Characteristics of slow-miniature endplate currents show a subunit composition

Hydration-sensitive gene expression in brain

Dehydration has a profound influence on neuroexcitability. The mechanisms remained, however, incompletely understood. The present study addressed the effect of water deprivation on gene expression in the brain. To this end, animals were exposed to a 24 hours deprivation of drinking water and neuronal gene expression was determined by microarray technology with subsequent confirmation by RT-PCR. As a result, water deprivation was followed by significant upregulation of clathrin (light polypeptide Lcb), serum/glucocorticoid-regulated kinase (SGK) 1, and protein kinase A (PRKA) anchor protein 8-like. Water deprivation led to downregulation of janus kinase and microtubule interacting protein 1, neuronal PAS domain protein 4, thrombomodulin, purinergic receptor P2Y - G-protein coupled 13 gene, gap junction protein beta 1, neurotrophin 3, hyaluronan and proteoglycan link protein 1, G protein-coupled receptor 19, CD93 antigen, forkhead box P1, suppressor of cytokine signaling 3, apelin, immunity-related GTPase family M, serine (or cysteine) peptidase inhibitor clade B member 1a, serine (or cysteine) peptidase inhibitor clade H member 1, glutathion peroxidase 8 (putative), discs large (Drosophila) homolog-associated protein 1, zinc finger and BTB domain containing 3, and H2A histone family member V. Western blotting revealed the downregulation of forkhead box P1, serine (or cysteine) peptidase inhibitor clade H member 1, and gap junction protein beta 1 protein abundance paralleling the respective alterations of transcript levels. In conclusion, water deprivation influences the transcription of a wide variety of genes in the brain, which may participate in the orchestration of brain responses to water deprivation.

Acetylcholine release at neuromuscular junctions of adult tottering mice is controlled by N-(cav2.2) and R-type (cav2.3) but not L-type (cav1.2) Ca2+ channels

The mutation in the alpha(1A) subunit gene of the P/Q-type (Ca(v)2.1) Ca(2+) channel present in tottering (tg) mice causes ataxia and motor seizures that resemble absence epilepsy in humans. P/Q-type Ca(2+)channels are primarily involved in acetylcholine (ACh) release at mammalian neuromuscular junctions. Unmasking of L-type (Ca(v)1.1-1.2) Ca(2+) channels occurs in cerebellar Purkinje cells of tg mice. However, whether L-type Ca(2+) channels are also up-regulated at neuromuscular junctions of tg mice is unknown. We characterized thoroughly the pharmacological sensitivity of the Ca(2+) channels, which control ACh release at adult tg neuromuscular junctions. Block of N- and R-type (Ca(v)2.2-2.3), but not L-type Ca(2+) channels, significantly reduced quantal content of end-plate potentials in tg preparations. Neither resting nor KCl-evoked miniature end-plate potential frequency differed significantly between tg and wild type (WT). Immunolabeling of Ca(2+) channel subunits alpha(1A), alpha(1B), alpha(1C), and alpha(1E) revealed an apparent increase of alpha(1B), and alpha(1E) staining, at tg but not WT neuromuscular junctions. This presumably compensates for the deficit of P/Q-type Ca(2+)channels, which localized presynaptically at WT neuromuscular junctions. No alpha(1C) subunits juxtaposed with pre- or postsynaptic markers at either WT or tg neuromuscular junctions. Thus, in adult tg mice, immunocytochemical and electrophysiological data indicate that N- and R-type channels both assume control of ACh release at motor nerve terminals. Recruitment of alternate subtypes of Ca(2+) channels to control transmitter release seems to represent a commonly occurring method of neuronal plasticity. However, it is unclear which conditions underlie recruitment of Ca(v)2 as opposed to Ca(v)1-type Ca(2+) channels.

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.

Effects of 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one on synaptic vesicle cycling at the frog neuromuscular junction

Inositol phospholipids are thought to play an important regulatory role in synaptic membrane traffic. We investigated the effects of perturbing 3-phosphoinositide metabolism on neurotransmission at the frog neuromuscular junction. We used the reversible phosphoinositide-3 kinase (PI3K) inhibitor 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one [LY294002 (LY)] and we examined its effects by intracellular recording, fluorescence imaging with styryl dyes (FM 1-43 and FM 2-10), calcium imaging, and electron microscopy. LY treatment reversibly inhibited vesicle cycling; electron micrographs indicated a dramatic reduction in the number of vesicles, balanced by the appearance of numerous cisternas. LY wash-off reverted the phenotype; terminals were refilled with vesicles, and they resumed normal FM 1-43 uptake and release. Surprisingly, LY treatment also enhanced the frequency of spontaneous release up to 100-fold in a calcium-independent manner. LY evoked similar effects in normal frog Ringer's solution, Ca-free Ringer's solution, and BAPTA AM-pretreated preparations; imaging of nerve terminals loaded with the calcium-sensitive fluorescent dye fluo-3 showed no significant change in fluorescence intensity during LY treatment. FM 1-43 imaging data suggested that LY evoked the cycling of 70-90% of all vesicles. The LY-induced effect on spontaneous release was reproduced by the casein kinase 2 inhibitor 5,6-dichlorobenzimidazole riboside but not, however, by the PI3K inhibitor wortmannin. Because LY has been shown recently to potently inhibit casein kinase 2 as well as PI3K, we hypothesize that casein kinase 2 inhibition is responsible for the enhancement of spontaneous release, whereas PI3K inhibition induces the block of vesicle cycling.

