[Subcellular fractionation of the electric organ of torpedo marmorata]

The electromotor system of Torpedo. A model cholinergic system

The electric organ of Torpedo, besides providing abundant amounts of cholinoceptive post-synaptic membrane for the isolation of the acetylcholine receptor protein, is a rich source of cholinergic nerve terminals. Using perfused, innervated tissue blocks from which synaptic vesicles in different functional states can be isolated, much information can be obtained about synaptic-vesicle dynamics. So far this is consistent with the view that the synaptic vesicles are the source of transmitter released on stimulation and that uptake of newly synthesized transmitter by the vesicles is dependent on their having discharged their previous charge of transmitter in at least one cycle of exo- and endocytosis. Studies of the protein composition of the vesicle membrane, especially when combined with similar information about the external presynaptic membrane, purified samples of which are now available from synaptosome (T-sac) preparations, promise to throw new light on the molecular mechanism underlying vesicle exo-/endocytosis.

Isolation of pure cholinergic nerve endings from Torpedo electric organ. Evaluation of their metabolic properties

Pure cholinergic nerve endings (synaptosomes) were isolated from the electric organ of Torpedo by a rapid procedure. These synaptosomes are approximately 3 micron in diameter. They contain an occasional mitochondrion, numerous synaptic vesicles, and sometimes an active zone is observed. No postynaptic membrane attachment is found. This nerve ending fraction is extremely pure as shown by morphological controls and biochemical data. It is rich in choline acetyltransferase (450 nmol/h per mg protein) and acetylcholine (ACh) (130 nmol/mg protein). The isolated endings retain their cytoplasmic components and they synthesize ACh and are stable in vitro for several hours, as shown by biochemical measurements and morphological analysis.

Metabolism of acetylcholine in the nervous system of Aplysia californica. IV. Studies of an identified cholinergic axon

[3H]Choline, injected directly into the major axon of the identified cholinergic neuron R2, was readily incorporated into [3H]acetylcholine. Its metabolic fate was similar to that of [3H]choline injected into the cell body of R2. Over the range injected, we found that the amounts of acetylcholine formed were proportional to the amounts injected; the synthetic capability was not exceeded even when 88 pmol of [3H]choline were injected into the axon. Newly synthesized acetylcholine moved within the axon with the kinetics expected of diffusion. We could not detect any selective orthograde or retrograde transport from the site of the injection. In contrast, as indicated by experiments with colchicine, 30% of the [3H]acetylcholine formed after intrasomatic injection was selectively exported from the cell body and transported along the axon. Most of the [3H]acetylcholine was recovered in the soluble fraction after both intra-axonal and intrasomatic injection of [3H]choline; only a small fraction was particulate. The significance of large amounts of soluble acetylcholine in R2 is uncertain, and some may occur physiologically. The concentrations of choline introduced by intraneuronal injection into both cell body and axon were, however, greater than those normally available to choline acetyltransferase in the cholinergic neuron; nevertheless, these large concentrations were efficiently converted into the transmitter. The synthetic capacity of the neuron supplied with injected choline may exceed the capacity of storage vesicles and of the axonal transport process.

Oscillation of acetylcholine during nerve activity in the Torpedo electric organ

The amount of transmitter in the electric organ of Torpedo was measured with a time resolution of 1 sec in the course of stimulation. In parallel, the modifications of the electrophysiological response were analysed by determining the conductance increase (deltaG) and the electromotive force of electroplaques. Large changes in the level of total acetylcholine (ACh) were seen during stimulation. These changes were two-fold: a slow wave and, superimposed on it, a rapid oscillation. The slow wave raised total ACh to the initial level, or even higher. It was probably related to modifications in the amount of ACh released since it corresponded to characteristic inflections in the evolution of the deltaG curve. The slow wave and this physiological parameter were similarly affected when the experiments were performed at a reduced temperature. The rapid oscillation had an amplitude of about 20-40% of the total ACh. It was undamped and its period was 4-5 sec. In contrast to the slow wave, no clear physiological change associated with the rapid oscillation has been observed. The slow wave and rapid oscillation occurred in the 'free pool' of ACh, whereas bound ACh, the fraction associated with synaptic vesicles, was not affected by these changes. A dynamic description of synaptic activity is proposed. The content of 'free' ACh is used and renewed completely after a few tens of impulses, so that transmission seems to imply the continual recycling of the same pool of transmitter rather than utilization of a large preloaded store. The release process must then be integrated in rapid metabolic loops.

