Vesicular storage and release of acetylcholine in Torpedo electroplaque synapses

Imaging single synaptic vesicles in mammalian central synapses with quantum dots

This protocol describes a sensitive and rigorous method to monitor the movement and turnover of single synaptic vesicles in live presynaptic terminals of mammalian central nerve system. This technique makes use of fluorescent semiconductor nanocrystals, quantum dots (Qdots), by their nanometer size, superior photoproperties, and pH-sensitivity. In comparison with other fluorescent probes like styryl dyes and pH-sensitive fluorescent proteins, Qdots offer strict loading ratio, multi-modality detection, single vesicle precision, and most importantly distinctive signals for different modes of vesicle recycling. This application is spectrally compatible with existing optical labels for synapses and thus allows multichannel and simultaneous imaging. With easy modification, this technique can be applied to other types of cells.

Imaging single synaptic vesicles in Mammalian central synapses with quantum dots

This protocol describes a sensitive and rigorous method to monitor the movement and turnover of single synaptic vesicles in live presynaptic terminals of mammalian central nervous system. This technique makes use of Photoluminescent semiconductor nanocrystals, quantum dots (Qdots), by their nanometer size, superior photoproperties, and pH-sensitivity. In comparison with previous fluorescent probes like styryl dyes and pH-sensitive fluorescent proteins, Qdots offer strict loading ratio, multi-modality detection, single vesicle precision, and most importantly distinctive signals for different modes of vesicle fusion. Qdots are spectrally compatible with existing fluorescent probes for synaptic vesicles and thus allow multichannel -imaging. With easy modification, this technique can be applied to other types of synapses and cells.

Kiss-and-run and full-collapse fusion as modes of exo-endocytosis in neurosecretion

Neurotransmitters and hormones are released from neurosecretory cells by exocytosis (fusion) of synaptic vesicles, large dense-core vesicles and other types of vesicles or granules. The exocytosis is terminated and followed by endocytosis (retrieval). More than fifty years of research have established full-collapse fusion and clathrin-mediated endocytosis as essential modes of exo-endocytosis. Kiss-and-run and vesicle reuse represent alternative modes, but their prevalence and importance have yet to be elucidated, especially in neurons of the mammalian CNS. Here we examine various modes of exo-endocytosis across a wide range of neurosecretory systems. Full-collapse fusion and kiss-and-run coexist in many systems and play active roles in exocytotic events. In small nerve terminals of CNS, kiss-and-run has an additional role of enabling nerve terminals to conserve scarce vesicular resources and respond to high-frequency inputs. Full-collapse fusion and kiss-and-run will each contribute to maintaining cellular communication over a wide range of frequencies.

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.

Role of mediatophore in connection with proteins of the active zone in synaptic transmission

Mediatophore is a protein purified from Torpedo electric organ synaptosomes, which translocates acetylcholine (ACh) upon calcium action after reconstitution in artificial membranes. After expression in transfected cells, it endows these cells with a calcium-dependent release mechanism displaying clear quantal properties. The role of mediatophore in synaptic transmission is discussed in relation to the ultrastructural organization of the active zone and the cytosolic high calcium microdomains that transiently appear after presynaptic membrane depolarization.

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.

Effect of N,N'-dicyclohexylcarbodiimide on compartmentation and release of newly synthesized and preformed acetylcholine in Torpedo synaptosomes

