Effects of an inhibitor of the synaptic vesicle acetylcholine transport system on quantal neurotransmitter release: an electrophysiological study

Temporal components of cholinergic terminal to dopaminergic terminal transmission in dorsal striatum slices of mice

Striatal dopamine (DA) is critically involved in major brain functions such as motor control and deficits such as Parkinson's disease. DA is released following stimulation by two pathways: the nigrostriatal pathway and the cholinergic interneuron (ChI) pathway. The timing of synaptic transmission is critical in striatal circuits, because millisecond latency changes can reverse synaptic plasticity from long-term potentiation to long-term depression in a DA-dependent manner. Here, we determined the temporal components of ChI-driven DA release in striatal slices from optogenetic ChAT-ChR2-EYFP mice. After a light stimulus at room temperature, ChIs fired an action potential with a delay of 2.8 ms. The subsequent DA release mediated by nicotinic acetylcholine (ACh) receptors had a total latency of 17.8 ms, comprising 7.0 ms for cholinergic transmission and 10.8 ms for the downstream terminal DA release. Similar latencies of DA release were also found in striatal slices from wild-type mice. The latency of ChI-driven DA release was regulated by inhibiting the presynaptic vesicular ACh release. Moreover, we describe the time course of recovery of DA release via the two pathways and that of vesicle replenishment in DA terminals. Our work provides an example of unravelling the temporal building blocks during fundamental synaptic terminal-terminal transmission in motor regulation.

Experience-Dependent Formation and Recruitment of Large Vesicles from Reserve Pool

The sizes and contents of transmitter-filled vesicles have been shown to vary depending on experimental manipulations resulting in altered quantal sizes. However, whether such a presynaptic regulation of quantal size can be induced under physiological conditions as a potential alternative mechanism to alter the strength of synaptic transmission is unknown. Here we show that presynaptic vesicles of glutamatergic synapses of Drosophila neuromuscular junctions increase in size as a result of high natural crawling activities of larvae, leading to larger quantal sizes and enhanced evoked synaptic transmission. We further show that these larger vesicles are formed during a period of enhanced replenishment of the reserve pool of vesicles, from which they are recruited via a PKA- and actin-dependent mechanism. Our results demonstrate that natural behavior can induce the formation, recruitment, and release of larger vesicles in an experience-dependent manner and hence provide evidence for an additional mechanism of synaptic potentiation.

Recycling and refilling of transmitter quanta at the frog neuromuscular junction

1. Fluorescent dyes have been used at the frog neuromuscular junction to label synaptic vesicular membrane. Retrieved membrane is reformed into vesicles, which are released along with pre-existing vesicles. Consequently, if vesicular refilling with acetylcholine (ACh) is depressed by inhibitors, two sizes of quanta should be released: normal and smaller. As recycling continues the fraction of smaller size quanta should increase exponentially. 2. We enhanced the rate of quantal release by elevating the K+ concentration. The principal inhibitors were (-)-vesamicol (VES), hemicholinium-3 (HC3), and NH4+. Quantal size measurements were fitted to one and to two cumulative lognormal probability distribution functions. When two fitted better, the statistical significance assessment took into account the three additional parameters used in calculating the fit. 3. After recycling in the presence of inhibitor, many sets were fitted better by two lognormal functions. As recycling continued, the fraction of the miniature endplate potential voltage-time integrals ( MEPPs) in the larger sub-population decreased exponentially. 4. The size of the releasable pool was estimated by counting the quanta released by carbonyl cyanide m-chlorophenylhydrazone (CCCP). This was compared to pool sizes calculated from the inhibitor experiments. The two estimates of pool size were indistinguishable, with mean values ranging from about 170,000 to 270,000. 5. With all of the treatments tested, the means of the sizes in the smaller sub-population of MEPPs were about 1/3 those of the larger sub-populations. 6. Recycling synaptic vesicles appear to be incorporated into the releasable pool from which they have roughly the same probability of release as the pre-existing vesicles.

