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Biomedical subjects

L G Magazanik

Publications and source records attributed to L G Magazanik.

At least 19 recordsLinked to original sources
PubMed1992

Selective presynaptic insectotoxin (alpha-latroinsectotoxin) isolated from black widow spider venom.

A homogenous protein of 120,000 mol. wt isolated from black widow spider (Lactrodectus mactans tredecimguttatus) venom and referred to as alpha-latroinsectotoxin was highly potent (4 nM) in the induction of an increase of the frequency of miniature excitatory postsynaptic potentials in blowfly (Calliphora vicina) larvae neuromuscular preparations. In the frog nerve ending, however, even 50 nM alpha-latroinsectotoxin failed to affect transmitter release. Pretreatment of insect preparations with alpha-latrotoxin or frog preparations with alpha-latroinsectotoxin did not prevent the specific effect of consequent applications of alpha-latroinsectotoxin (insect) and alpha-latrotoxin (frog), respectively. The binding of labelled [125I]alpha-latroinsectotoxin to insect and [125I]alpha-latrotoxin to bovine membrane preparations was saturable and highly specific. The presynaptic effect, but not the binding of alpha-latroinsectotoxin, was dependent on the presence of divalent cations in the external medium. Mg2+ could readily substitute for Ca2+ and increase of transmitter release induced by alpha-latroinsectotoxin also occurred in Ca(2+)-free solutions. Pretreatment of preparations with 300 micrograms/ml concanavalin A completely abolished both the presynaptic effect of alpha-latroinsectotoxin and its binding to insect membrane preparations. Thus, the phenomenology of alpha-latroinsectotoxin action on insects resembles in general that described for the action of alpha-latrotoxin on vertebrates. The selectivity of alpha-latrotoxin and alpha-latroinsectotoxin seems to be due to differences in the structure of neurotoxin receptors in nerve endings of vertebrates and insects, although the mode of presynaptic action has a great deal in common.

Animals↗
PubMed1992

Transduction mechanism for glutamate-induced potassium current in neurones of the mollusc Planorbarius corneus.

1. The potassium currents evoked by glutamate agonists on isolated and identified neurones of molluscan pedal ganglia were investigated using the voltage clamp technique. 2. Glutamate responses were not modified by increasing intracellular cyclic nucleotide concentrations (treatment with 8-Br-cAMP, 8-Br-cGMP, forskolin and/or the phosphodiesterase inhibitor isobutylmethylxantine, IBMX), whereas inward-going currents induced by the nucleotides were observed. It follows that glutamate currents are independent of intracellular cyclic nucleotide control. 3. Protein kinase C activation with phorbol esters or oleoylacetylglycerol induced a slowly developing outward current and reduced glutamate response amplitude. Staurosporine itself did not affect the glutamate responses but completely prevented the effects of phorbol esters and oleoylacetylglycerol. This indicated that protein kinase C was not involved in the transduction mechanism for the potassium component of the glutamate response. 4. The possible involvement of inositol-1,4,5-trisphosphate seems to be improbable because the glutamate responses were independent of intracellular calcium concentration. Intracellular injection of calcium buffer BAPTA, failed to affect any of the glutamate currents, although it effectively blocked the after-hyperpolarization following directly evoked action potentials. 5. Nordihydroguaiaretic acid (NDGA) and indomethacin, inhibitors of the lipoxygenase and cyclo-oxygenase pathways of arachidonic acid metabolism, correspondingly, did not change the glutamate responses of these neurones. 6. The failure to demonstrate the involvement of any known secondary messenger systems in glutamate response transduction favours two assumptions: (1) the receptor-G protein complex controls the potassium channel directly; or (2) some still unknown transduction system is used.

Animals↗
PubMed1991

Dimensions of the ion channel in neuronal nicotinic acetylcholine receptor as estimated from analysis of conformation-activity relationships of open-channel blocking drugs.

Relationship between the size of the molecule in the series of organic ions Et3+N--(CH2)5--+NR1R2R3 (Ri--alkyl or cycloalkyl substituents) and their abilities to block nicotinic acetylcholine receptors (AChRs) due to their open-channel blockade in the neurons of autonomic ganglia and in frog end-plate was analyzed. All low-energy equilibrium conformations of the drugs were calculated by the molecular mechanics method. A unique rectangular channel profile 6.1 x 8.3 A, for which the best correlation between blocking activity of the drugs and total population of their conformations being able to penetrate into the channel, was deduced from all those tested.