Using isolated cholinergic synaptosomes prepared from Torpedo electric organ, we studied the effects of N,N'-dicyclohexylcarbodiimide (DCCD) on acetylcholine (ACh) synthesis, compartmentation, and release after stimulation. Whereas ACh synthesis was unchanged, ACh compartmentation inside synaptosomes was affected by the presence of DCCD. In resting conditions, the uptake into the synaptic vesicle pool of newly synthesized ACh (i.e., [14C]ACh synthesized in the presence of the drug) was progressively and markedly inhibited as the duration of DCCD preincubation was increased, whereas compartmentation of endogenous ACh was unchanged in the presence of DCCD. After stimulation, the release of endogenous ACh from DCCD-treated synaptosomes was similar to that of control, in contrast to the release of [14C]ACh, which was markedly inhibited. This inhibition was observed whatever the conditions of stimulation used (gramicidin D, calcium ionophore A23187, or KCl depolarization). The study of the compartmentation of [14C]ACh during stimulation revealed a transfer of highly labeled ACh from the free to the bound ACh compartment in the presence of DCCD, suggesting the existence of several ACh subcompartments within the free and bound ACh pools. The present results are discussed in comparison with the previously reported effects of vesamicol (AH5183) on ACh compartmentation and release.

Acetylcholine transport, storage, and release

ACh is released from cholinergic nerve terminals under both resting and stimulated conditions. Stimulated release is mediated by exocytosis of synaptic vesicle contents. The structure and function of cholinergic vesicles are becoming known. The concentration of ACh in vesicles is about 100-fold greater than the concentration in the cytoplasm. The AChT exhibits the lowest binding specificity among known ACh-binding proteins. It is driven by efflux of protons pumped into the vesicle by the V-type ATPase. A potent pharmacology of the AChT based on the allosteric VR has been developed. It has promise for clinical applications that include in vivo evaluation of the density of cholinergic innervation in organs based on PET and SPECT. The microscopic kinetics model that has been developed and the very low transport specificity of the vesicular AChT-VR suggest that the transporter has a channel-like or multidrug resistance protein-like structure. The AChT-VR has been shown to be tightly associated with proteoglycan, which is an unexpected macromolecular relationship. Vesamicol and its analogs block evoked release of ACh from cholinergic nerve terminals after a lag period that depends on the rate of release. Recycling quanta of ACh that are sensitive to vesamicol have been identified electrophysiologically, and they constitute a functional correlate of the biochemically identified VP2 synaptic vesicles. The concept of transmitter mobilization, including the observation that the most recently synthesized ACh is the first to be released, has been greatly clarified because of the availability of vesamicol. Differences among different cholinergic nerve terminal types in the sensitivity to vesamicol, the relative amounts of readily and less releasable ACh, and other aspects of the intracellular metabolism of ACh probably are more apparent than real. They easily could arise from differences in the relative rates of competing or sequential steps in the complicated intraterminal metabolism of ACh rather than from fundamental differences among the terminals. Nonquantal release of ACh from motor nerve terminals arises at least in part from the movement of cytoplasmic ACh through the AChT located in the cytoplasmic membrane, and it is blocked by vesamicol. Possibly, the proteoglycan component of the AChT-VR produces long-term residence of the macromolecular complex in the cytoplasmic membrane through interaction with the synaptic matrix. The preponderance of evidence suggests that a significant fraction of what previously, heretofore, had been considered to be nonquantal release from the motor neuron actually is quantal release from the neuron at sites not detected electrophysiologically.(ABSTRACT TRUNCATED AT 400 WORDS)

Mobilization of the readily releasable pool of acetylcholine from a sympathetic ganglion by tityustoxin in the presence of vesamicol

The present experiments tested whether preganglionic stimulation and direct depolarization of nerve terminals by tityustoxin could mobilize similar or different pools of acetylcholine (ACh) from the cat superior cervical ganglia in the presence of 2-(4-phenylpiperidino)cyclohexanol (vesamicol, AH5183), an inhibitor of ACh uptake into synaptic vesicles. In the absence of vesamicol, both nerve stimulation and tityustoxin increased ACh release. In the presence of vesamicol, the release of ACh induced by tityustoxin was inhibited, and just 16% of the initial tissue content could be released, a result similar to that obtained with electrical stimulation under the same condition. When the impulse-releasable pool of ACh had been depleted, tityustoxin still could release transmitter, amounting to some 10% of the ganglion's initial content. This pool of transmitter seemed to be preformed in the synaptic vesicles, rather than synthesized in response to stimuli, as tityustoxin could not release newly synthesized [3H]ACh formed in the presence of vesamicol, and hemicholinium-3 did not prevent the toxin-induced release. In contrast to the results with tityustoxin, preganglionic stimulation could not release transmitter when impulse-releasable or toxin-releasable compartments had been depleted. Our results confirm that vesamicol inhibits the mobilization of transmitter from a reserve to a more readily releasable pool, and they also suggest that, under these experimental conditions, there might be some futile transmitter mobilization, apparently to sites other than nerve terminal active zones.

