[Presynaptic mechanisms of zinc effects on the neuromuscular transmission]

The effect of zinc ions on presynaptic currents and transmitter release was studied at the neuromuscular junction of the frog cutaneous pectoris muscle preparation with using an extracellular microelectrode. It has been shown that zinc (100 mkM) amplified MEPP frequency at first, but suppressed it later. Zinc affected the presynaptic spike waveform and transmitter release in a concentration-dependent manner. Depending on concentration and time of exposure zinc increased or suppressed transmitter release. Increase of transmitter release was shown to be resulted by blockade voltage gated and calcium activated potassium channels in nerve ending, leading to broad of both presynaptic spike and action potential. Strong change of presynaptic spike waveform after high concentration zinc treatment supposed that under this condition zinc depressed voltage gated calcium and sodium channel leading to decrease of transmitter release. It was concluded that the final and irreversible depression of acetylcholine release by zinc was due to alteration of whole ion conductances in nerve ending and to change of configuration of proteins included in structure of ion channels. It is discussed possible mechanisms of various effects of zinc ions at the neuromuscular synapse.

[Effect of sodium nitroprusside on the mediator release and functional properties of the postsynaptic membrane at the neuromuscular junction]

The effect of nitric oxide donor sodium nitroprusside on the end-plate currents was studied under two-electrode voltage-clamp condition at frog neuro-muscular junction. Sodium nitroprusside (10(-4) M) reduced to the half the amplitude of end-plate currents while did not change miniature end-plate currents indicating the presynaptic nature of end-plate depression. In keeping with such suggestion sodium nitroprusside essentially (to 33%) suppressed the frequency of miniature end-plate currents but did not affect the decay time constant and voltage-dependence of miniature end-plate decay. In contrast to another presynaptic inhibitors sodium nitroprusside rather reduced than increased the presynaptic facilitation and did not change postsynaptic potentials. Thus, nitric oxide is the powerful inhibitor of both evoked and spontaneous transmitter release and did not change postsynaptic potential.

[The physiological role of nitric oxide]

The review is devoted to exposition of a physiological role of a nitric oxide (NO), free radical gas, in various physiological functions. The number of those NO involvements is extremely high: bacteriocidal, cytotoxic and antitumor leukocyte effects, a relaxation of smooth-muscle cells of both vessels and gastrointestinal tract, the name just a few. The scheme of NO formation in various biological systems and its targets were shown and neuromodulator functions of NO in a brain were analyzed by the review presented. The findings of own researches on a role of NO in function of neuro-muscular synapse were included by the authors.

[GABA--the basic mediator of excitation in the early stages of hippocampal development]

GABA is the principal neurotransmitter of inhibition in the adult mammalian brain. However, at early stages of development, including embryonic period and first week of postnatal life, GABA plays the role of main neurotransmitter of excitation. The paradoxical excitatory effect of GABA is due to an inversed chloride gradient and therefore a depolarizing direction of GABA-A receptor mediated responses. In addition, another type of GABAergic inhibition mediated by postsynaptic GABA-B receptors is not functional at early stage of life. In the neonatal rat hippocampus, GABA, acting via GABA-A receptors, activates voltage gated sodium and calcium channels and potentiates the activity of NMDA receptors by reducing their voltage dependent Mg2+ block. The temporal window when GABA exerts excitatory actions coincides with a particular pattern of activity of hippocampal neuronal network that is characterized by periodical giant depolarizing potentials (GDPs) reminiscent of interictal-like epileptiform discharges. Recent studies have shown that GDPs result from the synchronous discharge of GABAergic interneurons and principal glutamatergic pyramidal cells and are mediated by the synergistic excitatory actions of GABA-A and glutamate receptors. GDPs provide synchronous intracellular Ca2+ oscillations and may therefore be implicated in hebbian modulation of developing synapses and activity-dependent formation of the hippocampal network.

Effect of phenol on the ion currents of frog nerve ending

The effect of phenol on the transmitter release was studied at the neuromuscular junction of a frog cutaneous pectoris muscle using the extracellular microelectrode technique. It was shown that phenol (0.5 mM) enlarged the quantal content of the end-plate currents (EPCs), and it caused the increase of duration of the second negative and third positive phases of the triphasic response observed experimentally. The amplitude of the third phase (outward K+ current) decreased, and one of the second phase was unchanged. The effect of phenol remained in the presence of the Ca(2+)-channel blockers (Co2+, Mn2+), and disappeared in the presence of the tetraethylammonium (TEA) and 4-aminopyridine (4-AP). The results suggest that phenol changes the kinetics of the voltage-dependent K+ current, which is one of the mechanisms of the phenol facilitating effect at the synaptic transmission.

