The chemical structure of synaptic membranes

Light and electron microscopic demonstration of cholesterol distribution in membrane structures of the rat auditory cortex

As part of our investigations on the changes in the cortex of different stages of ontogeny, the aim of this study was to analyse the cholesterol distribution in the auditory cortex of adult rats. The light microscopic Schultz reaction as well as electron microscopic thin sections and freeze-etching combined with a cholesterol specific marker were applied to cholesterol demonstration. High and low cholesterol areas were found in the plasma membrane and membranes of some organelles. A low cholesterol content was observed in the membranes of the Golgi apparatus, mitochondria and junctional contacts. A very low cholesterol content was found in the pre- and postsynaptic membranes. High cholesterol contents were present in the neuronal and glial non-junctional plasma membranes. The cholesterol distribution in the membranes of the endoplasmic reticulum and nuclear envelope appeared to be different.

G-proteins in Torpedo marmorata electric organ. Differential distribution in pre- and post-synaptic membranes and synaptic vesicles

The nature of the G-proteins present in the pre- and post-synaptic plasma membranes and in the synaptic vesicles of cholinergic nerve terminals purified from the Torpedo electric organ was investigated. In pre- and post-synaptic plasma membranes, Bordetella pertussis toxin, known to catalyze the ADP-ribosylation of the alpha-subunit of several G-proteins, labels two substrates at 41 and 39 kDa. The 39 kDa subunit detected by ADP-ribosylation in the synaptic plasma membrane fractions was immunologically similar to the Go alpha-subunit purified from calf brain. In contrast to bovine chromaffin cell granules, no G-protein could be detected in Torpedo synaptic vesicles either by ADP-ribosylation or by immunoblotting.

Protein organization of rat synaptic plasma membranes and synaptic vesicles: a one- and two-dimensional study

The protein organization of rat brain synaptic plasma membranes (SPM) and synaptic vesicles (SV) was investigated by surface iodination and one- and two-dimensional electrophoresis. Polypeptides of molecular weights (MWs, in Kilodaltons) 170 K, 135 K, 96-86 K, 68-64-61 K, 56 K, 52 K, 38 K, 35-33 K, and 18 K are predominantly or exclusively exposed on the extracellular side of synaptosomes. Several polypeptides of MW between 70 K and 40 K are exclusively exposed on the cytoplasmic side of SPM. The use of two-dimensional electrophoresis allowed to recognize that, for some classes of MW, there are polypeptides of nearly the same MW and different isoelectric points exposed on both sides of SPM. The synaptosomal membrane shows a predominance of acidic proteins on the extracellular side and more neutral and basic proteins on the cytoplasmic side. With respect to SPM, SV are particularly enriched with polypeptides of MW 71 K, 56 K, 39-38 K, 32 K, 16 K, and 15 K. One of them, a doublet of MW 39-38 K, is the most highly labeled species upon surface iodination and is similar, but not identical, with a doublet located on the cytoplasmic side of SPM.

Ultrastructural localization of calcium in the CNS of vertebrates

The ultrastructural localization of calcium in synaptic areas of the CNS of fish was investigated. Prefixation with phosphate-buffered glutaraldehyde followed by post-fixation with osmium/potassium-bichromate was used to precipitate and visualize endogenous calcium without the addition of external calcium. The presence of calcium in the electron-dense precipitates produced using this method was demonstrated by electron spectroscopic imaging using a Zeiss EM-902 transmission electron microscope, and in various control experiments using the calcium chelator EGTA. In the optic tectum of fish, electron dense precipitates containing calcium were found not only in intracellular compartments, e.g. the smooth endoplasmic reticulum, mitochondria and synaptic vesicles, but also at extracellular locations, particularly in synaptic clefts. In the extracellular sites, only chelate complexes of ionic calcium were found. This would seem to be in agreement with electrophysiological and biochemical data reported in earlier studies. Thus, using the present method, it should be possible to obtain further ultrastructural information concerning the mechanisms of synaptic transmission.

