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I Parnas

Publications and source records attributed to I Parnas.

At least 37 records · Page 2Linked to original sources
PubMed1996

Activation of GABAB receptors at individual release boutons of the crayfish opener neuromuscular junction produces presynaptic inhibition.

1. Presynaptic inhibition in crustaceans involves the activation of gamma-aminobutyric acid-A (GABAA) receptors that produce an increase in chloride conductance at excitatory axon terminals. Such inhibition produced by single inhibitory pulses is blocked by picrotoxin, a GABAA antagonist. 2. Presynaptic inhibition produced by bath application of GABA was not blocked by picrotoxin. Measurements of the membrane resistance of the excitatory axon terminals revealed that substantial presynaptic inhibition still persisted after 50 microM picrotoxin had completely blocked the increase in conductance produced by 10 microM GABA. 3. Baclofen, a GABAB agonist, reduced release from the excitatory nerve terminals, and 20H-Saclofen, a GABAB antagonist, blocked the effect of baclofen and the presynaptic inhibition produced by 10 microM GABA. 4. 20H-Saclofen alone did not block presynaptic inhibition produced by 100 microM GABA, and the combined action of both 20H-Saclofen and picrotoxin was required to block such effects. 5. The excitatory nerve terminals seem to contain GABAA and GABAB receptors. The GABAB receptors are preferentially activated at lower GABA concentrations (in the microM range), whereas both the GABAA and GABAB receptors are activated at high GABA concentrations.

Animals↗
PubMed1996

Differential activation of two distinct mechanisms for presynaptic inhibition by a single inhibitory axon.

1. Presynaptic inhibition of excitatory transmitter release evoked by inhibitory axon stimulation was studied at individual release boutons of the crayfish opener neuromuscular junction. 2. Presynaptic inhibition was maximal (approximately 30%) when a single inhibitory action potential preceded the excitatory test action potential by 1-2 ms. This inhibition lasted at most 5 ms. It was blocked by 50 microM picrotoxin, and is probably mediated mainly by gamma-aminobutyric acid-A (GABAA) receptors. 3. Presynaptic inhibition produced by a brief train of inhibitory action potentials (5 pulses at 100 Hz) was maximal (approximately 60%) when the last inhibitory action potential (of the train) preceded the excitatory test action potential by 10 ms. This inhibition lasted up to 50 ms. It seems that in this case GABAB receptors were activated as well, because the combined action of picrotoxin (50 microM) and 20H-Saclofen (100 microM) was required to block the inhibition. 4. We thus show that one and the same inhibitory release bouton can differentially activate two distinct mechanisms for presynaptic inhibition by activating GABAA and GABAB receptors.

Action Potentials↗
PubMed1995

The double-ticker: an improved fast drug-application system reveals desensitization of the glutamate channel from a closed state.

The present study describes a modification of the fast drug-application technique (ticker) which combines two fast-application systems, 'the double-ticker'. With the double-ticker, drugs can be applied to excised patches from either one of the tickers permitting switching among three different solutions in the sub-millisecond range. We made use of this advantageous feature of the double-ticker to study two aspects of the glutamate receptor channel in crayfish muscle. The first concerns revealing the number of glutamate binding sites from measurements of a dose-response relation (2-3 sites). The other relates to the state from which the receptor undergoes desensitization. For the quisqualate-sensitive glutamate receptor desensitization occurs from a closed state. This is in addition to desensitization from an open state.

Animals↗
PubMed1994

Glutamate and N-methyl-D-aspartate affect release from crayfish axon terminals in a voltage-dependent manner.

In the crayfish neuromuscular junction, the excitatory transmitter is glutamate. The present study shows that at concentrations as low as 5 x 10(-7) M, glutamate affects the depolarization-evoked release of neurotransmitter. Furthermore, the effect of glutamate on release is voltage-dependent and depends on the level of the depolarizing pulse. Nerve terminals were exposed to 5 x 10(-7) M tetrodotoxin and then depolarized to different levels by a macropatch electrode. Depending on the amplitude of the depolarizing pulse, glutamate (5 x 10(-7) to 1 x 10(-5) M) had a dual effect on release. At small depolarizing pulses, glutamate reduced release, whereas at large depolarizing pulses, it enhanced it. Glutamate at 10(-6) M had no significant effect on action-potential-induced release. At 10(-4) M glutamate, the action-potential-induced release was always inhibited. N-Methyl-D-aspartate was found to mimic one of the effects of glutamate: N-methyl-D-aspartate (10(-7) to 10(-5) M) reduced release at small depolarizing pulses but had no effect with larger depolarizations. 2-Amino-5-phosphonovaleric acid blocked the effect of N-methyl-D-aspartate.