[The effect of asynchronism in transmitter secretion on the amplitude-time parameters of an evoked postsynaptic current and potential in a neuromuscular synapse]

In the model of end-plate currents (EPC) and potential (EPP), no change was induced by an increase in the transmitter secretion asynchronism. The latter exerted no effect on the EPP amplitude or the maintenance of reliability of the synaptic transmission.

[The effect of phenol on the ion currents of the frog motor nerve ending]

Phenol was shown to enhance the quantum content of the end-plate currents and to increase the duration of the nerve ending response. The effect could be abolished by the potassium channel blockers. The data obtained suggest that alteration of the kinetics of the potential-dependent potassium current of the nerve terminal is one of the mechanisms of the phenol facilitating effect on neuro-muscular transmission.

[Kinetics of nerve ending ion currents with nonhomogeneous distribution of ion channel density]

The Hodgkin-Huxley's equations of unmyelinated nerve fiber's electric behaviour has been used to create model of nerve endings electrogenesis. The Eler's integration method has been applied to solve the partial differential equation system which describes the spread of excitation in nerve ending. The effect of nonhomogeneous distribution densities of ionic channels along nerve ending on the ionic currents were simulated. The comparison of computed curves with nerve ending responses recorded by extracellular electrodes was performed. It was shown that the better match between simulated and recorded responses observes during densities' decrease potential-dependent sodium, potassium and calcium-activated potassium channels according to exponent along the nerve ending without changes of calcium channels density. As the result the amplitude and time-course of spread action potential, calcium current and transmitter release along the nerve ending appear to be changed.

[The mechanisms of the recovery of nerve ending functions during the reinnervation of skeletal muscle]

At the experiments on the frog cutaneous-pectoris muscle the nerve terminal functions in course of reinnervation process were investigated by electrophysiological and morphological methods. At the 20-25th days after the nerve crushing the nerve terminal response, which reflect the nerve terminal currents, formed the propagated action potential, were restored and the arising of evoked transmitter secretion occurred. The regenerating terminals are characterized by a low amplitude and altered shape of responses, by small velocity of the excitation propagation and the low level of evoked transmitter release. The 4-aminopyridine effect at the new formed nerve terminals was quite another, than at the intact nerve terminals. This data concluded, that the membrane of regenerating nerve terminal have a low density and a small gradient of sodium channels along the terminal and have not a calcium-activated potassium channels. It was proposed, that different kinds of channels are inserted into the nerve endings membrane at the different periods of the reinnervation process.

The effect of acetylcholine and related drugs on currents at the frog motor nerve terminal

Acetylcholine, acetylthiocholine, carbachol, suberyldicholine, propionylcholine, succinylcholine, methylfurmethide and F 2268 were tested on motor nerve ending currents recorded with an extracellular microelectrode. The isolated and transversally cut cutaneous pectoris muscle of frog Rana ridibunda was used. Only acetylcholine and acetylthiocholine affected the spike waveforms in a concentration-dependent manner. Lower concentrations (1-6 x 10(-4) M) prolonged the inward Na+ current and increased the outward K+ current at the proximal and central parts of the nerve terminal. Most remote parts of the terminal were not affected. At 7 x 10(-4) M and higher, both drugs further prolonged the Na+ current and eliminated the K+ component of the spike. The potentiating effect of acetylcholine and acetylthiocholine on the K+ phase of nerve terminal current disappeared after treatment with tetraethylammonium and 4-aminopyridine. The effect also disappeared when synaptic cholinesterase was inhibited by the anticholinesterases or by treatment with collagenase. Reactivation of cholinesterase by dipyroxime restored the facilitating effect of acetylcholine. Choline and slight acidification to pH 6.8 did not mimic the acetylcholine action on the terminal currents. Facilitation of the K+ current by acetylcholine was not calcium-dependent. The results indicate that lower acetylcholine concentrations inhibit the delayed rectifier only, whereas 7 x 10(-4) M and higher concentrations of acetylcholine depress all outward currents of the terminal.