Isolation of pure cholinergic nerve endings from the electric organ of Torpedo marmorata

A rapid method for the preparation of highly purified cholinergic nerve endings from the electric organ of Torpedo is described. The endings retain their cytoplasmic components, as shown by biochemical and morphological observations. The homogeneity of these synaptosomes make them a useful tool for further studies.

Isolation of synaptic vesicles from Narcine brasiliensis electric organ: some influences on release of vesicular acetylcholine and ATP

A simple density gradient centrifugation technique for separating electric organ cholinergic synaptic vesicles from other organelles and membrane fragments is described. Frozen, ground electric organ is extracted with a solution of similar density to the vesicles; during the subsequent centrifugation, vesicles remain suspended in the extraction medium and heavier contaminating structures sediment out. In confirmation of results obtained with mammalian central nervous system vesicles, a biphasic pattern of efflux of bound ACh is demonstrated. Low levels of phospholipase A2 (EC 3.1.1.4.) induce efflux of ACh from the vesicle fraction; it is shown that the concomitant fall in vesicle bound ATP is due to ATP efflux rather than ATP hydrolysis within the vesicle.

Phospholipid turnover in Torpedo marmorata electric organ during discharge in vivo

One electric organ of anaesthetized Torpedo marmorata was stimulated through electrodes placed on the electric lobe of the brain. Nerves to the other electric organ were cut to provide an unstimulated control. Glucose 6-[32P]phosphate was injected into each organ 16h before electrical stimulation. After stimulation for 10 min at 5 Hz, the organs were removed homogenized and centrifuged on a density gradient for the preparation of subcellular fractions. Stimulation increased the incorporation of 32P into phosphatidate, phosphatidylinositol and phosphatidylcholine. The increased phosphatidate labelling, but not that of the other two lipids, was seen in fractions rich in synaptic vesicles. Stimulation had no effect on ATP labelling. The phosphatidate content of most fractions fell slightly after stimulation, but amounts of other phospholipids were not affected.

A radioautographic analysis in the light and electron microscope of identified Aplysia neurons and their processes after intrasomatic injection of L-(3H)fucose

We have studied radioautographically the distribution and fate of 3H-glycoproteins within the single identified neurons L10 and R12 of Aplysia californica after intrasomatic pressure injection of [3H]fucose. Silver grains were localized to intracytoplasmic membranes in both cell body and axon 3 h after injection of the cholinergic neuron L10. Grains also appear at this time over presumptive synapses. In the cell body the Golgi apparatus was labeled, as were vesicles, multivesicular bodies, pigment granules, smooth endoplasmic reticulum, mitochondria and peroxisomes. The Golgi apparatus is the most intensely labelled organelle (relative specific activity 9,5). Over 50% of the silver grains are associated with the Golgi apparatus and with vesicles. In the axons, vesicles were labeled most intensely, having a relative specific activity of 40.4 (% silver grains/% area), an intensity 10 times that of similar appearing somatic vesicles, and 4.5--10 times that of other organelles (multivesicular bodies, mitochondria, smooth endoplasmic reticulum) in the axon. At least 32% of the silver grains are associated with vesicles. It appears that the biosynthetic machinery of these neurons is heavily involved in the production of vesicle membrane destined for transport along axons and to terminals. The preponderant labeling of vesicles in the axon parallels the rapid and perferential transport of glycoprotein components described by Ambron et al. and may indicate that specific glycoprotein molecules can be identified as components of these vesicles. After injection of the cholinergic neuron R2 transport of radioactivity was restricted to the axonal tree of the injected neuron. After injection of L10, one other neuron was invariably labeled. By varying the conditions of injection, as many as 5 other neuron cell bodies could be labeled. These are located in the position of cells known to be electrically coupled to L10, and they probably became labeled by transneuronal movement of fucose across electrotonic junctions. Since restriction of label to the injected neuron is easily determined in each experiment, this technique makes possible the identification of chemical and perhaps electrical synapses of identified cells with optimal preservation of fine structure.