Ca2+o-independent veratridine-evoked acetylcholine release from striatal slices is not inhibited by vesamicol (AH5183): mobilization of distinct transmitter pools

The effect of 2-(4-phenylpiperidino)cyclohexanol (AH5183 or vesamicol), a compound known to block the uptake of acetylcholine (ACh) into cholinergic synaptic vesicles, on the release of endogenous and [14C]ACh from slices of rat striatum was investigated. ACh release was evoked either by electrical stimulation or by veratridine. The effect of electrical stimulation was entirely dependent on external Ca2+. By contrast, veratridine (40 microM) also enhanced ACh release in the absence of Ca2+. Indeed, with veratridine two components were clearly distinguished: one dependent on external Ca2+ and the other not. Vesamicol inhibited [14C]ACh release evoked by both veratridine and electrical stimulation in the presence of external Ca2+, provided it was added to the tissue prior to loading with [14C]choline. With the same treatment vesamicol only slightly affected the release of endogenous ACh. Under the same conditions the Ca2(+)-independent [14C]ACh release evoked by veratridine was not prevented by vesamicol. The differential responsiveness to vesamicol suggests that ACh pools involved in Ca2+o-dependent ACh release are different from those mobilized during Ca2+o-independent ACh release.

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.

Complementary DNAs for choline acetyltransferase from spinal cords of rat and mouse: nucleotide sequences, expression in mammalian cells, and in situ hybridization

Complementary DNA clones containing the entire coding region of choline acetyltransferase (ChAT) were isolated from the spinal cords of rat and mouse. The cDNAs of rat and mouse coded for 640 and 641 amino acids, respectively, and showed 95% mutual homology and 80% homology with the cDNA of porcine ChAT. Northern blot analysis revealed a single band of 4.4 kb in the spinal cord and brain in each species. Introduction of the cDNAs into Chinese hamster ovary cells and neuron-derived cell lines, N1E115 and NG108-15, expressed a high ChAT activity, which was inhibited by a specific ChAT inhibitor. In situ hybridization using the rat cRNA probe revealed specific labeling of the motoneurons in the spinal cord and neurons in various forebrain nuclei of the rat where the existence of cholinergic neurons has been demonstrated immunohistochemically.

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.

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.

Effect of 2-(4-phenylpiperidino)cyclohexanol on acetylcholine release and subcellular distribution in rat striatal slices

These experiments measured the effect of 2-(4-phenylpiperidino)cyclohexanol (AH5183) on the release of acetylcholine (ACh) and its subcellular distribution in slices of rat striatum incubated in vitro. The AH5183, a drug that blocks the uptake of ACh by isolated synaptic vesicles, reduced the release of ACh from slices stimulated to release transmitter in response to K+ depolarization. Tissue stimulated in the presence of AH5183 contained more ACh in a nerve terminal cytoplasmic fraction than did tissue stimulated in the drug's absence, but stimulation in AH5183's presence reduced the amount of ACh measured in fractions containing synaptic vesicles. The depletion of ACh caused by stimulating tissue in the presence of AH5183 was more evident in the fraction of nerve terminal ACh occluded within synaptic vesicles as isolated by gradient centrifugation (fraction D) than it was in other nerve terminal occluded stores. It is concluded that the synaptic vesicles isolated as fraction D under the present experimental conditions likely contain releasable transmitter. The AH5183 also depressed the spontaneous release of ACh from incubated slices of striatum and this effect was evident in the presence or the absence of medium Ca2+. It is suggested that this effect might indicate that the process of spontaneous ACh release measured neurochemically results, in part, from an AH5183-sensitive carrier-mediated process.