Topographical atlas of the gangliosides of the adult human brain

Forty different brain samples, consisting of neocortical, archicortical, and paleocortical areas; telencephalic, diencephalic, and mesencephalic subcortical nuclei; and the cerebellum as well as some of the corresponding white matter bundles were analyzed with respect to total content of ganglioside-sialic acid and the ganglioside pattern. The total content of gangliosides seems to depend mainly on the proportions of gray and white matter. Thus, neocortical areas, which are rich in gray matter, have a four- to fivefold higher ganglioside content (per milligram of protein) than white matter-rich samples such as optic chiasm, capsula interna, or corpus callosum. White matter-rich regions, although very heterogeneous in ganglioside composition, are further characterized by appreciable amounts of the myelin-enriched GM4. In the neocortex a remarkable degree of regional pattern differences was revealed. In the frontal and parietal areas there is a moderate, and in the temporal region a strong preponderance of sialic acid bound to gangliosides of the a-pathway (GD1a, GM1). In contrast, the occipital cortex favors the b-pathway of ganglioside synthesis (GQ1b, GT1b, GD1b). A predominance of "b-gangliosides" was found in all structures that are related to the visual system (optic chiasm, pulvinar-thalamus, superior colliculi, visual cortex) as well as in the cerebellum and the nucleus ruber. All diencephalic nuclei tend to favor slightly "b-gangliosides," while the mesencephalic nuclei are very heterogeneous in their ganglioside composition. A preponderance of "a-gangliosides" was found in the periamygdalar cortex, putamen, inferior colliculi, substantia nigra, frontal white matter, internal capsule, globus pallidus, basal nucleus of Meynert, and corpus callosum as well as in the frontal, parietal, and temporal cortices. An exceptional predominance of GM1 and GD1a was revealed for the hippocampal archicortex and the amygdala, suggesting a possible functional correlation to glutaminergic synaptic transmission.

Poststimulatory endocytosis, microvesicle repopulation and changes in smooth endoplasmic reticulum in nerve endings of the median eminence superfused in vitro

Mediobasal hypothalami of adult rats were superfused in vitro. A single 5 min pulse of 60 mM KCl-containing medium was infused, followed by either 15, 30, 45, 60 or 75 min superfusions with standard medium. In some experiments, 5 or 10% dextran was added followed by a 15 min recovery. Morphologically, two recovery phases were recognized. The early phase (15-30 min) was characterized by two features: (1) A clear-cut increase in the quantity of large, pleomorphic vacuoles occupying the axoplasm of nerve endings; these vacuoles were observed to be connected to caveolae of the same diameter in the axolemma and they were either coated or uncoated. (2) Progressive increase in the quantity of microvesicles (synaptic vesicles) from an initial depleted state. The vacuoles were found to contain dextran aggregates. Microvesicle-like protrusions bulged from the membrane of vacuoles. The late phase, from 45 min poststimulation onward, was typically identified after the appearance of tubules of smooth endoplasmic reticulum at the most distal segments of the nerve terminals. During this period, large vacuoles tended to decrease in quantity. Granular vesicles remained scant during the entire observation period. Images suggesting formation of microvesicles from tubules of smooth endoplasmic reticulum were observed. These results open the possibility that endocytosis of patches of membranes forming large vacuoles be an important mechanism for retrieving the membranes belonging to microvesicles and granular vesicles. Some of these large vacuoles may contribute to the early regeneration of microvesicles. More microvesicles could later be produced from the smooth endoplasmic reticulum.

