Differences in the kinetics of axonal transport for individual lipid classes in rat sciatic nerve

Axonal regeneration, but not myelination, is partially dependent on local cholesterol reutilization in regenerating nerve

A recycling pathway in peripheral nerve permits cholesterol from degenerating myelin to be salvaged by macrophages and resupplied to myelinating Schwann cells by locally produced lipoproteins. A similar reutilization of cholesterol by regenerating axons has been proposed but not demonstrated. Neurites in culture, however, do take up cholesterol and cholesterol-containing lipoproteins, where these molecules are found to promote neurite extension. To test the requirement for cholesterol reutilization in axon regeneration and myelination, we examined 2 models of blocked intracellular cholesterol transport: 1) bone marrow transplants from Niemann-Pick C mice into wild-type recipient mice, and 2) imipramine treatment. Following nerve crush in these models, we found that unusually large, debris-filled macrophages appeared and persisted for many weeks. A morphometric analysis of regenerating nerves revealed that myelination proceeded at a normal rate (normal g-ratios), but that axon growth was retarded (decreased fiber numbers and diameters) in these animals. Cholesterol synthesis was elevated in these nerves, indicating that Schwann cells compensated for the decreased exogenous supply of cholesterol by up-regulating de novo synthesis to support myelination. These data indicate that Schwann cells are not dependent on cholesterol reutilization to support myelination, but that optimal axonal regeneration is dependent on a local supply of cholesterol.

Regional distribution of phospholipids and polyphosphatidyl inositides in the rabbit's spinal cord

The plasticity of the membrane phospholipids in general and stimulated phosphoinositides turnover in particular are the subjects in a variety of neural paradigms studying the molecular mechanisms of neuronal changes under normal and pathological conditions. The regional modifiability of phospholipids (SM, PC, PS, PI, PA + DG, PE), polyphosphatidylinositides (PI, PIP, PIP2) and diacylglycerol-dependent incorporation of CDP-choline into phosphatidylcholine in the gray matter, white matter, dorsal horns, intermediate zone and ventral horns of the rabbit's spinal cord was studied. We have found 1. a significant increase in the concentration of SM, PC, PS, DG + PA and PE in the white matter in comparison to the gray one, 2. the highest concentration of the outer membrane leaflet-bound phospholipids in the dorsal horns and the inner membrane phospholipids in the intermediate zone in comparison to the gray matter, 3. a substantial amount of labeled polyphosphatidylinositides (poly-PI(s)) in the spinal cord white matter with descending order PIP > PI > PIP2, 4. similar incorporation of myo-2-[3H]inositol into all poly-PI(s) in ventral horns and intermediate zone, but a different, lower incorporation into PI and PIP and higher into PIP2 in the dorsal horns, 5. higher diacylglycerol-dependent incorporation of CDP-choline into PC in the regionally undivided gray matter than in the white matter taken as a whole, 6. the high proportion of diacylglycerol-dependent incorporation of CDP-choline into PC in both the ventral and dorsal horns, whereas that in the intermediate zone remained low.

Locally synthesized phosphatidylcholine, but not protein, undergoes rapid retrograde axonal transport in the rat sciatic nerve

