Subunit composition of the spontaneous miniature end-plate currents at the mouse neuromuscular junction

Carrier-mediated GABA Release: Is There a Functional Role?

Neurotransmitters are normally released from neurons via calcium-dependent exocytosis of synaptic vesicles. However, after blockade of vesicular release by removal of calcium, or treatment with tetanus toxin, neurotransmitter release can still occur. In the case of GABA, nonvesicular release results from reversal of its uptake transporter, found on both neurons and glia. These GABA transporters are sodium-dependent and electrogenic, and therefore can be induced to operate in reverse by cell depolarization or by breakdown of the sodium gradient. Although demonstrated biochemically, less is known about whether this form of release occurs in vivo or whether it results in electrophysiological effects. Because conditions that favor reversal of the GABA transporter occur during high-frequency firing, nonvesicular GABA release may occur with excessive neuronal activity, such as during seizures. NEUROSCIENTIST 3:151-157, 1997

Porocytosis: Fusion Pore Array Secretion of Neurotransmitter

We believe that there is sufficient experimental evidence to support the premise that transmitter is secreted by the simultaneous activation of arrays of fusion pores at docked vesicles. This process is initiated by the action potential that activates calcium channels to increase the number of cytoplasmic calcium ions. Calcium ions trigger fusion pores to flicker open causing transmitter to diffuse from vesicular stores. We define the term porocytosis to identify this process and use the term synaptomere to indicate the anatomical and physiological functional unit of the synapse or junction. Our model shows that the simultaneous flicker of fusion pores in an array can generate unitary-end plate potentials (u-EPPs) and miniature end plate potentials (MEPPs) and that activation of all fusion pores produces EPPs. U-EPPs and EPPs generated with the model show mean values and coefficients of variation similar to experimental observations. The model is robust in that the number of docked vesicles can vary and these can be full to empty depending on nerve frequencies and vesicular traffic. The model shows that the overall process of excitation-secretion coupling is highly deterministic. At the neuromuscular junction, secretion from arrays of fusion pores ensures that a muscle fiber action potential is always produced over a range of frequencies because all transmitter release sites are activated. Our model shows that transmission at the synaptomere guarantees fidelity of information transfer at different frequencies. This characteristic shows a dynamic relationship of the secretory process to memory and learning.

Secretion without membrane fusion: porocytosis

We have recently proposed a mechanism to describe secretion, a fundamental process in all cells. That hypothesis, called porocytosis, embodies all available data and encompasses both forms of secretion, i.e., vesicular and constitutive. The current accepted view of exocytotic secretion involves the physical fusion of vesicle and plasma membranes; however, that hypothesized mechanism does not fit all available physiological data. Energetics of apposed lipid bilayers do not favor unfacilitated fusion. We consider that calcium ions (e.g., 10(-4) to 10(-3) M calcium in microdomains when elevated for 1 ms or less), whose mobility is restricted in space and time, establish salt bridges among adjacent lipid molecules. This establishes transient pores that span both the vesicle and plasma membrane lipid bilayers; the diameter of this transient pore would be approximately 1 nm (the diameter of a single lipid molecule). The lifetime of the transient pore is completely dependent on the duration of sufficient calcium ion levels. This places the porocytosis hypothesis for secretion squarely in the realm of the physical and physical chemical interactions of calcium and phospholipids and places mass action as the driving force for release of secretory material. The porocytosis hypothesis that we propose satisfies all of the observations and provides a framework to integrate our combined knowledge of vesicular and constitutive secretion.

Presynaptic quantal plasticity: Katz's original hypothesis revisited

Changes in the amplitudes of signals conveyed at synaptic contacts between neurons underlie many brain functions and pathologies. Here we review the possible determinants of the amplitude and plasticity of the elementary postsynaptic signal, the miniature. In the absence of a definite understanding of the molecular mechanism releasing transmitters, we investigated a possible alternative interpretation. Classically, both the quantal theory and the vesicle theory predict that the amount of transmitter producing a miniature is determined presynaptically prior to release and that rapid changes in miniature amplitude reflect essentially postsynaptic alterations. However, recent data indicates that short-term and long-lasting changes in miniature amplitude are in large part due to changes in the amount of transmitter in individual released packets that show no evidence of preformation. Current representations of transmitter release derive from basic properties of neuromuscular transmission and endocrine secretion. Reexamination of overlooked properties of these two systems indicate that the amplitude of miniatures may depend as much, if not more, on the Ca(2+) signals in the presynaptic terminal than on the number of postsynaptic receptors available or on vesicle's contents. Rapid recycling of transmitter and its possible adsorption at plasma and vesicle lumenal membrane surfaces suggest that exocytosis may reflect membrane traffic rather than actual transmitter release. This led us to reconsider the disregarded hypothesis introduced by Fatt and Katz (1952; J Physiol 117:109-128) that the excitability of the release site may account for the "quantal effect" in fast synaptic transmission. In this case, changes in excitability of release sites would contribute to the presynaptic quantal plasticity that is often recorded.

Porocytosis: a new approach to synaptic function

We propose a new approach to address the question of how a single quantum of neurotransmitter is secreted from a presynaptic terminal whose clustered secretory vesicles are locally bathed in high levels of calcium ions [Proceedings of the Symposium on Bioelectrogenesis (1961) 297-309; The Physiology of Synapses (1964) Chapters 1, 4, 5, 6; How the Self Controls its Brain (1994) Chapters 1, 4, 5, 6; Science 256 (1992) 677-679]. This hypothesis, which we term 'porocytosis', posits that the post-synaptic quantal response results from transmitter secreted through an array of docked vesicle/secretory pore complexes. The transient increase in calcium ions, which results from the voltage activated calcium channels, stimulates the array of secretory pores to simultaneously flicker open to pulse transmitter. Porocytosis is consistent with the quantal nature of presynaptic secretion and transmission, and with available biochemical, morphological and physiological evidence. It explains the frequency dependency of quantal size as a function of the secretion process. It permits a signature amount of transmitter release for different frequencies allowing a given synapse to be employed in different behavioral responses. The porocytosis hypothesis permits fidelity of secretion and the seemingly apposed characteristic of synaptic plasticity. The dynamics inherent in an array insure a constant quantal size as a function of the number of units within the array. In this hypothesis, plasticity is a consequence of concurrent pre- and post-synaptic changes due to a change in array size. Changes in the number of docked vesicle-secretory pore complexes composing the array can explain facilitation, depletion, graded excitation-secretion and long term plasticity.

