem transmission times (BTTs) between IS + PUFA and IDA + PUFA siblings (n = 10/group); the neural synchrony was also similar in both groups. Despite the absence of differences in auditory thresholds, IDA + PUFA siblings demonstrated a sensorineural hearing loss in the extreme range of frequencies (32, 4, and 2 kHz), as well as modified brain FA profiles compared to the IS + PUFA siblings.

DISCUSSION

The present study reveals that siblings born from dams exposed to a moderate IDA diet including balanced physiological LC-PUFA levels during pregnancy and lactation demonstrate minor impairments of ABR compared to the control siblings, particularly on the auditory acuity, but not on neural synchrony, auditory nerve velocity and BTT.

Use of phosphatide precursors to promote synaptogenesis

New brain synapses form when a postsynaptic structure, the dendritic spine, interacts with a presynaptic terminal. Brain synapses and dendritic spines, membrane-rich structures, are depleted in Alzheimer's disease, as are some circulating compounds needed for synthesizing phosphatides, the major constituents of synaptic membranes. Animals given three of these compounds, all nutrients-uridine, the omega-3 polyunsaturated fatty acid docosahexaenoic acid, and choline-develop increased levels of brain phosphatides and of proteins that are concentrated within synaptic membranes (e.g., PSD-95, synapsin-1), improved cognition, and enhanced neurotransmitter release. The nutrients work by increasing the substrate-saturation of low-affinity enzymes that synthesize the phosphatides. Moreover, uridine and its nucleotide metabolites activate brain P2Y receptors, which control neuronal differentiation and synaptic protein synthesis. A preparation containing these compounds is being tested for treating Alzheimer's disease.

Unoprostone isopropyl rescues retinal progenitor cells from apoptosisin vitro

PURPOSE

Unoprostone isopropyl is an ocular hypotensive that was originally produced as a prostaglandin F2alpha analogue and is eventually recognized as a synthetic docosanoid. The compound is recently suggested to have potent neuroprotective ability in the retina. The purpose of this study is to test whether and how the biologically active metabolites of unoprostone isopropyl rescue retinal neuro-glial progenitor cells from apoptosis.

METHODS

R28 cells were deprived of serum for 24 hr with or without varying concentrations of unoprostone metabolite M1 or M2 or vehicle in the presence or absence of specific inhibitors against several types of signal transduction proteins. Immunocytochemistry against activated caspase-3 with Hoechst nuclear staining was performed.

RESULTS

Up to 15%of R28 cells became pyknotic and activated caspase-3 immunoreactive after 24-hr serum withdrawal. M1, but not M2, significantly reduced apoptotic cells in a dose-dependent fashion with a maximal effect at 100 microM (p < .0001). LY294002, the phosphatidylinositol 3-OH kinase (PI3K) inhibitor, and KT5823, the protein kinase G (PKG) inhibitor, reversed the antiapoptotic effect of M1.

CONCLUSIONS

The unoprostone metabolite M1 protects retinal neuro-glial progenitor R28 cells from apoptosis induced by serum deprivation via the PI3K and PKG pathways.

Synapse formation is enhanced by oral administration of uridine and DHA, the circulating precursors of brain phosphatides

OBJECTIVE

The loss of cortical and hippocampal synapses is a universal hallmark of Alzheimer's disease, and probably underlies its effects on cognition. Synapses are formed from the interaction of neurites projecting from "presynaptic" neurons with dendritic spines projecting from "postsynaptic" neurons. Both of these structures are vulnerable to the toxic effects of nearby amyloid plaques, and their loss contributes to the decreased number of synapses that characterize the disease. A treatment that increased the formation of neurites and dendritic spines might reverse this loss, thereby increasing the number of synapses and slowing the decline in cognition.

DESIGN SETTING, PARTICIPANTS, INTERVENTION, MEASUREMENTS AND RESULTS

We observe that giving normal rodents uridine and the omega-3 fatty acid docosahexaenoic acid (DHA) orally can enhance dendritic spine levels (3), and cognitive functions (32). Moreover this treatment also increases levels of biochemical markers for neurites (i.e., neurofilament-M and neurofilament-70) (2) in vivo, and uridine alone increases both these markers and the outgrowth of visible neurites by cultured PC-12 cells (9). A phase 2 clinical trial, performed in Europe, is described briefly.