Histochemical detection of anionic components in the cephalopod brain

Brains obtained from three species of mediterranean cephalopods (Loligo, Sepia, Octopus) were fixed in Bouin's solution. Paraffin sections were cut sequentially at the frontal plane and used for Alcian blue staining (critical electrolyte concentration method), colloidal iron hydroxide staining procedure, the periodic acid Schiff's reagent method, and the lead tetra-acetate-Schiff method. The stained sections were evaluated at 2 regions of different histological composition: the palliovisceral ganglion and the optic lobe. A high concentration of anionic components was found in synaptic regions of the neuropil whereas neuronal cell bodies showed a relatively weak staining of these constituents. There was a significant reaction of the perineuronal glia nets in the cellular rind of the palliovisceral ganglion. From the comparison of staining patterns obtained with the 4 methods in this study and literature data it can be concluded that the detected anionic sites are mainly carboxyl groups of acidic proteins and/or glycoproteins. Sulphate groups may be present in lower concentrations. Their distribution reveals that the role of anionic components other than sialic acids in the invertebrate brain might be discussed in the context of synaptic transmission similar to that in vertebrates. The possible involvement of the glia cell population has to be taken into consideration.

Brain gangliosides in hibernating dormice (Glis glis) and cold-exposed laboratory mice

The concentration of proteins, sialo-glycoproteins and gangliosides and the ganglioside composition of 8 brain regions from normothermic and hibernating fat dormice (Glis glis) and from laboratory mice being acclimated to 6, 22 and 28 degrees C were investigated. During hibernation the concentration of sialo-glycoproteins and gangliosides decreased significantly in brain of dormice; the protein content remained uninfluenced. Cold-exposure of laboratory mice yielded generally a slightly decreased sialo-glycoprotein concentration in brain; the data on ganglioside concentration in the CNS were not uniform. The ganglioside composition of brain of laboratory mice being kept at different environmental temperatures did not show any alterations. The brain gangliosides of hibernating dormice in contrast to their normothermic counterparts are more polar (higher amount of GTlb and GQlb.). Most striking is the complete absence of a distinct ganglioside fraction (O-acetylated-GTlb) during hibernation. Brain gangliosides of normothermic dormice were found to be more sensitive against neuraminidase treatment than those of hibernating animals. The results are discussed with regard to modulatory functions of neuronal gangliosides for the process of synaptic transmission during seasonal adaptation.

Developmental changes in the composition of, and precursor incorporation into, polypeptides of rat brain slices

Developmental changes in both polypeptide composition and incorporation of L-[3,4(n)-3H]valine into rat brain slice polypeptides have been monitored by SDS polyacrylamide gel electrophoresis coupled with fluorography. Six polypeptide bands (mol. wt 280, 210, 117, 66, 55 and 53 k) showed developmental decreases in relative amount whilst eleven others (mol. wt 230, 156, 95, 77, 68, 62, 48, 38, 34, 28 and 18 k) showed developmental increases. The majority of these changes were accompanied by corresponding, but not parallel changes in precursor incorporation. At least one polypeptide, mol. wt 44 k, showed a developmental decrease in precursor incorporation, but little change in relative amount. Localisation of these polypeptides in SPM, myelin and mitochondrial enriched fractions has been studied.

Development of cerebellar cells in neuron-enriched cultures: cell surface proteins

Lactoperoxidase catalyzed 125I-iodination of 8-day-old rat cerebellar cultures enriched in interneurons, mainly granule cells, was studied during a period 1-8 days in vitro, when the mature appearance of the cultures develop. Autoradiography of the surface iodinated constituents after separation by SDS-polyacrylamide gel electrophoresis showed a limited number of heavily labeled bands, including polypeptides of apparent molecular weight (X 10(3] of 140, 88, 68, 58 and 53 daltons. Their relative proportion in terms of 125I-content changed during the development of the cultures. Initially, the labeled 140 kdaltons band (P140) was dominant. Using crossed immunoelectrophoresis with an antiserum raised against immature rat cerebellar plasma membrane preparations (anti-BPM serum) that primarily recognizes one neuronal surface antigen (D2)33, it was established that the P140 comprises the D2 protein. In contrast to the amount of D2, which increases during the 8-day culture period, the labeling of P140 decreased sharply after 2 DIV. This decline coincided with a developmental change in the molecular forms of D2 involving desialylation. Treatment of 2 DIV cultures with neuraminidase, which reproduces the D2 developmental change, prior to 125I-iodination resulted in a marked reduction in the labeling of P140, whereas the other major labeled group of polypeptides in the 50 kdalton range were little affected. Further experiments showed that the D2 protein is phosphorylated in the plasma membrane. It was found that some of the surface labeled proteins, including P140, are released into the culture medium, but apparently in a non-phosphorylated form. Thus it would appear that a significant part of the polypeptide chain of D2, which is an integral membrane constituent, is exposed on the cell surface, and that either D2 has an anchorage within the membrane that is phosphorylated but is not released or D2 is rapidly dephosphorylated when it is shed from the membrane.