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.

Acetylcholine synthesis and release by a sympathetic ganglion in the presence of 2-(4-phenylpiperidino) cyclohexanol (AH5183)

These experiments measured the release and the synthesis of acetylcholine (ACh) by cat sympathetic ganglia in the presence of 2-(4-phenylpiperidino) cyclohexanol (AH5183), an agent that blocks the uptake of ACh into synaptic vesicles. Evoked transmitter release during short periods of preganglionic nerve stimulation was not affected by AH5183, but release during prolonged stimulation was not maintained in the drug's presence, whereas it was in the drug's absence. The amount of ACh releasable by nerve impulses in the presence of AH5183 was 194 +/- 10 pmol, which represented 14 +/- 1% of the tissue ACh store. The effect of AH5183 on ACh release was not well antagonized by 4-aminopyridine (4-AP), and not associated with inhibition of stimulation-induced calcium accumulation by nerve terminals. It is concluded that AH5183 blocks ACh release indirectly, and that the proportion of stored ACh releasable in the compound's presence represents transmitter in synaptic vesicles available to the release mechanism. The synthesis of ACh during 30 min preganglionic stimulation in the presence of AH5183 was 2,448 +/- 51 pmol and in its absence it was 2,547 +/- 273 pmol. Thus, as the drug decreased ACh release it increased tissue content. The increase in tissue content of ACh in the presence of AH5183 was not evident in resting ganglia; it was evident in stimulated ganglia whether or not tissue cholinesterase was inhibited; it was increased by 4-AP and reduced by divalent cation changes expected to decrease calcium influx during nerve terminal depolarization.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of the acetylcholine transport blocker 2-(4-phenylpiperidino) cyclohexanol (AH5183) on the subcellular storage and release of acetylcholine in mouse brain

The effect of the acetylcholine (ACh) transport blocker 2-(4-phenylpiperidino) cyclohexanol (AH5183) on the subcellular storage and release of acetylcholine was studied in mouse forebrain. Results indicated that AH5183 reduced the amount of ACh released from mouse forebrain minces by high K+ and veratridine over the identical concentration range as it inhibits the active transport of ACh into synaptic vesicles isolated from the electric organ of Torpedo. However, AH5183 did not block the K+- or veratridine-induced reduction of cytoplasmic (S3) ACh. Also, it did not block the loss of vesicular (P3) ACh caused by these depolarizing agents. It did, however, cause a disappearance of nerve ending ACh which was partially matched by a selective gain in the choline content of the P3 fraction. When minces of mouse forebrain were pretreated in high K+ to deplete the S3 and P3 fractions of their ACh content and then subsequently incubated in normal Krebs with [14C]choline, AH5183, at a concentration which reduces ACh release by 50%, did not affect the repletion of P3 stores with newly synthesized [14C]ACh. At somewhat higher concentrations, however, AH5183 reduced the amount of [14C]ACh in the P3 fraction without affecting the amount of [14C]ACh in the S3 fraction. At these concentrations it did not inhibit extracellular choline transport or ChAT activity. These results suggest that AH5183 may reduce the amount of ACh released from central cholinergic nerve terminals in response to depolarization through a combination of effects: (1) it may facilitate the breakdown or loss of ACh stored in the vesicular fraction; (2) it may also block the transport of newly synthesized ACh into the vesicular fraction.

Is propionylcholine present in or synthesized by electric organ?

