Purification of small dense vesicles from stimulated Torpedo electric tissue by glass bead column chromatography

Synaptic vesicle pools and plasticity of synaptic transmission at the Drosophila synapse

Our knowledge on the Drosophila neuromuscular synapse is rapidly expanding. Thus, this synapse offers an excellent model for studies of the molecular mechanism of synaptic transmission and synaptic plasticity. Two synaptic vesicle (SV) pools have been identified and characterized using a fluorescent styryl dye, FM1-43, to stain SVs. They are termed the exo/endo cycling pool (ECP), which corresponds to the readily releasable pool (RRP) defined electrophysiologically, and the reserve pool (RP). These two pools were identified first in a temperature-sensitive paralytic mutant, shibire, and subsequently confirmed in wild-type larvae. The ECP participates in synaptic transmission during low frequency firing of presynaptic nerves and locates in the periphery of presynaptic boutons in the vicinity of release sites, while SVs in the RP spread toward the center of boutons and are recruited only during tetanic stimulation. These two pools are separately replenished by endocytosis. Cyclic AMP facilitates recruitment of SVs from the RP to the ECP. Activation of presynaptic metabotropic glutamate receptors recruits SVs from the RP and enhances SV release by elevation of the cAMP level. Memory mutants that have defects in the cAMP/PKA cascade, dunce and rutabaga, exhibit reduced levels of recruitment of synaptic SVs from the RP to the ECP and have limited short-term synaptic plasticity. SV mobilization between the two pools could be a key step for changes in synaptic efficacy. Since a variety of mutants that have distinct defects in synaptic transmission are available for detailed studies of synaptic function, this direction of approach in Drosophila seems promising.

Loading and recycling of synaptic vesicles in the Torpedo electric organ and the vertebrate neuromuscular junction

In vertebrate motor nerve terminals and in the electromotor nerve terminals of Torpedo there are two major pools of synaptic vesicles: readily releasable and reserve. The electromotor terminals differ in that the reserve vesicles are twice the diameter of the readily releasable vesicles. The vesicles contain high concentrations of ACh and ATP. Part of the ACh is brought into the vesicle by the vesicular ACh transporter, VAChT, which exchanges two protons for each ACh, but a fraction of the ACh seems to be accumulated by different, unexplored mechanisms. Most of the vesicles in the terminals do not exchange ACh or ATP with the axoplasm, although ACh and ATP are free in the vesicle interior. The VAChT is controlled by a multifaceted regulatory complex, which includes the proteoglycans that characterize the cholinergic vesicles. The drug (-)-vesamicol binds to a site on the complex and blocks ACh exchange. Only 10-20% of the vesicles are in the readily releasable pool, which therefore is turned over fairly rapidly by spontaneous quantal release. The turnover can be followed by the incorporation of false transmitters into the recycling vesicles, and by the rate of uptake of FM dyes, which have some selectivity for the two recycling pathways. The amount of ACh loaded into recycling vesicles in the readily releasable pool decreases during stimulation. The ACh content of the vesicles can be varied over eight-fold range without changing vesicle size.

Two distinct pools of synaptic vesicles in single presynaptic boutons in a temperature-sensitive Drosophila mutant, shibire

In a temperature-sensitive Drosophila mutant, shibire, synaptic vesicles are completely depleted in nerve terminals after stimulation at 34 degrees C, but upon returning to 22 degrees C, endocytosis resumes. In this study, synaptic vesicles in the boutons of nerve terminals at the mutant neuromuscular junction were loaded with a fluorescent dye, FM1-43, during vesicle reformation at 22 degrees C after complete depletion at 34 degrees C. We found two distinct pools of synaptic vesicles, namely an exo/endo cycling pool, located in the periphery of the bouton, and a reserve pool, located in its center. Cytochalasin D treatment eliminated the reserve pool and reduced synaptic transmission evoked by high frequency stimulation. Thus, the reserve pool may play a crucial role for sustaining high frequency synaptic transmission.

