Polymodal distribution of synaptic delays in the frog neuromuscular junction

Synaptic delay of single-quantum response with low mean quantal size (0.05–1) was measured during experiments on preparations of frog neuromuscular junctions using extracellular focal recording of presynaptic action potentials and endplate currents. It was found that distribution of these synaptic delays is of a polymodal nature and mean intermodal interval equaled 0.22±0.01 msec over 13 experiments. An increase in quantal size produced only a redistribution of mode weighting, while mean modal interval remained unchanged. A reduction in temperature induced an increase in the modal interval with the temperature coefficient Q₁₀=2.42±0.14 (n=15). The explanation is suggested that the process of quantal transmitter release is determined by interaction between the calcium-dependent mechanism for raising the likelihood of release on the one hand and the rhythmic operation of the system producing transmitter release on the other. The latter stage in the process depends on temperature, not intracellular Ca²⁺ concentration. The polymodal distribution of synaptic delay reflects the rhythmic operation of the transmitter release zone.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 18, No. 6, pp. 748–756, November–December, 1986.     

The Quantal Nature of Synaptic Transmission at the Neuromuscular Junction of a Spider

Spontaneous and evoked release of transmitter at neuromuscular junctions in three different leg muscles of a tarantula (Dugesiella hentzi) was investigated. In most cases the spontaneous miniature potentials were released independently, although bursts from single synaptic junctions occasionally occurred. In contrast to recent findings in other arthropod muscles, focal extracellular recording from junctional areas revealed that the evoked release of transmitter quanta followed Poisson's theorem at low quantal content synaptic junctions in arachnid muscles.     

Numerical characteristics of synaptic vesicular apparatus structure in the dorsal horn of the cat spinal cord

Statistical parameters characterizing the structure of the vesicular apparatus of presynaptic endings (PE) were determined from the findings of ultrastructural morphometric analysis of 135 synapses in the dorsal horn of the cat spinal cord. Quantitative estimates of vesicles in the PE (averaging about 470) were obtained, based on stereometric principles. The bimodal pattern of distribution of distances from the center of each vesicle to the nearest portion of the active zone was demonstrated — viewed as the structural correlate of the two-pool model of transmitter storage. The possibility of classifying PE according to the sign of vesicle spatial distribution is discussed as well as the relationship between this distribution and parameters of transmitter mobilization and synaptic release.Dnepropetrovsk State University. A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 21, No. 5, pp. 597–605, September–October, 1989.     

Analysis of factors restricting the use of binomial statistics for studying transmitter release in neuromuscular junctions

Binomial parameters of transmitter secretion were calculated on the basis of analysis of synaptic potentials in the frog sartorius muscle. Negative values of the parameter p were found in some synapses. This happened most often in low Ca²⁺ concentrations and with low amplitude of miniature end-plate potentials. The results were interpreted in terms of a model assuming spatial heterogeneity of probability of transmitter quantum release at different release points. Simulation of transmitter secretion by computer showed that the appearance of negative values of the parameter p and incorrect estimates of n experimentally are connected with the form of distribution of probability of transmitter quantum release in the synapse and with the amplitude of miniature potentials.S. V. Kurashov Kazan' Medical Institute, Ministry of Health of the RSFSR. Translated from Neirofiziologiya, Vol. 16, No. 2, pp. 182–189, March–April, 1984.     

Effects of ethimizol on frog neuromuscular junction

The effects were studied of ethimizol, a substance activating memory processes, on features of synaptic transmission during experiments on frog cutaneous pectoris muscle. It was found that the presynaptic action of ethimizol consists of raising the frequency of miniature potentials, when used at a concentration of 0.5–10 mM, and modulating quantal content of synaptic transmission due to changes in binomial quantal release parameters p and n when 0.5–2 mM ethimizol was used. This substance facilitated transmission at synapses with a low initial level of transmitter release. This substance facilitated transmission at synapses with a low initial level of transmitter release. Ethimizol was also found to have a postsynaptic action, consisting of reducing amplitude at a concentration of 5–10 mM and prolonging synaptic currents and potentials when concentrations of 0.5–10 mM were used. The latter effect produced a considerable increase in the time integral of endplate potentials. The postsynaptic action of ethimizol is perhaps seen in its effects on features of postsynaptic ionic channels. The effects of ethimizol are discussed with a view to how it may act within the central nervous system as a nonspecific modulator.A. A. Zhdanov Leningrad State University. Translated from Neirofiziologiya, Vol. 17, No. 6, pp. 757–763, November–December, 1985.     