Animals↗
PubMed1991

Different types of glutamate receptors in isolated and identified neurones of the mollusc Planorbarius corneus.

1. The membrane currents evoked by glutamate agonists on isolated and identified neurones of molluscan pedal ganglia were investigated using the voltage clamp technique. 2. The fast chloride current (Er (reversal potential) = -41 mV) evoked in a Ped-9 neurone by application of glutamate, quisqualate and ibotenic acid could be blocked by furosemide (0.1 mM). The slow potassium current (Er = -85 mV) evoked in Ped-8 and Ped-9 neurones by glutamate, quisqualate and kainate could be blocked by tetraethylammonium (50 microM). 3. N-Methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-proprionic acid (AMPA) failed to induce a response in neurones studies. 4. The spider venoms argiopine and argiopinine III (50-500 nM) selectively inhibited quisqualate-induced potassium current, but had no influence on glutamate-, ibotenate- or quisqualate-induced chloride and kainate-induced potassium currents. Glutamate-induced potassium current was partially inhibited by argiopine and argiopinine III. 5. The existence of several types of distinct glutamate receptors was confirmed in cross-desensitization experiments, and a lack of interaction was observed between quisqualate and kainate. 6. Potassium currents induced both by quisqualate and kainate strongly depended on temperature and could be blocked by pertussis toxin. Intracellular injection of the calcium chelator, EGTA, did not affect quisqualate and kainate responses. 7. In neurones loaded with non-hydrolysable GTP analogues, GTP-gamma-S (guanosine-5'-O-(3-thio)triphosphate) or GppNHp (5'-guanylylimidodiphosphate), the potassium current was gradually induced in the absence of agonists. As this current progressed, the magnitude of the glutamate- or kainate-evoked current transients became smaller and finally negligible. The GTP-gamma-S-induced current was not inhibited by argiopine. 8. These data indicate that in the molluscan neurones studied there are at least three pharmacologically distinct glutamate receptors: (1) a receptor of quisqualate-ibotenate type which directly controls chloride channel; (2) quisqualate and (3) kainate receptors which control in calcium-independent manner the common potassium channel by activation of GTP-binding protein.

Animals↗
PubMed1991

[The postsynaptic effects of substance P in the frog neuromuscular synapse].

The effect of substance P on the end-plate currents (EPC) and miniature EPC (MEPC) was studied in the "cut" sartorius muscle of the frog using voltage-clamp technique after acetylcholinesterase inhibition. Substance P in the concentration 5.10(-7)-1.10(-6) mol/l had no effect on the amplitude and time course of the single EPC and MEPC, but promoted significant prolongation of EPC decay during repetitive nerve stimulation (10/s), which indicated development of postsynaptic potentiation. Elevation of the substance P concentration to 5.10(-6) mol/l has led to the shortening of single EPS decay and more significant depression of the EPC amplitude in trains. This effect was connected with a decrease of the postsynaptic membrane sensitivity to acetylcholine, i. e. development of desensitization.

Animals↗
PubMed1990

Changes in the time course of miniature endplate currents induced by bath-applied acetylcholine.

Bath application of 0.5 and 2 microM acetylcholine (ACh) slowed the decay phase of miniature endplate currents (MEPC) recorded in isolated, voltage-clamped and prostigmine-treated frog sartorius muscle. Washout of ACh led to a decrease of the decay time constant of the MEPC to 72 +/- 5% (n = 5) and 51 +/- 3% (n = 6) of initial values, respectively, followed by very slow and incomplete recovery. MEPC amplitude changed slightly and recovered relatively fast. This discrepancy in the recovery rates is suggested to be due to a 'trapping' ability of desensitized receptors which can compete with the free receptors for ACh molecules and prevent repetitive binding. Thus the high affinity of desensitized receptors to ACh may partially compensate the absence of acetylcholinesterase activity.

Acetylcholine↗
PubMed1990

Identification of two types of excitatory monosynaptic inputs in frog spinal motoneurones.

