Biomedical subjects

H Parnas

Publications and source records attributed to H Parnas.

At least 73 records · Page 4Linked to original sources

PubMed1989

Neurotransmitter release: development of a theory for total release based on kinetics.

According to the calcium-voltage hypothesis for the control of neurotransmitter release, a molecule (or molecular complex) must be activated by membrane depolarization, after which the activated molecule can bind calcium and initiate release. In this study, we have examined properties of the kinetics of phasic release resulting from a set of differential equations that characterize the calcium-voltage hypothesis. It was found that, in accord with experiments, an important feature is the approximate constancy of the shape of the graph for the kinetics of phasic release at various depolarizations and extracellular calcium concentrations. The shape constancy allowed us to obtain an explicit and relatively simple analytical formula for the total transmitter release (quantal content) by approximating the differential equations of the model. This formula shows a saturating sigmoidal dependence on both intracellular and extracellular calcium concentrations. The formula thus agrees with various experiments. Moreover, it agrees with, and provides meaning to, earlier phenomenological expressions for the dependence of release on calcium concentration. In particular, the formula provides an expression for the maximal release in terms of kinetic parameters from the calcium-voltage model, and thereby allows one to supplement earlier kinetic tests of the calcium-voltage hypothesis with further tests focused upon the dependence of total release on depolarization.

Animals↗

PubMed1989

Facilitation as a tool to study the entry of calcium and the mechanism of neurotransmitter release.

We have shown the usefulness of using facilitation as an indirect tool for measuring release-related processes that cannot be measured directly. Most of the findings obtained by measuring facilitation have been verified (by various groups) by direct measurements in systems where such measurements could be carried out. This provides reassurance that the methodology is sound. Using facilitation one can gain insight into numerous processes that together govern the dependence of release on the intracellular calcium concentration C. The physiological conclusions have been listed in the text. We reiterate some of these conclusions here, in order to emphasize certain additional matters. For 20 years it has been known that release is a saturating cooperative function of the extracellular Ca2+ concentration Ce (Dodge and Rahamimoff, 1967). Yet several questions remained open, such as whether this behavior in fact reflected the dependence of the release on the intracellular Ca2+ concentration C, of entry on Ce, or of a combination of these individual possibilities. We have found, using short-term facilitation as the investigatory tool, that release is a saturating cooperative function of C. It is obvious that saturation will eventually take place as C increases. What is important to emphasize is that the saturation occurs at physiological values of C. Such values typically correspond to the amount of Ca2+ that enters following only one or very few pulses. Various aspects of facilitation F can be used to characterize the processes responsible for the removal of the calcium that enters the nerve terminal. Here we emphasized the role of the duration of F, but information can also be obtained by examining other aspects of facilitation (Parnas and Segel, 1980; H. Parnas et al., 1982; I. Parnas et al., 1982a). The principal conclusion is that removal shows saturation kinetics.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1989

Effect of Ca2+ diffusion on the time course of neurotransmitter release.

The three-dimensional (3D) diffusion model of Fogelson, A. L., and R. S. Zucker (1985. Biophys. J. 48: 1003-1017) has been employed as the basis of a refined version of the "Ca theory" for neurotransmitter release. As such, it has been studied here as to its ability to predict the time course of release under various conditions. In particular, conditions were chosen in which the temporal variations in intracellular Ca2+ concentration, the sole factor controlling the release according to the Ca theory, were modified and tested experimentally. The predictions of this model were compared with the experimental results. It is shown that the 3D diffusion model, similarly to earlier simpler versions of the Ca theory, predicts that the time course of release is highly sensitive to both the level of depolarization and the level of the resting concentration of intracellular Ca2+ Moreover, the 3D diffusion model predicts that the time course of release is insensitive to changes in temperature. In contrast, the experimental results show that the time course of release is invariant to the level of depolarization and to the resting level in intracellular Ca2+, but highly sensitive to variations in temperature.

Animals↗

PubMed1989

Sequential model to describe the nicotinic synaptic current.

An analytical formula is derived to describe the synaptic end plate current (epc) at the nicotinic receptor. Various concurrently occurring underlying processes, including (a) diffusion, (b) hydrolysis of acetylcholine, and (c) its binding to the dimeric receptor, were considered in order to develop the equation. Numeric solution of the equations that describe the events underlying the epc showed that these events occur in sequence, rather than concurrently. This sequential occurrence of the processes allowed for simplifications, which were used as the basis for the new description of the epc. The resulting formula serves as a tool for evaluating the relative contribution of the various processes in formation of the natural occurring transient epc.

