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H Parnas

Publications and source records attributed to H Parnas.

At least 55 records · Page 3Linked to original sources
PubMed1994

Diffusion cannot govern the discharge of neurotransmitter in fast synapses.

In the present work we show that diffusion cannot provide the observed fast discharge of neurotransmitter from a synaptic vesicle during neurotransmitter release, mainly because it is not sufficiently rapid nor is it sufficiently temperature-dependent. Modeling the discharge from the vesicle into the cleft as a continuous point source, we have determined that discharge should occur in 50-75 microseconds, to provide the observed high concentrations of transmitter at the critical zone.

Animals↗
PubMed1993

A basic biophysical model for bursting neurons.

Presented here is a basic biophysical cell model for bursting, an extension of our previous model (Av-Ron et al. 1991) for excitability and oscillations. By changing a limited set of model parameters, one can describe different patterns of bursting behavior in terms of the burst cycle, the durations of oscillation and quiescence, and firing frequency.

Animals↗
PubMed1993

A molecular scheme for the reaction between acetylcholine and nicotinic channels.

In outside-out patches of mouse-muscle membrane, embryonic-like channels were activated by pulses of acetylcholine (ACh). On increasing the ACh concentration, the rate of desensitization, 1/tau d, increased linearly with the peak open probability, indicating desensitization from the open state. Desensitization had only one time constant tau d at each ACh concentration. Recovery from desensitization was only approximately 10 times slower than desensitization, whereas the probability of steady-state channel opening, declined to < 0.01 with > 10(-6) M ACh. The peak probability of opening in > 10(-4) M ACh pulse was close to 1. A linear reaction scheme was not compatible with these results. The scheme had to be expanded resulting in a circular scheme with two additional ACh binding steps to desensitized channel states. The approximate rate constants of all reaction steps in the circular scheme could be determined using computer simulations. The model predicted that clusters of channel opening had the average duration tau d at the respective ACh concentration. In cell-attached patches on intact muscle fibers, similar average cluster durations were observed at the respective ACh concentration. This indicates that tau d in the intact muscle fibers has similar values as in outside-out patches.

Acetylcholine↗
PubMed1993

Spatial facilitation and depression within one motor nerve terminal of frogs.

1. Perfused macropatch electrodes were used to stimulate and simultaneously measure release from two sites on the same terminal of the frog cutaneous pectoris muscle. 2. It was found that release occurring at one site often affected release at an adjacent site 50 microns away, either enhancing it ('spatial facilitation') or depressing it ('spatial depression'). Spatial facilitation (or depression) was defined as the release produced by a test pulse at the second site (test electrode) when preceded by a pulse at the first site (prepulse electrode) divided by the release produced by the test pulse alone. 3. Spatial facilitation varied with the time interval between the prepulse and the test pulse. Peak spatial facilitation, which on the average was 2.14, occurred with an interval of 1-3 ms. With longer intervals spatial facilitation decayed with a time constant between 3-6 ms. When the time interval between the prepulse and the test pulse was zero (no delay), the release after the test pulse was always depressed. 4. When Ca2+ was omitted from the perfusate of the prepulse electrode, spatial facilitation was abolished. When a brief hyperpolarizing pulse followed the depolarizing prepulse with zero delay spatial facilitation was also abolished. 5. Electrotonic spread or Ca2+ diffusion within the axon terminal are excluded as coupling agents for spatial facilitation. It is suggested that the coupling agent may possibly be related to a hypothetical release-promoting factor.

Animals↗
PubMed1992

Recovery from the rapid desensitization of nicotinic acetylcholine receptor channels on mouse muscle.

Pulses of acetylcholine (ACh) applied to outside-out patches of embryonic-like muscle membrane elicited channel currents which declined rapidly (tau d = 10-60 ms) due to desensitization. Recovery from desensitization was determined by pulse pairs, varying the pulse interval. When the pulse interval was about 300 ms, the response to the second pulse was about half that to the first pulse, i.e. about half of the channels had recovered from desensitization. The results are discussed in the frame of a cyclic reaction scheme. If this scheme includes high affinity binding of ACh to desensitized receptors, it can also explain the finding that low ACh concentrations (less than or equal to 1 microM) largely desensitize the receptors, but elicit very little channel opening.

Acetylcholine↗
PubMed1992

Neurotransmitter release: facilitation and three-dimensional diffusion of intracellular calcium.