Synaptic responses induced in motoneurones by stimulation of dorsal root (DR) and reticular formation (RF) were recorded intracellularly in the isolated frog spinal cord. Argiopine (spider neurotoxin, a selective blocker of glutamate receptors of the non-N-methyl-D-aspartate (non-NMDA) type) in concentrations from 3 x 10(-7) to 1 x 10(-5) M effectively inhibited di- and polysynaptic components of excitatory postsynaptic potentials (EPSPs) induced by DR and RF stimulation. The monosynaptic component of the RF response was inhibited by argiopine, whereas the monosynaptic component of DR response was actually enhanced. All types of EPSPs studied were blocked by kynurenate (1-2 x 10(-3) M). D,L-2-Amino-5-phosphonovaleric acid (1 x 10(-4) M) slightly diminished the amplitude only of polysynaptic DR responses. A special type of argiopine-resistant, non-NMDA glutamate receptor is assumed to be involved in the generation of an EPSP by the monosynaptic DR input.

Animals↗
PubMed1990

[Identification and isolation of the protein insect toxin (alpha-latroinsectotoxin from venom of the spider Latrodectus mactans tredecimguttatus].

The crude venom of spider Latrodectus mactans tredecimguttatus was fractionated by the combination of anion exchange and hydrophobic chromatography. The biological activity of fraction was tested by means of: 1) estimation of toxicity for housefly larva; 2) intracellular recording of miniature excitatory potentials (MEPSPs) in blowfly larvae muscle fibres. As a result of sequential procedures of chromatography separation a homogeneous protein of 120 kilodalton molecular weight was obtained. This protein referred to alpha-latroinsectotoxin produced: 1) a great increase of the frequency of MEPSPs in the dose of 4.2.10(-10) M and its paralytic dose for fly larva was approximately 20 ng/species; 2) no influence of the MEPSPs after application in the dose of 1.2.10(-7) M to the neuromuscular junction of the frog.

Amino Acids↗
PubMed1989

Development of desensitization during repetitive end-plate activity and single end-plate currents in frog muscle.

1. The amplitudes of end-plate currents (EPCs) in short trains of fifteen to seventeen EPCs at 10 Hz were depressed in the presence of 10 microM-proadifen when acetylcholinesterase (AChE) was inhibited. 2. The proadifen-induced EPC depression was voltage-dependent and the effect was more pronounced at negative membrane potentials. 3. In the presence of proadifen, the mean amplitude of miniature end-plate currents (MEPCs) was reduced by 36% 5 s after the EPC train as compared with MEPCs before the train. 4. Without proadifen, but with inhibited AChE, an increase of temperature from 20 to 26 degrees C and elevation of external Ca2+ from 1.8 to 2.5 mM led to EPC amplitude depression in the train, which was also potential-dependent. 5. After AChE inhibition, proadifen (10 microM) progressively shortened MEPC decay without significant reduction of amplitude up to 40 min of exposition. MEPCs were not affected by proadifen when AChE was active. 6. It is concluded that these postsynaptic effects of proadifen can be explained neither by its action on the resting acetylcholine receptors (AChR) nor on open ion channels but are due to its desensitization-promoting action.

Animals↗
PubMed1989

[Simulation of the action of ion channel blockaders on post-synaptic currents].

Numerical model showing action of open channel blockers on the neuromuscular junction is described. Quantitative aspects of the simple blocking action are analyzed and criteria which allow discriminating between channel blocking action and channel kinetics modification are suggested. The results of simulation are compared with experimental data. Some peculiarities of channel blocking drug action after acetylcholinesterase inhibition are discussed.

Animals↗
PubMed1988

Intense non-quantal release of glutamate in an insect neuromuscular junction.

Large inward currents (tens of nanoamperes) were induced in voltage-clamped muscle fibres of blowfly larvae (Calliphora vicina) by inhibition of glutamate (Glu) uptake by means of substitution of sodium by lithium in the perfusion, or reduction of temperature from 23 to 10 degrees C. These currents appeared to be due to accumulation of (non-quantal) Glu in the synaptic cleft: (1) the amplitude of the current depended on desensitization to Glu, as it could be enhanced by concanavalin A pretreatment; (2) the current was inhibited by a specific blocker of Glu-activated channels, argiopin; (3) the current presented a power spectrum which was identical to the spectrum induced by glutamate application. It was shown the non-quantal release is not compensated by Glu uptake and under normal conditions (no lithium, room temperature) there was some Glu in the synaptic cleft, whose concentration was sufficient to induce significant current noise.

Animals↗
PubMed1988

[Analysis of a model of the miniature endplate current].