DISCUSSION AND CONCLUSION

Uridine and DHA are circulating precursors for the phosphatides in synaptic membranes, and act in part by increasing the substrate-saturation of enzymes that synthesize phosphatidylcholine from CTP (formed from the uridine, via UTP) and from diacylglycerol species that contain DHA: the enzymes have poor affinities for these substrates, and thus are unsaturated with them, and only partially active, under basal conditions. The enhancement by uridine of neurite outgrowth is also mediated in part by UTP serving as a ligand for neuronal P2Y receptors. Moreover administration of uridine with DHA activates many brain genes, among them the gene for the m-1 metabotropic glutamate receptor [Cansev, et al, submitted]. This activation, in turn, increases brain levels of that gene's protein product and of such other synaptic proteins as PSD-95, synapsin-1, syntaxin-3 and F-actin, but not levels of non-synaptic brain proteins like beta-tubulin. Hence it is possible that giving uridine plus DHA triggers a neuronal program that, by accelerating phosphatide and synaptic protein synthesis, controls synaptogenesis. If administering this mix of phosphatide precursors also increases synaptic elements in brains of patients with Alzheimer 's disease, as it does in normal rodents, then this treatment may ameliorate some of the manifestations of the disease.

Excess and deficient omega-3 fatty acid during pregnancy and lactation cause impaired neural transmission in rat pups

Omega-3 fatty acids (omega-3 FA) consumption during pregnancy and lactation is beneficial to fetal and infant growth and may reduce the severity of preterm births. Thus, scientists and clinicians are recommending increasingly higher omega-3 FA doses for pregnant women and nursing babies for advancing the health of preterm, low birth weight, and normal babies. In contrast, some studies report that over-supplementation with omega-3 FA can have adverse effects on fetal and infant development by causing a form of nutritional toxicity. Our goal was to assess the effects of omega-3 FA excess and deficiency during pregnancy and lactation on the offspring's neural transmission as evidenced by their auditory brainstem responses (ABR). Female Wistar rats were given one of three diets from day 1 of pregnancy through lactation. The three diets were the Control omega-3 FA condition (omega-3/omega-6 ratio approximately 0.14), the Deficient omega-3 FA condition (omega-3/omega-6 ratio approximately 0%) and the Excess omega-3 FA condition (omega-3/omega-6 ratio approximately 14.0). The Control diet contained 7% soybean oil, whereas the Deficient diet contained 7% safflower oil and the Excess diet contained 7% fish oil. The offspring were ABR-tested on postnatal day 24. The rat pups in the Excess group had prolonged ABR latencies in comparison to the Control group, indicating slowed neural transmission times. The pups in the Excess group also showed postnatal growth restriction. The Deficient group showed adverse effects that were milder than those seen in the Excess group. Milk fatty acid profiles reflected the fatty acid profiles of the maternal diets. In conclusion, excess or deficient amounts of omega-3 FA during pregnancy and lactation adversely affected the offspring's neural transmission times and postnatal thriving. Consuming either large or inadequate amounts of omega-3 FA during pregnancy and lactation seems inadvisable because of the potential for adverse effects on infant development.

Oral administration of circulating precursors for membrane phosphatides can promote the synthesis of new brain synapses

Although cognitive performance in humans and experimental animals can be improved by administering omega-3 fatty acid docosahexaenoic acid (DHA), the neurochemical mechanisms underlying this effect remain uncertain. In general, nutrients or drugs that modify brain function or behavior do so by affecting synaptic transmission, usually by changing the quantities of particular neurotransmitters present within synaptic clefts or by acting directly on neurotransmitter receptors or signal-transduction molecules. We find that DHA also affects synaptic transmission in mammalian brain. Brain cells of gerbils or rats receiving this fatty acid manifest increased levels of phosphatides and of specific presynaptic or postsynaptic proteins. They also exhibit increased numbers of dendritic spines on postsynaptic neurons. These actions are markedly enhanced in animals that have also received the other two circulating precursors for phosphatidylcholine, uridine (which gives rise to brain uridine diphosphate and cytidine triphosphate) and choline (which gives rise to phosphocholine). The actions of DHA aere reproduced by eicosapentaenoic acid, another omega-3 compound, but not by omega-6 fatty acid arachidonic acid. Administration of circulating phosphatide precursors can also increase neurotransmitter release (acetylcholine, dopamine) and affect animal behavior. Conceivably, this treatment might have use in patients with the synaptic loss that characterizes Alzheimer's disease or other neurodegenerative diseases or occurs after stroke or brain injury.

