Biomedical subjects

I Parnas

Publications and source records attributed to I Parnas.

At least 55 records · Page 3Linked to original sources

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

Augmented synaptic release by one excitatory axon in regions in which a synergistic axon was removed in lobster muscle.

1. In the lobster, every fibre of the lateral abdominal extensor muscle is innervated by two excitatory axons. When one of the excitatory axons (the common excitor) was removed chronically by intracellular injection of pronase, terminals of the remaining axon (the specific L1 excitor) showed augmented transmitter release. 2. Evidence as to the mechanism of this strengthening can be obtained taking advantage of the peculiar innervation pattern of the abdominal extensors. The L1 excitor axon of one segment sends a branch to part of the next posterior segment. The common excitor axon innervates only muscle fibres of its own segment. 3. 10-20 days after removing the common excitor axon of segment II, the quantum content of release of terminals of the L1 excitor axon was measured in segments I, II and III. Terminals of the L1 excitor axon of segment I which innervate segment II released much more transmitter than controls, while the terminals of the same axon innervating segment I remained normal. Similarly, terminals of the L1 excitor axon of segment II became 'stronger' in segment II but remained normal in segment III. 4. It is concluded that only those terminals of one axon which innervate targets with reduced innervation increased the average release rate. It seems that the signal for synaptic strengthening, after removal of a synergistic axon, is generated and acts locally in partially denervated muscle fibres.

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↗

PubMed1987

Strengthening of synaptic inputs after elimination of a single neurone innervating the same target.

The problem of 'competition' between neurones innervating the same target can be studied in simple neural systems such as the central nervous system of the leech and the lobster neuromuscular junction. Intracellular injection of pronase to kill selectively a single neurone shows that, in the leech, removal of one neurone is a sufficient signal to produce compensatory changes. After removal of a given neurone, only neurones of the same function respond to innervate the 'vacant territory'. This was shown both for a motor neurone (annulus erector) and sensory neurones (T or N). Thus the response is very specific. The lobster neuromuscular junction, with its multiple excitatory and inhibitory innervation, has advantages for the study of changes in synaptic efficacy of the remaining neurones after removal of a defined neurone releasing the same or a different transmitter. Killing the inhibitory neurone produced prolongation of the excitatory synaptic current because of a prolonged channel open time. When an excitatory axon is killed the remaining excitatory axon releases more transmitter. Over a period of 10 days, there is first a strengthening of existing synapses, then the appearance of new release sites and sprouting. Only those terminals of a neurone that innervate a territory with reduced innervation become stronger, while other terminals of the same axon remain normal. Cutting of axons produces different responses from those seen after killing single neurones.

Animals↗

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

Nonlinear cable properties of the giant axon of the cockroach Periplaneta americana.

The steady state nonlinear properties of the giant axon membrane of the cockroach Periplaneta americana were studied by means of intracellular electrodes. The resistivity of this membrane markedly decreases in response to small subthreshold depolarizations. The specific slope resistance is reduced by twofold at 5 mV depolarization and by a factor of 14 at 20 mV depolarization. As a result, the spatial decay, V(X), of depolarizing potentials is enhanced when compared with the passive (exponential) decay. This enhancement is maximal at a distance of 1-1.5 mm from a point of subthreshold (0-20 mV) depolarizing perturbation. At that distance, the difference between the actual potential and the potential expected in the passive axon is approximately 30%. The effects of membrane rectification on V(X) were analyzed quantitatively with a novel derivation based on Cole's theorem, which enables one to calculate V(X) directly from the input current-voltage (I0-V) relation of a long axon. It is shown that when the experimental I0-V curve is replotted as (I0Rin)-1 against V (where Rin is the input resistance at the resting potential), the integral between any two potentials (V1 greater than V2) on this curve is the distance, in units of the resting space constant, over which V1 attenuates to V2. Excellent agreement was found between the experimental V(X) and the predicted value based solely on the input I0-V relation. The results demonstrate that the rectifying properties of the giant axon membrane must be taken into account when the electrotonic spread of even small subthreshold potentials is studied, and that, in the steady state, this behavior can be extracted from measurements at a single point. The effect of rectification on synaptic efficacy is also discussed.

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

Conduction block in a branching axon innervating two muscles under physiological conditions.