Adenosine triphosphate in cholinergic vesicles isolated from the electric organ of Electrophorus electricus

Synaptic vesicles have been isolated from the electirc organ of the bony fish Electrophorus electricus using sucrose step gradients and zonal centrifugation. Although the acetylcholine (ACh) content of the Electrophorus electric organ is only 2% of that of Torpedo, ACh and ATP can readily be measured in the peak fractions using the leech microassay and the firefly luciferin luciferase assay respectively. The protein content of the vesicle fraction in experiments with Electrophorus was much higher than with Torpedo, but a possible contamination of this fraction with mitochondrial or cytoplasmic particles could be excluded. The ACh to ATP ratio of 10.8 is close to that found for cholinergic vesicles isolated from Torpedo and also to that of other amine storing granules.

Metabolism of acetylcholine in the nervous system of Aplysia californica. III. Studies of an indentified cholinergic neuron

[3H] choline and [3H] acetyl CoA were injected into the cell body of an identified cholinergic neuron, the giant R2 of the Aplysia abdominal ganglion, and the fate and distribution of the radioactivity studied. Direct eveidence was obtained that the availabliity of choline to the enzymatic machinery limits synthesis. [3H] choline injected intrasomatically was converted to acetylcholine far more efficiently than choline taken up into the cell body from the bath. Synthesis from injected [3H] acety CoA was increased more than an order of magnitude when the cosubstrate was injected together with a saturating amount of unlabeled choline. In order to study the kinetics of acetylcholine synthesis in the living neuron, we injected [3H] choline in amounts resulting in a range of intracellular concentrations of about four orders of magnitude. The maximal velocity was 300 pmol of acetylcholine/cell/h and the Michaelis constant was 5.9 mM [3H] choline; these values agreed well with those previously reported for choline acetyltransferase assayed in extracts of Aplysia nervous tissue. [3H] acetylcholine turned over within the injected neuron with a half-life of about 9 h. The ultimate product formed was betaine. Subcellular distribution of [3H] acetylcholine was studied using differential and gradient centrifuagtion, gel filtration, and passage through cellulose acetate filters. A small portion of acetylcholine was contained in particulates the size and density expected of cholinergic vesicles.

Metabolism of acetylcholine in the nervous system of Aplysia californica. II. Reginal localization and characterization of choline uptake

The choline required for synthesis of acetylcholine is derived exogenously by Aplysia ganglia. Under physiological conditions choline was taken up primarlily by neuropile and nerves and not by cholinergic cell bodies. In addition, compared with their contents of choline acetyltransferase, those components of nervous tissue which contain nerve terminals and axons synthesized acetylcholine far more efficiently. Choline was accumulated by high and low affinity uptake processes; the high affinity process appeared to be characteristic of cholinergic nuerons (Swartz, J. H., M. L. Eisenstadt, and H. Cedar.1975. J. Gen. Physiol. 65:255). The two uptake processes were similarly affected by temperature with a Q10 of 2.8. Both were dependent on a variety of ions in a complicated manner. High affinity uptake seemed to be more dependent on Na+, showed greater inhibition by ouabain, and was selectively inhibited by oxotremorine. We found that the functional state of neurons did not alter uptake of radioactive choline by either process, nor did it change the conversion to radioactive acetylcholine.

Cellular specificity of serotonin storage and axonal transport in identified neurones of Aplysia californica

1. [(3)H]D,L-5-hydroxytryptophan ([(3)H]5HTP) was injected under pressure into cell bodies of identified cholinergic and serotonergic neurones in the central nervous system of the marine mollusc, Aplysia californica.2. Both serotonergic and cholinergic neurones converted [(3)H]5HTP to [(3)H]5-hydroxytryptamine ([(3)H]5HT).3. The fate of [(3)H]5HT in the two types of neurones differed. In serotonergic cells, 5HT was present primarily in particulate form; the transmitter readily moved from cell bodies into nerves by selective transport. In contrast, 5HT remained free in the cytoplasm of the cholinergic neurone, and was not transported from the cell body.4. Treatment of Aplysia with reserpine decreased the proportion of [(3)H]5HT associated with particulate material, and also decreased the amount of [(3)H]5HT recovered.5. Serotonergic neurones possess specific mechanisms for the storage and axonal transport of 5HT which are absent in cholinergic cells.

