Decrease of quantal size and quantal content during tetanic stimulation detected by focal recording

Synaptic vesicle distribution and release at rat diaphragm neuromuscular junctions

Synaptic vesicle release at the neuromuscular junction (NMJ) is highly reliable and is vital to the success of synaptic transmission. We examined synaptic vesicle number, distribution, and release at individual type-identified rat diaphragm NMJ. Three-dimensional reconstructions of electron microscopy images were used to obtain novel measurements of active zone distribution and the number of docked synaptic vesicles. Diaphragm muscle-phrenic nerve preparations were used to perform electrophysiological measurements of the decline in quantal content (QC) during repetitive phrenic nerve stimulation. The number of synaptic vesicles available for release vastly exceeds those released with a single stimulus, thus reflecting a relatively low probability of release for individual docked vesicles and at each active zone. There are two components that describe the decline in QC resulting from repetitive stimulation: a rapid phase (<0.5 s) and a delayed phase (<2.5 s). Differences in the initial rapid decline in QC were evident across type-identified presynaptic terminals (fiber type classification based on myosin heavy chain composition). At terminals innervating type IIx and/or IIb fibers, the initial decline in QC during repetitive stimulation matched the predicted depletion of docked synaptic vesicles. In contrast, at terminals innervating type I or IIa fibers, a faster than predicted decline in QC with repetitive stimulation suggests that a decrease in the probability of release at these terminals plays a role in addition to depletion of docked vesicles. Differences in QC decline likely reflect fiber-type specific differences in activation history and correspond with well-described differences in neuromuscular transmission across muscle fiber types.

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.

Short-term synaptic plasticity

Synaptic transmission is a dynamic process. Postsynaptic responses wax and wane as presynaptic activity evolves. This prominent characteristic of chemical synaptic transmission is a crucial determinant of the response properties of synapses and, in turn, of the stimulus properties selected by neural networks and of the patterns of activity generated by those networks. This review focuses on synaptic changes that result from prior activity in the synapse under study, and is restricted to short-term effects that last for at most a few minutes. Forms of synaptic enhancement, such as facilitation, augmentation, and post-tetanic potentiation, are usually attributed to effects of a residual elevation in presynaptic [Ca(2+)]i, acting on one or more molecular targets that appear to be distinct from the secretory trigger responsible for fast exocytosis and phasic release of transmitter to single action potentials. We discuss the evidence for this hypothesis, and the origins of the different kinetic phases of synaptic enhancement, as well as the interpretation of statistical changes in transmitter release and roles played by other factors such as alterations in presynaptic Ca(2+) influx or postsynaptic levels of [Ca(2+)]i. Synaptic depression dominates enhancement at many synapses. Depression is usually attributed to depletion of some pool of readily releasable vesicles, and various forms of the depletion model are discussed. Depression can also arise from feedback activation of presynaptic receptors and from postsynaptic processes such as receptor desensitization. In addition, glial-neuronal interactions can contribute to short-term synaptic plasticity. Finally, we summarize the recent literature on putative molecular players in synaptic plasticity and the effects of genetic manipulations and other modulatory influences.

Mechanisms controlling bursting activity induced by disinhibition in spinal cord networks

Disinhibition reliably induces regular synchronous bursting in networks of spinal interneurons in culture as well as in the intact spinal cord. We have combined extracellular multisite recording using multielectrode arrays with whole cell recordings to investigate the mechanisms involved in bursting in organotypic and dissociated cultures from the spinal cords of embryonic rats. Network bursts induced depolarization and spikes in single neurons, which were mediated by recurrent excitation through glutamatergic synaptic transmission. When such transmission was blocked, bursting ceased. However, tonic spiking persisted in some of the neurons. In such neurons intrinsic spiking was suppressed following the bursts and reappeared in the intervals after several seconds. The suppression of intrinsic spiking could be reproduced when, in the absence of fast synaptic transmission, bursts were mimicked by the injection of current pulses. Intrinsic spiking was also suppressed by a slight hyperpolarization. An afterhyperpolarization following the bursts was found in roughly half of the neurons. These afterhyperpolarizations were combined with a decrease in excitability. No evidence for the involvement of synaptic depletion or receptor desensitization in bursting was found, because neither the rate nor the size of spontaneous excitatory postsynaptic currents were decreased following the bursts. Extracellular stimuli paced bursts at low frequencies, but failed to induce bursts when applied too soon after the last burst. Altogether these results suggest that bursting in spinal cultures is mainly based on intrinsic spiking in some neurons, recurrent excitation of the network and auto-regulation of neuronal excitability.

Safety factor at the neuromuscular junction

Reliable transmission of activity from nerve to muscle is necessary for the normal function of the body. The term 'safety factor' refers to the ability of neuromuscular transmission to remain effective under various physiological conditions and stresses. This is a result of the amount of transmitter released per nerve impulse being greater than that required to trigger an action potential in the muscle fibre. The safety factor is a measure of this excess of released transmitter. In this review we discuss the practical difficulties involved in estimating the safety factor in vitro. We then consider the factors that influence the safety factor in vivo. While presynaptic transmitter release may be modulated on a moment to moment basis, the postsynaptic features that determine the effect of released transmitter are not so readily altered to meet changing demands. Different strategies are used by different species to ensure reliable neuromuscular transmission. Some, like frogs, rely on releasing a large amount of transmitter while others, like man, rely on elaborate postsynaptic specialisations to enhance the response to transmitter. In normal adult mammals, the safety factor is generally 3-5. Both pre- and postsynaptic components change during development and may show plasticity in response to injury or disease. Thus, both acquired autoimmune and inherited congenital diseases of the neuromuscular junction (NMJ) can significantly reduce, or even transiently increase, safety factor.