Neurotransmitter release at rapid synapses

The classical concept of the vesicular hypothesis for acetylcholine (ACh) release, one quantum resulting from exocytosis of one vesicle, is becoming more complicated than initially thought. 1) synaptic vesicles do contain ACh, but the cytoplasmic pool of ACh is the first to be used and renewed on stimulation. 2) The vesicles store not only ACh, but also ATP and Ca(2+) and they are critically involved in determining the local Ca(2+) microdomains which trigger and control release. 3) The number of exocytosis pits does increase in the membrane upon nerve stimulation, but in most cases exocytosis happens after the precise time of release, while it is a change affecting intramembrane particles which reflects more faithfully the release kinetics. 4) The SNARE proteins, which dock vesicles close to Ca(2+) channels, are essential for the excitation-release coupling, but quantal release persists when the SNAREs are inactivated or absent. 5) The quantum size is identical at the neuromuscular and nerve-electroplaque junctions, but the volume of a synaptic vesicle is eight times larger in electric organ; at this synapse there is enough ACh in a single vesicle to generate 15-25 large quanta, or 150-200 subquanta. These contradictions may be only apparent and can be resolved if one takes into account that an integral plasmalemmal protein can support the formation of ACh quanta. Such a protein has been isolated, characterised and called mediatophore. Mediatophore has been localised at the active zones of presynaptic nerve terminals. It is able to release ACh with the expected Ca(2+)-dependency and quantal character, as demonstrated using mediatophore-transfected cells and other reconstituted systems. Mediatophore is believed to work like a pore protein, the regulation of which is in turn likely to depend on the SNARE-vesicle docking apparatus.

Transmitter quantal size in Torpedo electrocytes is determined by frequency of release

Miniature end-plate potentials (MEPPs) were focally recorded from the cytoplasmic surface of electrocytes in isolated columns of the Torpedo electric organ. Double electrode studies showed that the junctional area was restricted to 12 micron2. MEPP frequencies ranging from 1/min to 400/s were controlled with electrode advancement against the cytoplasmic surface. Stable membrane potentials and noise levels indicated constant intracellular, focal recording conditions. Focal MEPPs are only 1-3 mV and MEPP amplitudes smoothly decreased with an increase in MEPP frequency which demonstrates a process that meters quantal size at moment of release. Thus, release if not from a prepackaged store. MEPP interval analyses showed that events are weakly interactive at low frequencies and periodic at higher frequencies. The interdependency of MEPP amplitudes and intervals indicates that the mechanism of release controls both rate and quantal size. We propose that the amplitude and frequency dependencies of MEPPs at the Torpedo nerve-electrocyte junction are best described by a membrane channel (e.g., mediatophore, Israël and Dunant, Neurochem. Int. 28 (1996) 1-9) that meters transmitter from a presynaptic store.

Miniature EPSPs and sensory encoding in the primary afferents of the vestibular lagena of the toadfish, Opsanus tau

The synaptic activity transmitted from vestibular hair cells of the lagena to primary afferent neurons was recorded in vitro using sharp, intracellular microelectrodes. At rest, the activity was composed of miniature excitatory postsynaptic potentials (mEPSPs) at frequencies from 5 to 20/s and action potentials (APs) at frequencies betwen 0 and 10/s. mEPSPs recorded from a single fiber displayed a large variability. For mEPSPs not triggering APs, amplitudes exhibited an average coefficient of variance (CV) of 0.323 and rise times an average CV of 0.516. APs were only triggered by mEPSPs with larger amplitudes (estimated 4-6 mV) and/or steeper maximum rate of rise (10.9 mV/ms, +/- 3.7 SD, n=4 experiments) compared to (3.50 mV/ms, +/-0.07 SD, n=6 experiments) for nontriggering mEPSPs. The smallest mEPSPs showed a fast rise time (0.99 ms between 10% and 90% of peak amplitude) and limited variability across fibers (CV:0.18) confirming that they were not attenuated signals, but rather represented single-transmitter discharges (TDs). The mEPSP amplitude and rise-time relationship suggests that many mEPSPs represented several, rather than a single pulse of secretion of TDs. According to the estimated overall TD frequency, the coincidence of TDs contributing to the same mEPSP were not statistically independent, indicating a positive interaction between TDs that is reminiscent of the way subminiature signals group to form miniature signals at the neuromuscular junction. Depending on the duration and intensity of efferent stimulation, a complete block of AP initiation occurred either immediately or after a delay of a few seconds. Efferent stimulation did not significantly change AP threshold level, but abruptly decreased mEPSP frequency to a near-complete block that followed the block of APs. Maximum mEPSP rate of rise decreased during, and recovered progressively after, efferent stimulation. After termination of efferent stimulation, mEPSP amplitude did not recover instantly and for a few seconds the amplitude distribution of synaptic events showed fewer large-amplitude events than during the control period. This confirms that mEPSP amplitude and rate of rise properties, which are critical for triggering afferent APs, are modified by efferent activity. The depression of afferent AP firing during efferent stimulation corresponded to a decrease in mEPSP frequency and, to a lesser extent, a decrease in mEPSP amplitude and rate of rise, suggesting, a decrease in the level of interaction among TDs contibuting to a mEPSP.