[The early postdenervation changes in the properties of the postsynaptic membrane of amphibian muscle fiber]

The amplitude-temporal characteristics of miniature end-plate currents (MEPC) were studied in denervated frog cutaneous-pectoral muscles. After the nerve cutting, the initial shortening of MEPC decay time was followed by its prolongation. In 6-8 days after the denervation, the MEPC sensitivity to membrane potential decreased, while the average open time of ACh-activated channels increased 2-fold. The autocorrelation function of ACh-induced current noise revealed two components at some of the synapses. The denervation of the frog cutaneous-pectoral muscle seems to make the postsynaptic membrane to contain two types of ACh receptors (AChR): normal synaptic AChR which gradually disappear, and new synthesized AChRs which open the channels with much longer mean time and a lesser sensitivity to membrane potential changes.

[The single nerve ending of the frog sartorius muscle: its ultrastructural characteristics and mediator secretion]

Electron microscopy and extracellular recordings were used for the investigation of structural and functional peculiarities of single frog sartorius muscle nerve terminal. It has been found that the diameter, length of the synaptic contact and quantity of synaptic vesicles decreased from proximal to distal parts of the nerve terminal. A number of varicosities, separated from each other by schwann cells, have been revealed along the course of the nerve terminal. This indicates the existence of an interrupted synaptic contact. Both the evoked and spontaneous transmitter release decreased from the initial to the end parts of the nerve terminal. The data obtained suggest that there is a correlation between structural heterogeneity and the differences in the transmitter release.

[Periodic changes in evoked mediator secretion at the neuromuscular synapse]

Extra- and intracellular recordings of the frog m. sartorius postsynaptic potentials with low concentration of Ca2+ ions (0.2-0.6 mmol/l) aided to study the dynamics of quanta transmitter release in prolonged motor nerve stimulation (0.5-10.0 imp/s). The statistical analysis showed that with intracellular recording only, time intervals between transmitter release were distributed in an evenly manner. Estimation of the correlation function and the spectral density revealed the periodical fluctuations of evoked transmitter release with periods from a few seconds to 1-2 min. With intracellular recording, oscillations of the transmitter release occurred in about 1/3 of cases whereas with extracellular recording they were always present. Analyses of a single release site activity by means of two extracellular electrodes inserted into a single nerve terminal, showed considerable changes of the cyclic transmitter secretion. The evoked transmitter release in a single release site seems to fluctuate periodically.

[Characteristics of electrical activity in different sections of nerve endings in the frog]

In experiments on frog sartorius neuromuscular preparations, the evoked electrical responses of nerve endings were recorded by extracellular microelectrodes. It was shown that in proximal parts of the nerve ending, the three-phase response (+ - +) with a high amplitude negative phase occurred due to motor nerve stimulation. With movement of the extracellular electrode in distal direction a certain increase of the initial positive phase and a significant decrease of the negative one were observed. At the end of the terminal that response transformed to the monophasic one (+). On local iontophoretic application of tetrodotoxin (TTX) to the recording site two components of the nerve ending response were revealed: TTX-insensitive and TTX-sensitive. A significant decrease of the TTX-sensitive component occurred along the course of the nerve ending. That component was absent from the distal synaptic areas. It is concluded that in frog nerve ending, the action potential propogates with decrement while depolarization of the end parts of the terminal is passive in nature.

[Electrophysiological evaluation of mediator secretion and pool at an individual point of neuromuscular synapse release in the frog]

The time-course of one-quantum end plate potentials during repetitive stimulation of the motor nerve (5-10 imp/sec) was studied under iontophoretic application of Ca2+ to the nerve ending with the aid of high-resistance exocellular microelectrodes. It was found that long-term stimulation was first accompanied by rapid and then by a slower decrease of transmitter release. After 2 to 3 minutes of stimulation the level of secretion reached a plateau. Blockade of acetylcholine synthesis by hemicholinium-3 did not affect the time-course of transmitter secretion drop but led to the disappearance of the plateau state. Electrophysiological study of transmitter exchange in an individual point of release provided the following values: the available transmitter pool 32-47 quanta, the mobilization pool 120-320 quanta, the rate of transmitter synthesis 0.3-1.0 quanta/s.

