Cholinergic synaptic vesicles isolated from Torpedo marmorata: demonstration of acetylcholine and choline uptake in an in vitro system

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.

Cloning and expression of the vesamicol binding protein from the marine ray Torpedo. Homology with the putative vesicular acetylcholine transporter UNC-17 from Caenorhabditis elegans

Complementary DNA clones corresponding to a messenger RNA encoding a 56 kDa polypeptide have been obtained from Torpedo marmorata and Torpedo ocellata electric lobe libraries, by homology screening with a probe obtained from the putative acetylcholine transporter from the nematode Caenorhabditis elegans. The Torpedo proteins display approximately 50% overall identity to the C. elegans unc-17 protein and 43% identity to the two vesicle monoamine transporters (VMAT1 and VMAT2). This family of proteins is highly conserved within 12 domains which potentially span the vesicle membrane, with little similarity within the putative intraluminal glycosylated loop and at the N- and C-termini. The approximately 3.0 kb mRNA species is specifically expressed in the brain and highly enriched in the electric lobe of Torpedo. The Torpedo protein, expressed in CV-1 fibroblast cells, possesses a high-affinity binding site for vesamicol (Kd = 6 nM), a drug which blocks in vitro and in vivo acetylcholine accumulation in cholinergic vesicles.

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)

The regulation of quantal size

Quantal size can be altered experimentally by numerous treatments that seem to lack any common thread. The observations may seem haphazard and senseless unless clear distinctions are made from the outset. Some treatments shift the size of the entire population of quanta. These quanta are released by nerve stimulation. Other treatments add quanta of abnormal size or shape--monstrosities--to the population (4.0). Usually, perhaps even invariably, the monstrosities are not released by nerve stimulation. 6.1. POPULATION SIZE INCREASES. 6.1.1. Quantal size must be regulated. The size of the entire quantal population can be experimentally shifted to a larger size, with the mean rising two- or even four-fold. Before these observations, it was reasonable to suppose that quantal size was relatively fixed, with little room for maneuver. A logical picture is that synaptic vesicles have a maximum transmitter capacity, and usually they are filled to the brim. This picture is wrong. The quantity of transmitter packaged in the quantum must be regulated by the neuron, so depending on circumstances, quantal size can be increased or decreased. Figure 18 makes the case for regulation more strongly than words. We are beginning to identify some of the signals for up and down regulation, and the first steps have been made in discovering the signal transduction pathways, but we are far from a true understanding. This is hardly surprising, because our information about how transmitter molecules are assembled into quantal packages is still imperfect. Until we understand the engine, it may be difficult to picture the accelerator or the brake. 6.1.2. Signals that up regulate size. Stimulation of the presynaptic neuron increases quantal size at the NMJ, at synapses in autonomic ganglia and in hippocampus. The stimulus parameters necessary to elicit the quantal size increase have not been explored sufficiently in any of these cases, and all deserve further investigation. At both frog and mouse NMJs quantal size is roughly doubled following exposure to hypertonic solutions, which elevate the rate of spontaneous quantal release. This discovery, coupled with the increases caused by tetanic stimulation, suggested that the signal for up regulation is a period of greatly enhanced quantal output. The size increase takes about 15 min in hypertonic solution in mouse and about 60 min in frog. Highly hypertonic solutions do not increase the rate of quantal release in frog; they also do not increase quantal size. This supported the idea that quantal release rate is the signal for up regulation.(ABSTRACT TRUNCATED AT 400 WORDS)

AH5183 and cetiedil: two potent inhibitors of acetylcholine uptake into isolated synaptic vesicles from Torpedo marmorata

Synaptic vesicles purified on a sucrose-KCl sedimentation gradient were tested for their ability to accumulate [1-14C]acetylcholine ([1-14C]ACh) in the absence and in the presence of AH5183 and cetiedil. Kinetic studies of ACh transport showed that it was time dependent and saturable as a function of ACh concentration, with a KT of 1.2 mM. The protein-modifying agents N-ethylmaleimide and 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole were powerful inhibitors of ACh uptake. In agreement with other studies, AH5183 was found to be a potent inhibitor of ACh uptake by synaptic vesicles. Inhibition was of the mixed noncompetitive type, and the inhibition constant was 45.2 +/- 3.4 nM. Cetiedil, a drug that resembles ACh, was previously shown on intact nerve endings to inhibit the translocation of newly synthesized ACh into the synaptic vesicle compartment, and we demonstrate here that cetiedil is indeed an efficient blocker of ACh uptake by isolated synaptic vesicles. It acted as a competitive inhibitor, with a Ki of 118.5 +/- 9.5 nM. Neither ATP-dependent calcium uptake nor Mg2+-ATPase activity was affected by the drugs, a finding showing their specificity toward the ACh uptake process. The binding of L-[3H]AH5183 to intact vesicles was characterized in the absence or the presence of ACh or cetiedil. Saturation experiments showed a total binding capacity of approximately 126 pmol/mg of vesicular protein and a dissociation constant of 19.9 +/- 4.1 nM under control conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantal analysis of action of hemicholinium-3 studied at a central cholinergic synapse of Aplysia

