The incorporation of [1-14C]linolenate into lipids of developing rat brain during essential fatty acid deprivation

Dietary fish oil replacement with lard and soybean oil affects triacylglycerol and phospholipid muscle and liver docosahexaenoic acid content but not in the brain and eyes of surubim juveniles Pseudoplatystoma sp

Triplicate groups of juvenile suribim were fed for 183 days one of four different isonitrogenous (47.6% crude protein) and isolipidic (18.7% lipid) diets formulated using three different lipid sources: 100% fish oil (FO, diet 1); 100% pig lard (L, diet 2); 100% soybean oil (SO, diet 3), and FO/L/SO (1:1:1, w/w/w; diet 4). The tissue levels of fatty acids 18:2n-6 and 18:3n-3 decreased relative to corresponding dietary fatty acid values. The 20:5n-3 and 22:6n-3 composition of muscle and liver neutral lipids were linearly correlated with corresponding dietary fatty acid composition. In contrast, the 22:6n-3 composition of the brain and eye were similar among treatments. The 22:6n-3 level was enriched in all tissues, particularly in the neural tissues. Similar results were observed for tissue polar lipids: fatty acids content reflected dietary composition, with the exception of the 22:6n-3 level, which showed enrichment and no differences between groups. Given these results, the importance of the biochemical functions (transport and/or metabolism) of 22:6n-3 in the development of the neural system of surubim warrants further investigation.

Essential fatty acid uptake and metabolism in the developing rodent brain

Studies were carried out to determine whether the brain takes up and metabolizes essential fatty acids during early postnatal development in rodents. Rats and mice were dosed with deuterium-labeled linoleic and linolenic acids either by intraperitoneal injection or by gavage. Animals were killed at different times thereafter, and organs were removed. Brains, livers, and blood were analyzed by gas chromatography--negative-ion-mass spectrometry for labeled fatty acids. To determine whether fatty acids were present in the brain apart from cerebral blood, a subset of animals was exsanguinated by perfusion with buffered saline, and the brain was then fractionated into subcellular components. Results demonstrated that the brain took up both labeled essential fatty acids within 8 h from the time of dosing. There was on average a greater uptake of linolenic acid into the cerebellum than into the cerebral cortex during the first 8 d of life in rats. The amount of linoleic acid taken into either region was similar, however. Docosahexaenoic acid intermediates, 20:5n-3 and 22:5n-3, were also found labeled in the brain. Time-course labeling experiments indicated that these intermediates may be converted to 22:6n-3 within the brain. A rise of labeled 22:6n-3 in the brain at 24 h appeared to be due to uptake of this fatty acid from the blood. The amount of labeled 22:6n-3 in the brain continued to increase beyond 24 h, and this did not appear to be correlated with its blood concentration. These results suggest that, during development in the rodent, different regions within the brain may vary in their capacity to synthesize 22:6n-3, and this may be correlated with regional growth rates.

Intravenous injection of [1-14C]arachidonate to examine regional brain lipid metabolism in unanesthetized rats

We examined the metabolic disposition and brain distribution of an unsaturated fatty acid, [1-14C]arachidonate, between 5 and 240 min following its intravenous bolus injection in unanesthetized adult rats. Injected [1-14C]arachidonate was cleared rapidly from plasma, with less than 10% remaining by 2 min. Total brain radioactivity, 0.2% of the injected dose, was near maximal by 5 min, reached a peak by 15 min, then slowly declined. Radioactivity in brain lipids constituted greater than 82% of the total brain radioactivity at all times. Radioactivity in aqueous-soluble metabolites was greatest at 5 min (13% of total) and declined to 5% by 240 min. Protein pellet-associated radioactivity gradually rose to a peak of 7% by 120 min. Within the lipid fraction, more than 92% of radioactivity was in glycerolipids, with greater than 81% in phospholipids. Radioactivity in inositol phosphoglyceride was maximal at 5 min (47% of phospholipid radioactivity); and declined to 34% by 20 min, whereas radioactivity in choline phosphoglyceride peaked at 15 min (41% of phospholipid radioactivity) and was constant thereafter. In contrast, radioactivity in ethanolamine phosphoglycerides increased from 7 to 17% during the course of the experiment. Quantitative autoradiography of brain sections indicated incorporation of [1-14C]arachidonate into gray-matter regions was 1.5- to threefold that into white-matter regions. The data were analyzed in terms of a model for brain fatty acid uptake from plasma. Estimates of unidirectional transfer constants, k, for [1-14]arachidonate from plasma to brain regions with an intact blood-brain barrier ranged from 0.0005 to 0.0015 ml.sec-1.g-1 and were correlated with those for [9,10-3H]palmitate. The results indicate that brain phospholipid metabolism in awake animals can be examined regionally and quantitatively using intravenous injection of [1-14C]-arachidonate combined with quantitative autoradiography and biochemical analysis.

Sources for brain arachidonic acid uptake and turnover in glycerophospholipids

Brain arachidonic acid comes from linoleic acid and arachidonic acid in the blood. Part of the brain arachidonic acid is elongated to adrenic acid, 22:4 (n = 6), especially in higher animals. With labeled arachidonic acid injected into cerebral ventricles of mice, the highest specific radioactivity was in triacylglycerols. The highest labeling in PtdCho and PtdIns was found at 15 to 60 minutes. Labeling of PtdEtn was much less. The molecular species with 16:0 and 18:1 were labeled better than those with 18:0. Adrenic acid was preferred by alkylacyl-GroPEtn. The highest level of labeling by arachidonic acid was found in the choline plasmalogens and the alkylacyl-GroPCho at 24 hours after injection. The PtdCho arachidonic acid turned over several times within 24 hours. Part of this turnover probably represents the transfer of labeled arachidonic acid to unlabeled ether-linked choline glycerophospholipids, including 1-alkyl-2-lyso-GroPCho, also known as lyso platelet activating factor. The energy-independent transfer of arachidonic acid from PtdCho to ether-linked choline glycerophospholipids may follow removal of their arachidonic acid by phospholipase A2 due to receptor activation. The lack of pulse labeling of ether-linked choline glycerophospholipids complicates the study of their function.