[Electrophysiological and morphological study of the synaptic organization of the tonic muscle fibers in the frog]

The histochemical and electrophysiological investigation of tonic muscle fibers was performed in the frog m. cruralis. Cholinesterase activity studies in tonic fibers revealed 4 to 14 synapses aggregated in groups. A number of synapses were in positive correlation with the muscle fibers length. In MPSP recording, the distribution of amplitude, periods of growth and of half-time decay had a nonsymmetrical form. At the scatter diagrams of MPSP amplitude recorded by two intracellular microelectrodes, two to six signal groups were revealed in single synapses at different distances from the site of recording. The groups were represented by different number of signals of various duration. Similar groups of signals were also found in evoked PSP, the number of such groups depending upon the intensity of nerve stimulation. The synapses formed by a single motor axon seem to congregate in certain areas of tonic fibers and to be functionally heterogeneous. Intracellular recording revealed only a part of the tonic fibre synapses.

[Analysis of polymodality in the distribution of latent periods and amplitudes of single-quantum synaptic potentials in the frog neuromuscular junction]

Intracellular recording with low concentrations of Ca2+ ions as analysed in the histograms of latency values and amplitude distributions, revealed 4-7 peaks in unitary potentials from the neuromuscular junction in the frog sartorius muscle. The increase of Ca2+ ions concentration led to disappearance of the multimodality. Extracellular recording revealed but a single mode in histograms of distributions. The number of modes in latency values distributions was found to have a positive correlation with values of the binominal parameter n and the number of peaks in histograms of the EP amplitudes. The peaks in histograms of distribution of the latency values and amplitudes of unitary EPs seem to reflect the action of transmitter release in the nerve terminals.

[Multimodality in the amplitude distribution of miniature endplate potentials due to the action of spatially discrete areas of transmitter release]

Miniature end plate potentials (MEPPs) were simultaneously recorded in frog sartorius muscle by two intracellular microelectrodes. Some isolated groups of points (clouds) were found on the diagram of MEPPs scatter. Several peaks each of which was composed of signals from certain clouds (or from several clouds) on the scatter diagram were found on the histograms of MEPPs amplitude distribution. It is assumed that the clouds on the scatter histogram and the peaks on the histogram of MEPPs amplitudes are formed at the cost of secretion of quantum acetylcholine from spatial separate areas of transmitter release. The data obtained do not correspond with the subquantum hypothesis of transmitter release in neuromuscular junctions.

[Mediator secretion in the proximal and distal portions of a nerve ending of the sartorius muscle in the frog]

In experiments on neuromuscular preparations of the frog sartorius muscle the evoked transmitter release was investigated by extracellular recording. A decrease of the evoked transmitter release from proximal to distal parts of nerve terminals was observed related to a decrease in probability of transmitter quantum release. The terminal part of synapse showed smaller sensitivity to an increase in extracellular Ca2+ concentration as compared with the proximal part, smaller facilitation of transmitter release at double and repetitive stimulation, deeper and more quickly developing depression. A conclusion is made that these peculiarities of the transmitter release in the terminal parts of the synapse are due to smaller transmitter reserve and smaller permeability of the presynaptic membrane to Ca2+ ions.

[Mechanism of facilitated transmitter release in the neuromuscular synapse]

In the frog m. sartorius preparation, the intracellular recording at large concentrations of Ca2+ ions revealed two components in the facilitation following a single stimulus applied to the motor nerve. A decrease in the [Ca0] concentration led to appearance of an additional very early component of the facilitation. Extracellular recording always revealed all three components. In rhythmical stimulation, the early facilitation component is summed up according to the power model of facilitation summation with the power index 2 to 5 depending on initial probability of a quantum transmitter release. The first and the second components are summed up according to the linear facilitation model.

[Statistical analysis of calcium accumulation in nerve endings during rhythmic stimulation]

The extracellular recording was used to examine the evoked transmitter release in neuromuscular junctions of the frog sartorius muscle at pair and repetitive stimulations. The repetitive stimulation with low initial level of secretion (the quantum content 0.1-0.5) led to the increased transmitter release. Statistical analysis showed that the probability of the quantum transmitter release in response to each stimulus was increased. The method of conditioned probabilities revealed a decrease in the probability of the quantum release provided the previous stimulus had induced the secretion of the acetylcholine quantum. It is assumed that at the moment of the quantum transmitter release (or promptly after it) a certain quantity of Ca2+ leaves the nerve terminal cytoplasm, thereby significantly reducing the intracellular concentration of calcium.