Adenosine triphosphate. A constituent of cholinergic synaptic vesicles

1. Synaptic vesicles separated by density-gradient centrifugation from extracts of the cholinergic nerve terminals of the electric organ of Torpedo marmorata were found to contain appreciable amounts of ATP as well as acetylcholine. 2. Vesicular ATP was stable in the presence of concentrations of apyrase and myokinase that rapidly destroyed equivalent amounts of endogenous or added free ATP; pre-treatment of cytoplasmic extracts of electric tissue with these enzymes destroyed endogenous free ATP, but did not affect the vesicular ATP. 3. When [U-(14)C]ATP was added to electric tissue at the time of comminution and extraction of the vesicles, all the radioactivity was associated with soluble components in the subsequent fractionation: none was associated with vesicles or membrane fragments; thus it is unlikely that vesicular ATP can be accounted for by the sequestration of endogenous free ATP within any vesicles formed during comminution and extraction of the tissue. 4. When synaptic vesicles were passed through iso-osmotic columns of Bio-Gel A-5m, which separates vesicles from soluble proteins and small molecules, all the recovered ATP and acetylcholine passed through together in the void volume. 5. Regression analysis showed that vesicular ATP content was highly correlated with vesicular acetylcholine content in different experiments, the molar ratio acetylcholine/ATP being 5.32+/-(s.e.m.) 0.45 (21 expts.) for the peak density-gradient fraction. The ratio varied, however, somewhat across the density-gradient peak suggesting some degree of chemical heterogeneity in the vesicle population.

The heterogeneity of bound acetylcholine and synaptic vesicles

Synaptic vesicles containing radioactive acetylcholine have been isolated from slices of Torpedo electric organ incubated with radioactive choline. The recently synthesized radioactive acetylcholine is preferentially removed from the vesicles by iso-osmotic gel filtration. There is therefore a small compartment of loosely bound recently synthesized acetylcholine within the monodisperse vesicle fraction. The specific radioactivity of this compartment correlates most closely with the ;free' acetylcholine of electric organ that is lost when the tissue is homogenized. Membrane-associated vesicles did not contain any particular enrichment of this compartment. On standing at 6 degrees C the loosely bound compartment stabilizes so that it survives iso-osmotic filtration. A study of this phenomenon revealed that it was proportional to the extent of the loss of tightly bound acetylcholine from the vesicles. Incubation with Ca(2+), at pH5.5, or partial hypo-osmotic shock, caused losses of tightly bound acetylcholine and proportional increases in the stabilization of loosely bound acetylcholine of vesicles. Incubation at 20 degrees C caused less loss of tightly bound, and less stabilization of loosely bound, acetylcholine. A theoretical treatment of these exchanges also shows that the random factors promoting loss of tightly bound acetylcholine are statistically correlated with those which cause stabilization of loosely bound acetylcholine. The reciprocal relationship between the exchanges is inconsistent with there being two distinct populations of vesicles, one containing recently synthesized, loosely bound acetylcholine and the other containing tightly bound acetylcholine. It is proposed that all the vesicles contain a core of tightly bound acetylcholine and a surface layer of loosely bound acetylcholine. The origin of the extravesicular acetylcholine and also of the acetylcholine released on stimulation is discussed in the light of these results.

Morphology of subcellular fractions derived from the electric organ of Torpedo

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The isolation of pure cholinergic synaptic vesicles from the electric organs of elasmobranch fish of the family Torpedinidae

1. Zonal centrifuging permitted the separation, on the milligram scale and in a form largely free from contamination by soluble cytoplasmic protein or membrane fragments derived from other structures, of synaptic vesicles from the purely cholinergic terminals of the electric organ of Torpedo. Up to 100g of tissue could be processed in a single run. 2. As much as 46% of the bound acetylcholine from the original tissue preparation was recovered as a single peak of density equivalent to 0.38m-sucrose-0.21m-NaCl and with a concentration of up to 680nmol of acetylcholine/mg of protein. 3. The limiting concentration of acetylcholine in isolated vesicles when allowance had been made for non-vesicular protein appeared to be about 600nmol/mg of protein. 4. Vesicle counts by a ;bead-tagging' procedure indicated an acetylcholine content of about 360mumol/ml of vesicles; thus the vesicle protein content would be about 60% (w/v). 5. Calculations showed that the core of the vesicle, accounting for about 55% of the vesicle volume, could be largely filled with acetylcholine and protein.