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.

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.

Constitutive secretion of exogenous neurotransmitter by nonneuronal cells: implications for neuronal secretion

Fibroblasts in cell culture were loaded with exogenous neurotransmitter acetylcholine (ACh). ACh secretion from loaded cells was detected by whole-cell patch clamp recordings from Xenopus myocytes manipulated into contact with ACh-loaded cells. Two different approaches were used for ACh loading. In the first approach, fibroblasts were incubated in the culture medium containing ACh. Recordings from myocytes revealed fast inward currents that resemble miniature endplate currents found at neuromuscular synapses. The currents observed in recordings from myocytes were due to exocytosis of ACh-containing vesicles. Although exogenous ACh penetrated through the plasma membrane of fibroblasts during incubation and was present in the cytoplasm at detectable levels, cytoplasmic ACh did not contribute to the quantal ACh secretion. In the second approach, exogenous ACh was loaded into the cytoplasm of fibroblasts by microinjection. Under these experimental conditions, fibroblasts also exhibited spontaneous quantal ACh secretion. Analysis of the exocytotic events in fibroblasts following two different protocols of ACh loading revealed that the vesicular compartments responsible for uptake of exogenous ACh are associated with the endocytic recycling pathway. Extrapolation of our results to neuronal cells suggest that in cholinergic neurons, in addition to genuine synaptic vesicles, ACh can be secreted by the vesicles participating in endosomal membrane recycling.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

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 transmitter release at somatic motor-nerve terminals: stochastic analysis of the subunit hypothesis

Here we analyze the problem of determining whether experimentally measured spontaneous miniature end-plate currents (MEPCs) indicate that quanta are composed of subunits. The properties of MEPCs at end plates with or without secondary clefts at the neuromuscular junction are investigated, using both stochastic and deterministic models of the action of a quantum of transmitter. It is shown that as the amount of transmitter in a quantum is increased above about 4000 acetylcholine (ACh) molecules there is a linear increase in the size of the MEPC. It is possible to then use amplitude-frequency histograms of such MEPCs to detect a subunit structure, as there is little potentiation effect above 4000 ACh molecules. Autocorrelation and power spectral analyses of such histograms establish that their subunit structure can be detected if the coefficient of variation of the subunit size is less than about 0.12 or, if electrical noise is added, about 0.1. Positive gradients relate the rise time and half-decay times of MEPCs to their amplitude, even in the absence of potentiating effects; these gradients are shallower at motor nerve terminals that possess secondary clefts. The effect of asynchronous release of subunits is also investigated. The criteria determined by this analysis for identifying a subunit composition in the quantum are applied to an amplitude-frequency histogram of MEPCs recorded from a small group of active zones at a visualized amphibian motor-nerve terminal. This did not provide evidence for a subunit structure.

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.

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.

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.

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.

Two classes of spontaneous GABA-mediated miniature synaptic currents in cultured rat hippocampal neurons

Amplitude and time course of spontaneous gamma-aminobutyric acid (GABA)-mediated miniature postsynaptic currents (MPSCs), recorded in cultured embryonic hippocampal neurons in presence of either tetrodotoxin (TTX) or increased external [Mg2+/Ca2+] ratio, revealed that they form two classes. The distribution of the most commonly recorded MPSCs was skewed both in terms of peak amplitude and rise-time (skew-MPSCs, mode: 70-120 pS). Another, less frequent class (mode: 1-3 nS) formed bell-shaped (bell-MPSCs) amplitude and rise-time distributions. MPSC initial slope did not correlate with rise time, indicating that smaller MPSCs were not electrotonically attenuated. Bell-MPSCs did not result from the integration of skew-MPSCs and both classes appeared to be composed of subunits.

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.

Miniature endplate potentials induced by ammonium chloride, hypertonic shock, and botulinum toxin

Intracellular recordings were made at the neuromuscular junction (NMJ) of the mouse diaphragm to study alteration of miniature endplate potential (MEPP) amplitude and rise time after different treatments. Following either hyperosmotic shock or 3 to 5 min of incubation in 10 to 50 mM ammonium chloride (NH4Cl) (replacing NaCl, a treatment which is known to raise intracellular pH) MEPP frequencies increased and the amplitudes of MEPPs decreased. These treatments as well as type A botulinum toxin (BoTx) gradually prolonged the rising phase of some MEPPs, which increased their time-to-peak (slow-MEPPs; Vautrin and Kriebel: Neuroscience 41:71-88, 1991) and increased eventually their amplitude. Fasciculation after hyperosmotic shock or during NH4Cl challenge was blocked by D-tubocurarine and was due to large slow-MEPPs that reached threshold for the muscle fiber action potential. The development of fasciculation provided the time course for the development of giant-MEPPs. Increased frequency of giant MEPP is accompanied by a block of the nerve-evoked muscle contraction. Effects of BoTx on spontaneous release were functionally antagonized either by NH4Cl or hyperosmotic shock. NH4Cl delayed BoTx blockage of bell-MEPPs. Data suggest that BoTx alters the formation of transmitter packets gradually but similarly to other treatments which increase incidence of skew-MEPPs.

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.