Hypertonic treatment reversibly increases the ratio of giant skew-miniature endplate potentials to bell-miniature endplate potentials

Miniature endplate potentials were recorded from single frog muscle fibers before, during and after treatment with hypertonic saline (200-500 mM NaCl or Na gluconate added to frog saline). Miniature endplate potential amplitude distributions were plotted from small muscle fibers so that the modes and ratios of the skew-miniature endplate potential to bell-miniature endplate potential classes could be defined. Muscle fibers were voltage clamped with two electrodes to determine the input resistance before, during and after treatment. Input resistance increased from two to 100 times during treatment and rapidly fell towards control values (no more than 30% greater) when preparations were returned to normal frog saline. Short duration treatments with 200-300 mM hypertonic salines immediately increased frequencies (100-fold) of both skew-miniature endplate potential and bell-miniature endplate potential classes. Preparations when returned to normal frog saline after a few minutes of treatment showed control miniature endplate potential distributions within minutes. One to two hour treatments left only the skew-miniature endplate potential class and with hour-long recovery periods bell-miniature endplate potentials reappeared and ratios of skew-miniature endplate potential to bell-miniature endplate potential classes returned to control values. Treatment with 500 mM NaCl added to frog saline immediately increased the percentage of skew-miniature endplate potentials (from 2 to 50%) with little or no increase in overall miniature endplate potential frequencies. The mode of the skew-miniature endplate potential class was unchanged after hypertonic treatment, whereas that of the bell-miniature end plate potential class either remained about the same size or decreased depending on the duration of treatment. The number and percentage of giant-miniature endplate potentials belonging to the skew-miniature endplate potential class increased as a function of the duration of 200-300 mM hypertonic saline treatments. Most giant-miniature endplate potentials had a slow rising phase with a foot and/or breaks demonstrating a composite structure. Sequentially recorded giant-miniature endplate potentials had similar initial slopes indicating either repetitive releases from single sites or releases from cooperative sites. After hypertonic treatment the bell-miniature endplate potential size was never more than that expected with the increase (under 30%) in input resistance. The results presented here are completely different from those of Yu and Van der Kloot [(1991) J. Physiol. 433, 677-704] who reported that the bell-miniature endplate potential amplitude was increased two- to four-fold after hypertonic treatment. The wide range of results in the ratio of skew-miniature endplate potential to bell-miniature endplate potential classes is discussed in regards to the quantal hypothesis which is based on a single class of immutable amounts of transmitter; and, a hypothesis based on a dynamical process that meters transmitter in subunit amounts to control miniature endplate potential size and class during release.

Quantal secretion from visualized boutons on rat pelvic ganglion neurones

Synaptic transmission from single preganglionic hypogastric nerves innervating monopolar pelvic ganglion neurones has been studied with intracellular electrodes to record transmission from all the boutons and with extracellular electrodes placed over boutons visualized with DiOC2(5) in order to record transmission from selected boutons. Intracellular electrodes revealed spontaneous excitatory postsynaptic potentials (EPSPs) with amplitude histograms that show increasing numbers of large EPSPs as the external calcium ([Ca2+]o) was increased from 0.15 to 1.0 mM. These histograms were in general well fitted by a Poisson mixture of gamma distributions. Extracellular electrodes placed over visualized boutons revealed evoked excitatory postsynaptic potentials (extracellular EPSPs) with amplitude histograms that were best described by single gamma distributions in most cases in low [Ca2+]o (less than 0.5 mM). The standard deviation of these gammas was not much larger than that of the electrical noise. In a minority of extracellular recordings the amplitude histogram of evoked extracellular EPSPs was best described by a gamma distribution in which the standard deviation was much greater than that of the noise. Confocal microscopy of boutons orthogradely labelled with dextran-rhodamine showed that about 30% of these formed closely apposing pairs on the surface of the neurones. These observations are discussed in terms of the hypothesis that multiquantal release at boutons occurs as a consequence of the coupled secretion from closely apposed boutons.

Variation in GABA mini amplitude is the consequence of variation in transmitter concentration

Miniature postsynaptic currents (minis) in cultured retinal amacrine cells, as in other central neurons, show large variations in amplitude. To understand the origin of this variability, we have exploited a novel form of synapse in which pre- and postsynaptic receptors sample the same quantum of transmitter. At these synapses, mini amplitudes measured simultaneously in the 2 cells show a strong correlation, accounting for, on average, more than half of the variance in amplitude. Two pieces of evidence support the conclusion that variations in the amount of transmitter in different quanta underlie this correlation. First, diazepam, which enhances GABA binding, increases mini amplitude, implying therefore that transmitter concentration is not saturating. Second, we show that amplitude distributions from all cells, even those with a small number of release sites, have the same shape, implying that most or all variance is intrinsic to each release site.

Probabilistic secretion of quanta: spontaneous release at active zones of varicosities, boutons, and endplates

The amplitude-frequency histogram of spontaneous miniature endplate potentials follows a Gaussian distribution at mature endplates. This distribution gives the mean and variance of the quantum of transmitter. According to the vesicle hypothesis, this quantum is due to exocytosis of the contents of a single synaptic vesicle. Multimodal amplitude-frequency histograms are observed in varying degrees at developing endplates and at peripheral and central synapses, each of which has a specific active zone structure. These multimodal histograms may be due to the near synchronous exocytosis of more than one vesicle. In the present work, a theoretical treatment is given of the rise of intraterminal calcium after the stochastic opening of a calcium channel within a particular active zone geometry. The stochastic interaction of this calcium with the vesicle-associated proteins involved in exocytosis is then used to calculate the probability of quantal secretions from one or several vesicles at each active zone type. It is shown that this procedure can account for multiquantal spontaneous release that may occur at varicosities and boutons, compared with that at the active zones of motor nerve terminals.