Animals↗

PubMed1988

Long-term facilitation of synaptic transmission demonstrated with macro-patch recording at the crayfish neuromuscular junction.

Recordings of synaptic currents from the crayfish opener muscle were made with a macro-patch recording technique, permitting clear detection of neurotransmitter quanta at individual nerve terminals before and after induction of long-term facilitation (LTF). Depolarization of the terminal by propagated action potentials or by local intracellular pulses induced LTF. The quantal content was increased on average by 93%. Binomial analysis indicated increased probability of release and also increased number of available quantal units. The increase occurred regardless of a blockade of sodium, calcium or potassium channels by appropriate pharmacological agents. Presynaptic recording with an intracellular microelectrode showed no change in presynaptic electrical properties. Also, there were no changes in the synaptic delay. It is concluded that LTF results from a depolarization-dependent alteration of synaptic release sites.

Action Potentials↗

PubMed1988

The 'Ca-voltage' hypothesis for neurotransmitter release.

The 'Ca-voltage' hypothesis for neurotransmitter release was reinvestigated by studying the kinetics of neurotransmitter release. These were independent of changes in intracellular or extracellular Ca2+ concentration. It is concluded that initiation and termination of release do not result from rapid entry and removal of Ca2+ although Ca2+ is essential for release. Quantal release of transmitter requires depolarization-dependent transformation of a membrane molecule from an inactive form T to a Ca2+-binding form S. The depolarization-dependent T----S transformation initiates release in the presence of Ca2+. The S----T transformation upon repolarization stops release even though the Ca2+ concentration at release sites is still high.

Animals↗

PubMed1988

Presynaptic effects of d-tubocurarine on neurotransmitter release at the neuromuscular junction of the frog.

1. Presynaptic effects of d-tubocurarine on neurotransmitter release were examined at the frog neuromuscular junction, using intracellular and extracellular recording techniques. 2. d-Tubocurarine in concentrations of 10(-7)-10(-6) M decreased the quantal content (m) measured by the coefficient of variation and failure methods. 3. d-Tubocurarine produced a shift to the right of the curve relating log quantal content to log [Ca2+]o without changing the slope. 4. The duration of twin-impulse facilitation was not affected by 5 x 10(-7) M-d-tubocurarine. Early facilitation was higher in d-tubocurarine. 5. d-Tubocurarine altered the synaptic delay histogram. The peak of the histogram was shifted to longer delays. Prolongation of the minimal delay was seen in most but not all experiments. 6. These results suggest that d-tubocurarine inhibits release of neurotransmitter by affecting a stage in the process of release, which occurs after the entry of Ca2+ ions.

Animals↗

PubMed1987

Quantal currents evoked by graded intracellular depolarization of crayfish motor axon terminals.

1. Quantal transmitter release was examined at nerve terminals of the excitatory motor axon of the crayfish opener muscle. The magnitude of synaptic currents, recorded with macro-patch electrodes at a nerve terminal, served as a measure of quantal size. Transmitter release was initiated by pulses of depolarizing current applied intracellularly to the axonal terminals after application of tetrodotoxin. Quantal release was altered by a variety of methods and the resulting quantal output and quantal size were measured. 2. Amplitude distributions of quantal events were obtained during experimental manipulations which altered the rate of quantal release by up to 25-fold. These manipulations consisted of: varying pulse amplitude or pulse duration; facilitating the release by prolonged depolarization; and application of a potassium channel blocker, 4-aminopyridine. 3. The amplitude of quantal events is impervious to marked changes in presynaptic depolarization and is not affected by experimental procedures which promote accumulation of calcium ions in the terminals. The vesicular mechanism of release, in which transmitter substance is prepackaged in vesicles which individually undergo exocytosis at a release zone, could account for the observed results.

4-Aminopyridine↗

PubMed1987

Influence of depolarizing pulse duration on the time course of transmitter release in lobster.

1. Experiments have been made at lobster neuromuscular synapses to study synaptic delays and in particular the phenomenon known as 'latency shift'. Earlier work had suggested that synaptic delay becomes prolonged when pulses of long duration are applied to presynaptic terminals. 2. By observing single quanta, prolonged depolarizing pulses at low and moderate amplitudes have been shown to shift the peak of the synaptic delay histogram. There is however no increase in the minimal delay. 3. The apparent differences between these and earlier results have been shown to depend on differences in experimental procedures. In particular, in the present study equal numbers of pulses are applied at the various pulse durations and the number of quanta that had been released is presented.