Lipids of nervous tissue: composition and metabolism

As indicated in the Introduction, the many significant developments in the recent past in our knowledge of the lipids of the nervous system have been collated in this article. That there is a sustained interest in this field is evident from the rather long bibliography which is itself selective. Obviously, it is not possible to summarize a review in which the chemistry, distribution and metabolism of a great variety of lipids have been discussed. However, from the progress of research, some general conclusions may be drawn. The period of discovery of new lipids in the nervous system appears to be over. All the major lipid components have been discovered and a great deal is now known about their structure and metabolism. Analytical data on the lipid composition of the CNS are available for a number of species and such data on the major areas of the brain are also at hand but information on the various subregions is meagre. Such investigations may yet provide clues to the role of lipids in brain function. Compared to CNS, information on PNS is less adequate. Further research on PNS would be worthwhile as it is amenable for experimental manipulation and complex mechanisms such as myelination can be investigated in this tissue. There are reports correlating lipid constituents with the increased complexity in the organization of the nervous system during evolution. This line of investigation may prove useful. The basic aim of research on the lipids of the nervous tissue is to unravel their functional significance. Most of the hydrophobic moieties of the nervous tissue lipids are comprised of very long chain, highly unsaturated and in some cases hydroxylated residues, and recent studies have shown that each lipid class contains characteristic molecular species. Their contribution to the properties of neural membranes such as excitability remains to be elucidated. Similarly, a large proportion of the phospholipid molecules in the myelin membrane are ethanolamine plasmalogens and their importance in this membrane is not known. It is firmly established that phosphatidylinositol and possibly polyphosphoinositides are involved with events at the synapse during impulse propagation, but their precise role in molecular terms is not clear. Gangliosides, with their structural complexity and amphipathic nature, have been implicated in a number of biological events which include cellular recognition and acting as adjuncts at receptor sites. More recently, growth promoting and neuritogenic functions have been ascribed to gangliosides. These interesting properties of gangliosides wIll undoubtedly attract greater attention in the future.(ABSTRACT TRUNCATED AT 400 WORDS)

Incorporation of [1-14C[palmitic acid into neutral lipids and phospholipids of rat cerebral cortex in vitro

Incorporation of [1-14C]palmitic acid into neutral lipids and phospholipids of rat cerebral cortex was examined in vitro in normal Krebs--Ringer bicarbonate buffer containing 3% (wt/vol) albumin and 0.75 mM palmitic acid. Under standard assay conditions, radioactivity in the triacylglycerol fraction increased rapidly during the first 30 min, and then decreased after 60 min, with corresponding increase in radioactivity in phosphatidyl choline, phosphatidyl ethanolamine, and a fraction of phosphatidyl inositol plus phosphatidyl serine. Diacylglycerol was shown to be an intermediate metabolite. Radioactivity increased in triacylglycerol, and decreased in phosphatidyl choline and phosphatidyl ethanolamine throughout incubation under N2 gas. In the fraction of phosphatidyl inositol plus phosphatidyl serine, radioactivity decreased after 30 min during incubation under N2 gas. A possible acylation--deacylation cycle, in which triacylglycerol could be a source of free fatty acids for phospholipids, is discussed.

The incorporation of radioactive fatty acids into the phospholipids of nerve-cell-body membranes in vivo

1. Nerve cell bodies were isolated in bulk from cerebral cortices of 15 day-old rabbits after intrathecal injections of [(3)H]plamitate, [(3)H]oleate or [(3)H]arachidonate and [(14)C]glycerol. 2. Nuclear, microsomal and two mitochondrial fractions were isolated from homogenates of the radioactively labelled nerve cell bodies by using differential and discontinuous-gradient centrifugation. 3. After 7.5min in vivo, a high percentage (>80%) of the total (3)H-labelled fatty acid radioactivity was found in the membrane fractions of the nerve cell bodies, whereas after 60min in vivo 50% of the total [(14)C]glycerol radioactivity was found in the high-speed supernatant. 4. The specific radioactivities of phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol, and the radioactivity in neutral lipid and non-esterified fatty acid fractions were determined in the four subfractions, as were the distributions of several marker enzymes and nucleates. 5. With respect of (3)H-labelled fatty acid, the phospholipids of the nuclear fraction had the highest specific radioactivities of the four subfractions. However, for [(14)C]glycerol labelling, generally the (14)C specific radioactivities for individual phospholipids were comparable in the four subfractions. This latter observation suggests transport of phospholipids synthesized de novo between membranes of the nerve cell body. 6. Double-labelling experiments demonstrated that individual phospholipids and the combined neutral lipids of the nuclear fraction had higher labelling ratios of (3)H-labelled fatty acid/[(14)C]glycerol than did the corresponding lipids of the microsomal or mitochondrial fractions. 7. On the basis of the labelling results and the marker studies, it is proposed that it is indeed the nuclei of the nuclear fraction that have these lipids highly labelled with (3)H-labelled fatty acid, and the existence of nuclear acyl transferases that are responsible for this fatty acid incorporation is suggested.