The origin of Gaussian distributions of synaptic potentials

Spontaneous synaptic potentials were identified at the motor endplate 40 years ago. These were shown to possess amplitudes that could be described by a Gaussian distribution as could the amplitudes of evoked synaptic potentials under conditions of very low probability for secretion. As these Gaussians were identical, the idea of a unit or quantum of transmission was conceived. The failure to obtain similar Gaussian distributions for both spontaneous and low-probability evoked potentials during development of endplates indicated that a unit of transmission was not operating. However both the spontaneous and very low-probability evoked potentials could each be described by mixtures of Gaussians indicating a subunit of transmission might be operative. There are no ganglionic or central synapses at which comparisons have been made between spontaneous and low-probability evoked potentials that show each can be described by a Gaussian distribution, let alone that these are the same indicating a unit of transmission as originally conceived. There is some evidence that mixtures of Gaussians can be used to describe both spontaneous and very low-probability evoked synaptic potential amplitudes, opening up the possibility for a subunit of transmission at these synapses. The vesicle hypothesis, that the quantum of transmission at the endplate is due to the exocytosis of the contents of a synaptic vesicle, was also enunciated nearly 40 years ago. The existence of subunits of transmission has required reconsideration of this hypothesis. Three alternatives are considered: in one, the calcium-transient hypothesis, the subunit of secretion is due to the release of calcium from one of several calcium stores in the nerve terminal, so that several subunits are released when a number of these calcium stores are engaged in a regenerative response to the terminal action potential; a second alternative, the mediatophore hypothesis, is that a subunit of secretion occurs when a single transmitter transport protein channels transmitter across the terminal membrane, several such mediatophore proteins acting in concert then give multiple subunit release; finally, there is the vesicle fusion-pore hypothesis, in which individual transient openings of a fusion-pore channel joining a synaptic vesicle to the terminal membrane are responsible for secretion of a transmitter subunit, with multiple transients giving several subunits. Perhaps we will have distinguished between these possibilities before the quantal hypothesis is 50 years old.

Dynamic responses of presynaptic terminal membrane pools to electrical stimulation

The anatomical tenets of the quantal-vesicular hypothesis of neurotransmission are a 1:1 ratio between numbers of releasable quanta and vesicles, a reciprocal response between vesicle and terminal membrane pools and constancy of the total membrane pool. We have used electrical stimulation and morphometry to study these relationships in the cholinergic presynaptic terminals of Torpedo electric organ. Our results show that during neurotransmission changes in vesicle numbers do not correlate with quantal release, vesicle and terminal membranes do not change in reciprocal fashion and total nerve terminal membrane does not remain constant. We conclude that these vesicular tenets of quantal release are not verifiable at the Torpedo electric organ junction.

Fast presynaptic GABAA receptor-mediated Cl- conductance in cultured rat hippocampal neurones

1. Hippocampal neurones cultured from the 18-day-old embryonic rat for 3 days to 3 weeks were recorded with Cl(-)-filled patch pipettes. Spontaneous synaptic currents, which reversed at the equilibrium potential for Cl- ions (ECl) and were blocked by the GABAA (gamma-aminobutyric acid) receptor antagonists bicuculline or picrotoxin, were recorded in every culture. At 25 degrees C and -80 mV they decayed with a time constant > or = 20 ms that invariably increased at positive potentials. After 2 weeks, 50-75% of all neurones were GABA immunoreactive. 2. In pairs-recordings, coincident synaptic currents in both cells were either spontaneous or evoked by stimulation of one cell. In the presence of tetrodotoxin and using pipettes containing lidocaine (lignocaine) N-ethyl bromide, coincident spontaneous Cl- transients still occurred in both neurones far more frequently than expected by chance. 3. Holding the potential of one neurone at a positive value reversed the synaptic transients in that cell and, in half of the cells, increased the frequency of coincident events in both cells. 4. In neurones where depolarization increased the frequency of coinciding events and all regenerative current apparent at the soma was abolished, short depolarizing pulses occasionally evoked all-or-none, pre- and postsynaptic currents with matching transmission failures and identical delays in transmission. 5. The results suggest that the same pulse of GABA simultaneously activates GABAA receptor-coupled Cl- channels on both sides of the same synaptic cleft, producing immediate auto-transmission in the absence of collaterals or interneurones.

Space and time characteristics of transmitter release at the nerve-electroplaque junction of Torpedo

1. A loose patch electrode was used to stimulate axon terminals and to record evoked electroplaque currents (EPCs) in a limited area of innervated membrane of the electric organ of Torpedo marmorata. Electrophysiological signals were compared to the predictions of a semi-quantitative model of synaptic transmission which was designed to simulate the release of several packets of neurotransmitter molecules, at the same or at different sites of the synapse, synchronously or with various temporal patterns. 2. The amplitude distribution of EPCs evoked by activation of nerve terminals showed quantal steps. The time to peak of EPCs was in most cases independent of amplitude, but in their decaying phase a positive correlation was seen between half-decay time and amplitude. Comparison with the model suggested that (i) a dynamic interaction occurred at the end of the EPC between the fields of postsynaptic membrane activated by individual quanta, and (ii) the sites of quantal release in the electric organ are separated from each other by 600-1000 nm. 3. Spontaneous miniature electroplaque potentials (MEPPs) were recorded externally with the same type of loose patch electrode. The majority (75%) of external MEPPs displayed a homogeneous and rapid time course. This fast MEPP population had a mean time to peak of 0.43 ms, a half-decay time of 0.45 ms and a time constant of decay of 0.35 ms. 4. Despite homogeneous characteristics of time course, fast MEPPs exhibited a wide amplitude distribution with a main population which could be fitted by a Gaussian curve around 1 mV, and another population of small amplitude. Both the time-to-peak and the half-decay time of fast MEPPs showed a positive correlation with the amplitude from the smallest to the largest events. Acetylcholinesterase was not blocked. 5. In addition to the fast MEPPs, spontaneous signals exhibiting a slow rate of rise, or a slow rate of decay, or both were observed. They occurred at any time during the experiment, independently of the overall frequency. Approximately 15% of the total number of events had a slow rise but their decay phase was nevertheless rapid and could be ascribed to the kinetics of receptors. These slow-rising MEPPs exhibited a variety of conformations: slow but smooth rise, sudden change of slope and sometimes several bumps or inflexions. Their average amplitude was significantly smaller than that of the main population of fast MEPPs. 6. Composite MEPPs with multiple peaks as well as bursts of small MEPPs were often encountered, even during periods of low frequency.(ABSTRACT TRUNCATED AT 400 WORDS)

