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H Parnas

Publications and source records attributed to H Parnas.

At least 37 records · Page 2Linked to original sources
PubMed1998

Functional and immunocytochemical identification of glutamate autoreceptors of an NMDA type in crayfish neuromuscular junction.

Functional and immunocytochemical identification of glutamate autoreceptors of an NMDA type in crayfish neuromuscular junction. J. Neurophysiol. 80: 2893-2899, 1998. N-Methyl--aspartate (NMDA) reduces release from crayfish excitatory nerve terminals. We show here that polyclonal and monoclonal antibodies raised against the mammalian postsynaptic NMDA receptor subunit 1 stain specifically the presynaptic membrane of release boutons of the crayfish neuromuscular junction. In crayfish ganglionic membranes, the polyclonal antibody recognizes a single protein band that is somewhat larger (by approximately 30 kD) than the molecular weight of the rat receptor. Moreover, the monoclonal (but not the polyclonal) antibody abolishes the physiological effect of NMDA on glutamate release. The monoclonal antibody did not prevent the presynaptic effects of glutamate, which also reduces release by activation of quisqualate presynaptic receptors. Only when 6-cyano-7-nitroquinoxatine-2,3,dione (CNQX) was added together with the monoclonal antibody was the presynaptic effect of glutamate blocked. These results show that presynaptic glutamate receptors of the crayfish NMDA type are involved in the regulation of neurotransmitter release in crayfish axon terminals. Although the crayfish receptor differs in its properties from the mammalian NMDA receptor, the two receptors retained some structural similarity.

Animals↗
PubMed1997

Voltage-dependent interaction between the muscarinic ACh receptor and proteins of the exocytic machinery.

1. Release of neurotransmitter into the synaptic cleft is the last step in the chain of molecular events following the arrival of an action potential at the nerve terminal. The neurotransmitter exerts negative feedback on its own release. This inhibition would be most effective if exerted on the first step in this chain of events, i.e. a step that is mediated by membrane depolarization. Indeed, in numerous studies feedback inhibition was found to be voltage dependent. 2. The purpose of this study is to investigate whether the mechanism underlying feedback inhibition of transmitter release resides in interaction between the presynaptic autoreceptors and the exocytic apparatus, specifically the soluble NSF-attachment protein receptor (SNARE) complex. 3. Using rat synaptosomes we show that the muscarinic ACh autoreceptor (mAChR) is an integral component of the exocytic machinery. It interacts with syntaxin, synaptosomal-associated protein of 25 kDa (SNAP-25), vesicle-associated membrane protein (VAMP) and synaptotagmin as shown using both cross-linking and immunoprecipitation. 4. The interaction between mAChRs and both syntaxin and SNAP-25 is modulated by depolarization levels; binding is maximal at resting potential and disassembly occurs at higher depolarization. 5. This voltage-dependent interaction of mAChRs with the secretory core complex appears suitable for controlling the rapid, synchronous neurotransmitter release at nerve terminals.

Animals↗
PubMed1997

"First step" negative feedback accounts for inhibition of fast neurotransmitter release.

This paper is concerned with feedback inhibition of neurotransmitter release by the neurotransmitter itself. We put forward the idea that, similar to multistep biochemical processes, feedback inhibition acts on the initial step in the chain of events that lead to release. Using experimental results carried out on glutamatergic synapses in crayfish, we show that the "first step" hypothesis can account for all experimental results. Our modeling suggests that the biochemical implementation of this inhibition involves the formation of a second messenger, whose production is triggered by binding of transmitter to the autoreceptor. We argue that the autoreceptor is a key part of the release-inducing machinery.

Animals↗
PubMed1997

Simultaneous measurement of intracellular Ca2+ and asynchronous transmitter release from the same crayfish bouton.

1. A technique has been developed to monitor neurotransmitter release simultaneously with intracellular Ca2+ concentration ([Ca2+]i) in single release boutons whose diameters range from 3 to 5 microns. 2. Using this technique, we have found a highly non-linear relationship between the rate of asynchronous release and [Ca2+]i. The Hill coefficient lies between 3 and 4. 3. The affinity (Kd) of the putative release-related Ca2+ receptor for asynchronous release was calculated to be in the range of 2-4 microM. 4. The same range of values of Hill coefficient and Kd were obtained when [Ca2+]i was elevated both by bath application of ionomycin and by repetitive stimulation at high frequency. 5. Our results show that the Ca2+ receptor(s) associated with asynchronous release exhibits high affinity for Ca2+.