Cholinergic modulation of the synaptic transmission in the frog spinal cord

It was found during experiments on isolated frog spinal cord involving extracellular recording from the dorsal roots (sucrose bridging) and intracellular recording from motoneurons by microelectrodes that 10 mM of the M-cholinomimetic arecoline produces motoneuronal depolarization which is matched by depolarizing electronic ventral root potentials and a rise in motoneuronal input resistance. Arecoline changes synaptic transmission by increasing the amplitude of postsynaptic potentials during intracellular recording and that of motoneuronal reflex discharges in the ventral roots but reduces the duration of dorsal root potentials. In the presence of arecoline, L-glutamate-induced motoneuronal response increases. Facilitation of synaptic transmission produced by arecoline in the spinal cord is bound up with cholinergic M₂- activation, since it is suppressed by atropine but not by low concentrations of pirenzipine; it is also coupled with a reduction in adenylcyclase activity. When motoneuronal postsynaptic response has been suppressed, as in the case of surplus calcium or theophylline, arecoline produces an inhibitory effect on the amplitude of motoneuronal monosynaptic reflex discharges which is suppressed by pirenzipine at a concentration of 1×10^–7 M. This would indicate the presence at the primary afferent terminals of presynaptic cholinergic M₁ receptors which mediate its inhibition of impulses of transmitter release. This effect is independent of changes in cyclic nucleotide concentration.A. M. Gorkii Medical Institute, Donetsk. Translated from Neirofiziologiya, Vol. 19, No. 3, pp. 399–405, May–June, 1987.     

Regulation of tetanus toxin-inhibited transmitter secretion in the neuromuscular junction in a calcium-free medium

The effect of Ca2+ removal from the external medium on regulation of the release of the synaptic transmitter in the tetanus toxin (TT)-inhibited neuromuscular junctions was studied on a rat phrenicodiaphragmal preparation with the aid of the conventional microelectrode technique of recording synaptic activity. As the external concentration of calcium was decreased from 2 to 0 mM, the frequency of miniature end plate potentials remained unchanged in the preparations isolated 3 to 3.5 h after intramuscular injection of TT (10(5) MLD for mouse). TT considerably reduced activation of the transmitter release, caused in intact synapses by ouabain (0.1 mM) and repetitive stimulation of the diaphragmatic nerve (50 imp/s). The data obtained indicate that in the TT-inhibited motor nerve terminals, the level of the transmitter release does not depend on the external concentration of calcium and that TT damages some of the intracellular sources of calcium.     

On mechanisms of depression in pathways of presynaptic inhibition

We investigated inhibition of the N₁-component of the spinal cord dorsal potential (CDP) evoked by experimental stimulation of the n. peroneus in spinal cats. Stimulation was carried out following two conditioning stimuli administered at different time intervals to the same or different cutaneous nerves. The interval between the last conditioning stimulus and the experimental one remained constant (20 msec). It is demonstrated that there is no dependence between weakening of inhibitory action of the second conditioning stimulus and inhibition of the dorsal horn interneurons excited by it that generate the N₁-component of the CDP. It is hypothesized that mechanisms which act on the principle of negative feedback are present in the vincinity of the synaptic junctions of cutaneous afferent fibers with neurons of the substantia gelationsa, and that these mechanisms restrict the development of presynaptic inhibition during inflow of a series of afferent impulses into the cord.Dnepropetrovsk State University. Translated from Neirofiziologia, Vol. 1, No. 3, pp. 253–261, November–December, 1969.     

Reticulospinal and propriospinal monosynaptic influences on motoneurons in frogs

Monosynaptic effects evoked by electrical stimulation of suprasegmental structures and the ventral and lateral columns were recorded intracellularly from motoneurons of the lumbar and cervical enlargements after isolation of the spinal cord and medulla in frogs. Reticulospinal fibers arising from cells of the medial reticular formation of the medulla and running in the ventro-lateral columns evoke monosynaptic excitation of cervical and lumbar motoneurons. The reticulo-motoneuronal E PSPs do not exceed 2–3 mV in amplitude and do not reach the threshold for action potential generation. Division of the spinal cord and interaction between all synaptic inputs tested in chronic experiments showed that monosynaptic E PSPs evoked by direct stimulation of the ventral and lateral columns are due to activation of the descending system of propriospinal fibers. By transmembrane polarization experiments the equilibrium potentials of the reticulo-motoneuronal and propriospinal monosynaptic E PSPs could be determined.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Leningrad. Translated from Neirofiziologiya, Vol. 5, No. 2, pp. 164–173, March–April, 1973.     