Quantal and subquantal GABAergic transmissions in cultured rat hippocampal neurons

At the neuromuscular junction, spontaneous miniature excitatory synaptic currents mediated by acetylcholine are considered elementary, "quantal" transmissions. These miniature conductances can be quantitatively dichotomized into a large-mode class whose mode is the mean of a normal, bell-shaped distribution and a small-mode class whose distribution is skewed to lower values with its mode being a fraction of the large-mode class. The large-mode class constitutes the population of synaptic signals originally utilized to formulate tenets of "quantal" transmission, which have been tacitly adopted in more recent studies of fast transmission at central synapses. Large- and small-mode conductance classes of inhibitory synaptic elementary conductances mediated by GABA have now been recorded in cultured hippocampal neurons (Vautrin J, Schaffner AE, Barker JL, 1991, Neurosci Lett 138:67). Pairs of hippocampal neurons were patch-recorded at optimal signal-to-noise and, using time course analysis, two elementary fluctuations (0.1-0.3 nS and 1-2 nS) were found within synaptic conductances evoked either by presynaptic action potentials or by presynaptic terminal stimulation. These results were interpreted with a simple model that shows how different frequencies of unitary GABA release can generate either small-mode, skew-distributed conductance (0.5-3 kHz) or large-mode, normally-distributed conductances (> or = 10 kHz). Only the latter satisfies the original tenets of the classic quantal theory.

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)

Frequency modulation of transmitter release

In 1952 Fatt and Katz recorded at a frog neuromuscular junction while stimulating the nerve and found "... that successive endplate potential responses varied in a step-like manner, corresponding to units of miniature endplate potentials" (J Physiol 117, 109-128). This led them to propose that fast neuromuscular transmission is 'quantal'. Quantal release is now commonly ascribed to a vesicular form of neurosecretion since vesicles have routinely been visualized in presynaptic terminals. The vesicular hypothesis (Del Castillo and Katz, 1955) assumes that quanta, or 'transmitter packets of standard size', are assembled and stored in the numerous vesicles routinely identified in micrographs of virtually all central and peripheral presynaptic nerve terminals. Simply stated, this model predicts that each one of the miniature synaptic signals (MSSs) follows from the exocytosis of one vesicle's contents. However, the time required for membrane fusion preceding exocytosis (Almers and Tse, 1990) and the variability in MSS amplitude and time course (Vautrin et al, 1992a,b) cannot readily be reconciled by a simple, exocytotic model of quantal release from preloaded vesicles. These difficulties with the original model have led us to re-evaluate MSSs generated at the classical peripheral synapse, the cholinergic neuromuscular junction of the mouse diaphragm, as well as at central synapses between embryonic hippocampal neurons mediated by gamma-aminobutyric acid (GABA). At these synapses, the release of GABA is also assumed to have classical quantal properties like peripheral acetylcholine release (Edwards et al, 1990). Our results show that at both synapses, progressive alterations in elementary signal properties can be induced in a remarkably rapid manner. The original report of preferred amplitudes and intervals in the spontaneous miniature signals (Fatt and Katz, 1952) has repeatedly been confirmed and is here incorporated into a dynamic model of fast synaptic transmission. Although MSSs exhibit variable rise-times and peak amplitudes, they can both be described in terms of synchronization of transmitter release. We have reviewed many experimental findings, which together strongly suggest that the original interpretation of Fatt and Katz (1952) regarding MSSs as reflecting the non-propagated 'neurogenic' activity of 'terminal spots' may be a useful concept to pursue since it may help to explain part of the underlying molecular basis of quantal release.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of calcium on the dynamic process of transmitter release which generates either skew- or bell-MEPPS

Miniature endplate potential (MEPP) amplitudes, MEPP frequencies and ratios of skew:bell-MEPPs were determined as well as synaptic vesicle diameters and densities at the mouse diaphragm neuromuscular endplate during exposure to elevated calcium concentrations. Additions of external Ca2+ had variable effects on MEPP frequencies and percentages of skew-MEPPs, regardless of concentrations used (1-25 mM). Nevertheless, changes in MEPP amplitudes were most sensitive (4-fold decrease) to low value increases of Ca2+. Changes in MEPP frequencies produced by an increase in Ca2+ were very sensitive to initial frequencies as well as the initial calcium concentration. An increase in Ca2+ usually increased MEPP frequency (providing skew-MEPPs were measured). Changes in the percentage of skew-MEPPs were extremely variable (4-90%) and these changes depended on initial frequencies, initial skew- to bell-MEPP ratios and age of the mouse. With a change in Ca2+ concentration, synaptic vesicle diameters and densities remained constant during changes in MEPP frequencies and large changes in the skew:bell-MEPP ratios; and, vesicle numbers were sometimes slightly increased. Because of the wide range in MEPP frequencies and amplitudes, this study demonstrates that the effect of various treatments should be evaluated on identified endplates and that analyses of randomly selected endplates must consider the large variability between endplates. These results show that the skew-MEPP class must not be ignored in studies of spontaneous MEPP release, and that initial frequencies and age of the mouse are also important in evaluating changes in skew-MEPP to bell-MEPP ratios. The rapid changes in skew- to bell-MEPP classes indicate that MEPP class and size are determined at the moment of release by the state of the release process as proposed by Kriebel et al. (1990). Because changes in calcium concentration can immediately alter the ratio of skew- to bell-MEPPs we conclude that the release process has two states to generate the two classes of MEPPs, and that the release process is very sensitive to conditions so that states are easily changed. We propose that the release process meters transmitter in subunit amounts to form both classes of MEPPS and that the calcium ions modulate the process.