Influence of electrical stimulation and deprivation on the electric organ discharge behaviour and metabolism of neuronal gangliosides of the tapirfish (Gnathonemus petersi, Mormyridae, Teleostei)

The influence of electrical stimulation ("attack"-frequency of 40 Hz, 2 V, 2 days) and of social and electrical deprivation on the metabolism of gangliosides of various brain structures and the electric organ of the weakly electric tapirfish (Gnathonemus petersi, Mormyridae) was investigated. After stimulation the daily average discharges of the electric organ increased from 9.4 to 11.1 Hz, whereas after deprivation they decreased to 7.9 Hz as compared with controls. There were significant and structure specific differences in some ganglioside-fractions (GM1, GD3, GD1a, GD1b and GP1) in concentration and in specific radioactive NeuAc-labelling between stimulated and deprived animals respectively, compared with controls.

Studies on the mechanisms of the epileptiform activity induced by U186661. I Gross alteration of the lipids of synaptosomes and myelin

U18666A, an inhibitor of desmosterol reductase (a terminal enzyme in cholesterol synthesis), has been found to produce chronic epileptiform activity in laboratory animals. Since desmosterol might substitute for cholesterol in neuronal membranes without detriment, the present study was undertaken to examine the possibility that this drug-induced epilepsy was related to changes in other brain lipids. Chronic treatment of rat with U18666A, beginning at one day of age, resulted in pronounced decreases in the concentration of phospholipids and increases in gangliosides of brain microsomal, synaptosomal, and crude myelin fractions. Since total sterol levels were not changed, the ratio of sterols to phospholipids also increased. If drug treatment was stopped at 4 weeks of age, brain lipids of all subcellular fractions examined returned to normal levels by 8 weeks, and no epileptiform activity was detected. However, following 8 weeks of continuous treatment, epileptiform activity was present, and the changes in brain lipids were focused in the myelin fraction. Phospholipid levels and the sterol:phospholipid ratio of microsomes and synaptosomes, in contrast to myelin, were near normal; however, gangliosides were still clearly elevated in all fractions. A reported ability to induce epileptiform activity in rats by treatment with antiserum to brain gangliosides could indicate a special significance of the altered myelin and synaptic gangliosides to the U18666A-induced epilepsy. We suggest that some epileptiform conditions could be directly related to alterations in the lipid composition of critical neuronal structures.

A new brain cell surface glycoprotein identified by monoclonal antibody

Of 207 monoclonal antibodies produced against cultured mouse cerebellar cells, 16 reacted with cerebellar cell surfaces and 4 reacted with glycoproteins. One of them, called an anti-BSP-3 (Brain cell Surface Protein-3) defines a 48,000 molecular weight protein which can be iodinated at the surface of cultured cerebellar cells. Lectin-binding and sugar incorporation studies established the glycoprotein nature of the antigen. Astroglia (glial fibrillary acidic protein-positive cells) in primary cerebellar cultures were labelled intensely for this antigen by the indirect immunofluorescence method while neuronal cells and their processes were more weakly labelled. Fibronectin-positive cells were negative for BSP-3. In cerebellar sections using the immunoperoxidase method at both the optical and electron microscope levels, the difference in staining intensity between astrocytes and neuronal cells was not significant: in Purkinje cells and in the large neurones present in the deep cerebellar nuclei the immunoperoxidase percipitate was confined to the plasma, membrane while in both astrocytes and granule cells cytoplasmic labelling was also observed. Oligodendrocytes do not appear to react with the anti-BSP-3 monoclonal antibody; neither do endothelial or leptomeningeal cells. The availability of a monoclonal antibody produced by a stable hybridoma line will be a powerful tool in attempts to purify the BSP-3 antigen and to elucidate its function.