Synaptic vesicles have two distinct recycling pathways

In this paper, evidence is presented that two distinct synaptic vesicle recycling pathways exist within a single terminal. One pathway emanates from the active zone, has a fast time course, involves no intermediate structures, and is blocked by exposure to high Mg2+/low Ca2+ saline, while the second pathway emanates at sites away from the active zone, has a slower time course, involves an endosomal intermediate, and is not sensitive to high Mg2+/low Ca2+. To visualize these two recycling pathways, the temperature-sensitive Drosophila mutant, shibire, in which vesicle recycling is normal at 19 degrees C but is blocked at 29 degrees C, was used. With exposure to 29 degrees C, complete vesicle depletion occurs as exocytosis proceeds while endocytosis is blocked. When the temperature is lowered to 26 degrees C, vesicle recycling membrane begins to accumulate as invaginations of the plasmalemma, but pinch-off is blocked. Under these experimental conditions, it was possible to distinguish the two separate pathways by electron microscopic analysis. These two pathways were further characterized by observing the normal recycling process at the permissive temperature, 19 degrees C. It is suggested that the function of these two recycling pathways might be to produce two distinct vesicle populations: the active zone and nonactive zone populations. The possibility that these two populations have different release characteristics and functions is discussed.

Purification of active synaptic vesicles from the electric organ of Torpedo californica and comparison to reserve vesicles

At least two distinguishable forms of synaptic vesicles exist, the active and reserve, but the reserve form is studied most because it has been difficult to purify the active vesicles. In the work reported here the active vesicles (termed VP2) were highly enriched from the electric organ of Torpedo californica by an improved method developed for the reserve vesicles (termed VP1) with the addition of density gradient centrifugation based on Percoll. No significant differences between the vesicular types were found in the amounts of SV1, SV2, and SV4 epitopes and P-type and V-type ATPase activities. The buoyant densities (g/ml) of VP1 and VP2 vesicles were determined by centrifugation in isosmotic sucrose (1.051, 1.069), Percoll (1.034, 1.040), and glycerol (1.087, 1.090) gradients. The radii were determined by dynamic quasi-elastic laser light-scattering to be (56.6 +/- 10.8) nm and (55.0 +/- 12.7) nm. For both vesicular types the volume of excluded sucrose is only about 37% of the volume of excluded Percoll, indicating that the surfaces are rough. Approx. 51% of the VP1 and 32% of the VP2 vesicular volumes are 'osmotically active' water that is exchangeable with glycerol. The different buoyant densities and amounts of osmotically active water in VP1 and VP2 vesicles probably are due to the different internal solutes. Previously observed differences in acetylcholine active transport and vesamicol binding by VP1 and VP2 synaptic vesicles cannot be explained by major alterations in the protein composition or conformation of the membranes in the two types of vesicles.

Vesamicol blocks the recovery, by recycling cholinergic electromotor synaptic vesicles, of the biophysical characteristics of the reserve population

The effect of vesamicol on the ability of recycling cholinergic synaptic vesicles to recover, during a period of post-stimulation rest, the biophysical properties of the reserve pool was studied in prestimulated perfused blocks of the electric organ of the electric ray, Torpedo marmorata, a tissue rich in cholinergic synapses. The effect of the drug was analysed by high-resolution centrifugal density-gradient fractionation in a zonal rotor of the extracted vesicles. The two vesicle fractions were identified by their ATP and acetylcholine content and the recycled vesicles by their acquisition of [3H]acetylcholine derived from [3H]acetate in the perfusate. Vesamicol (10 microM) blocked the uptake of tritiated acetylcholine by recycled vesicles and also prevented them from rejoining the reserve pool. This is consistent with a previously formulated model of the recovery process, whereby the increase in the acetylcholine and ATP content of the recycled vesicles which takes place during a post-stimulus period of rest increases their osmotic load and thus their content of free water. Vesamicol, by blocking acetylcholine uptake, also blocks rehydration of the recycled vesicles and thus the accompanying decrease in their density to the value characteristic of fully charged vesicles.