Early postnatal changes in presynaptic potassium sensitivity

Amplitude histograms of miniature endplate potentials (MEPPs) and the overall frequency of skew-MEPPs and bell-MEPPs were examined in 5 and 15 mM potassium solutions at postnatal day (PD) 3, PD 10 and PD 27 neuromuscular junctions. Temporal non-uniformities in spontaneous release produced clusters of bell-MEPPs at PD 0-PD 3 junctions. PD 3 nerve terminals that preferentially released skew-MEPPs (5 mM potassium) were significantly (P less than 0.01) less sensitive to elevations in potassium than more mature (PD 10) junctions that preferentially released bell-MEPPs. Increases in the potassium concentration at PD 3 junctions increased the frequency of bell-MEPPs and altered the MEPP amplitude distribution profile by significantly (P less than 0.01) reducing the percentage of skew-MEPPs. Although the potassium sensitivity of PD 10 and PD 27 preparations were as expected for adult preparations, there was an increase in overall MEPP frequency in 5 mM potassium between PD 10 and PD 27. These results suggest that early postnatal increases in the number of presynaptic calcium channels establish adult levels of depolarization sensitivity and promote the preferential spontaneous release of bell-MEPPs. Since these changes occur during an early period of synapse elimination, they may play a critical role in synapse stabilization.

Regulation of single quantal efficacy at the snake neuromuscular junction

1. Postsynaptic responses to spontaneous quantal transmitter release have been compared among neuromuscular junctions in a thin snake muscle. For each junction the type, diameter, and input conductance, G(in) of the postsynaptic muscle fibre were determined. Particularly among fibres of a given type, G(in) was directly correlated with fibre diameter. 2. Miniature endplate potentials (MEPPs) were recorded intracellularly near endplates visualized with Nomarski optics. Mean MEPP amplitude decreased with increasing G(in) among fibres in one muscle. However, the dependence of mean amplitude upon G(in) was not ohmic, as would be expected if the underlying single quantal currents (miniature endplate currents, MEPCs) were of similar amplitude at all junctions. Instead, the relation between MEPPs and G(in) suggested that mean MEPC amplitudes, calculated as mean MEPP amplitude x G(in), increased with increasing G(in). 3. MEPCs were recorded directly using the two-microelectrode voltage clamp technique. Mean MEPC amplitudes depended systematically on G(in), again such that MEPCs were on average larger in fibres with higher G(in). 4. MEPCs were recorded extracellularly from small regions of endplates (underlying a few nerve terminal boutons). Amplitudes of MEPCs depended on G(in) or fibre diameter in the same manner as amplitudes of MEPCs recorded by intracellular voltage clamp. 5. When the anticholinesterase agent neostigmine was added to the bath, amplitude and duration of MEPPs, MEPCs, and extracellular MEPCs increased. However, the systematic dependence of mean MEPC amplitude on G(in) or fibre diameter remained. 6. Evoked subthreshold endplate potentials (EPPs) were recorded under conditions of low extracellular Ca2+. Endplate currents (EPP amplitudes x G(in)) were systematically larger in fibres with larger G(in), indicating regulation of evoked synaptic current in the muscle. The regulation was found to be due to a combination of increased quantal content and larger single quantal currents in larger (higher G(in)) fibres. 7. Synaptic size, assessed either by area of cholinesterase staining or number of terminal boutons, increased with increasing fibre diameter. Assuming that quantal content is proportional to synaptic size, this relation was sufficient to account for the observed increase in quantal content with increasing G(in) among fibres in the muscle, but was not alone sufficient to account for the observed regulation of evoked current. 8. It is concluded that the efficacy of individual transmitter quanta released at the snake neuromuscular junction is regulated such that large muscle fibres receive larger single quantal currents. Regulation of single quantal current contributes substantially to overall regulation of synaptic strength in the muscle.

Further evidence for the dynamic formation of transmitter quanta at the neuromuscular junction

Fatt and Katz (Nature 166:597-598, 1950; J Physiol 117:109-128, 1952) attributed miniature endplate potentials (MEPPs) to the action of a standard quantity of transmitter, the quantum (Del Castillo and Katz, J Physiol 124:560-573, 1954). Quantal packets of transmitter were proposed to be preformed (Del Castillo and Katz, In CNRS Paris (Ed): "Microphysiologie comparée des éléments excitables" 67:245-258, 1957) and stored in large numbers in the motor nerve terminal. Statistical analyses of intervals between MEPPs and numbers of quanta composing small endplate potentials indicated that quantal release was a random process and that release sites functioned independently of each other. With the discovery of synaptic vesicles it was proposed that each contained one quantum of transmitter. The quantal-vesicular hypothesis (Del Castillo and Katz, as cited above) fails, however, to explain amplitude distributions of MEPPs that are skewed and/or that show multiple peaks (Kriebel et al., Brain Res Review 15:167-178, 1990). The drop formation process (Shaw, "The Dripping Faucet as a Model Chaotic System," Santa Cruz, CA: Aerial Press, Inc., 1984) was shown to generate amplitude classes of drops that were similar to classes of MEPPs which suggested that rapid changes in quantal size and ratios of skew- to bell-MEPPs could be explained with a simple dynamic process which determines quantal size at the moment of release (Kriebel et al., as cited above, 1990). Further similarities between miniature endplate currents (MEPCs) and the formation of drops are reported here. We found that rapid changes in MEPC amplitudes and time courses, which accompany an increase in frequency, mimic changes in drop sizes that accompany increases in flow rate. MEPC intervals have a minimum and their distributions are comparable to those of drop intervals. During an increased rate of transmitter release, MEPP amplitudes and intervals were positively correlated. The results suggest that spontaneously released transmitter "packets" are formed at the moment of release and that transmitter supply to the process that forms packets is continuous.