Glycoproteins from adult rat brain synaptic vesicles. Fractionation on four immobilized lectins

Glycoproteins obtained from large amounts of highly purified synaptic vesicles isolated from adult rat brain were fractionated by sequential affinity chromatography in the presence of SDS on four different immobilized lectins: concanavalin A, Ulex europeus lectin, Ricinus sanguinis lectin and wheat germ agglutinin. 83% of the total protein-bound sugar of synaptic vesicles can be adsorbed on the lectins and separated from the bulk of carbohydrate free proteins. Nine fractions containing only glycoproteins and differing by their terminal sugars were separated by analysed for their carbohydrate composition and electrophoretic profiles. A considerable heterogeneity of the glycoprotein population was observed which cannot be explained solely by the microheterogeneity of the glycans of the synaptic vesicle glycoproteins.

Distribution of six synaptic membrane antigens in subcellular fractions of rat brain cortex

The subcellular distribution in rat brain cortex of six synaptic membrane antigens (56K, 58K, 62K, 63K, 64K, 66K) was studied by rocket immunoelectrophoresis, using antiserum to a highly purified synaptic plasma membrane fraction. Initial analysis of the insoluble portion of subcellular fractions showed that these antigens were also present in smooth microsomes, rough microsomes, and synaptic vesicles; that only traces were present in synaptic junctions; and that none was present in nuclei, mitochondria, and myelin. A trace amount of activity was also present in synaptic vesicle cytosol, but none in whole brain cytosol. Quantitative measurements of synaptic plasma membranes, smooth microsomes, and synaptic vesicles showed that all six antigens were present in synaptic plasma membranes and smooth microsomes, but that the 66K antigen was absent from synaptic vesicles. The 56K, 58K, 62, 63K, and 64K antigens were present in highest concentration in synaptic plasma membranes, whereas the 66K antigen content was highest in smooth microsomes. Only the 58K, 62K, and 63K antigen were detectable in the membrane fraction of whole brain. Their enrichments in synaptic plasma membranes were 10.9, 5.4, and 5.9, respectively. We conclude that the 56K, 58K, 62K, 63K and 64K antigens are primary components of synaptic plasma membranes. The presence of synaptic plasma membrane antigens in smooth microsomes and synaptic vesicles probably represents material being actively transported, consistent with the hypothesis that proteins of synaptic plasma membranes and synaptic vesicles are hypothesis that proteins of synaptic plasma membranes and synaptic vesicles are transported via smooth endoplasmic reticulum.

[Histochemical demonstration of sialic acid-containing compounds in the CNS of mice and goldfish by means of the mPAT ("mild" periodic acid-thionine)-method (author's transl)]

The "mild" periodic acid-thionine (mPAT)-method was examined for the quantitative research of histochemical localization of sialic acid (NeuAc)-containing compounds in brain samples of goldfish and albino mice in comparison to salivary gland (Glandula submandibularis) of the mouse. Biochemical determinations of NeuAc contents of the tissues showed that during histological treatment a significant decrease in the amount of lipid-bound NeuAc, especially in the brain had occurred, because of its high content of lipid-bound NeuAc. A previous KOH-treatment caused in an increase of the colour reaction, which indicated mainly the amount of O-acylated NeuAc just as possibly lactone containing NeuAc-compounds. The intensity of staining, especially in the optic tectum of goldfish, decreased to 45% after neuraminidase-treatment. The low concentration of NeuAc-compounds in the brain in comparison to the salivary glands, results in only a faint staining. Therefore, the use of the mPAT-method seems to be not very suitable for a specific and quantitative staining of all NeuAc-compounds in the CNS.