Quantitative mapping of transverse relaxivity (1/T(2)) in hepatic iron overload: a single spin-echo imaging methodology

Recent research into the non-invasive assessment of hepatic iron concentrations using magnetic resonance imaging has shown that the proton transverse relaxivity (1/T(2)) varies linearly with liver iron concentration. However, the development of an image-based system for the assessment of hepatic iron distribution has been confounded by the presence of motion induced artifacts in the T(2)-weighted images. We report on the development of a single spin-echo imaging methodology that enables the generation of transverse relaxivity maps over the liver. A simple smoothing technique is used to accommodate the image intensity perturbations caused by abdominal motion. The relaxivity maps are consistent with the variation of iron concentration throughout the liver. A Parzen density estimate and histogram of the relaxivity distribution are generated to assist in the visual assessment of the degree and variability of T(2) shortening with liver iron loading. It was found that one or two Gaussian functions could be used to characterize the relaxivity distributions with a small number of parameters. We propose that this methodology may be used in the clinical setting to monitor hepatic iron concentrations in the advent of an accurate transverse relaxivity calibration curve.

Regulation of quantal size by presynaptic mechanisms

Quantal size is often modeled as invariant, although it is now well established that the number of transmitter molecules released per synaptic vesicle during exocytosis can be modulated in central and peripheral synapses. In this review, we suggest why presynaptically altered quantal size would be important at social synapses that provide extrasynaptic neurotransmitter. Current techniques used to measure quantal size are reviewed with particular attention to amperometry, the first approach to provide direct measurement of the number of molecules and kinetics of presynaptic quantal release, and to CNS dopamine neuronal terminals. The known interventions that alter quantal size at the presynaptic locus are reviewed and categorized as (1) alteration of transvesicular free energy gradients, (2) modulation of vesicle transmitter transporter activity, (3) modulation of fusion pore kinetics, (4) altered transmitter degranulation, and (5) changes in synaptic vesicle volume. Modulation of the number of molecules released per quantum underlies mechanisms of drug action of L-DOPA and the amphetamines, and seems likely to be involved in both normal synaptic modification and disease states. Statistical analysis for examining quantal size and data presentation is discussed. We include detailed information on performing nonparametric resampling statistical analysis, the Kolmogorov-Smirnov test for two populations, and random walk simulations using spreadsheet programs.

Comparison of vesicular volume and quantal size in bovine chromaffin cells

Electrochemical measurements of vesicular content released were compared with the morphometric measurements of vesicular size in bovine chromaffin cells. Cross-sectional vesicular diameters were determined from electron micrographs. Two methods were used to determine the frequency histograms of "true" vesicular diameters (i.e. diameters of the vesicles in the equatorial plane): (i) "peeling off" method [Coupland R. E. (1968), Nature 217, 384-388], and (ii) summation of individual probabilities of "true" vesicular diameters. Quantal size was estimated from the area under the spontaneous current spike detected electrochemically. The frequency histograms of "true" vesicular diameters are found to be skewed (thus not well described by a Gaussian function) irrespective of the method used to calculate them, as are the frequency histograms of the cube roots of the quantal sizes. Furthermore, we also find that the frequency histograms of electrochemical measurements (the cube roots of quantal sizes) have lower skews and coefficients of variation than those of morphometric measurements ("true" vesicular diameters), with discrepancy being especially pronounced for noradrenaline-secreting cells. Such a difference in both coefficients of variation and skews suggests that the intravesicular catecholamine concentration is not uniform, but that it is lower for vesicles of larger size. In conclusion a variety of factors--vesicular volume, vesicular surface area to volume ratio, binding capacity of chromogranin and/or ATP, likely determines the amount of catecholamine stored in the vesicle.

Kinetics of synaptic depression and vesicle recycling after tetanic stimulation of frog motor nerve terminals

We measured the time courses of two key components of the synaptic vesicle cycle during recovery from synaptic depression under different conditions, and used this and other information to create a kinetic model of the vesicle cycle. End plate potential (EPP) amplitudes were used to follow recovery from synaptic depression after different amounts of tetanic stimulation. This provided an estimate of the time course of vesicle mobilization from the reserve pool to the docked (readily releasable) pool. In addition, FM1-43 was used to measure the rate of membrane retrieval after tetanic stimulation, and the amount of membrane transferred to the surface membrane. This provided a measure of the rate of refilling of the reserve pool with recycled vesicles. The time courses of both synaptic depression and endocytosis were slowed by prolonged tetanic stimulation. This behavior could be fitted by a simple model, assuming a first-order kinetics for both vesicle endocytosis and mobilization. The results show that a nearly 20-fold decrease in the rate constant of endocytosis greatly delays refilling of the depleted reserve pool. However, to fully account for the slower recovery of depression, a decrease in the rate constant of vesicle mobilization from the reserve pool of about sixfold is also required.