Focal, extracellular recording of slow miniature junctional potentials at the mouse neuromuscular junction

Miniature endplate potentials (MEPPs) with slow rising phase can be attributed either to burst of transmitter releases or to distortion of conduction from remote releasing sites. The spontaneous activity of neuromuscular junctions recorded extracellularly at mouse diaphragms using sharp electrodes was analyzed to test these two hypotheses. The miniature junctional potentials (MEJPs) frequencies observed intracellularly as compared to MEPP frequency measured intracellularly in controls indicate that most events recorded extracellularly are induced by the presence of the electrode. All types of MEPPs (bell-MEPPs, skew-MEPPs, slow-, and giant MEPPs) previously described with intracellular recording methods (Vautrin and Kriebel, Neuroscience 41:71-88, 1991) were observed extracellularly and showed similar characteristics. This means that the presynaptic and postsynaptic zones that generate these synaptic events are restricted within areas of a few micrometers squared of synaptic contact. Long rise times of extracellularly recorded synaptic spontaneous events may be explained by multiple transmitter releases at intervals shorter than the rise time of individual events, which postsynaptic responses fuse into a single peak.

On the mechanism of spontaneous recovery of neuromuscular transmission after acetylcholinesterase inhibition in the rat neuromuscular junction

Neuromuscular transmission shows a significant degree of spontaneous recovery after being impeded by acetylcholinesterase inhibition. Part of this recovery can be ascribed to de novo synthesis of acetylcholinesterase but another part is independent of enzyme activity. To unravel the mechanism underlying this synaptic adaptation to acetylcholinesterase inhibition we have compared a number of electrophysiological parameters in diaphragms taken from animals that were sacrificed within 15 min after a 2 x LD50 dose of the acetylcholinesterase inhibitor diisopropylfluorophosphate and from similarly treated animals killed after being kept alive for 3 h under artificial respiration. We found no differences in the quantal content. There was a significantly smaller degree of endplate potential rundown at tetanic stimulation and the miniature endplate potential amplitude was smaller in the 3-h adapted animals. In addition, the desensitization induced by carbachol appeared to be less in this group. It is likely that these synaptic changes, demonstrating the plasticity of the neuromuscular synapse, are involved in the spontaneous recovery of neuromuscular transmission after acetylcholinesterase inhibition.

Temperature dependence of carbachol-induced modulation of miniature end-plate potential frequency in rats

In the rat soleus, the frequency of miniature end-plate potentials (MEPP) did not change after application of 10(-5) M of the cholinomimetic drug carbachol between 18 degrees C and 34 degrees C but decreased by 40% at physiological temperatures of 37-38 degrees C. The carbachol-induced decrease in MEPP frequency was not eliminated by 10(-7) to 10(-8) M atropine or 3 x 10(-7) (+)-tubocurarine similarly as had been previously found at frog neuromuscular junction.

Characteristics of slow-miniature endplate currents show a subunit composition

The normal neuromuscular junction shows two classes of spontaneous miniature endplate potentials. These classes are based on a discontinuity in the profile of miniature endplate potential amplitude distributions. The amplitude of one class of miniature endplate potentials from a bell-shaped amplitude distribution and the remaining miniature endplate potentials compose a population which forms a left-hand skew distribution with a mode 1/7 to 1/10 that of the bell-miniature endplate potentials [Kriebel M. E. and Gross C. E. (1974) J. gen. Physiol, 64, 85-103]. Some skew-miniature endplate potentials have a slow time-to-peak and show breaks on the rising phase. Most treatments that alter the miniature endplate potential frequency change the ratio of skew-miniature endplate potentials/bell-miniature endplate potentials [Kriebel M. E. et al. (1976) J. Physiol. 262, 553-581]. The time characteristics of miniature endplate currents were readily altered in the isolated frog and mouse neuromuscular junctions with several agents known to increase the percentage of slow-miniature endplate potentials (heat, botulinum toxin, 4-aminoquinoline and increases in bath osmolarity). The slow-miniature endplate potential amplitudes were a continuum of amplitudes from skew- to giant miniature endplate potentials. The rising phases of miniature endplate potentials were a continuum from smooth to many with breaks and offsets. In a series of sequentially recorded slow-miniature endplate currents, many had congruent rising phases of constant slope regardless of amplitude or of time-to-peak. The rising phases of congruent slow-miniature endplate currents which showed a change in slope deviated at similar amplitudes. The least value of the slope of a slow-miniature endplate current was that of the sub-miniature endplate current; and, miniature endplate currents with overall lower slope values showed a wave pattern and/or irregular breaks which suggests summation of sequentially delayed sub-miniature endplate currents. Plots of the amplitude vs time-to-peak of miniature endplate currents from identified junctions demonstrated that the normal percentage of slow-miniature endplate currents was greatly increased with the treatments used here and that the time-to-peak of giant miniature endplate currents usually was longer than that of normally occurring bell-miniature endplate currents. Giant miniature endplate currents with short time-to-peak values are probably from two miniature endplate currents occurring, by chance, almost simultaneously. During and/or after treatments, miniature endplate currents formed clusters of similar size miniature endplate currents, not randomly distributed in time, which graded from distinct miniature endplate currents to giant miniature endplate currents.(ABSTRACT TRUNCATED AT 400 WORDS)