The localization of lectin binding sites during photoreceptor synaptogenesis in the chick retina

Carbohydrate-containing macromolecules have been localized in the outer plexiform layer of the embryonic and hatchling chick retina by horseradish peroxidase-conjugated wheat germ agglutinin (WGA) and Ricinus communis agglutinin (RCA) lectins. Both WGA and RCA binding sites are present along developing photoreceptor synaptic membranes in the embryonic retina and the plasma membranes of developing neurites and glia. After photoreceptor synapse formation, RCA staining is restricted to non-synaptic membranes, but WGA staining is present on the pre- and post-synaptic membranes of receptor ribbon synapses in addition to non-synaptic membranes. These differing results between the accessibility of RCA and WGA binding sites on mature synaptic membranes in the chick retina suggests that RCA receptors on synaptic membranes are somehow masked after synapse formation and maturation, but that WGA receptors remain accessible. The effects of enzymatic digestion on WGA and RCA binding has been studied after prior treatment with neuraminidase. RCA staining of developing synaptic and non-synaptic membranes in the embryo remains the same after treatment with the enzyme, but in the hatchling RCA staining of non-synaptic membranes is enhanced, which suggests that galactosyl residues are relatively exposed on immature membranes but inaccessible to the lectin on mature membranes until neuraminidase acts to expose them by removing the terminal sialic acid residues. WGA staining on developing synaptic and non-synaptic membranes in the embryo is greatly diminished after neuraminidase pretreatment which suggests that a considerable amount of staining at this time is due to sialic acid in addition to N-acetylglucosamine. In the hatchling, photoreceptor synaptic membranes are no longer labeled with WGA and non-synaptic membrane staining is reduced after neuraminidase digestion, which suggests that after synapse formation synaptic membrane WGA labeling is primarily to sialyl residues, whereas most of the non-synaptic labeling is to N-acetylglucosamine residues.

The incorporation of intrastriatally injected [3H]fucose into electrophoretically separated synaptosomal glycoproteins. II. The influence of passive avoidance training

Following intrastriatal injections of [3H]L-fucose, male albino rats served as (a) trained subjects in a step-down passive avoidance task, (b) stress controls receiving inescapable shock or (c) handled controls. At a series of time points after treatment the animals were sacrificed and the P2 fraction of the injected neostriatum was isolated. This tissue was electrophoresed on SDS-polyacrylamide gels and radioactivity profiles were constructed from 1 mm gel slices. The profiles of trained subjects were compared to shocked and handled control subjects from the same time point group. No differences in total [3H]fucose incorporation into neostriatal glycoproteins were detected as a result of the behavioral treatment used, nor was an incorporation into the majority of electrophoresed peaks altered. Three radioactive gel peaks were significantly altered as a function of experience. At the one day time point, trained subjects exhibited a significant increase in the tritium content of a 70,000 dalton fucosyl-glycoprotein peak. At the 5 day time point, increased label was detected in a 180,000 dalton peak in both trained and shocked subjects, while a significant increase in a 140,000 dalton peak was observed only in trained animals. The relation of the present findings to perviously reported training related differences in glycoprotein metabolism are discussed.

Incorporation of intrastriatally injected[3H]fucose into electrophoretically separated synaptosomal glycoproteins. I. Turnover and molecular weight estimations

The radioactivity profiles of electrophoresed neostriatal P2 fraction glycoproteins were examined at a series of times (2.5, 3 and 4 h; 1, 5 and 10 days) following intracranial injections of [3H]fucose into the neostriatum. Ten major fucosylglycoprotein peaks were discerned in these profiles and certain aspects of their metabolism were characterized. The half-life of fucosylglycoproteins in the P2 fraction was estimated to be 9.7 days. The half-lives of the individual glycoprotein peaks ranged from 4.9 to 17.9 days. The apparent molecular weights of the glycoprotein peaks obtained by our procedures ranged from 32,000 to 180,000 daltons. One peak (peak VIII) incorporated radioactivity primarily at short intervals following the injection. The time course of [3H] fucose incorporation into this peak suggests involvement in the transport, activation and/or incorporation of fucose in brain. Since intracranial injections of [3H]fucose are incorporated into proteins in the cell body, synaptosomal fractions from caudate neurons alone are labeled by this technique. This may be useful in separating pre- and postsynaptic glycoprotein biochemistry. Finally, we tentatively propose that the glycoprotein peaks observed in neostriatum may be identical to previously isolated glycoproteins of known function or subcellular location.