The heterogeneity of vesicular acetylcholine storage in cholinergic nerve terminals

The vesicular hypothesis of quantal acetylcholine release describes the process by which discrete packages (or quanta) of the transmitter are released from nerve terminals through the exocytosis of the content of synaptic vesicles. However, cholinergic synaptic vesicles can no longer be vaguely regarded as simple membrane bound 'sacks' of the transmitter. Modern molecular, biochemical, morphological and electrophysiological research has revealed them to be complex cellular structures with a heterogeneous mixture of functions. Thus, not all synaptic vesicle populations are formed under the same circumstances and there are variations in the releasability of synaptic vesicle populations. This review briefly outlines some of the experimental research that has lead to our current thinking on the heterogeneity of vesicular acetylcholine storage in cholinergic nerve terminals. In addition, a model for vesicular acetylcholine storage and release is presented that attempts to accommodate many of the modern ideas concerning cholinergic synaptic vesicle function and interaction.

Physiological and regenerative acetylcholine release from motor nerve: differential inhibitions by vesamicol and omega-agatoxin IVA

Stimulation of mammalian motor neurons can elicit Ca(2+)-dependent regenerative release of acetylcholine and prolonged endplate depolarization when the enzymatic degradation of the neurotransmitter is inhibited. Unlike physiological phasic release of acetylcholine, the regenerative release is sensitive to L-type Ca2+ channel blockers. We studied the effects of vesamicol (an inhibitor of active transport of acetylcholine into synaptic vesicles) and omega-agatoxin IVA (a blocker of the motor nerve P-type Ca2+ channel) on these two types of acetylcholine release to compare the vesicle pools and Ca2+ channels responsible for the release. When coupled with repetitive stimulations, vesamicol decreased mean amplitude of miniature endplate potentials, resulting in a skewed distribution to lower amplitude, reduced quantal content of endplate potentials and decreased immediate available pool of acetylcholine. omega-Agatoxin IVA had no effect on miniature endplate potential but inhibited quantal content of endplate potential. The mean inhibitory concentration was around 5-10 nM. Vesamicol and omega-agatoxin IVA decreased the probability of triggering regenerative release. However, the magnitude and duration of regenerative release, once triggered, were not depressed by either agent. It appears that the majority of Ca2+ necessary for regenerative release is translocated via omega-agatoxin IVA-insensitive Ca2+ channels, which can be activated by prolonged depolarization of nerve terminals induced by accumulated acetylcholine. The results suggest that different Ca2+ channels are activated in the regenerative (L-type) and phasic (P-type) acetylcholine release, which utilize different pools of synaptic vesicles.

Effects of vesicular acetylcholine uptake blockers on frequency augmentation-potentiation in frog neuromuscular transmission

Vesamicol inhibits the vesicular loading of acetylcholine molecules. The effects of vesamicol and similarly acting compounds on neuromuscular transmission in frogs were investigated to determine whether these inhibitors-inhibit the frequency augmentation-potentiation of transmitter release. Various vesicular acetylcholine transport blockers suppressed the stimulation frequency-related release parameter, k, in a dose-dependent manner. Artane, cetiedil, chloroquine, ethodin, quinacrine, vesamicol and its benzyl-analogue, 2-(4-benzylpiperidino)cyclohexanol, had strong effects, while those of aminacrine, chlorpromazine, fluphenazine, imipramine, pyrilamine and thioridazine were weak. A significant correlation was observed between the biochemically reported values of IC50 and the electrophysiological inhibitory potencies on k at 20 microM. Contrary to expectations from the biochemical data, however, vesamicol and its benzyl-analogue showed equipotent inhibitory actions on the electrophysiological frequency augmentation-potentiation relation. Low sensitivity and low selectivity of the frequency augmentation-potentiation for vesamicol and its benzyl-analogue lead us to conclude that the vesicular acetylcholine transporter is not the site of the electrophysiological action of vesamicol and similarly acting chemicals.

The chemical nature of the main central excitatory transmitter: a critical appraisal based upon release studies and synaptic vesicle localization