When small blocks comprising four columns of electrocytes were excised from electric organs of Torpedo marmorata after stimulation in vivo via the electric lobe at 1 Hz for 1 h and allowed to recover at 20-22 degrees C for several hours in medium containing 100 microM d4 choline and 500 microM propionate, small quantities of propionylcholine amounting to no more than 1% of the endogenous acetylcholine of the tissue could be detected in tissue extracts by gas chromatography-mass spectrometry (GCMS). Kinetic studies demonstrated that there was no nonexchangeable propionylcholine in the tissue and in the absence of added propionate, propionylcholine levels were less than 0.2% of tissue acetylcholine. Vesicular propionylcholine amounted to less than 0.5% of vesicular acetylcholine and the distribution of d0 and d4 propionylcholine suggested that an appreciable proportion (up to one-third) of this could be an artifact of preparation for GCMS determinations. Propionylcholine formation during extraction and demethylation of an artificial mixture of acetylcholine, choline, and propionate was indeed detected. It is concluded that propionylcholine has no significance as an endogenous or as a false transmitter at this terminal, in conformity with the work of Sheridan et al. [Z. Zellforsch. 74, 281-307 (1966)] but in contrast to the report of O'Regan [J. Neurochem. 39, 764-772 (1982)].

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.

Postmortal decay of acetylcholine levels in the spinal cord of rat, chicken and frog

The decay of acetylcholine (ACh) after death of an animal has been estimated in the cervical spinal cord of rat, chicken and frog. The level of ACh in the frog (19.90 nmol/g wet weight) shows no variation from 20 to 500 s after death. In the rat and chicken, there is a decrease in the first 100 s after death to lower values; 4.35 nmol/g wet weight in the rat and 4.60 nmol/g wet weight in the chicken. The levels of ACh in the cervical spinal cord of the rat an chicken at the time of death were estimated by extrapolation to time 0 of the curve of the decay of ACh in the first 100 s. The values obtained were: 121.64 nmol/g wet weight in the chicken and 34.19 nmol/g wet weight in the rat.

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.

Uptake of acetate and propionate by isolated nerve endings from the electric organ of Torpedo marmorata and their incorporation into choline esters

The uptake and incorporation into choline esters of acetate and propionate by electric organ synaptosomes were compared, with the aim of better understanding the basis for the selectivity of choline ester synthesis shown by this tissue for acetate. It was found that propionate uptake, like acetate uptake, was a temperature-dependent, saturable process. Both uptake mechanisms had similar affinities for their substrates, but the maximal velocity of propionate uptake was considerably lower than that of acetate uptake; and less of the accumulated propionate was used for choline ester synthesis than of the accumulated acetate. While acetate was a good inhibitor of propionate uptake, propionate was a very poor inhibitor of acetate uptake. This finding, in addition to the observation that the two uptakes were not affected in the same way by changes in pH, led to the suggestion that acetate uptake and propionate uptake reflect different processes. In both cases, however, the pH dependence of uptake indicated that these substrates cross the membrane as the charged species. Acetate uptake and acetylcholine synthesis remained closely associated under various experimental conditions, while propionate uptake could be dissociated from the synthesis of propionylcholine. Hence, it appears that acetate is taken up by a specific, high-velocity mechanism linked to acetylcholine synthesis, whereas propionate uptake may represent a less specific mechanism.

Morphological changes in neuromuscular junctions during exercise

Long lasting exercise produces several morphological changes in teleostean neuromuscular junctions (NJs), consisting of progressive synaptic vesicles (SVs) depletion and lamellar branching of the nerve endings. Exercised fishes kept swimming during 1 hr against a 3.5 1/min flow of oxygenated water in spite of the fact that the number of SVs was reduced in about 70% after 10 min of exercise. This observation indicates that the SVs formation fails to restore their original number and consequently, under such circumstances, the transmitter release may occur by a different mechanism.

