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

Influence of a nanorod molecular layer on the biological activity of neuronal cells. A semiclassical model for complex solid/liquid interfaces with carbon nanotubes

A general account of electric effects is given for a biological phase interacting with a nanorod molecular layer by means of the formed hard-soft and solid-liquid interfaces. In particular, the frequency enhancement previously detected for the spontaneous activity of neuronal cultures interfaced with carbon nanotubes is quantitatively explained upon a quantum/semiclassical description, where the duration of a biological signal is viewed as the (average) lifetime of a decaying state (or population of states), and the effect of the carbon phase as a linewidth broadening. Four contributions were principally accounted for, one biological, for the synaptic strength, one electrochemical, for the overall capacitance increase implied by the nanotube double layers, one geometric, for the typical scales ruling the electron and ion conduction mechanisms, and one electromagnetic-like, translating the membrane polarization changes. These calculations predict an enhancement factor equal on average to ≃6.39, against a former experimental value ≃6.08.

Antibody-mediated impairment and homeostatic plasticity of autonomic ganglionic synaptic transmission

Antibodies against ganglionic acetylcholine receptors (AChR) are implicated as the cause of autoimmune autonomic ganglionopathy (AAG). To characterize ganglionic neurotransmission in an animal model of AAG, evoked and spontaneous excitatory post-synaptic potentials (EPSP) were recorded from neurons in isolated mouse superior cervical ganglia (SCG). In vitro exposure of ganglia to IgG from AAG patients progressively inhibited synaptic transmission. After passive transfer of antibody to mice, evoked EPSP amplitude decreased, and some neurons showed no synaptic responses. EPSP amplitude recovered by day 7 despite persistence of ganglionic AChR antibody in the mouse serum. There was a more persistent (at least 14-day) reduction in miniature EPSP amplitude consistent with antibody-mediated reduction in post-synaptic AChR. Although the quantal size was reduced, a progressive increase in the frequency of spontaneous synaptic events occurred, suggesting a compensatory increase in presynaptic efficacy. The quantal size returned to baseline by 21 days while the frequency remained increased for at least four weeks. Ganglionic AChR antibodies cause an impairment of autonomic ganglionic synaptic transmission. Homeostatic plasticity in autonomic neurotransmission could help explain the spontaneous clinical recovery seen in some AAG patients and may also play an important role in regulating normal autonomic reflexes.

Fluctuation analysis of tetanic rundown (short-term depression) at a corticothalamic synapse

Hypothetical scenarios for "tetanic rundown" ("short-term depression") of synaptic signals evoked by stimulus trains differ in evolution of quantal amplitude (Q) and covariances between signals. With corticothalamic excitatory postsynaptic currents (EPSCs) evoked by 2.5- to 20-Hz trains, we found Q (estimated using various corrections of variance/mean ratios) to be unchanged during rundown and close to the size of stimulus-evoked "miniatures". Except for covariances, results were compatible with a depletion model, according to which incomplete "refill" after probabilistic quantal release entails release-site "emptying". For five neurons with 20 train repetitions at each frequency, there was little between-neuron variation of rundown; pool-refill rate increased with stimulus frequency and evolved during rundown. Covariances did not fit the depletion model or theoretical alternatives, being excessively negative for adjacent EPSCs early in trains, absent at equilibrium, and anomalously positive for some nonadjacent EPSCs. The anomalous covariances were unaltered during pharmacological blockade of receptor desensitization and saturation. These findings suggest that pool-refill rate and release probability at each release site are continually modulated by antecedent outputs in its neighborhood, possibly via feedback mechanisms. In all data sets, sampling errors for between-train variances were much less than theoretical, warranting reconsideration of the probabilistic nature of quantal transmitter release.

Loading and recycling of synaptic vesicles in the Torpedo electric organ and the vertebrate neuromuscular junction

In vertebrate motor nerve terminals and in the electromotor nerve terminals of Torpedo there are two major pools of synaptic vesicles: readily releasable and reserve. The electromotor terminals differ in that the reserve vesicles are twice the diameter of the readily releasable vesicles. The vesicles contain high concentrations of ACh and ATP. Part of the ACh is brought into the vesicle by the vesicular ACh transporter, VAChT, which exchanges two protons for each ACh, but a fraction of the ACh seems to be accumulated by different, unexplored mechanisms. Most of the vesicles in the terminals do not exchange ACh or ATP with the axoplasm, although ACh and ATP are free in the vesicle interior. The VAChT is controlled by a multifaceted regulatory complex, which includes the proteoglycans that characterize the cholinergic vesicles. The drug (-)-vesamicol binds to a site on the complex and blocks ACh exchange. Only 10-20% of the vesicles are in the readily releasable pool, which therefore is turned over fairly rapidly by spontaneous quantal release. The turnover can be followed by the incorporation of false transmitters into the recycling vesicles, and by the rate of uptake of FM dyes, which have some selectivity for the two recycling pathways. The amount of ACh loaded into recycling vesicles in the readily releasable pool decreases during stimulation. The ACh content of the vesicles can be varied over eight-fold range without changing vesicle size.

Calcium-dependent release specificities of various cell lines loaded with different transmitters

By loading cells in culture with acetylcholine (ACh) we have characterized a calcium-dependent release mechanism and shown that it was expressed independently of synthesis or storage of ACh. (Israël et al., 1994, Neurochemistry International 37, 1475-1483; Falk-Vairant et al., 1996a, Proc. Natl. Acad. Sci. U.S.A. 93, 5203-5207; Falk-Vairant et al., 1996b, Neuroscience 75, 353-360; Falk-Vairant et al., 1996c, Journal of Neuroscience Research 45, 195-201). The transmitter loading procedure was applied to two other transmitters, gamma-aminobutyric acid (GABA) and glutamate (Glu). We could then study the specificity of the release mechanism for the three transmitters in a variety of cell lines, including neural-derived cells. Four different calcium-dependent release phenotypes were identified: two were specific for ACh or GABA, and two co-released two transmitters ACh and GABA but not Glu, or ACh and Glu but not GABA. We conclude that release mechanisms having different specificities are expressed by the cell lines studied, they become functional after loading the cells with the relevant transmitters. These observations will help the identification of proteins controlling the specificity of release, and provide an interesting model for pharmacological studies.

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.

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.