The escape reflex of the lobster consists of a series of tail flips resulting from alternating activity of the abdominal flexor and extensor muscles. Electromyographic (EMG) activity was recorded from the medial (DEAM) and the lateral (DEAL1) deep abdominal extensor muscles during free swimming. During the escape response, the muscles were active either synchronously or separately, at frequencies of 100-120 Hz. This activity pattern could be generated either by central programming, or by a peripheral mechanism such as frequency-dependent differential conduction block into one of the two branches of the common excitor axon (C.Ex) innervating these muscles. In order to explore the latter possibility in a living animal, we left the DEAM and DEAL1 muscles innervated only by the C.Ex from the tested segment. This was accomplished by manually cutting all other axons in the nerve under visual control. During escape responses in six successfully dissected animals, we found 27 sudden failures of the DEAM responses and only three in DEAL1. The failures were usually preceded by an increase in the delay of the response. These findings strongly suggest that conduction block occurs in the M branch innervating the DEAM under physiological conditions.

Animals↗

PubMed1984

Excitability and depolarization-release characteristics of excitatory nerve terminals in a tail muscle of spiny lobster.

In the deep abdominal L1-extensor muscle of the spiny lobster (Panulirus penicillatus) quantal excitatory postsynaptic currents (EPSCs) were recorded through macro-patch-clamp electrodes. Release of transmitter quanta from terminals was also elicited by depolarizing current pulses given through the recording electrode. The majority of terminals were excitable: on increasing the depolarization pulses, release was triggered at a threshold in an all-or-nothing manner. If excitation was blocked by tetrodotoxin (TTX), release was graded with depolarization reaching the amplitude of the all-or-nothing response at pulse amplitudes several times higher than the former threshold level. Some inexcitable terminals were also found: in these, release was graded for increasing depolarization pulses, and TTX did not alter the depolarization-release relation. Among the other types of terminals studied with the same technique, the proportion of excitable terminals in this lobster tail muscle is higher than in the crayfish opener and lower than in the frog's cutaneous pectoris muscle. The contribution of the increase in intraterminal Ca concentration to the control of release was estimated using facilitation of a test EPSC as an indicator of Ca inflow during a preceding depolarization pulse. This facilitation was found to have a maximum at a certain pulse amplitude, PF, and to decline for larger depolarizations. Release, however, rose considerably for depolarizations larger than those effected at PF. It is concluded that, like in crayfish and frog motor terminals, release is controlled directly by depolarization in addition to the control by Ca-inflow.

Action Potentials↗

PubMed1984

Strengthening of synaptic contacts of an excitatory axon on elimination of a second excitatory axon innervating the same target.

In the deep abdominal extensor muscles of spiny lobsters (Panulirus-pennicillatus), the common excitor axon of segment II was eliminated by intracellular injection of pronase. At 1 to 23 days after the operation, the quantal content of excitatory postsynaptic currents (EPSCs), elicited by stimulation of the specific excitor of the L1 muscle, was determined in a specific area of the L1 muscle, both in the operated and in the contralateral control side. The EPSCs in the operated muscles had about a 5 times higher quantal content compared to those in the controls, the change developing within 1 to 2 days after operation. In camera lucida drawings of preparations stained with methylene blue, increased branching of the remaining excitatory axon was obvious at more than 4 days after the operation. To investigate the possibility of contribution of central mechanisms (Rotshenker, S. (1979) J. Physiol. (Lond.) 292: 535-547). to this effect, the bundle of five axons to the deep abdominal extensors of segment II was cut immediately after injection of pronase into the common excitor axon. This caused a reduction of the quantal content of EPSCs and shrinking of the field of innervation in the operated L1 muscle as compared to the control. Therefore, axonal continuity or central connections seem to be necessary for the development of an increased innervation by the specific excitor to L1 after eliminating the common excitor axon. Possible postsynaptic effects of the elimination of the common excitor axon were controlled by recording synaptic single channel currents elicited by the excitatory transmitter glutamate, using the patch clamp method. These single current events did not show appreciable changes in operated L1 muscles. Therefore, the presynaptic strengthening effect on the nerve terminals of the specific L1 excitor is predominant after elimination of the common excitor axon.

Animals↗