Retrograde axonal transport of phosphatidylcholine in the sciatic nerve has been demonstrated only after injection of lipid precursors into the cell body region. We now report, however, that after microinjection (1 microliter) of [methyl-3H]choline chloride into the rat sciatic nerve (35-40 mm distal to the L4 and L5 dorsal root ganglia), time-dependent accumulation of 3H-labeled material occurred in dorsal root ganglia ipsilateral, but not contralateral, to the injection site. The level of radioactivity in the ipsilateral dorsal root ganglia was minimal at 2 h after isotope injection but was significantly increased at 7, 24, 48, and 72 h after intraneural isotope injection (n = 3-8 per time point); at these time points, all of the radiolabel in the chloroform/methanol extract of the ipsilateral dorsal root ganglia was present in phosphatidylcholine. The radioactivity in the water-soluble fraction did not show a time-dependent accumulation in the ipsilateral dorsal root ganglia as compared with the contralateral DRGs, ruling out transport or diffusion of precursor molecules. In addition, colchicine injection into the sciatic nerve proximal to the isotope injection site prevented the accumulation of radiolabel in the ipsilateral dorsal root ganglia. Therefore, this time-dependent accumulation of radiolabeled phosphatidylcholine in the ipsilateral dorsal root ganglia is most likely due to retrograde axonal transport of locally synthesized phospholipid material. Moreover, 24 h after injection of both [3H]choline and [35S]-methionine into the sciatic nerve, the ipsilateral/contralateral ratio of radiolabel was 11.7 for 3H but only 1.1 for 35S, indicating that only locally synthesized choline phospholipids, but not protein, were retrogradely transported.

Relationships between the rapid axonal transport of newly synthesized proteins and membranous organelles

Rapid axonal transport is generally viewed as being exactly analogous to the secretory process in nonneuronal cells. The cell biology of rapid axonal transport is reviewed, the central concern being to explore those aspects that do not fit into the general secretory model and which may thus represent specific neuronal adaptations. Particular attention is paid to the relationship between the transport of newly synthesized proteins and of the membranous organelles that act as carriers. Sites in the transport sequence at which the behavior of axonal transport may differ from the secretory model are at the initiation of axonal transport at the trans-side of the Golgi apparatus, within the axon where molecules are deposited from the moving phase to a stationary phase, and at nerve terminals or axonal lesions where transport reversal takes place.

Theoretical analysis of lipid transport in sciatic nerve

We modify our previous mathematical model of axonal transport to analyze data on the fast transport of lipids in rat sciatic nerve given in Toews et al. (J. Neurochem. 40, 555-562 (1983)). The theoretical model accounts well for the shapes of the profiles of phosphatidylcholine, phosphatidylethanolamine, cholesterol and diphosphatidylglycerol. The parameters obtained support the qualitative conclusions of Toews et al. and provide quantitative estimates of the underlying processes, e.g., rates of vesicle and mitochondria translocation, rate constants for association and dissociation between vesicles, kinesin and microtubules, rates of deposition and rates of loss of each class of lipid from the nerve by leakage or via removal by the retrograde transport system. The analysis suggests that two classes of vesicles moving at different speeds may be involved in the transport of phosphatidylcholine and phosphatidylethanolamine.

Retrograde axonal transport of locally synthesized phosphoinositides in the rat sciatic nerve

Although autoradiography has demonstrated local incorporation of [3H]inositol into axonal phospholipids after intraneural injection, retrograde axonal transport of phosphatidylinositol has only been demonstrated after injection of lipid precursor into the cell body regions (L4 and L5 dorsal root ganglia) of the sciatic nerve. We now report the retrograde axonal transport of inositol phospholipids synthesized locally in the axons. Following microinjection of myo-[3H]inositol into the rat sciatic nerve (50-55 mm distal to L4 and L5 dorsal root ganglia), a time-dependent accumulation of 3H label occurred in the dorsal root ganglia ipsilateral to the injection site. The ratio of dpm present in the ipsilateral dorsal root ganglia to that in the contralateral dorsal root ganglia was not significantly different from unity between 2 and 8 h following isotope injection but increased to 10-12-fold between 24 and 72 h following precursor injection. By 24 h following precursor injection, the ipsilateral/contralateral ratio of the water-soluble label in the dorsal root ganglia still remained approximately 1.0, whereas the corresponding ratio in the chloroform/methanol-soluble fraction was approximately 20. The time course of appearance of labeled lipids in the ipsilateral dorsal root ganglia after injection of precursor into the nerve at various distances from the dorsal root ganglia indicated a transport rate of at least 5 mm/h. Accumulation of label in the dorsal root ganglia could be prevented by intraneural injection of colchicine or ligation of the sciatic nerve between the dorsal root ganglia and the isotope injection site. These results demonstrate that inositol phospholipids synthesized locally in the sciatic nerve are retrogradely transported back to the nerve cell bodies located in the dorsal root ganglia.