Chronic administration of docosahexaenoic acid or eicosapentaenoic acid, but not arachidonic acid, alone or in combination with uridine, increases brain phosphatide and synaptic protein levels in gerbils

Synthesis of phosphatidylcholine, the most abundant brain membrane phosphatide, requires three circulating precursors: choline; a pyrimidine (e.g. uridine); and a polyunsaturated fatty acid. Supplementing a choline-containing diet with the uridine source uridine-5'-monophosphate (UMP) or, especially, with UMP plus the omega-3 fatty acid docosahexaenoic acid (given by gavage), produces substantial increases in membrane phosphatide and synaptic protein levels within gerbil brain. We now compare the effects of various polyunsaturated fatty acids, given alone or with UMP, on these synaptic membrane constituents. Gerbils received, daily for 4 weeks, a diet containing choline chloride with or without UMP and/or, by gavage, an omega-3 (docosahexaenoic or eicosapentaenoic acid) or omega-6 (arachidonic acid) fatty acid. Both of the omega-3 fatty acids elevated major brain phosphatide levels (by 18-28%, and 21-27%) and giving UMP along with them enhanced their effects significantly. Arachidonic acid, given alone or with UMP, was without effect. After UMP plus docosahexaenoic acid treatment, total brain phospholipid levels and those of each individual phosphatide increased significantly in all brain regions examined (cortex, striatum, hippocampus, brain stem, and cerebellum). The increases in brain phosphatides in gerbils receiving an omega-3 (but not omega-6) fatty acid, with or without UMP, were accompanied by parallel elevations in levels of pre- and post-synaptic proteins (syntaxin-3, PSD-95 and synapsin-1) but not in those of a ubiquitous structural protein, beta-tubulin. Hence administering omega-3 polyunsaturated fatty acids can enhance synaptic membrane levels in gerbils, and may do so in patients with neurodegenerative diseases, especially when given with a uridine source, while the omega-6 polyunsaturated fatty acid arachidonic acid is ineffective.

Reduced auditory acuity in rat pups from excess and deficient omega-3 fatty acid consumption by the mother

Consumption of the nutrients omega-3 fatty acids (omega-3 FA) during pregnancy and lactation is considered beneficial to fetal and infant development. It may also reduce the incidence and severity of preterm births by prolonging gestational length. However several recent human and animal studies have reported that over-supplementation with omega-3 FA, especially in the form of fish oil, can have adverse effects on fetal and infant development and the auditory brainstem response (ABR). Our goal was to assess further the effects of omega-3 FA excess and deficiency during pregnancy and lactation on the offspring's auditory acuity as evidenced by their ABR thresholds. Female Wistar rats were given diets that were either deficient, adequate (control) or excess in omega-3 FA from day 1 of pregnancy through lactation. The offspring were ABR-tested at the postnatal age of 24 days. The rat pups in the Excess treatment condition had significantly elevated (worse) ABR thresholds, postnatal growth restriction, and a trend for increased postnatal mortality in comparison to the Control group. The Deficient group was intermediate. In conclusion, excess or deficient amounts of omega-3 FA during pregnancy and lactation in the laboratory rat adversely affected the offspring's auditory acuity. Postnatal thriving was also adversely affected. Consuming or administering large or inadequate amounts of omega-3 FA during pregnancy and lactation seems inadvisable because of the potential for adverse effects on infant development.

Docosahexaenoic acid therapy in docosahexaenoic acid-deficient patients with disorders of peroxisomal biogenesis

A patient with classic Zellweger syndrome was treated with docosahexaenoic acid ethyl ester (DHA-EE) for three months. Five other patients with Zellweger variants (four of them less than one-year-old and a five-year-old) were treated with DHA-EE until normalization of the DHA levels in erythrocytes. When arachidonic acid (AA) concentration decreased, AA was added to the diet. Thereafter, a combined treatment with DHA plus AA followed, in a variable proportion that allowed the high levels of DHA in erythrocytes to be maintained. In the patient with Zellweger syndrome, DHA therapy produced an increase in plasmalogen and a decrease in 26:0 and 26:1. No clear clinical improvement could be detected in this patient during the short period of treatment with DHA-EE. The most consistent clinical effect produced by DHA therapy in the other patients with disorders of peroxisomal biogenesis was visual improvement, even in those patients that were virtually blind before the treatment. In general, the developmental curve began to accelerate. The infants became more alert, acquired better visual and social contact and muscular tone improved, with the beginning of good head control. The liver tests tended to normalize and some patients showed a reduction of hepatomegaly. All these favorable changes occurred when the patients were taking the DHA-EE alone. In some of the patients, muscular tone seemed to improve further after introducing AA supplements. From the biochemical point of view, the plasmalogen levels increased in most cases in erythrocytes, and the two ratios 26:0/22:0 and 26:1/22:0 decreased in plasma. In some patients there was a tendency for 26:1 to increase in plasma and for 18:0 plasmalogen to decrease in erythrocytes when AA was introduced in the diet. The significance of these findings remains to be elucidated, but they stress the importance of strict monitoring and control of the polyunsaturated fatty acids status during DHA therapy.