Synthesis of acetylcholine from acetate in a sympathetic ganglion

The present experiments tested whether acetate plays a role in the provision of acetyl-CoA for acetylcholine synthesis in the cat's superior cervical ganglion. Labeled acetylcholine was identified in extracts of ganglia that had been perfused for 20 min with Krebs solution containing choline (10(-5) M) and [3H], [1-14C], or [2-14C]acetate (10(-3) M); perfusion for 60 min or with [3H]acetate (10(-2) M) increased the labeling. The acetylcholine synthesized from acetate was available for release by a Ca2+-dependent mechanism during subsequent periods of preganglionic nerve stimulation. When ganglia were stimulated via their preganglionic nerves or by exposure to 46 mM K+, the labeling of acetylcholine from [3H]acetate was reduced when compared with resting ganglia. The reduced synthesis of acetylcholine from acetate during stimulation was not due to acetate recapture, shunting of acetate into lipid synthesis, or the transmitter release process itself. In ganglia perfused with [2-14C]glucose, the amount of labeled acetylcholine formed was clearly enhanced during stimulation. An increase in acetylcholine labeling from [3H]acetate was shown during a 15-min resting period following a 60-min period of preganglionic nerve stimulation (20 Hz). It is concluded that acetate is not the main physiological acetyl precursor for acetylcholine synthesis in this sympathetic ganglion, and that during preganglionic nerve stimulation there is enhanced delivery of acetyl-CoA to choline acetyltransferase from a source other than acetate.

Proton gradient linkage to active uptake of [3H]acetylcholine by Torpedo electric organ synaptic vesicles

It has been confirmed that cholinergic synaptic vesicles isolated from the electric organ of Torpedo californica exhibit adenosine 5'-triphosphate (ATP) dependent active uptake of [3H]acetylcholine. Active uptake can be completely inhibited by low concentrations of the mitochondrial uncouplers carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone, nigericin, gramicidin, valinomycin, and A 23187. Under similar conditions uncouplers stimulate the vesicle adenosinetriphosphatase (ATPase) by from 40 to 80%. ATP-supported uptake of [3H]acetylcholine increases greatly as the external pH is increased from 6.6 to 7.6 and remains approximately constant from pH 7.8 to pH 8.6. The uptake also becomes more selective for [3H]acetylcholine compared to [14C]choline as the pH is increased from 6.6 to 7.6, achieving 12-fold selectively, in a manner similar to the increase in the amount of [3H]acetylcholine taken up. Bicarbonate stimulates both the amount and selectivity of [3H]acetylcholine uptake over the lower pH range, but it has no effect over the higher pH range. Exogenous ammonium ion completely inhibits active [3H]acetylcholine uptake, with lower concentrations of ammonium ion required at higher pH values in a manner consistent with ammonia being the active species. Adenosine 5'-diphosphate and a nonhydrolyzable ATP analogue do not support active [3H]acetylcholine uptake. It is concluded that an ATPase pumps protons into the cholinergic synaptic vesicle to produce an internally acidic and positively charged proton gradient that is linked to [3H]acetylcholine uptake.

Differential labeling of depot and active acetylcholine pools in nondepolarized and potassium-depolarized rat brain synaptosomes

To test the hypothesis that a pool of newly synthesized acetylcholine (ACh) turns over independently of performed ACh, compartmentation and K+-evoked release of ACh were examined in perfused synaptosomal beds intermittently stimulated by 50 mM K+. In resting synaptosomes, endogenous and labeled ACh was distributed between synaptic vesicles and the cytoplasm in a dynamic equilibrium ratio of 4:6. In the absence of new ACh synthesis, five sequential K+-depolarizations caused a decremental release of performed labeled ACh totaling 30% of the initial transmitter store. Further depolarization evoked little additional release, despite the fact that 60% of the labeled ACh remained in these preparations. Release of the performed [14C]ACh was unaltered while new ACh was being synthesized from exogenous [3H]choline. Since the evoke release of [3H]ACh was maintained while that of [14C]ACh was decreasing, the [3H]ACh/[14C]ACh ratio in perfusate increased with each successive depolarization. This ratio was six to ten times higher than the corresponding ratio in vesicles or cytoplasm. These results indicate that the newly synthesized ACh did not equilibrate with either the depot vesicular or cytoplasmic ACh pools prior to release.