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.

Acrylamide-induced increases in deposition of axonally transported glycoproteins in rat sciatic nerve

The axonal transport of proteins, glycoproteins, and gangliosides in sensory neurons of the sciatic nerve was examined in adult rats exposed to acrylamide via intraperitoneal injection (40 mg/kg of body weight/day for nine consecutive days). The L5 dorsal root ganglion was injected with either [35S]methionine to label proteins or [3H]glucosamine to label, more specifically, glycoproteins and gangliosides. At times ranging from 2 to 6 h later, the sciatic nerve and injected ganglion were excised and radioactivity in consecutive 5-mm segments determined. In both control and acrylamide-treated animals, outflow profiles of [35S]methionine-labeled proteins showed a well defined crest which moved down the nerve at a rate of approximately 340 mm/day. Similar outflow profiles and transport rates were seen for [3H]glucosamine-labeled glycoproteins in control animals. However, in animals treated with acrylamide, the crest of transported labeled glycoprotein was severely attenuated as it moved down the nerve. This finding suggests that in acrylamide-treated animals, axonally transported glycoproteins were preferentially transferred (unloaded or exchanged against unlabeled molecules) from the transport vector to stationary axonal structures. We also examined the clearance of axonally transported glycoproteins distal to a ligature on the nerve. The observed impairment of clearance in acrylamide-treated animals relative to controls is supportive of the above hypothesis. Acrylamide may directly affect the mechanism by which axonally transported material is unloaded from the transport vector. Alternatively, the increased rate of unloading might reflect an acrylamide-induced increase in the demand for axonally transported material.

Biosynthesis of membrane cholesterol during peripheral nerve development, degeneration and regeneration

Biosynthesis of peripheral nerve cholesterol was investigated by the in vivo and in vitro incorporation of [1-14C]-acetate into sciatic endoneurium of normal rats during development, degeneration and regeneration. Labeled sterols were rapidly formed (less than 10 min) within the endoneurial portion of sciatic nerve after [1-14C]acetate administration by intraneural injection. The majority of labeled sterols were initially found in lanosterol and desmosterol. After six hr, the 14C-labeling in both precursors was decreased to minimum, whereas cholesterol became the major labeled product of sterol. As myelination proceeded, the incorporation of [1-14C]acetate into endoneurial cholesterol decreased rapidly and reached a minimum after six mo. In mature adult nerve, an increased proportion of biosynthesis of lanosterol and desmosterol also was demonstrated. The in vitro incorporation of [1-14C]acetate into cholesterol was inhibited during Wallerian degeneration. Instead, cholesteryl esters were labeled as the major sterol product. Such inhibition, however, was not observed in the adult Trembler nerve (Brain Res. 325, 21-27, 1985), which is presumed to be due to a primary metabolic disorder of Schwann cells. The cholesterol biosynthesis was gradually resumed in degenerated nerve by either regeneration of crush-injured nerve or reattachment of the transected nerve. These results suggest that cholesterol biosynthesis in peripheral nerve relies on the axon to provide necessary substrates. De novo synthesis appears to be one of the major sources of endoneurial cholesterol that forms and maintains peripheral nerve myelin.