Action Potentials↗

PubMed1987

Latency of transmitter release at crayfish motor nerve endings examined by intracellular depolarization.

Latency of release of individual quanta of transmitter was studied at neuromuscular junctions of a crayfish (Procambarus clarkii). Postsynaptic quantal currents were recorded at individual motor nerve endings with a macropatch electrode while the subterminal axon branch was depolarized by current passed through an intracellular microelectrode. For depolarizing currents of moderate size, the latency of transmitter release did not change when the duration of the depolarizing current was altered. Previous studies in which a contrary result was obtained may have been compromised by artefacts or by the sampling methods employed. The present results do not support the hypothesis of a depolarization-induced "repressor" of quantal release.

Animals↗

PubMed1986

On the quantal hypothesis of neurotransmitter release: an explanation for the calcium dependence of the binomial parameters.

Results for quantal neurotransmitter release can be explained by assuming a binomial distribution with a population of N elements each with a probability p to release a quantum in a given trial. The binomial parameter N was unexpectedly observed to depend on external calcium concentration and (to a lesser extent) on the frequency of stimulation. This observation is explained here by the hypothesis that the release population is not homogeneous. It is shown that the same hypothesis can also account for other experimental findings. A possible cause for this inhomogeneity is suggested.

Calcium↗

PubMed1986

A new method for determining co-operativity in neurotransmitter release.

It has been accepted for some time that neurotransmitter release exhibits a co-operative dependence on calcium. Here we suggest a new procedure for estimating the co-operativity, based on the early rise of synaptic delay histograms of induced release at low quantal content. Measurements of such histograms at the lobster neuromuscular junction are reported. On the basis of this data, and also of data from the literature for other species, a re-examination is made of the conventional hypothesis that the kinetics of release is primarily determined by the time course of entry and removal of calcium ions. Two major new hypotheses for the nature of co-operativity are discussed, both containing the additional feature that membrane depolarization activates a molecule or complex that only then can bind calcium and induce release. The measurements confirm the hypothesis that the co-operativity arises from the action of several complexes between calcium and a depolarization-activated molecule to initiate the release of a vesicle. The co-operativity exponent is estimated to be between three and five in lobster neuromuscular junction and also in crayfish, macrobrachium, and frog.

Allosteric Regulation↗

PubMed1986

Neurotransmitter release and its facilitation in crayfish. VII. Another voltage dependent process beside Ca entry controls the time course of phasic release.

Quantal synaptic currents were recorded at nerve terminations on the opener muscle of crayfish using a macro-patch-clamp electrode, and the release was elicited by depolarizing current pulses applied to the terminal through the same electrode. After 2 ms depolarization pulses at low temperature, release started with about 2 ms delay after the onset of depolarization, and the maximum rate of release occurred at about 4 ms delay. Large variations in Ca inflow during the pulses were concluded from the facilitation of test EPSCs. The time course of release proved to be remarkably invariant in spite of large changes in release. If a conditioning train of depolarization pulses preceded the test pulse, release due to the test pulse was facilitated up to 60-fold, but the shapes of distributions of quantal delays were practically not affected by this facilitation. Facilitation by the conditioning trains must have raised the [Ca]i level at the onset of the test pulse. The invariance of the time course of release with respect to the level of [Ca]i cannot be explained by theories in which [Ca]i alone controls the time course of release. The time courses of reactions controlling release were explored by mathematical analysis and simulation. A reaction scheme in which the activation of "release sites" directly by depolarization had rate limiting control on the release reactions, in which rise of [Ca]i only was a promoting cofactor, and in which a cooperative reaction involving the complex of release sites and Cai, (SCai) was one of the final steps eliciting release, was able to predict the delayed onset of release and the substantial latency between the end of the depolarization pulse and the maximum of the rate of release. Reaction schemes in which the direct effect of depolarization on release occurred at one or more steps following the entry of Ca could be excluded generally by showing conflict with the experimental findings.

Animals↗

PubMed1986

Neurotransmitter release and its facilitation in crayfish. VIII. Modulation of release by hyperpolarizing pulses.