Acetylcholine changes underlying transmission of a single nerve impulse in the presence of 4-aminopyridine in Torpedo

1. Transmission of a single nerve impulse has been investigated at the nerve-electroplaque junction of Torpedo marmorata in the presence of 4-aminopyridine (4-AP), a drug which powerfully potentiates evoked transmitter release.2. Three methodological approaches were used conjointly. These were (i) electrophysiological recording of the compound electroplaque potential (e.p.p.), (ii) radiochemical measurement of evoked acetylcholine (ACh) release and (iii) analysis of the content of ACh and ATP in the tissue at brief time intervals during the course of the e.p.p. and soon after. The last was achieved by using a stimulator coupled to a rapid tissue freezer.3. In the response to a single stimulus, 4-AP enhanced in a dose-dependent manner the size of the e.p.p., increasing the duration much more than the amplitude. At 10(-4) M-4-AP, this resulted in the generation of a characteristic ;giant e.p.p.' whose area (in V x ms) was approximately 120 times greater than that of a normal e.p.p.4. The giant e.p.p. consisted of an initial peak, lasting for some 100 ms, a late rebound at about 300 ms, and finished between 500 and 1000 ms after the stimulus. Temperature changes greatly affected the shape of the giant e.p.p., modifying particularly the amplitude and time course of the late rebound.5. The amount of ACh released in response to a single stimulus was measured radiochemically and was found to greatly increase in the presence of 4-AP, explaining the potentiation of the e.p.p. With 4-AP concentrations ranging from 10(-6) M to 10(-4) M, the augmentation of ACh release showed a close correlation with increase of the e.p.p. area.6. The large potentiation of evoked transmitter release occurred in spite of a reduction of ACh stores. After treatment with 10(-4) M-4-AP, the total ACh content was reduced by 30-40% in the absence of any electrical stimulation. The reduction affected to a similar extent the vesicular and extravesicular compartments of ACh. This was accompanied by a general increase in the resting rate of ACh turnover.7. Synaptic vesicles were isolated from small fragments of electric organ, rapidly frozen with our device. Compartmental analysis was carried out by labelling the transmitter pools with a radioactive precursor and it was confirmed that vesicular ACh has a relatively low metabolic rate, whereas free ACh (most probably cytoplasmic ACh) turns over more rapidly. The same finding was obtained after treatment with 4-AP, but the starting levels of ACh and the yield of synaptic vesicles were lower.8. The total ACh content was measured at 30 and 100 ms intervals during the course of the giant e.p.p., and soon after. We found characteristic and significant changes which were (i) an initial fall of total ACh occurring within 100-150 ms, (ii) a transient ACh increase which occurred later and seemed to correspond to the late rebound of the giant e.p.p. and (iii) a steady 20% lowering of total ACh, observed from the end of the giant e.p.p. and lasting for more than 1 s.9. The ATP content of the tissue, during and after the giant e.p.p., followed a time course which was remarkably similar to that of total ACh. A significant ATP/ACh relationship was found in most experiments separately, and in the pooled results with a higher degree of significance.10. Vesicular ACh did not exhibit any significant change during and after the giant e.p.p. Neither the transient initial variations of total ACh nor its later lowering were reflected in similar changes of vesicular ACh. It was therefore the extravesicular pool of ACh which was concerned in the characteristic pattern of changes of total ACh.11. Compartmental analysis of transmitter stores was performed during the course of transmission, after labelling ACh in the tissue with a radioactive precursor. It was found that no detectable transfer of ACh occurred from cytoplasm to vesicles, either during the giant e.p.p., or within the following second.12. The following conclusions were reached. The effect of 4-AP is to cause a very strong and long-lasting potentiation of ACh release, resulting in a giant and complex electrical discharge. Transmitter release under these conditions was not only due to sudden liberation of the preformed, available ACh but also to a marked contribution of new ACh made during the giant e.p.p. These changes in ACh content were very significant and took place exclusively in the extravesicular pool of transmitter.