Effect of crush lesion on radiolabelling of ganglioside in rat peripheral nerve

Left sciatic nerves of adult male Sprague-Dawley rats were crushed and allowed to recover for 0, 1, 2, 4, 7, or 14 days. At each of these times both L-5 dorsal root ganglia were injected with 100 microCi of [3H]glucosamine. Two days later, dorsal root ganglia, lumbosacral trunks, and sciatic nerves were removed bilaterally. The amounts of radiolabelled ganglioside in crushed lumbosacral trunks were consistently higher than in the controls, with the largest difference occurring within 2 days from simultaneous crush and injection to killing (specimens labelled day 0). The largest difference in the amount of radiolabelled ganglioside between crushed and control sciatic nerve (4-9 days from crush to killing) occurred later than that of lumbosacral trunk, but no significant difference occurred within the first 3 days following crush. There was only a slightly higher radioactivity in gangliosides totalled from all three anatomical specimens of crushed than in control nerves. The neutral nonganglioside lipid and acid-precipitable fraction followed patterns of synthesis and accumulation similar to those of the gangliosides. These findings indicate that after nerve crush gangliosides, glucosamine-labelled neutral nonganglioside lipids, and glycoproteins accumulate close to the proximal end of the regenerating axon. This accumulation could serve as a reservoir to increase the ganglioside concentration in the growth cone membrane.

Axonal transport characteristics of gangliosides in sensory axons of rat sciatic nerve

The distribution of axonally transported gangliosides and glycoproteins along the sciatic nerve was examined from 3 h to 4 weeks following injection of[3H]glucosamine into the fifth lumbar dorsal root ganglion of adult rats. Incorporation of labeled precursor into these glycoconjugates reached a maximal level in the ganglion within 6 h. Outflow patterns of radioactivity for glycoproteins showed a well-defined crest with a transport rate of approximately 330 mm/day. In contrast, the crest of transported gangliosides was continuously attenuated, implying a significant deposition along the axon, and an alternative method of calculating velocity was required. Analysis of accumulation of labeled material at double ligatures demonstrated both anterograde and retrograde transport of glycoproteins and gangliosides and allowed for the calculation of an anterograde transport rate of about 270 mm/day for each. Additional evidence of ganglioside transport is provided in that the TLC pattern of transported radioactive gangliosides accumulating at a ligature is significantly different from the pattern seen in the dorsal root ganglion or following intraneural administration of the labeled precursor. These data indicate that gangliosides are transported at the same rapid rate as glycoproteins but are subject to a more extensive exchange with stationary material than are glycoproteins.

Bidirectional transport of gangliosides, glycoproteins and neutral glycosphingolipids in the sensory neurons of rat sciatic nerve

Bidirectional axonal transport of glycoconjugates was studied in the sensory axons of rat sciatic nerve following injection of radiolabelled precursors into L4 and L5 dorsal root ganglia. After varying time intervals, gangliosides and neutral glycosphingolipids were isolated from anterograde and retrograde accumulation segments and radioactivity determined. Radiolabelled glycoproteins were measured in delipidated residues. These glycoconjugates were shown to undergo both anterograde and retrograde transport, accumulation occurring in roughly parallel manner for the three classes. The velocity of anterograde transport was collectively estimated at approximately 360 mm/day. Neutral glycosphingolipids, previously unknown to be axonally transported, were present in sensory axons and transported in roughly equivalent amounts as gangliosides--as judged by levels of transported radioactivity. TLC-radioautography revealed a number of molecular species in the general region of tetra- and larger glycosylceramides. Fractionation of gangliosides according to sialic acid content demonstrated the presence of mono-, di- and polysialo species at the anterograde site.

Effect of aging on the rate of axonal transport of choline-phosphoglycerides

The anterograde axonal transport of choline-phosphoglycerides was studied in sciatic nerve motoneurons of adult (3-month-old) and aged (24-month-old) rats. After the spinal cord injection of [2-3H]glycerol, choline-phosphoglycerides; the major phospholipid class was transported along the nerve. The axonal transport rate was determined by plotting the distance covered by the front of transported radioactivity as a function of the time employed. In aged animals the rate of the choline-phosphoglyceride anterograde axonal transport was about 68% lower than that of adults; furthermore, the rate slowed down along the nerve in the proximal-distal direction. This altered axonal transport mechanism might contribute to the degenerative processes observed in distal regions of peripheral nerve fibers of aged animals.