Polypeptides of the synaptic membrane antigens D1, D2, and D3

The rat brain synaptic membrane antigens D1, D2, and D3 were labelled by 125I and precipitated by antibodies in a crossed immunoelectrophoresis. The precipitates were stained, scraped off, reduced, and analysed by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate. The D1 antigen was composed of two polypeptide chains, apparent molecular weights 50 300 and 116 000 D2 of only one polypeptide chain, apparent molecular weight 139 000, and D3 of three polypeptides, apparent molecular weights 14 100, 23 500, and 34 400. Higher apparent molecular weight polypeptides were present in variable amounts in the D3 precipitate, except when the synaptic membrane extracts had been pre-treated with phospholipase D.

Isolation of synaptosomal plasma membranes from cholinergic nerve terminals and a comparison of their proteins with those of synaptic vesicles

Plasma membranes were purified from purely cholinergic nerve endings (synaptosomes) isolated from the electric organ of Torpedo marmorata. Synaptosomes were lysed, membranes recovered and further separated by density gradient centrifugation. A fraction was obtained enriched in 5'-nucleotidase, Na+, K+-activated ATPase and acetylcholine esterase. Morphological examination showed abundant membrane fragments of the size range of synaptosomes and few of vesicle size. The fraction has a characteristic protein composition upon gel electrophoresis. Five reproducible major bands with apparent Mr of 100000, 75000, 52000, 42000 and 35000--33000 are found. A gel-electrophoretic comparison with proteins from synaptic vesicles from the same source (major bands Mr 160000, 147000, 34000 and 25000) was made. Comigration of major bands was detected in one-dimensional gel electrophoresis with the 42000-Mr, 35000--33000-Mr and 34000-Mr components. Upon two-dimensional gel electrophoresis the 42000-Mr component comigrates with a similar component in vesicles, recently characterized as actin; the other components are different. The presence of tubulin-like polypeptides is unlikely. Beside actin, all major vesicle proteins are often detected in small amounts in the plasma membrane preparation. It cannot be decided if they result from fused or contaminating vesicle membranes, but since they are essentially absent in some preparations, it seems that the plasma membrane does not contain vesicle proteins.

Nerve terminal proteins of the rabbit visual relay nuclei identified by axonal transport and two-dimensional gel electrophoresis

The proteins in nerve terminals can be uniquely identified by two-dimensional gel electrophoresis of proteins labeled during synthesis in the cell body and then transported intra-axonally to the terminals. We have explored the potential of the identification procedure by comparing the proteins which are transported from the retina to the lateral geniculate nucleus (LGN) and the superior colliculus (SC) of the rabbit. We have been able to identify between 150 and 200 proteins which ate common to both LGN and SC nerve terminals, very few of which are present at significantly different concentrations in one nucleus relative to the other. The similarity between proteins sent from the retina along two neural pathways subserving different functions illustrates the subtlety of biochemical changes that must underlie physiological differences. Only a small fraction of the labeled proteins are major proteins of the relay nuclei as judged by Coomassie-staining, and some of these arise from in situ nonspecific labeling with blood-borne radioactivity, rather than by transport to the terminals. We have shown that about 5 times more proteins are transported at fast than at intermediate transport rates. More than 50% of the fast proteins turn over rapidly and are gone in 24 h. Few intermediate proteins turn over rapidly. Since only 6% of the proteins in the relay nuclei (at 36 h) could not be detected in the optic tract at that time, transsynaptic labeling by breakdown and resynthesis must be small, if it occurs at all.