In order to account for the time courses of both evoked release and facilitation, in the framework of the Ca2+ hypothesis, Fogelson and Zucker (1985, Biophys. J. 48, 1003-1017) suggested treating diffusion of Ca2+, once it enters through the Ca2+ channels, as a three-dimensional process (three-dimensional diffusion model). This model is examined here as a refined version of the "Ca(2+)-theory" for neurotransmitter release. The three-dimensional model was suggested to account for both the time course of release and that of facilitation. As such, it has been examined here as to its ability to predict the dependence of the amplitude and time course of facilitation under various experimental conditions. It is demonstrated that the three-dimensional diffusion model predicts the time course of facilitation to be insensitive to temperature. It also predicts the amplitude and time course of facilitation to be independent of extracellular Ca2+ concentration. Moreover, it predicts that inhibition of the [Na+]o in equilibrium with [Ca2+]i exchange does not alter facilitation. These predictions are not upheld by the experimental results. Facilitation is prolonged upon reduction in temperature. The amplitude of facilitation declines and its duration is prolonged upon increase in extracellular Ca2+ concentration. Finally, inhibition of the [Na+]o in equilibrium with [Ca2+]i exchange prolongs facilitation but does not alter the time course of evoked release after an impulse.

Animals↗
PubMed1991

Effects of intra-axonal injection of Ca2+ buffers on evoked release and on facilitation in the crayfish neuromuscular junction.

Ca2+ buffers were injected into the excitatory axon of the crayfish opener muscle. The magnitude and time course of evoked release and of facilitation were measured. EGTA (on-rate about 10(6) M-1S-1) had no effect on evoked release but reduced facilitation. BAPTA and nitr-5, buffers with similar Kd's but faster on-rates, reduced both evoked release and facilitation. However, these buffers had no effect on the time course of evoked release. These results show that fast Ca2+ buffers reduce the Ca2+ transient associated with evoked release and also the level of residual Ca2+ involved in facilitation. However, Ca2+ buffering is not the mechanism which controls the time course of release.

Animals↗
PubMed1991

Evoked phasic release in frog nerve terminals obtained after block of Ca2+ entry by Cd2+.

Cutaneous pectoris muscles of frogs were isolated, mounted in a chamber and superfused with Ringer's solution. With a macro-patch-clamp electrode placed on a section of a motor nerve terminal, quantal synaptic currents were elicited by depolarizing pulses and recorded. The electrode tip and the section of the terminal recorded from were perfused rapidly by Ringer's solution alone or containing 20-500 microM Cd2+ to block Ca2+ inflow. Separate superfusion of the muscle and the rest of the terminal with normal or elevated Ca2+ Ringer's solution provided a sufficiently high resting Ca2+ concentration in the terminal even when Ca2+ was blocked by Cd2+. The depolarization level of maximal Ca2+ inflow into the terminal was found by measuring maximal test pulse facilitation, Fc. In control solution as well as in the case of Cd2+ block, the rate of phasic release after depolarizing pulses rose further when depolarization was increased past the level of Fc, and reached a saturation level which was maintained at estimated depolarizations up to +200 mV. Block of Ca2+ inflow by Cd2+ decreased release substantially, but did not suppress it. The depression of release was greater in the range of large Ca2+ inflow (around Fc) than for very large depolarizations. The time course of phasic release was unaltered by blockage of Ca2+ inflow. It is concluded that Ca2+ inflow contributes to the promotion of evoked release only in the depolarization range in which Ca2+ inward current is large.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1991

Evaluation of the time course of neurotransmitter release from the measured PSC and MPSC.

A method for obtaining delay histograms for the time course of neurotransmitter release is presented. The delay histogram is derived from the measured psc (or the sum of several psc's) and the mpsc (obtained experimentally or otherwise) by means of a simple, quick, mathematical procedure. The procedure may be automated for the greater part. No approximation of the mpsc shape is performed, and the method is applicable to all quantal contents. For low and medium quantal contents, the delay histograms obtained by the method are compared to those obtained by direct analysis. A reasonable agreement is achieved. An experiment of high quantal content, for which direct analysis is impossible, is then analysed using the new method. Difficulties which may arise when applying the procedure and methods to overcome them are discussed at length. Other methods are set forth in the Discussion.

Animals↗
PubMed1991

Kinetic constants of the acetylcholine (ACh) receptor reaction deduced from the rise in open probability after steps in ACh concentration.