A ganglioside species (GD1 alpha) migrates at a slow rate and CMP-sialic acid severalfold faster in Xenopus sciatic nerve: fluorographic demonstration

The ninth dorsal root ganglion of adult Xenopus laevis was labeled with N-acetyl-D-[6-3H]mannosamine, and intraaxonal migration of gangliosides was examined by analysis of the chloroform/methanol extract of each of 5-mm consecutive nerve segments by TLC coupled with fluorography. A unique disialoganglioside (GD1 alpha), which amounted to up to 83% of the total ganglioside in this nerve, migrated at 1-2 mm/day at 15 degrees C. This contrasts with the rapid transport of other ganglioside species previously reported in the optic systems of goldfish, rabbits, chickens, and rats. Fluorographic analysis also revealed a trichloroacetic acid-soluble substance migrating at a velocity of approximately 8 mm/day at 15 degrees C. The substance was considered to be CMP-sialic acid on the basis of observations that it comigrates with authentic CMP-N-acetylneuraminic acid in TLC developed with two different solvent systems, it is very labile to weak acid but resistant to neuraminidase from Vibrio cholerae, it is converted to N-acetylmannosamine when treated first with weak acid and subsequently with N-acetylneuraminic acid aldolase, and it has a beta-sialosyl group in its structure. Because CMP-sialic acid is believed to be the sole sialosyl donor in the cells, its migration in axons toward terminals, together with the previous demonstration of sialyltransferase activity in the synaptosomal plasma membrane, strongly supports the possibility that sialosylation of gangliosides and probably of other sialoglycoproteins is not confined to the Golgi apparatus, but can also occur after the compounds are committed to axonal transport.

Studies of sorbinil on axonal transport in streptozotocin-diabetic rats

Deficits of axonal transport in short-term experimental diabetes may be a consequence of increased sorbitol pathway flux and may contribute to the development of degenerative neuropathies. Therefore, we studied the effect of the aldose reductase inhibitor sorbinil on the axonal transport of choline acetyltransferase (ChAT) in the cholinergic neurons of the sciatic nerve of rats with short-term streptozotocin diabetes. In addition, to examine the extent of axonal transport deficits, we studied the axonal transport of choline containing lipids in sensory neurons of the sciatic nerve of similarly diabetic rats and the effects of sorbinil thereon. In experimentally diabetic animals, sorbinil both prevented and reversed deficits of the axonal transport of ChAT and prevented a deficit in the axonal transport of choline containing lipids.

Theoretical analysis of radioactivity profiles during fast axonal transport: effects of deposition and turnover

In a preceding study [Blum, J.J., and Reed, M.C. (1985): Cell Motil. 5:507-527], factors responsible for the shape and velocity of the leading edge of the radiolabeled organelle profile were analyzed, but processes that might influence the shape of the plateau-like region behind the advancing wave were ignored. It is now shown that deposition of material from the fast transport system into membrane-associated structures, degradation of such deposited material and its return to the soma by the retrograde transport system, or leakage of radiolabeled material from the axon can account for the shape of the plateau. Furthermore, these processes are compatible with the maintenance of such structural inhomogeneities as the nodes of Ranvier.