Controlled ultraviolet irradiation generates endothelial damage without affecting the nerve terminals of the cerebral artery in cats

The vesicles of adventitial autonomic nerve terminals were examined quantitatively under an electron microscope in controlled ultraviolet ray (UV)-irradiated cerebral vessels. Five cats whose basilar arteries were irradiated with UV (UV group) and 5 cats whose basilar arteries were irradiated with visible rays (control group) were compared. Endothelial vacuolation was observed only in the UV group. There was no statistically significant difference in the diameters of the dense-cored vesicles, related to noradrenaline, and clear vesicles, related to acetylcholine, between the two groups. It is concluded that controlled UV irradiation which generates endothelial damage does not affect the vascular adventitia ultrastructurally.

Moderate hypoglycemia induces ultrastructural changes in perivascular nerve terminals of cat cerebral arteries

A quantitative morphological analysis of the perivascular nerve terminals of cerebral arteries during moderate hypoglycemia was performed. 5-Hydroxydopamine (5-OHDA) was applied to discriminate dense-cored vesicles, related to noradrenaline, and clear vesicles, related to acetylcholine, under the electron microscope. Five hypoglycemic and 5 normoglycemic cats, all receiving 5-OHDA, were compared. In both the middle cerebral artery and vertebral artery, the dense-cored vesicles were significantly smaller and clear vesicles were significantly larger in hypoglycemia than in normoglycemia. These morphological changes in the vesicles may indicate hyperactivity of the sympathetic system and hypoactivity of the parasympathetic system of the cerebral vessels during hypoglycemia.

Recycled synaptic vesicles contain vesicle but not plasma membrane marker, newly synthesized acetylcholine, and a sample of extracellular medium

To monitor the fate of the synaptic vesicle membrane compartment, synaptic vesicles were isolated under varying experimental conditions from blocks of perfused Torpedo electric organ. In accordance with previous results, after low-frequency stimulation (0.1 Hz, 1,800 pulses) of perfused blocks of electric organ, a population of vesicles (VP2 type) can be separated by density gradient centrifugation and chromatography on porous glass beads that is denser and smaller than resting vesicles (VP1 type). By simultaneous application of fluorescein isothiocyanate-dextran as extracellular volume marker and [3H]acetate as precursor of vesicular acetylcholine, and by identifying the vesicular membrane compartment with an antibody against the synaptic vesicle transmembrane glycoprotein SV2, we can show that the membrane compartment of part of the synaptic vesicles becomes recycled during the stimulation period. It then contains both newly synthesized acetylcholine and a sample of extracellular medium. Recycled vesicles have not incorporated the presynaptic plasma membrane marker acetylcholinesterase. Cisternae or vacuoles are presumably not involved in vesicle recycling. After a subsequent period of recovery (18 h), all vesicular membrane compartments behave like VP1 vesicles on subcellular fractionation and still retain both volume markers. Our results imply that on low-frequency stimulation, synaptic vesicles are directly recycled, equilibrating their luminal contents with the extracellular medium and retaining their membrane identity and capability to accumulate acetylcholine.

A morphometric analysis of isolated Torpedo electric organ synaptic vesicles following stimulation

The electric organ of Torpedo has been stimulated with 1800 pulses at 0.1 Hz to produce biochemical and morphological heterogeneity of its synaptic vesicle population. This was verified by biochemical and morphometric analyses of the synaptic vesicle population isolated by sucrose density gradient zonal separation following stimulation. Biochemical or metabolic heterogeneity was verified using 2 established criteria: the appearance of a second peak of acetylcholine (ACh) in denser fractions of the zonal gradient and a corresponding overlapping peak of incorporated radiolabelled ACh. Morphologic heterogeneity was deduced by the presence in this second peak of a subclass of synaptic vesicles having a mean diameter of 68 nm i.e., a diameter 20-25% smaller than the 90 nm subclass that represents the most prominent subclass of the intact terminal population. Despite having satisfied these 3 criteria, functionally relevant heterogeneity cannot be assumed. One reason is due to our failure to recover the 90 nm subclass of vesicle which provides the physical basis to explain the 2 ACh peaks along the gradient. Because of this, the point is raised whether the stimulation-induced ACh peak is not merely an artifact due to inadequate sampling. On the other hand, radioactive labelling of the ACh pool provides a more convincing demonstration of the existence of 2 metabolically different subclasses. We conclude that morphological heterogeneity of the ACh vesicle population has never been established and that metabolic heterogeneity, as it has been studied to date, pertains to a single-sized subclass population of vesicles measuring 68 nm in diameter.