Polyunsaturated fatty acids in the developing human brain, erythrocytes and plasma in peroxisomal disease: therapeutic implications

Patients with Zellweger syndrome and related peroxisomal disorders have profound changes in the polyunsaturated fatty acid (PUFA) patterns in brain and other tissues, with a constant decrease in docosahexaenoic acid (DHA, 22: 6omega3) concentration. Arachidonic acid (AA, 20: 4omega6) concentration is normal or increased and linoleic acid (LA, 18: 2omega6) is increased in the brain of Zellweger patients. In the retina of these patients, the levels of DHA are extremely low. Since these alterations are reflected elsewhere, they can be detected in vivo in patients with generalized peroxisomal disorders by measuring the PUFA content of plasma and erythrocytes, which show very low concentrations of DHA. The concentration of AA is low in plasma in generalized peroxisomal patients, although it is within normal limits in erythrocytes. Patients with X-linked adrenoleukodystrophy (X-ALD) or adrenomyeloneuropathy (AMN) have a normal DHA and AA content in both plasma and erythrocytes, unless they receive extremely low-PUFA diets. Given the probable role of DHA deficiency in the pathogenesis of Zellweger syndrome (ZS), it is important to normalize concentrations of DHA, at least in blood, in an attempt to correct the DHA deficiency in brain. DHA ethyl ester was given orally to two infants with a peroxisome deficiency disorder for a year, and some favourable biochemical changes were produced in erythrocytes and plasma. Normalization of the DHA concentrations in erythrocytes was obtained in about 2 months, and the ratios 26: 0/22: 0 and 26: 1/22: 0 decreased markedly in plasma in the two patients. The plasmalogen ratio 18: 0 dimethyl acetal/18: 0 in erythrocytes increased to virtually normal values in both patients. There was a clear clinical improvement in the two patients, which paralleled the increase in blood DHA. The concentrations of AA and other PUFAs were closely monitored and, when necessary, AA was added to the diet. Such a DHA therapy, given under close biochemical and clinical control, and accompanied by a diet rich in other long-chain PUFA, is strongly recommended in all patients with peroxisomal disorders in whom a DHA deficiency is detected in blood.

Inhibition by n-3 fatty acids of arachidonic acid metabolism in a primary culture of astroglial cells

Docosahexaenoic acid (22:6 n-3) was present in low concentrations in a primary culture of rat brain astroglial cells, when compared to brain cortex. We have thus supplemented these cells with this fatty acid and investigated the effects of its incorporation in cell phospholipids on the conversion of arachidonic acid, 20:4 n-6, through the cyclo and lipoxygenase pathways, after cell stimulation. Docosahexaenoic acid-enriched cells produced less thromboxane B2 and 6-keto-Prostaglandin F1 alpha and markedly less 12-hydroxyeicosatetraenoic acid than unsupplemented cells, after stimulation with the Ca(2+)-ionophore A23187. The production of 15-hydroxyeicosatetraenoic acid from arachidonic acid was slightly increased in docosahexaenoic acid-supplemented cells. We have also supplemented these cells with eicosapentaenoic acid (20:5 n-3) and, in addition to accumulation of this fatty acid in cell phospholipids, we found elevation of 22:5 n-3 and some increment of 22:6, confirming that glial cells are able to convert eicosapentaenoic acid to the long chain, more unsaturated derivatives. In conclusion, n-3 fatty acids, when supplemented to glial cells, appear to modulate the arachidonic acid cascade and to be converted through the elongation and desaturation pathways.