The effects of hemicholinium-3 (HC-3) on cholinergic transmission were studied on central identified inhibitory (H-type post-synaptic cell, Cl- channels) and on excitatory (D-type post-synaptic cell, cationic channels) synapses of Aplysia californica. In the H-type post-synaptic cell, the amplitude and the decay time of miniature post-synaptic currents (m.p.s.c.s.) were calculated by statistical analysis of long duration induced post-synaptic current (l.d.i.p.s.c.) due to 3 s depolarizations of the presynaptic neurone in the presence of tetrodotoxin. On H-type receptors, with respect to acetylcholine (ACh), HC-3 acted as an agonist and a blocker whereas on D-type receptors, it acted only as a blocker. At low concentration of bath-applied HC-3, in the H-type synapse, the decay time of the evoked inhibitory post-synaptic current (i.p.s.c.) as well as that of the m.p.s.c. was lengthened. These changes were rapidly reversible by wash. The decay time of excitatory post-synaptic current (e.p.s.c.) at the D-type synapse was not affected. On the inhibitory synapse, HC-3 applied in the bath at the concentration of 10(-5) M, reduced considerably the size of the m.p.s.c.s whereas the evoked i.p.s.c.s and the l.d.i.p.s.c.s were only slightly affected pointing to an increase of the quantal content of both responses. After wash, both i.p.s.c.s and l.d.i.p.s.c.s showed a clear facilitation which persisted for several tens of minutes. The presence of presynaptic receptors was considered. Similar facilitation of e.p.s.c.s by HC-3 was observed at the D-type synapse. The comparison of the degree of depression by HC-3 of the m.p.s.c.s and of the responses to ionophoretically applied ACh, indicated that the size of the quantum was not changed. Intracellular injection of HC-3 into the presynaptic neurone of the H-type synapse led to a decrease of transmitter release which affected solely the quantal content of the responses. As the synaptic transmission could not be restored by injection of exogenous ACh into the presynaptic neurone, it was concluded that the depression of transmission was not due to a decrease of ACh synthesis.

Effects of 2,4-dithiobiuret treatment in rats on cholinergic function and metabolism of the extensor digitorum longus muscle

Effects of 2,4-dithiobiuret (DTB) treatment in rats on neuromuscular transmission and the disposition of cholinergic substances, acetylcholine (ACh) and choline (Ch), were examined in a combined electrophysiological/biochemical study using an in vitro extensor digitorum longus (EDL) muscle-peroneal nerve preparation. EDL muscle preparations isolated from rats treated with DTB (1 mg/kg/day X 5 days, ip) displayed a 49% depression in the frequency of miniature end-plate potentials (MEPPs) and a 21% depression in mean MEPP amplitude. Statistical analysis of evoked end-plate potentials (EPPs) measured in curarized preparations indicated that the mean quantal content (m) was significantly depressed in EDL muscles from DTB-treated rats. At stimulation rates of 1, 10, 20, and 50 Hz the estimated values of m in EDL preparations from DTB-treated rats were, respectively, 21, 25, 45, and 51% of that in control preparations. Biochemical determinations of ACh and Ch revealed a significant DTB-induced increase in endogenous ACh and Ch content in EDL preparations fixed for extraction of ACh and Ch immediately after dissection from the treated rats. In vitro, however, there were negligible changes in overall ACh synthesis since the total (tissue and medium) tracer ACh (2H4-ACh) synthesized from tracer Ch (2H4-Ch; 10 microM) supplied in the perfusion medium was similar in EDL preparations from DTB-treated and control rats. Also, in EDL muscles from DTB-treated rats the resting release of ACh was not affected, but when exogenous Ch (2H4-Ch) was not supplemented in the medium the evoked release (via peroneal nerve stimulation) of ACh was depressed. Thus, decreases in spontaneous quantal ACh release, as detected in the electrophysiological experiments, were not reflected by changes in the biochemically determined ACh resting release. The biochemical determination of evoked ACh release, however, correlated with the decrease in quantal content detected in the electrophysiological analysis of evoked EPPs when exogenous Ch was not supplemented in the perfusion medium. Significant and consistent increases (two to three times) in both Ch content and efflux occurred in the EDL muscles from DTB-intoxicated rats. These results indicate that DTB induces a prejunctional impairment of neuromuscular transmission that is not specifically directed at ACh synthesis. Rather those processes by which ACh is incorporated into or released from vesicles appear to be altered.