Cholinergic synaptic vesicles are metabolically and biophysically heterogeneous even in resting terminals

The metabolic heterogeneity of synaptic vesicles in the cholinergic nerve terminals of the electromotor neurons of Torpedo marmorata has been studied in resting tissue by evaluating the molecular acetylcholine content (MAC) of synaptic vesicles after extraction from frozen and crushed tissue and high-resolution centrifugal density gradient separation in a zonal rotor. Although vesicular acetylcholine was distributed in the gradient as a single, more or less symmetrical peak, 3 subpopulations of synaptic vesicles could be identified: a small, relatively light subpopulation of low MAC on the ascending limb of the acetylcholine peak, designated V0, a main population of fully charged vesicles designated V1, and a small, denser subpopulation also of low MAC on the descending limb of the acetylcholine peak, designated V2. The mean proportions and MACs of the 3 pools were: V0, 13%, 58,000; V1, 53%, 246,000; V2, 34%, 79,000. When tritiated acetate was perfused through excised blocks of electric organ for 1-2 h before vesicle isolation, the specific radioactivity of the acetylcholine in the V0 and V2 pools was 10-30 times higher than in the V1 pool. This suggests that both the V0 and V2 pools are not generated by the isolation procedure but are present in the intact endings and are functionally active. On the basis of their density and uptake of newly synthesized acetylcholine, the V0 and V2 pools were identified with the previously described VP0 pool of axonal vesicles and the VP2 pool of recycling vesicles in stimulated nerve terminals respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

A morphometric analysis of Torpedo synaptic vesicles isolated by iso-osmotic sucrose gradient separation

The presynaptic terminal vesicle population of Torpedo electric organ is heterogeneous in size, consisting of two prominent subpopulations that comprise 80% of the total. The use of standard iso-osmotic sucrose gradients with zonal centrifugation to isolate vesicle fractions that co-localize with the acetylcholine (ACh) peak results in the recovery of: (1) 10% of the total estimated vesicle population; and (2) a single 68-nm diameter vesicle size class. The whereabouts of the major 90-nm subclass, which accounts for 60% of the total terminal population and which has long been considered to represent the resident ACh population, has been investigated. Assuming this subclass to have undergone severe osmotic stress, the effects of hypo- and hyper-osmotic salines, buffers and fixatives were examined and found to produce only negligible changes on vesicle size. Isolation of vesicles by hypo-osmotic shocking of synaptosomes purified on a Ficoll gradient, however, resulted in a reasonable approximation of the in situ distribution. As the iso-osmotic sucrose gradient procedure utilizes frozen blocks of electric tissue, this step is suspected of being involved in the loss, perhaps because of the slow freezing rates employed. These findings indicate that the 90 nm subclass is lost rather than transformed during isolation by sucrose gradient separation and that dimensionally, the cholinergic vesicle is a constant-sized and relatively stable structure.

Two classes of spontaneous miniature excitatory junction potentials and one synaptic vesicle class are present in the ray electrocyte