Animals↗
PubMed1997

A mechanism for discharge of charged excitatory neurotransmitter.

Excitatory neurotransmitter is charged, so that emptying of a transmitter-containing vesicle (discharge) would seem to require considerable energy. Even if the energy problem is surmounted and discharge thereby made possible, there is still a problem of making the discharge fast enough (considerably less than 1 ms). Proposed here is a mechanism wherein discharge of charged transmitter is accompanied by the influx of cocharged ions or coefflux of counter-charged particles (ion interchange). It is shown theoretically that ion interchange obviates the necessity for a separate energy source and can provide the observed rapid vesicle discharge.

Acetylcholine↗
PubMed1996

Open channel and competitive block of the embryonic form of the nicotinic receptor of mouse myotubes by (+)-tubocurarine.

1. Embryonic-like nicotinic channels were studied in mouse myotubes. Channel currents were measured by patch clamping outside-out excised patches to which pulses of agonists and drugs could be applied by a liquid filament switch. The holding potential of the patches was generally around-10 to-40 mV. 2. Pulses of 100 microM or 1 mM acetylcholine (ACh) elicited average channel currents which reached a maximum open probability of 0.93 within 0.5-1.0 ms, decayed with a time constant of desensitization of 20-80 ms, and fell rapidly to zero at the end of the pulse. When such pulses together with increasing concentrations of (+)-tubocurarine (TC) were applied to outside-out patches, the time constant of current decay, tau, decreased beginning at concentrations of TC added to the test solution of > 10 microM, and the peak amplitude of the current decreased markedly at concentrations of TC of > 30 microM due to an open channel block of nicotinic channels by TC. 3. When the outside-out patches were pre-incubated with TC, the peak current elicited by pulses of 100 microM ACh or 1 mM ACh + TC decreased markedly, beginning with concentrations of TC > 30 nM due to a competitive block. 4. The results could be quantitatively modelled by computer calculations based on a circular reaction scheme containing desensitization. TC blocked the open state as well as the unliganded closed state of the embryonic-like nicotinic receptors of mouse myotubes. Also the blocked open channel was subject to desensitization. 5. The rates of block and unblock of the open channel were 3 x 10(6) M-1 S-1 and 0.8 S-1, respectively, and those of the competitive block were 0.5 x 10(6) M-1 S-1 and 0.1 S-1, respectively (at 20 degrees C).

Acetylcholine↗
PubMed1996

Relationship between burst properties and sensitivity to input: a theoretical analysis.

This paper examines the sensitivity of endogenous bursters to a brief input pulse. The interneurons of the lobster cardiac ganglion were selected as a case study. Using a mathematical model specifically developed for the neurons in the cardiac ganglion of the lobster (Av-Ron et al., 1993), we show a tight link between burst characteristics and certain other parameters. We show that cells with different burst properties differ in their sensitivity to an input of a brief pulse. Irrespective of these differences, all cells display a bimodal response to a brief pulse applied during the quiescent period. During the first three-quarters of the quiescent period, they respond by producing a single spike at most. During the remaining one-quarter, the brief pulse can initiate the cells' intrinsic burst. Our predictions fit experimental results obtained by Tazaki and Cooke (1979). The results obtained herein are discussed with respect to fault tolerance considerations.

Animals↗
PubMed1996

Parallel computation enables precise description of Ca2+ distribution in nerve terminals.

Parallel computation employing a domain decomposition method was used to calculate precisely without approximations the spatio-temporal distribution of Ca2+ in nerve terminals. The results showed, contrary to expectations, that for equal admitted Ca2+ currents at low (one channel open) and high (four channels open) depolarization, the average Ca2+ concentration at the release area is higher at the low depolarization. These calculations provide additional support for the Ca(2+)-voltage hypothesis for neurotransmitter release.