The numerical model of the miniature end-plate current MEPC described earlier was systematically analyzed to obtain an optimal set of parameters. This set permits simulating a number of experimental effects: time course of normal MEPC, cholinesterase inhibition, alpha-bungarotoxin action, potential dependence of MEPC, etc. The time course of the simulated "giant" MEPC fitted the experimental data only partially. A good correspondence between the model and experimental data underlies the conclusion that this model reflects well the relative contribution of several processes to MEPC generation.

Animals↗
PubMed1987

Argiopin blocks the glutamate responses and sensorimotor transmission in motoneurones of isolated frog spinal cord.

Argiopin, a low-molecular weight component of the spider Argiope lobata venom, inhibited depolarizations of motoneurones induced by glutamate in experiments on the frog isolated spinal cord, but had no effect on aspartate-induced responses. Half of the blocking effect (ED50) was seen at 7.5 +/- 3.7 x 10(-8) M argiopin. The same concentrations of argiopin (7.5 x 10(-8) M to 2.3 x 10(-7) M) suppressed the responses of the ventral root to electrical stimulation of the dorsal root. The results suggest that argiopin selectively blocks only one population of the excitatory amino acid receptors on motoneurones, and these argiopin-sensitive receptors are found to be involved in sensorimotor synaptic transmission in the spinal cord.

Animals↗
PubMed1986

[Phenomenon of asynchrony of induced mediator release at the neuromuscular junction of the frog].

"Hump-like" distortions of evoked end-plate currents were observed with the help of extracellular focal recordings in sartorius and cutaneous frog muscle preparations at 20 degrees C. These distortions resembled spontaneous or unitary evoked responses by their amplitude and time course. Statistical analysis allowed rejecting the simplified assumption that these "humps" were the result of spontaneous nervous signals superimposed on the evoked ones. The asynchronous release of single quanta forming the multiquantal response seemed to be a more plausible explanation. A distinct correlation between the distribution of synaptic delay values of unitary responses (at low quantal content) and observed asynchronous responses (at high quantal content) was found. A polymodal pattern of distribution of synaptic delays was shown in both cases. It is concluded that the presence of asynchronous responses and discrete character of variations in synaptic delays are an intrinsic property of the transmitter release mechanism.

Acetylcholine↗
PubMed1986

[Effect of changes in membrane potential and temperature on the post-synaptic potential on the neuromuscular junction of the frog].

The decay time-constants of the 2nd and 1st nerve-evoked paired end-plate currents (epc) were recorded in transversely cut muscle preparations of frog. After the inhibition of synaptic acetylcholinesterase by prostigmine (3 X 10(-6) mol/l) the decay of the 2nd epc was 39 +/- 8% slower than the decay of the 1st epc (the interstimulus interval being 100 ms) due to postsynaptic potentiation (PSP). It was found that PSP does not depend on the membrane potential level in the range of-30-120 mV. A drop in temperature from 22 degrees to 12 degrees resulted in several effects: an increase in the decay time constant of epc and meps; a slight decrease in mepc amplitude; a fall of epc quantal content. The comparison of paired epc of equal quantal content showed that PSP was more pronounced at lower temperature. The temperature coefficient (Q10) for the ratio of decay time constants of the 2nd and the 1st epc was 2.0 +/- 0.2. Evidently, the trace of preceding activity of the transmitter does not depend on the membrane potential level but becomes stronger with a fall of the temperature.

Acetylcholinesterase↗
PubMed1986

[Postsynaptic potentiation and desensitization at the myoneural synapse of the frog induced by rhythmic stimulation of a motor nerve].

The contribution of postsynaptic potentiation (PSP) and desensitization (DS) to the changes in the amplitude and time course of miniature end-plate currents (mepc) recorded after 10 Hz repetitive stimulation of the motor nerve during 5 or 60 s was studied in the experiments on "cut" nerve-muscle preparation with inhibited acetylcholinesterase. After the short (5 s) train the mepc amplitude did not differ from the initial one while the decay time constant (tau mepc) increased by 32% (indication of PSP). The decrease of mepc amplitude by 23% was observed after the end of the long (60 s) train, while tau mepc did not differ from the original one (indication of DS). Similar but more marked two-phase changes occurred in the time course of end-plate currents. These effects were not observed when acetylcholinesterase was active. The PSP and DS manifestations were reproduced with acetylcholine addition into the bath. It was possible to change the ratio of PSP and DS under the action of aprodifen.

Acetylcholine↗