Cross sections (1-2 mm thick) of the ray (Raja) tail were secured to a dish and immersed in elasmobranch saline. Spontaneous miniature excitatory junction potentials (MEJPs) were recorded by advancing a 50 k omega, KCl filled electrode into the electric organ (20 microV peak-to-peak baseline noise). Data were filmed, and/or recorded on magnetic tape for computer analyses. Intracellularly recorded MEJP amplitude histograms showed a peak at 60 microV and had a right-hand skew with MEJPs up to 0.5 mV. The small peak amplitude and the skewed amplitude distribution of intracellularly recorded MEJPs result from the relatively low input resistance and the short space constant of the electrocyte coupled with the dispersed synapses on the electrocyte. At 23 degrees C the intracellularly recorded MEJP frequency ranged from 1-10 MEJPs/s. The MEJPs became larger and became focally recorded as the electrode was advanced against the intracellular surface of the innervated membrane of the electrocyte. Focal extracellular MEJPs (reversed polarity) were also recorded with the electrode positioned against the outside surface of the innervated side of the electrocyte. The frequency of focally recorded intracellular MEJPs was increased (up to 40/s) when the electrode was pushed against the membrane. Focal MEJP frequencies decreased to a few/min within 5-10 min but the mean amplitude of 3-5 mV remained constant. Decreases in amplitude and frequency in focally recorded intracellular MEJPs are attributed to changes in electrode pressure against the membrane. Amplitude histograms were constructed from focally recorded intracellular or extracellular MEJPs which showed the same time characteristics. The focal MEJP amplitude histograms have two distinct classes, each forming a bell-shaped distribution. It is concluded that both classes are generated at the electrode tip. The smaller class of MEJPs has a mean 1/10th that of the larger class and composes about 2% of the MEJPs. The small class is analogous to the sub-MEPP class found in the frog sartorius (Kriebel and Gross 1974) and mouse diaphragm (Kriebel et al. 1976, 1982). Distributions of synaptic vesicle diameters are slightly log normal (right hand skew) such that the mean diameter (57 nm) is slightly larger than the modal value (52 nm). Vesicles touching the membrane were of the same size and diameter distribution as the entire vesicle population. The profiles of the distributions are smooth and suggest only 1 class of synaptic vesicle based on diameter.

Acetylcholine, ATP, and proteoglycan are common to synaptic vesicles isolated from the electric organs of electric eel and electric catfish as well as from rat diaphragm

Cholinergic synaptic vesicles were isolated from the electric organs of the electric eel (Electrophorus electricus) and the electric catfish (Malapterurus electricus) as well as from the diaphragm of the rat by density gradient centrifugation followed by column chromatography on Sephacryl-1000. This was verified by both biochemical and electron microscopic criteria. Differences in size between synaptic vesicles from the various tissue sources were reflected by their elution pattern from the Sephacryl column. Specific activities of acetylcholine (ACh; in nmol/mg of protein) of chromatography-purified vesicle fractions were 36 (electric eel), 2 (electric catfish), and 1 (rat diaphragm). Synaptic vesicles from all three sources contained ATP in addition to ACh (molar ratios of ACh/ATP, 9-12) as well as binding activity for an antibody raised against Torpedo cholinergic synaptic vesicle proteoglycan. Synaptic vesicles from rat diaphragm contained binding activity for the monoclonal antibody asv 48 raised against a rat brain 65-kilodalton synaptic vesicle protein. Antibody asv 48 binding was absent from electric eel and electric catfish synaptic vesicles. These antibody binding results, which were obtained by a dot blot assay on isolated vesicles, directly correspond to the immunocytochemical results demonstrating fluorescein isothiocyanate staining in the respective nerve terminals. Our results imply that ACh, ATP, and proteoglycan are common molecular constituents of motor nerve terminal-derived synaptic vesicles from Torpedo to rat. In addition to ACh, both ATP and proteoglycan may play a specific role in the process of cholinergic signal transmission.

A kinetic study of stimulus-induced vesicle recycling in electromotor nerve terminals using labile and stable vesicle markers

The kinetics of recovery, by recycling electromotor synaptic vesicles, of the biophysical parameters of the reserve population has been studied in perfused blocks of electric organ of Torpedo marmorata prestimulated in vivo, followed by density gradient separation of the extracted vesicles in a zonal rotor using labile (acetylcholine and ATP) and stable (proteoglycan) vesicle markers. Stimulation in vivo at 0.15 Hz for 3.3 h depleted tissue acetylcholine much less than stimulation at 1 Hz for 1 h but nevertheless generated a much larger pool of recycled vesicles that recovered more slowly. At the lower rate of stimulation, recovery of the biophysical characteristics of the reserve population by the recycled vesicles, identified by their content of newly synthesized transmitter, was essentially complete by 8 h. The stable proteoglycan marker was immunochemically assayed and was bimodally distributed in the vesicle-containing portion of the density gradient even in experiments with unstimulated or recovered tissue. The second peak corresponded with that of newly synthesized transmitter and was thus identified as containing the recycled vesicles. Its normalized acetylcholine/proteoglycan ratio was lower than that of the first peak, which is consistent with earlier findings that recycled vesicles, before recovery, are only partially loaded with transmitter. However, as expected, the proportion of total vesicular proteoglycan and acetylcholine associated with the recycled vesicle fraction was very much lower in preparations derived from unstimulated or recovered tissue than in those from recently stimulated tissue.