Outside-out patches of enzymatically dissociated adult and denervated mouse muscle fibers were superfused repetitively by pulses of acetylcholine (ACh) containing solution. Up to 300 channels opened simultaneously 300 microseconds after the beginning of a 1,000 microM ACh pulse corresponding to a peak current i of almost -1 nA. Single responses to ACh were averaged and the concentration dependence of i and of the rise time tr from 0.1 i to 0.9 i was measured. In adult receptors, i increased proportional to the second to third power of ACh concentration, whereas in embryonic-type receptors it was proportional to the first to the second power. tr increased from approximately 0.3 ms at 1,000 microM ACh to a plateau value of approximately 5 ms for adult and of approximately 10 ms for embryoniclike receptors at concentrations less than 10 microM ACh. The concentration dependence of i and tr was simulated using the standard model of ACh binding with different combinations of rate constants and two and three binding sites for ACh. The calculated curves were compared to the measurements and a set of well fitting rate constants was determined for adult and embryoniclike receptors. Three binding sites for ACh were necessary to fit the dose response for i for adult receptors. A method for deriving rate constants in a model of ACh-receptor interaction is described that avoids analysis of open-closed kinetics of single channels, which in rapid systems, as the ones studied here, are at the limit of the frequency response of the current measurement.

Acetylcholine↗
PubMed1990

Release kinetics as a tool to describe drug effects on neurotransmitter release.

A procedure is developed wherein the time course of neurotransmitter release is used to discern the mechanism of effects of drugs on the release process. It is shown, in agreement with experiments, that the time course of release is insensitive to the temporal distribution of intracellular Ca2+ concentration. The time course is also invariant to the steps in the release process that are Ca2+ dependent. The most influential events in determining the time course of release are shown to be the steps associated with the slowest forward rate constant and all the backward steps. The procedure was examined for its ability to explain the effects on release of the poison curare and those of temperature. It appears that curare probably blocks release by lowering the rate constant associated with the rate limiting step in release. Increasing temperature is predicted to increase both the forward and the backward rate constants, but not to the same extent.

Animals↗
PubMed1990

Blockage of synaptic release by brief hyperpolarizing pulses in the neuromuscular junction of the crayfish.

1. Synaptic currents were evoked at the neuromuscular junction of the deep extensor abdominal muscle of the crayfish by direct depolarization of motor nerve endings. 2. Quantal content and time course of neurotransmitter release were determined from delay histograms of unitary release events recorded with a macropatch clamp technique. 3. Synaptic facilitation was elicited by pairing depolarizing pulses at intervals ranging from 10 to 200 ms. At 14 degrees C the duration of facilitation was about 50 ms. Reducing activity of the Nao(+)-Cai2+ exchange by lowering [Na+]o by 50% resulted in prolonged facilitation, which lasted approximately 150 ms. 4. Normalized synaptic delay histograms at normal [Na+]o and 50% [Na+]o were the same for the first and the facilitated second response, indicating that activity of the Na(+)-Ca2+ exchange does not determine the time course of release. 5. The application of a hyperpolarizing post-pulse after the first depolarizing stimulus reduced release and altered its time course to a similar extent both in normal and in 50% [Na+]o. However, it did not affect the level and the time course of release of the facilitated response. 6. A hyperpolarizing post-pulse given after the first and second pulses of a pair reduced release to the same extent for the two depolarizing pulses. 7. These results indicate that whereas manipulations thought to increase [Ca2+]i (i.e. reducing activity of the Nao(+)-Cai2+ exchange or facilitation) affect the quantal content, they do not influence the time course of release. However, changes of membrane potential do affect the quantal content, and more importantly the time course of release, thus suggesting a contributory role of membrane potential in the control of synaptic release.

Action Potentials↗
PubMed1989

Membrane depolarization evokes neurotransmitter release in the absence of calcium entry.

The discovery that Ca2+ is necessary for the release of neurotransmitter, the primary means by which nerve cells communicate, led to the calcium hypothesis of neutransmitter release, in which release is initiated after an action potential only by an increase in intracellular Ca2+ concentration near the release sites and is terminated (1-2 ms) by the rapid removal of Ca2+. Since then, the calcium-voltage hypothesis has been proposed, in which the depolarization of the presynaptic terminals has two functions. First, in common with the calcium hypothesis, the Ca2+ conductance is increased, thereby permitting Ca2+ entry. Second, a conformational change is induced in a membrane molecule that renders it sensitive to Ca2+, and then binding of Ca2+ to this active form triggers release of neurotransmitter. When the membrane is repolarized, the molecule is inactivated and release is terminated, regardless of the local Ca2+ concentration at that moment. This hypothesis, in contrast to the calcium hypothesis, accounts for the insensitivity of the time course of release to experimental manipulations of intracellular Ca2+ concentration. Furthermore, it explains rapid termination of release after depolarization, even though Ca2+ concentration may still be high. Here we describe experiments that distinguish between these two hypotheses and find that our results support the calcium voltage hypothesis.

Action Potentials↗