Acetylcholine measured at short time intervals during transmission of nerve impulses in the electric organ of Torpedo

1. The amounts of total acetylcholine (ACh) and ATP, and of vesicle-bound ACh were measured at short time intervals in the electrogenic tissue of Torpedo marmorata. The aim of this study is to approach with biochemical analysis the speed of electrophysiological phenomena.2. A stimulator coupled to a rapid freezer device was used to quench a number of tissue samples simultaneously, at different time intervals during transmission of a brief train of impulses at 100 Hz.3. The level of total ACh decreased significantly with the first ten impulses. Then a rapid but transient increase in total ACh occurred, reaching a maximum value by the fifteenth to sixteenth impulse.4. Vesicle-bound ACh did not exhibit any changes parallel to those of total ACh, and did not decrease beyond the control level during transmission of twenty impulses at 100 Hz.5. The amount of ATP in the tissue varied in close relation to that of total ACh. No significant phase shift was observed between the transmitter and the nucleotide and the ACh/ATP molar ratio was not significantly different from 1.6. The shortest time interval investigated in this work was 10 ms. The rate at which the pieces of tissue are quenched for biochemical measurements when plunged into a liquid at low temperature has been estimated. It has also been evaluated to what extent the freezing rate may distort measurements of the biochemical changes occurring in the tissue.7. It is concluded that fast freezing appears to be a valuable approach for investigating the rapid biochemical changes underlying cholinergic transmission; a better time resolution might be reached at the price, however, of greatly reducing the size of the samples. The second conclusion is that transmission of a brief train of impulses is accompanied by significant changes in the amount of extravesicular ACh.

Free and bound acetylcholine in frog muscle

1. Frog sartorius muscles were divided into end-plate containing (e.p.) and end-plate-free (non-e.p.) segments or homogenized in Ringer solution at 0 degrees C in the presence or absence of added acetylcholinesterase from electric eel. ACh was extracted from the tissue or from the homogenates and measured by mass fragmentography. 2. The concentration of ACh in non-e.p. segments was about six times lower than that in e.p. segments. 3. Homogenization of muscles in Ringer caused the hydrolysis of a small fraction ('free-1') of total ACh; addition of extra acetylcholinesterase caused hydrolysis of another, greater, fraction ('free-2' ACh). The esterase-resistant ('bound') ACh was stable at 0 degrees C up to 15 min of incubation. 4. Denervation for 15 days, which caused the disappearance of the nerve terminals, did not influence ACh in non-e.p. segments, but reduced total and bound ACh by about 75%, and free-2 ACh by 90%. 5. Treatment with La3+ ions, which caused the disappearance of synaptic vesicles, did not influence total ACh, but reduced bound ACh by 75%, whereas free-1 and free-2 ACh were increased. 6. Electrical stimulation of the nerve at 5 sec-1 or incubation with 50 mM-KCl did not affect ACh in the non-e.p. segments, but reduced by roughly 60% total, bound, and free ACh. 7. It is concluded that about 75% of bound ACh derives from synaptic vesicles, corresponding to 11,000 molecules per vesicle, and 25% from non-neural ACh; that free-1 and free-2 ACh derive mainly from the nerve terminal cytoplasm, although they may be contaminated by vesicular ACh.

The subsynaptosomal distribution and release of [3H]acetylcholine synthesized by rat cerebral cortical synaptosomes

Synaptosomes were prepared from rat cerebral cortex and incubated in [3H]choline for periods ranging from 1 to 90 min. The [3H]ACh synthesized during this period was found only in the cytoplasm and in a membrane-associated fraction. A negligible amount of the newly formed [3H]ACh was recovered in the vesicular fraction despite concerted efforts to protect a hypothetical population of labile vesicles. The specific activity of the membrane-associated component, accounting for 21% of the total [3H]ACh, was by far the highest. This membrane-associated fraction was not released by hypotonic shock or homogenization and apparently was not in association with the monodisperse synaptic vesicles. The [3H]ACh was released in a calcium dependent manner. This investigation has determined that the ACh synthesized by synaptosomes is localized in only two fractions, cytoplasmic and membrane-associated; that this newly synthesized ACh can be released from synaptosomes by a process consistent with physiological release; and that at least part of the ACh released was originally present in the cytoplasm.