The density and free water of cholinergic synaptic vesicles as a function of osmotic pressure

Synaptic vesicles from the cholinergic electromotor nerve terminals of Torpedo marmorata are among the most uniform subcellular organelles known and are osmotically sensitive. Changes in density accompanying osmotic perturbation have enabled changes in water content to be calculated; when referred to a standard state of known volume and water content, fractional and absolute water contents could be calculated for the perturbed states and compared with the fractional free water content as measured by the glycerol space. Under hyperosmotic conditions, discrepancies were found between these two estimates, the glycerol space falling more rapidly than the water space predicted from the density change. This is attributed to a failure of glycerol to displace water imbibed by the membrane as it collapses round an aqueous core of decreasing volume. 'Reserve' vesicles obeyed a relationship between density, osmotic load and osmolality derived for a perfect osmometer, and independent estimates of fractional free water content under standard conditions and osmotic load were made. The former of these agreed well with the glycerol space under standard conditions and the latter agreed with previous estimates of the osmotic load using morphological and analytical data and an assumed activity coefficient of 0.65. Finally, it was possible to model the interconversion of reserve and recycling vesicles more accurately than in previous work.

Separation of recycling and reserve synaptic vesicles from cholinergic nerve terminals of the myenteric plexus of guinea pig ileum

Acetylcholine-rich synaptic vesicles were isolated from myenteric plexus-longitudinal muscle strips derived from the guinea pig ileum by the method of Dowe, Kilbinger, and Whittaker [J. Neurochem. 35, 993-1003 (1980)] using either unstimulated preparations or preparations field-stimulated at 1 Hz for 10 min using pulses of 1 ms duration and 10 V . cm-1 intensity. The organ bath contained either tetradeuterated (d4) choline (50 microM) or [3H]acetate (2 muCi . ml-1); d4 acetylcholine was measured by gas chromatography-mass spectrometry. As with Torpedo electromotor cholinergic vesicle preparations made under similar conditions the distribution of newly synthesized (d4 or [3H]) acetylcholine in the zonal gradient from stimulated preparations was not identical with that of endogenous (d0, [1H]) acetylcholine, but corresponded to a subpopulation of denser vesicles (equivalent to the VP2 fraction from Torpedo) that had preferentially taken up newly synthesized transmitter. The density difference between the reserve (VP1) and recycling (VP2) vesicles was less than that observed in Torpedo but this smaller difference can be accounted for theoretically by the difference in size between the vesicles of the two tissues. At rest, a lesser incorporation of labelled acetylcholine into the vesicle fraction was observed, and the peaks of endogenous and newly synthesized acetylcholine coincided. Stimulation in the absence of label followed by addition of label did not lead to incorporation of labelled acetylcholine, suggesting that the synthesis and storage of acetylcholine in this preparation and its recovery from stimulation is much more rapid than in Torpedo.

Differences in the osmotic fragility of recycling and reserve synaptic vesicles from the cholinergic electromotor nerve terminals of Torpedo and their possible significance for vesicle recycling

In this study we demonstrate differences in the osmotic fragility of two metabolically and physically heterogeneous synaptic vesicle populations from stimulated electromotor nerve terminals. When synaptic vesicles isolated on sucrose density gradients are submitted to solutions of decreasing osmolarity 50% of VP2-type vesicles lysed at (mean + S.E. (number of experiments] 332 +/- 14 (4) mosM and 50% of VP1-type vesicles lysed at 573 +/- 8 (3) mosM. These results indicate that recycling vesicles are more resistant to hypo-osmotic lysis and they are consistent with our earlier conclusion that changes in water content on recycling are secondary to changes in the content of the osmotically active small-molecular-mass constituents acetylcholine and ATP.