High-affinity, sodium-gradient-dependent transport of choline into vesiculated presynaptic plasma membrane fragments from the electric organ of Torpedo marmorata and reconstitution of the solubilized transporter into liposomes

Vesiculated fragments of presynaptic plasma membranes have been isolated from the purely cholinergic electromotor nerve terminals of Torpedo marmorata. Synaptosomes, generated from the terminals by homogenization, were separated on a discontinuous Ficoll gradient and then lysed by osmotic shock at 2 degrees C, pH 8.5 in the presence of 0.1 mM MgCl2. These conditions for lysis were optimal for choline transport. Electron micrographs of lysed synaptosomes showed vesiculated membranes with diameters smaller than those of synaptosomes; occasionally, synaptic vesicles were observed attached to them. Intact mitochondria or synaptosomes and basal laminae were not present. High-affinity (KT = 1.7 microM) uptake of choline into these vesiculated membrane fragments showed: an absolute dependence on the Na+ gradient (outside greater than inside), a transient Na+-gradient-dependent accumulation of choline over the equilibrium concentration (over-shoot), electrogenicity and rheogenicity, since the uptake was further stimulated in the presence of a Na+ gradient by valinomycin, dependence on the presence of external Cl-, and partial dependence on a Cl- gradient (outside greater than inside), high-affinity (Ki = 25 nM) inhibition by hemicholinium-3 and temperature sensitivity. The plasma membranes were further purified by centrifugal density gradient fractionation on a 4-12% Ficoll gradient. Several enzymes and polypeptides copurified with the specific binding sites for choline present in the membranes. The fraction with the most binding sites was one denser than 12% Ficoll. This was also the fraction richest in acetylcholinesterase, 5'-nucleotidase and polypeptides of relative molecular mass, Mr (X 10(-3)) of greater than 200, 140, 68 (doublet), 57, 54 and 28. Acetylcholinesterase was positively identified as a Mr 68 000 component by immune blot. By contrast the ouabain-sensitive ATPase showed a negative correlation with choline binding sites. When the solubilized proteins of the vesiculated membranes were transferred to liposomes, they conferred on the latter the capacity to take up choline in a manner closely resembling its transport in natural membranes but with an initial (one minute) rate of uptake approximately 10-times greater per mg of protein. Several proteins were selectively transferred to the liposomes including ones of Mr (X 10(-3)) 34, 42, 47, 54, 60, 68, 92, 160 and greater than 200. The polypeptides of Mr (X 10(-3)) 140, 57 and 28 were lost in the transfer.(ABSTRACT TRUNCATED AT 400 WORDS)

Acetylcholine release from isolated synaptic vesicles related to ionic permeability changes: continuous detection with a chemiluminescent method

The effect of ionic permeability changes on acetylcholine (ACh) release from isolated cholinergic synaptic vesicles of Torpedo was studied using a chemiluminescent method for continuous ACh detection. Vesicles rendered freely permeable to potassium by valinomycin lost most of their ACh content in K+ media, if the accompanying anion was permeant; it thus appeared that ACh leakage occurred as the result of internal osmotic changes. Upon addition of ionophores that catalyse monovalent cation/H+ exchange (gramicidin D or a mixture of valinomycin plus protonophore FCCP), a rapid but transient ACh release was observed. Surprisingly, nigericin which also catalyses K+/H+ exchange, had no effect on ACh release. The divalent cation ionophore A23187 promoted ACh release only when calcium (and not magnesium) was introduced into the external medium in a millimolar concentration range. As the simultaneous addition of the protonophore FCCP and A23187 decreased this calcium-dependent ACh leakage, a releasing effect of A23187 through Ca2+/H+ exchange is suspected. The present results emphasise the role of internal protons for ACh retention inside synaptic vesicles.

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.

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.

Acetylcholine incorporation by cholinergic synaptic vesicles from Torpedo marmorata

The ability of cholinergic vesicles to incorporate acetylcholine (ACh) was studied using highly purified synaptic vesicles from Torpedo electric organ. Depleted vesicles were capable of rapidly taking up exogenous ACh. Evidence that this represented true incorporation was that labelled ACh comigrated with vesicular ATP on gel filtration and that vesicle-associated ACh was protected against enzymatic hydrolysis and was releasable under hypoosmotic conditions. The total amount of ACh incorporated depended on the ACh concentration up to 100 mM. A sudden fall in the external ACh concentration did not cause leakage of the ACh incorporated in vitro. Preliminary results indicated that retention of ACh inside the vesicle was pH-dependent. Choline was also taken up by vesicles, but the time pattern strongly suggested that it was not being retained. The magnitude of ach incorporation was estimated with respect to the intravesicular space.

On the specificity of uptake by isolated Torpedo synaptic vesicles

Synaptic vesicles were isolated from the electric organ of Torpedo marmorata in highly purified form. Their uptake properties were examined using a large number of small organic molecules as substrates. Following incubation at 26 degree C for 1 h, it was found that concentrative accumulation, indicated by a vesicle:medium concentration ratio greater than unity, was achieved by all the choline analogues used and by four biogenic amines, but not by a variety of purine and pyrimidine bases and nucleosides. Amino acids penetrated poorly, as did sugars, and of organic anions, acetate but not citrate or thiocyanate, was almost excluded. Thus Torpedo vesicles are relatively impermeable to compounds which cannot utilize the ACh or ATP carriers, but show a very high rate of amine uptake, which may be linked to a pH gradient.