The chemical nature of the central transmitter responsible for fast excitatory events and other related phenomena is analysed against the historical background that has progressively clarified the structure and function of central synapses. One of the problems posed by research in this field has been whether one or more of the numerous excitatory substances endogenous to the brain is responsible for fast excitatory synaptic transmission, or if such a substance is, or was, a previously unknown one. The second question is related to the presence in the CNS of three main receptor types related to fast excitatory transmission, the so-called alpha-amino-3-hydroxy-5-methylisoxazole propionic acid, kainate and N-methyl-D-aspartate receptors. This implies the possibility that each receptor type might have its own endogenous agonist, as has sometimes been suggested. To answer such questions, an analysis was done of how different endogenous substances, including L-glutamate, L-aspartate, L-cysteate, L-homocysteate, L-cysteine sulfinate, L-homocysteine sulfinate, N-acetyl-L-aspartyl glutamate, quinolinate, L-sulfoserine, S-sulfo-L-cysteine, as well as possible unknown compounds, were able to fulfil the more important criteria for transmitter identification, namely identity of action, induced release, and presence in synaptic vesicles. The conclusion of this analysis is that glutamate is clearly the main central excitatory transmitter, because it acts on all three of the excitatory receptors, it is released by exocytosis and, above all, it is present in synaptic vesicles in a very high concentration, comparable to the estimated number of acetylcholine molecules in a quantum, i.e. 6000 molecules. Regarding a possible transmitter role for aspartate, for which a large body of evidence has been presented, it seems, when this evidence is carefully scrutinized, that it is either inconclusive, or else negative. This suggests that aspartate is not a classical central excitatory transmitter. From this analysis, it is suggested that the terms alpha-amino-3-hydroxy-5-methylisoxazole propionic acid, kainate and N-methyl-D-aspartate receptors, should be changed to that of glutamate receptors, and, more specifically, to GLUA, GLUK and GLUN receptors, respectively. When subtypes are described, a Roman numeral may be added, as in GLUNI, GLUNII, and so on.

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 pharmacology of vesamicol: an inhibitor of the vesicular acetylcholine transporter

1. Vesamicol (2-[4-phenylpiperidino] cyclohexanol) inhibits the transport of acetylcholine into synaptic vesicles in cholinergic nerve terminals. 2. Recent pharmacological studies of the effects of vesamicol on skeletal neuromuscular transmission have revealed a pattern of activity for the compound consistent with the neurochemical observation of the mechanism of action of the compound. 3. Pharmacological manipulation of vesicular acetylcholine transport has been used to investigate the recycling and mobilization of synaptic vesicles within cholinergic nerve terminals. 4. In addition to its effects on vesicular acetylcholine transport, vesamicol also possesses some sodium channel and alpha-adrenoceptor blocking activity. 5. Vesamicol clearly represents a unique tool for investigating presynaptic mechanisms in cholinergic nerve terminals.

Acetylcholine recycling and release at rat motor nerve terminals studied using (-)-vesamicol and troxpyrrolium

1. The presynaptic mechanisms governing the release and recycling of synaptic vesicles have been studied by examining the effects of nerve stimulation, (-)-vesamicol (an inhibitor of acetylcholine transport into synaptic vesicles) and troxypyrrolium (an inhibitor of the high-affinity, sodium-dependent, choline uptake system) on endplate currents (EPCs) and miniature endplate currents (MECPs) recorded from motor endplates in cut rat hemidiaphragm preparations. 2. In control experiments, 5 min of 10 Hz nerve stimulation had no effect on either the mean or the distribution of MEPC amplitudes. 3. Nerve stimulation in the presence of (-)-vesamicol (25 nM-10 microM) revealed a population of MEPCs that was unaffected by the compound and a population of MEPCs whose mean amplitude was selectively reduced by the compound. 4. Nerve stimulation in the presence of troxypyrrolium (20 microM) produced a uniform reduction in the amplitude of all MEPCs with no change in the coefficient of variance of MEPC amplitudes. 5. The concentration-dependent effects of (-)-vesamicol on the amplitude of the evoked EPCs paralleled the concentration-dependent effects of the compound on MEPC amplitudes. 6. The results are consistent with the hypothesis that both recycled and performed synaptic vesicles are heterogeneously released from rat motor nerve terminals and that (-)-vesamicol acts selectively on recycling vesicles. In addition, a model of vascular loading that accounts for the different effects of nerve stimulation on MEPC amplitudes in the presence of (-)-vesamicol and troxypyrrolium is described.