Animals↗
PubMed1996

Glutamate depresses release by activating non-conventional glutamate receptors at crayfish nerve terminals.

The present study shows that release of glutamate from crayfish nerve terminals is inhibited at low depolarizing current pulses by glutamate, N-methyl-D-aspartate (NMDA) and quisqualate. These agonists elicit inhibitory effects at concentrations as low as 10(-8) M (quisqualate) and 10(-7) M (glutamate and NMDA). The NMDA-mediated inhibition is blocked by (+/-)-2-amino-5-phosphonovaleric acid (APV). The quisqualate-mediated inhibition is blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Both CNQX and APV are needed to block glutamate-mediated inhibition. The inhibition of release is not accompanied by a detectable change in presynaptic membrane conductance at the secondary branch. Using fura-2, Ca2+ accumulation during repetitive stimulation (100 Hz) was monitored in single release boutons. Inhibition of release, elicited by 10(-4) M glutamate, was not associated with a reduction in the accumulation of Ca2+. We show that the glutamate released from a single or a few release boutons during normal activity acts similarly to glutamate added externally, i.e. it inhibits its own release.

2-Amino-5-phosphonovalerate↗
PubMed1996

Open channel block by physostigmine and procaine in embryonic-like nicotinic receptors of mouse muscle.

Embryonic-like nicotinic channels were studied in mouse myotubes. Channel currents were measured by patch-clamping outside-out excised patches to which pulses of agonists and drugs could be applied by a liquid filament switch. The holding potential of the patches was generally around 40 mV. Pulses of 10(-4) M acetylcholine elicited average channel currents which reached a peak open probability, P(o,peak,) of 0.93 within 0.5 ms and decayed with a time constant of desensitization of 20-80 ms. When physostigmine (10(-5) to 10(-3) M) or procaine (3 x 10(-5) to 10(-3) M) was added to the acetylcholine pulses, a fast decay component of the current appeared which shortened to a time constant of 0.5 ms for the maximal drug concentrations. The fast decay was followed by a slow one which declined in amplitude with increasing concentrations of the drugs. After the end of pulses of 10 M acetylcholine plus 3 x 10(-4) M physostigmine the average current rose again, reaching a peak with approximately 5 ms delay, and then decayed slowly. The amplitude of this recovery current was approximately 0.4 P(o,peak) after 5 ms pulses and decreased with increasing pulse duration due to desensitization. The results can be quantitatively modelled based on a circular reaction scheme involving desensitization. Physostigmine and procaine bind to the open state to cause channel block. Also, the blocked channel was subject to desensitization. The rate constants of block were 6 x 10(6) M(-1) s(-1) for physostigmine and 2 x 10(6) M(-1) s(-1) for procaine, and the rate of unblocking was 200 s(-1) for both blockers (at -40 mV and 20 degrees C).

Animals↗
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↗
PubMed1995

Modulated excitability: a new way to obtain bursting neurons.

Classical burster models are based on a fast system that either oscillates or is quiescent, depending on temporarily fixed values of slow variables. In a study of the lobster heart ganglion, we found a new type of burster for which the fast system is globally stable for all relevant fixed values of the slow variables. We describe how this burster works and speculate on its biological significance.

Animals↗
PubMed1995

Neuronal growth via hybrid system of self-growing and diffusion based grammar rules: I.

The formation of neuronal networks requires axonal growth towards target neurons. A simple set of grammar rules is introduced to describe axonal growth towards target cells situated both at short and long distances from the growing neuron. Growth for short distances is described by growth following the highest gradient of a chemical compound (which is spread by diffusion from the targets). This approach fails to describe long-distance growth, which is addressed by adopting a graph grammar theory for growing trees. With these rules a flexible tool to draw network of neurons by computer can be developed.

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

Sequential approach for describing channel opening and desensitization.

A formula describing the time-course of activation and desensitization of nicotinic-like acetylcholine channels is developed. The method used to arrive at this simplified formula is based on a sequential approach. Based upon this approximated and simple formula, analytical expressions for various specific experimental aspects are derived. These expressions together with the corresponding experiments provide a method to evaluate in a sequential manner the various rate constants involved.

Acetylcholine↗
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↗