Transmitter release: prepackaging and random mechanism or dynamic and deterministic process

Stepwise variations in end-plate potential amplitudes that are also multiples of spontaneous miniature end-plate potentials (MEPPs) demonstrate a quantal nature of evoked transmitter release at the vertebrate neuromuscular junction. Both the number of quanta which form relatively small end-plate potentials (EPPs) and the time intervals between MEPPs were found to fit Poisson statistics. These observations suggested that the release process randomly liberates uniform quantities of transmitter. Initial studies showed that quantal size remained stable after seemingly high rates of release which was interpreted to indicate that a large store of equally sized, equally available, and independently releasable quanta are present in the nerve terminals. The observation of numerous presynaptic vesicles that contain transmitter provided a morphological basis for prepacked transmitter (i.e., quanta). However, physiological studies over the last 15 years have yielded data that are difficult to incorporate into the quantum-vesicle hypothesis. With normal conditions and during most treatments which increase the rate of release, two classes of MEPPs have been found and both show a substructure. The bell-MEPP class was characterized by Fatt and Katz and the smaller skew-MEPP class has been studied by Kriebel. The ratio of the two classes and substructure compositions of both classes are variable. Short series of MEPPs and unitary EPPs (U-EPPs) show preferred amplitudes and longer series of MEPPs and U-EPPs show stepwise variations in amplitude. Slow-MEPPs and giant MEPPs belong to the skew class and represent nearly synchronous bursts of smaller MEPPs. Transmitter packet formation, preferred amplitudes, stepwise variations in amplitudes, random-like distributions and organized bursts can be simulated by a simple deterministic system, the drop formation process, that is known for its periodic and chaotic behaviors which are determined by the single parameter of flow rate. MEPP intervals, sizes and classes, are also dependent on rates of release which demonstrate that the release process(es) is highly organized and sensitive to different conditions. We demonstrate that the processes of drop formation and release of a packet of transmitter have similar properties and that deterministic characteristics describe MEPP and U-EPP time dependencies and amplitude substructures. The data and model presented here suggest that packet size of acetylcholine may be determined at the moment of release.

Postsynaptic structure may account for variations in miniature endplate current shapes along frog neuromuscular junctions

Intracellular recordings were made in the presence of 6-9 microM neostigmine with an electrode placed halfway between the distal end and the proximal region of a long simple branch of a frog neuromuscular junction. During the intracellular recording period, an extracellular electrode was placed alternatively at a distal and at a proximal site. The temporal correspondence between the extracellular miniature endplate current and intracellular miniature endplate potential permitted us to identify two miniature endplate potential (MEPP) populations originating, respectively, from the distal and proximal regions. The frequency and intracellular amplitude of MEPPs were higher in the proximal than in the distal region. The rise time and the decay time of the extracellularly recorded MEPC were longer in the proximal region. A significant correlation was observed between the amplitude (in mV) and surface (in mV.ms) of MEPCs. The slope of this regression line was steeper for the proximal region. Furthermore, the correlation coefficient was lower for that region owing to variations of MEPC shape. Our observations suggest that in the presence of anticholinesterase, ACh rebinds more often to acetylcholine receptors in the proximal region because of the presence of longer postjunctional folds.

A morphometric analysis of synaptic vesicle distributions

Synaptic vesicle populations have been morphometrically analyzed for size and density. Populations composed of a single size class of vesicles are represented by normal (Gaussian) or positive (log-normal) skew histograms. Populations with multiple size classes generate negative (left) skew distributions. Fixatives containing aldehydes differentially affect these distribution patterns but vesicles are able to withstand tonic effects over a wide range. Reader bias' contribute the most error in the data-collecting process. But despite this, the sizing of vesicle populations can be accomplished with great accuracy. Vesicle density computations, on the other hand, vary over a wide range and are of less value for comparative purposes.

Reversible effect of depolarization by K-propionate on sub-miniature endplate potential to bell-miniature endplate potential ratios, on miniature endplate potential frequencies and amplitudes, and on synaptic vesicle diameters and densities in frog neuromuscular junctions

Miniature endplate potentials were recorded from edge muscle fibers of frog sartorius muscles during high frequencies induced with K-propionate and during recovery. The identified neuromuscular junctions were studied with the electron microscope and their ultrastructure was correlated with amplitude and numbers of miniature endplate potentials generated. Miniature endplate potential amplitudes were maintained during the first 10 min of depolarization. They then decreased during the next 2-3 h until the mode was lost to the noise. Miniature endplate potential frequency was greatly increased during the first hour and there was initial depletion of vesicles. Miniature endplate potential frequencies remained high (5 x 10(5)/h) for 3 h but vesicle densities returned to nearly normal values during the second to third hour of treatment. The conspicuous infolding of the presynaptic membrane noted during the first hour of treatment suggests that recycling of vesicles is initially slower than fusion. Calculated recycling time is shorter than 25 min. During recovery after prolonged K-propionate treatment, the sub-miniature endplate potential class reappeared within minutes but about 20 min were required before it returned to control size. Subsequently, the bell-miniature endplate potentials reappeared and slowly increased in amplitude. The ultrastructure returned to a normal state. There was no change in vesicle diameters. No significant difference was found between the diameters of "touching vesicles" (vesicles touching the presynaptic membrane) and the non-touching vesicles. By comparison, lanthanum ions (1 mM) released a smaller number of quanta which did not exceed the number of vesicles present at the start of the experiment. Variations of the subunit hypothesis of the quantum of transmitter release are discussed.