[Analysis of frequency facilitation in neuromuscular synapses of the sartorius muscle in frogs]

Changes in the quantum content of end plate potentials (EPPs) were studied in experiments on the frog sartorius muscle in the course of a gradual increase in stimulation frequency (frequency facilitation). It was shown that the frequency facilitation coefficient does not depend on the initial quantum content of EPPs but is in good agreement with the initial frequency of miniature EPPs and in bad agreement with their amplitude. The frequency facilitation is accompanied by the growth of the statistical binomial parameters P and n. The increase in P parameters is consequent of interference in P changes during the second component of facilitation after the single impulse. The growth of n is accounted for by the summation of the n increase during the first component of facilitation the time course of which is prolonged on repetitive stimulation.

[Effect of calcium ions on bimodal parameters of mediator liberation in frog sartorius muscle neuromuscular junctions]

In synaptic junctions of the frog sartorius muscle, the dependence of the power index of the EPPs quantum content (m) on external Ca ions concentration [Ca]0 is directly proportional to the initial miniature EPP frequency and can range from 2 to 5. The increase of m due to the action of Ca ions was accompanied by the growth of P and n binomial parameters. The P parameter depended upon the first power of (Ca)0 and n parameter - upon the second power of [Ca]0. Various dynamics of m under the influence of Ca ions in sartorius neuromuscular junctions seems to be due to different increases in power of available transmitter release store.

[Effect of chromium ions on the function of neuromuscular junctions]

It has been shown on neuro-muscular preparations of frog sartorius muscle that chromium ions in the concentrations 1-4 x 10(-6) g/ml strengthen spontaneous and evoked transmitter release. Cr3+ ions in the concentrations above 4 x 10(-6) g/ml decrease the membrane potential of muscle fibres, decrease the quantum content of the end plate potentials. Experiments on a single Ranvier node have shown that Cr3+ ions decrease the amplitude, increase the rate and duration of the action potential of a nerve fibre. It is concluded that chromium ions produce a pronounced effect on synaptic transmission, which differs significantly from the action of manganese, cobalt and nickel ions.

[Mediator liberation at the neuromuscular junctions of rhythmically stimulated frog sartorius muscles]

Synapses of large fibers seem to have a more obvious initial increase followed by a greater decrease in quantum content (m) of EPPs than the small fibers. The total amount of neurotransmitter released during EPPs reproduction was much greater in the large fibres. The calculation of binomial parameters of transmitter release showed that n parameter repeated the dynamics of m, while the P parameter gradually increased. The initial uptake of P and increase of n was more obvious in verve endings of the large muscle fibres. The findings suggest an unequal length of synaptic contact.

[Electrophysiologic, morphometric and histocytochemical characteristics of frog sartorius muscle fibers]

Three groups of muscle fibers (dark, light, and intermediate) were revealed in the fibers of the frog sartorius muscle in examination of the succinate dehydrogenase (SDH) activity. There was revealed a reverse relationship between the diameter of the muscle fibers an the SDH activity in them. The external surface of sartorius muscle is chiefly represented by dark muscle fibers, whereas the internal one--by light ones. Microelectrode study demonstrated that the fibers of the external surface were characterized, in comparison with those of the internal one, by lesser action potentials, prolonged trace negative potential, low quant composition of the end plate potentials, high amplitude and low frequency of the end plate miniature potentials. Analysis of the data obtained demonstrated definite interrelationship between the histochemical profile of the muscle fibers of the frog sartorius muscle and their electrophysiological characteristics.

[The functional heterogeneity of the synapses of the frog sartorius muscle]

Three main types of neuro-muscular junctions were revealed by means of the e.p.p. quantum content and the e.p.p. dynamics during high-frequency stimulation. Heterogeneity of synaptic apparatus seems to be mainly due to different intensities of quantum release. Subctances facilitating neuro-muscular transmission (Ca2+ ions, imidazol) were more effective in myoneural junctions with low e.p.p. quantum content, while depressing chemical agents (acetylcholine, tubocurarine) exerted a more potent effect in junctions with high quantum content.