2-Amino-5-phosphonovalerate↗
PubMed1994

Neurotransmitter release at fast synapses.

As stated at the beginning of this review, the mechanism of neurotransmitter release is not yet known. Keeping this in mind, we shall, nevertheless, attempt to speculate and outline a possible scenario of events as it emerges from the foregoing discussion. At resting membrane potentials, the release machinery is in a blocked state produced by the constant presence in the synaptic cleft of neurotransmitter at low concentrations. At resting potentials, Ca2+ channels are closed, but this is probably not associated with the presence of low levels of neurotransmitter. Upon arrival of the action potential at the nerve terminal, (as suggested by the Ca-voltage hypothesis) two things happen independently: The release machinery is relieved of its block, being activated and readied to trigger release. Concurrently, Ca2+ enters the presynaptic terminal, and together with specific Ca2+ binding proteins, it abolishes the hydration repulsive forces without which the intimate contact between the vesicle and the plasmatic release machinery is not possible. The biophysical meaning of triggering release is at present not known. There are several suggestions, the one most consistent with the arguments of this review being the mechanism discussed and modeled by Nanavati et al. (1992; see also review: Monck & Fernandez, 1992). According to that hypothesis, an activated scaffold of proteins forms a dimple in the plasma membrane upon stimulation. This dimple, which exhibits high tension--perhaps together with Ca(2+)--overcomes the repulsive forces of hydration, permitting the two membranes to "jump" into intimate contact. As a result, a single hemifused bilayer is formed. In this hemifused bilayer, a lipidic fusion pore opens. In the context of the lipidic fusion pore hypothesis, the role of the depolarization-dependent triggering could be to start those manipulations in the plasmatic membrane that result in increased lateral bilayer tension and formation of the dimple. Ca2+ could then, in view of reduced repulsive forces and increased attractive forces, be responsible for the intimate docking of the vesicle at the release site. Under such conditions, hemifusion could take place with the final formation of the lipidic fusion pore. Finally, once the fusion pore opens, discharge of the vesicular content takes place immediately and lasts for up to 50-70 microseconds. To be so fast, discharge must occur by a mechanism other than diffusion, possibly by ion-exchange (R. Khanin, H. Parnas and L. Segel, in preparation).(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1994

The magnitude and significance of Ca2+ domains for release of neurotransmitter.

It is now widely accepted that localized high concentrations of Ca2+ (Ca2+ domains) play a major role in controlling the time course of neurotransmitter release. In the present work we calculate the magnitude and the time course of Ca2+ domains that evolve in the vicinity of a Ca2+ channel and an adjacent release site. In the calculations we consider a accurately dimensioned Ca2+ channel. Moreover, the Ca2+ current is continuously adjusted with regard to the accumulated intracellular Ca2+ and, in addition, endogenous buffers are considered. The calculations, carried out by the software FIDAP, based on finite element method, show that the Ca2+ concentrations achieved near the release sites are significantly lower than claimed by other investigators. Furthermore, we present arguments indicating that the Ca2+ domains, regardless of their magnitude, do not play a role in controlling the time course of release of neurotransmitter.

Animals↗
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

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

Synaptic transmission in decentralized axons of rock lobster.

Axons of the lobster deep abdominal extensor muscles were cut, and the resulting effects on their synaptic properties were observed. Decentralized axons continued to conduct action potentials and to release transmitter for at least a year after the cut. In controls, the single quanta were brief, and their decay phase could be fit by a single exponent, with a time constant of about 2 msec. Quanta of "cut axons" were slower, and their decay phase could not be fit by a single exponent. At midamplitude, the duration of the cut-axon quanta varied between 1.6 and 5.8 msec, as opposed to 0.6-2.8 msec in controls. Synaptic delay histograms were taken as a measure of time course of evoked release. In controls, evoked release lasted less than 10 msec at 14 degrees C. In cut axons, release lasted up to 10 times longer. The duration of release was not affected by tetrodotoxin, membrane depolarization, or hyperpolarization. It appears that the basic mechanism that controls the time course of evoked release is altered in degenerating terminals.

Action Potentials↗
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↗
PubMed1989

Long-term survival of decentralized axons and incorporation of satellite cells in motor neurons of rock lobsters.

Previous electrophysiological experiments have shown that in the abdominal extensor muscles of rock lobsters, axons which were cut in surviving animals do not degenerate peripherally for several months, but conduct action potentials and release transmitter quanta on stimulation closely distal to the scar. Electron micrographs from the axon distal to the scar (in a reliably conducting region) show invasion of the axoplasmic space by nucleated cells, probably glia. After several months, the cell membranes of the invaders have vanished and apparently functional multiple nuclei remain. We suggest that decentralized axons may survive for months with the help of 'donated' nuclei.

Action Potentials↗
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↗