Purification and characterization of a nonvesicular vesamicol-binding protein from electric organ and demonstration of a related protein in mammalian brain

A protein that binds vesamicol has been purified from a soluble fraction of the Torpedo electric organ homogenate that does not contain synaptic vesicles. The purified vesamicol-binding protein (VBP) has a molecular mass of 470 kDa composed of 30- and 24-kDa subunits. Chemical deglycosylation yielded a single, heterogeneous protein of 24 kDa. The 30-kDa subunit is also sensitive to endo-beta-galactosidase. The dissociation constant of the VBP.vesamicol complex is 0.9 microM, and the Bmax is 5,500 pmol/mg. Antiserum raised to the 30-kDa subunit cross-reacts with the 24-kDa subunit, but not with synaptic vesicles. Drug binding studies and Western blot analysis show that VBP is present in other Torpedo tissues as well as mammalian brain. Immunofluorescence microscopy demonstrates that VBP-like immunoreactivity is not localized exclusively to the nerve terminal regions of the electric organ. Thermal stability, the pH dependence of vesamicol binding, and pharmacological comparisons demonstrate that the VBP is not the cholinergic synaptic vesicle receptor for vesamicol. The implications of this finding for current efforts to develop in vivo diagnostics of cholinergic nerve terminal status based on vesamicol are discussed.

Local anaesthetic activity of vesamicol in the electric organ of Torpedo

Synaptic transmission in intact pieces of the Torpedo electric organ treated with vesamicol (2-(4-phenylpiperidino)cyclohexanol, formerly AH5183) was elicited by trains of repetitive electrical stimulation at different frequencies. When the frequency of stimulation was increased from 10 to 50 or 100 Hz, micromolar concentrations of vesamicol enhanced the tetanic rundown of the successive tissue responses. This effect was already detectable with 10 microM vesamicol. It was dramatically potentiated with concentrations of 50 or 100 microM vesamicol, which caused complete failure of transmission after usually less than 10 responses. The drug was unequivocally demonstrated to act by depressing the evoked release of acetylcholine as a consequence of a highly frequency- and concentration-dependent impairment of Na+ channel function in afferent axons. It is concluded that, in the electric organ, vesamicol blocks transmission by acting as a local anaesthetic. This action of micromolar concentrations of vesamicol must be taken into account especially during high-rate nerve activity.

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)

The effects of L-vesamicol, an inhibitor of vesicular acetylcholine uptake, on two populations of miniature endplate currents at the snake neuromuscular junction

The actions of the active L-isomer of vesamicol, an inhibitor of the vesicular storage of acetylcholine, has been studied on spontaneous and evoked acetylcholine release at the snake neuromuscular junction. Miniature endplate currents and endplate currents were recorded from cut muscle fibres of the garter snake, Thamnophis sirtalis. In controls, prolonged periods of high frequency nerve stimulation produced a bimodal distribution of miniature endplate current amplitudes. The stimulation induced "small-mode" miniature endplate currents had a mean amplitude of around 40-55% of the pre-stimulation miniature endplate current. Relative to the normal-sized post-stimulation miniature endplate current, the proportion and, to a lesser extent, amplitude of the small-mode miniature endplate currents was related to both the frequency and duration of nerve stimulation and to the extracellular calcium ion concentration. In unstimulated preparations, L-vesamicol (2-5 microM) did not affect either endplate current quantal content or miniature endplate current amplitude or frequency. However, at these doses, the mean amplitude of the stimulation-induced, small-mode miniature endplate current was reduced by L-vesamicol in a concentration-dependent manner such that they were not visible at the highest dose. L-Vesamicol had no affect on the mean or coefficient of variance of amplitude of the larger, normal-sized miniature endplate current. Additionally, the stimulation-induced increase in overall miniature endplate current frequency seen in controls was abolished by 5 microM L-vesamicol. After prolonged 10 Hz nerve stimulation endplate current amplitude was markedly reduced in both controls (by 94%) and in the presence of 5 microM L-vesamicol (by 98%). Analysis of endplate current amplitude variance showed that in control the decrease was due to reductions in both quantal content and quantal size while in L-vesamicol the decrease was due entirely to a change in quantal content with no change in quantal size. Thus, we have observed that L-vesamicol selectively reduces the amplitude of a population of stimulation-induced small-mode quanta both as miniature endplate currents and as constituents of endplate currents. We suggest that these quanta are derived from a highly active, readily releasable pool. An action of L-vesamicol on this labile pool is consistent with previous observations on its ability to inhibit the vesicular storage of acetylcholine.