Retrograde axonal transport of phospholipid in rat sciatic nerve

Retrograde axonal transport of phospholipid was studied in rat sciatic motoneuron axons by placing collection crushes on the nerve at intervals after injection of [methyl-3H]choline into the lumbosacral spinal cord, and allowing labelled material undergoing anterograde or retrograde movement to accumulate adjacent to the collection crushes. Control experiments showed that the accumulations of label were not a result of local uptake of circulating precursor. The majority of the 3H label was associated with phosphatidylcholine. Accumulation of label at the distal collection crush, representing retrograde transport, was observed subsequent to the anterograde transport of phospholipid. In comparison with a previous study on retrograde transport of protein, the following points were noted: (1) onset of retrograde transport occurred at approximately the same time after precursor injection (10-20 h) for both protein and phospholipid; (2) retrograde transport of lipids was more prolonged: maximum retrograde transport occurred later for phospholipid (approximately 30 h) than for protein (15-20 h), and declined to half-maximum between 49 and 99 h, compared to a corresponding value of 24-28 h for protein; (3) the proportion of total anterograde-transported activity subsequently undergoing retrograde transport was less in the case of phospholipid, at least over the time interval studied (up to 99 h after precursor injection). The similar times of onset of retrograde transport of phospholipid and protein support the concept of retrograde transport as a recycling mechanism returning to the cell body membrane fragments that were earlier transported into the axon. Coordinated retrograde transport of labelled protein and phospholipid components of the recycled membranes would be predicted.(ABSTRACT TRUNCATED AT 250 WORDS)

Remodeling and sorting process of ethanolamine and choline glycerophospholipids during their axonal transport in the rabbit optic pathway

The existence of a mechanism by which the ester- and ether-linked aliphatic chains of the major phospholipids are retailored during their axonal transport and sorted to specific membrane systems along the optic nerve and tract was investigated. A mixture of [1-14C]hexadecanol and [3H]arachidonic acid was injected into the vitreous body of albino rabbits. At 24 h and 8 days later, the distribution (as measured by the 3H/14C ratio) and the positioning (as monitored by hydrolytic procedures) of radioactivity in the various phospholipid classes of retina, purified axons, and myelin of the optic nerve and tract were determined. At the two intervals after labeling, the 3H/14C ratios of each diradyl type of phosphatidylethanolamine and phosphatidylcholine were (a) substantially unchanged all along the axons within the optic nerve and tract and (b) markedly modified in comparison with those found in the retina and axons for molecular species selectively restricted to myelin sheath. Evidence is thus available that intraxonally moving ethanolamine and choline glycerophospholipids, among others, are added to axonal membranes most likely without extensive modifications. In contrast, they are transferred into myelin after retailoring. Through these two processes, the sorting and targeting of newly synthesized phospholipids to their correct membrane domains, such as axoplasmic organelles, axolemma, or periaxonal myelin, could be controlled.

Retrograde axonal transport of gangliosides and glycoproteins in the motoneurons of rat sciatic nerve

Axonal transport of glycoconjugates was studied in the motoneurons of rat sciatic nerve following injection of [3H]glucosamine into the lumbosacral spinal cord. After varying time intervals, the sciatic nerve was exposed, and two ligatures were tied for collection of materials undergoing anterograde and retrograde transport. Gangliosides and glycoproteins were found to undergo fast anterograde transport, estimated at 284-446 mm/day. Both classes underwent retrograde transport as well, with labeled glycoproteins returning slightly ahead of labeled gangliosides. Only minor quantities of labeled proteoglycans were detected. Purified gangliosides extracted from nerve segments were fractionated according to sialic acid number on diethylaminoethyl-Sephadex; the distributional pattern tended to resemble that of brain gangliosides. The similarity between anterograde and retrograde patterns suggested absence of metabolic changes in gangliosides entering and leaving the axon-nerve terminal structures.

Proteins in fast axonal transport are differentially transported in branches of sensory nerves

Radioactively labeled, fast-transported proteins were collected at ligatures placed on peripheral and central branches of spinal sensory nerves of the bullfrog. In agreement with previous studies, we found that the same species of proteins were transported down each branch. For each protein species analyzed we have measured the amount of radioactivity reaching each ligature, and calculated the ratio of radioactivity reaching the peripheral ligature to that reaching the central ligature. Not all protein species have the same ratio. This suggests that there may be differential transport of fast-transported proteins in axonal branches.

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)