Quantal postsynaptic currents were recorded by a macro-patch-clamp electrode from synaptic spots on the opener muscle of the walking leg of large crayfish at 0 degrees to 4 degrees C. Through the same electrode, current pulses were applied which de- or hyperpolarized the nerve terminal. The depolarizing test pulse of fixed amplitude and duration elicited EPSCs with an average quantum content m1. If the test pulse was preceded or followed immediately by a modulatory hyperpolarizing pulse, the quantum content m1p of the EPSC was reduced by factors m1/m1p up to 10. This modulation of release increased with amplitude and duration of hyperpolarization, reaching a saturation level for durations greater than 3 ms. If an interval was interposed between modulatory hyperpolarization and test pulse, the reduction of release decayed with increasing interval with a time constant of about 2 ms. The release m2 due to a second test pulse following the first one with 10 ms interval was facilitated, m2/m1 greater than 1. If release by the first test pulse was modulated by associated hyperpolarizing pre- or post-pulses, facilitation of the second test EPSC was not reduced in comparison to facilitation without modulation. The time course of phasic release (distribution of delays of quanta after test pulse) was not appreciably affected by hyperpolarizing pre-pulses, but such post-pulses shortened the delay of the maximum of release. The effects of modulatory pre- and post-pulses could be described by an extension of the model given in H. Parnas et al. (1986). Release 'sites', S, are assumed to have two inactive states, T1 and T.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1986

Calcium is essential but insufficient for neurotransmitter release: the calcium-voltage hypothesis.

A new Ca-voltage hypothesis for neurotransmitter release is proposed. Accordingly, membrane depolarization has two roles. To increase membrane conductance to calcium and to activate a molecule S from an inactive form T. Only the active form S binds calcium ions to start the chain of events leading to release. Four lines of experiments are described to support this hypothesis. Disassociation between Ca2+ entry and transmitter release. Loading of the terminal with Ca2+ and obtaining release with little additional entry of Ca2+. Measurements of kinetics of release. Modulation of release by changes in membrane potential. Recent criticism as to the validity of the experimental techniques used in the first two lines of experiments is analyzed and rejected.

Animals↗

PubMed1985

Dopaminergic modulation of neuromuscular transmission in the prawn.

The action of the putative crustacean neurohormone dopamine was examined in the fast extensor musculature of the prawn with intracellular and extracellular recording techniques. Dopamine produced a concentration-dependent (10(-7)-10(-5) M) decrease in the size of the excitatory junctional potential (e.j.p.). It had no effect on the muscle fibre resting membrane potential or input resistance. High concentrations (10(-5)M) of dopamine had no effect on the amplitude distribution or decay time of quantal unit currents, indicating that the agent does not act by blocking post-synaptic receptors or channels. Bath application of dopamine reduced the quantal content at single release sites with a similar time course and concentration dependence as that observed for the e.j.p. Dopamine had no effect on histograms of synaptic delays determined over a 10 degree C range, indicating that it does not modify the time course of phasic neurosecretion. Twin-impulse facilitation experiments showed a marked decrease in the duration of facilitation in the presence of dopamine. These results are interpreted according to recent theoretical and experimental findings as indicating that the dopamine-induced reduction in transmitter release is produced by a decrease in the entry of Ca during the nerve terminal action potential.

Action Potentials↗

PubMed1984

Depolarization dependence of the kinetics of phasic transmitter release at the crayfish neuromuscular junction.

Quantal synaptic currents were recorded at nerve terminals on the crayfish opener muscle by means of a macro-patch-clamp electrode. Release could be elicited by graded depolarization pulses through the recording electrode. At low temperature, distributions of delays of single quantal currents from the onset of depolarization were determined for depolarizations varying from threshold to saturation range. This time course of release was little affected by the amplitude of depolarization: There was a tendency for release to start earlier and to rise faster for larger depolarizations, while the termination of release showed no significant variations. The time course of release after an action potential in the motor axon was similar to that of release after a depolarization pulse. It is concluded that the time course of quantal release is rather independent of amplitude of depolarization and of the amount of calcium (Ca) inflow, which seems to rule out the control of the release after a depolarization by the time course of [Ca]i.

Animals↗

PubMed1984

Exhaustion of calcium does not terminate evoked neurotransmitter release.

Theories are considered which assume that termination of evoked release is caused by the exhaustion of intracellular Ca. It is shown that such theories predict, contrary to experiment, that total release is an unsaturated function of intracellular Ca whose duration depends strongly on extracellular Ca. These and other findings lead to the conclusion that termination must be due to the fast change of another parameter (not intracellular Ca).

Action Potentials↗