Evidence for brain docosahexaenoate recycling in the free-moving adult rat: implications for measurement of phospholipid synthesis

The specific activity (SA) of unesterified docosahexaenoic acid (22:6 n-3) in the brain and arterial plasma was measured after constant intravenous infusion of [3H] 22:6 n-3 in the free-moving rat. Within 40-105 min, an apparent steady state of labeled unesterified 22:6 n-3 in plasma and in brain was reached. However, the values of the brain to plasma 22:6 n-3 SA ratios ranged from 0.03 to 0.05, indicating that an isotopic equilibrium between brain and plasma was not attained. This suggests that a considerable endogenous source of unesterified 22:6 n-3 (95-97%) (likely derived from lipid metabolism) dilutes the SA of the tracer coming from plasma. Using the SA of 22:6 in plasma instead of brain would thus lead to a gross underestimation of the rate of phospholipid synthesis.

Incorporation of arachidonic and docosahexaenoic acids into phospholipids of rat brain membranes

The incorporation of [3H]arachidonic acid (20:4n-6) into rat brain membranes and its mobilization in response to norepinephrine, a relevant neuromediator were studied. The most efficient [3H]20:4n-6 incorporation was in inositol glycerophospholipids (PI) where it reached a plateau after 10 min incubation, while this incorporation was very weak in choline glycerophospholipids (PC). In contrast, the esterification of docosahexaenoic acid, another polyunsaturated fatty acid occurring at high level in brain, was similar in PI and PC, the incorporation in PI being 8-fold lower than that of 20:4n-6. The newly esterified [3H]20:4n-6 was exclusively found in the 1,2-diacyl subclasses of PI and PC. The bulk of incorporation was in the 18:0/20:4n-6 molecular species of 1,2-diacyl-glycerophosphoinositol and in 16:0/20:4n-6 + 18:1/20:4n-6 molecular species of 1,2-diacyl-glycerophosphocholine, which agrees with the usual location of 20:4n-6 in brain phospholipid classes. Upon norepinephrine treatment, [3H]20:4n-6 was not released from PC, but was dose-dependently decreased in PI, the release being significant from 10(-5) M of the agonist. These results suggest that 20:4n-6 exhibits a high specific turnover in brain PI and is mobilized from this class upon relevant neuromediator stimulation. The acellular system used preserved the specificity of enzymes catalyzing the polyunsaturated fatty acid incorporation and release and could be helpful for studying their turn over in brain.

n-6 polyunsaturated fatty acids increase the neurite length of PC12 cells and embryonic chick motoneurons

We have tested the action of three n-6 polyunsaturated fatty acids, either free or in the form of ethyl esters, on the neurite outgrowth in two neuronal models: a rat pheochromocytoma cell line (PC12) and embryonic chick motoneurons, after 7 days in culture. An inverted microscope coupled with the 'VIDS 4' software was used for measuring the neurite length. Free fatty acids were found to be cytotoxic at 10(-3) M and the maximal increase of the neurite length was obtained at 10(-5) M. In contrast, fatty acids in the form of ethylesters were not cytotoxic and at 10(-3) M induced the maximal increase in the neurite length. This increase (1.2 to 2 fold) significantly differed from the control and was dose-dependent. These results were discussed in relation to the action of fatty acids on enzyme activation and membrane fluidity.

Uptake of docosahexaenoic acid by microvessels from developing rat brain

Uptake of polyunsaturated fatty acids by the rat brain occurs mainly during the three weeks before weaning. Docosahexaenoic acid [22:6], the predominant polyunsaturated fatty acid in the adult brain, appears to be preferentially taken up from the circulation by both the adult and developing rat brain. To test the hypothesis that this preferential incorporation was mediated by the cerebral microvasculature, we compared the incorporation of 22:6 to a saturated fatty acid, palmitic acid [16:0], in freshly isolated rat brain microvessels from the pooled brains of entire litters of two-week-old rats (n = 8 litters). For each litter duplicate incubations with 2 microCi of [1-14C]22:6 or [1-14C]16:0 were performed in 60% autologous rat serum for 2 hr at 37 degrees C. [3H]Sucrose was included in each incubation, allowing correction for non-specific uptake and trapping. An average of 2.7 +/- 2.0% (SD) of the radioactivity from 16:0 was found in the microvessels after 2 hr, vs 0.9 +/- 0.6% for 22:6. This yielded a three-fold enrichment of 16:0 over 22:6 (P = 0.02, paired t-test). There was preferential incorporation of 22:6 into phosphatidylethanolamine and of 16:0 into phosphatidylcholine, although most of the label from either substrate remained as fatty acid after the 2 hr incubation. These results do not indicate that brain capillaries mediate the preferential incorporation of polyunsaturated fatty acids into brain tissue that was seen in intact young rats.

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.