Synaptic potentials of motoneurons of the masseter muscle

Postsynaptic potentials of 93 motoneurons of the masseter muscle evoked by stimulation of different branches of the trigeminal nerve were studied. Stimulation of the most excitable afferent fibers of the motor nerve of the masseter muscle evoked monosynaptic EPSPs with a latent period of 1.2–2.0 msec, changing into action potentials when the strength of stimulation was increased. A further increase in the strength of stimulation produced an antidromic action potential in the motoneurons with a latent period of 0.9 msec. In some motoneurons polysynaptic EPSPs and action potentials developed following stimulation of the motor nerve to the masseter muscle. The ascending phase of synaptic and antidromic action potentials was subdivided into IS and SD components, while the descending phase ended with definite depolarization and hyperpolarization after-potentials. Stimulation of cutaneous branches of the trigeminal nerve, and also of the motor nerve of the antagonist muscle (digastric) evoked IPSPs with a latent period of 2.7–3.5 msec in motoneurons of the masseter muscle. These results indicate the existence of functional connections between motoneurons of the masseter muscle and its proprioceptive afferent fibers, and also with proprioceptive afferent fibers of the antagonist muscle and cutaneous afferent fibers.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 1, No. 3, pp. 262–268, November–December, 1969.     

Mathematical theory of chemical synaptic transmission

Mathematical theory of chemical synaptic transmission is suggested in which the modes of operation of chemical synapses are given as consequencies of some fundamental theoretical principles presented in the form of systems of quantum and macroscopic postulates. These postulates establish transmitter transfer rules between 3 component parts — cytoplasmic, vesicular and external pools of neurotransmitter. The main features of the transfers are determined by special properties of the dividing membranes (synaptic and vesicle) which show high selectivity towards the direction of the transmitter quantum transfer. The formulation of a previously unknown effect of transmitter quantum transfer from the vesicular pool into the cytoplasmic one is introduced: it is postulated that each arriving presynaptic impulse not only releases a constant fraction of the current contents of the cytoplasmic pool into the synaptic cleft (external pool), but also realizes practically simultaneous transmitter transfer from the vesicular pool into the cytoplasmic one. Zone structure of the vesicular pool is postulated. In accordance with basic equations of the theory a nonlinear control system (dynamic synaptic modulator — DYSYM) of transmitter release from the terminal is constructed.Depending on the parameters relation two types of synapses are classified — those with rapid and slow demobilization. Analytical dependencies of the transmitter pools sizes on the stimulation frequency are introduced. By fitting the frequency dependencies to the empirical data model parameters are determined corresponding to a set of experimentally studied synaptic junctions. Different aspects of the chemical synapse behaviour under the influence of presynaptic stimulation are simulated.     

Location and functioning of transmitter release sites in frog neuromuscular junction

End-plate potentials (EPP) and miniature EPP (MEPP) were recorded in a single neuromuscular synapse of the frog sartorius muscle by means of two microelectrodes with a resistance of 0.5–2.0 M. Groups of signals (fields), reflecting transmitter secretion in spatially distinct release sites were identified by extracellular recording on MEPP amplitude scatter diagrams. Release sites in the nerve ending were found to be unevenly distributed, to be grouped in certain areas, and to differ in their probability of secretion of a quantum of transmitter. Comparison of fields on MEPP and uniquantal EPP amplitude scatter diagrams in solution with low Ca⁺⁺ concentration (0.2–0.4 mM) showed that ability to induce evoked and spontaneous transmitter release at the release site differs, and that sometimes a release site does not participate in evoked secretion. The results of simultaneous recording of synaptic potentials using extra- and intracellular electrodes indicate that transmitter secretion in spatially distinct groups of release sites leads to the appearance of polymodality in the distribution of amplitudes of intracellularly recorded MEPP and uniquantal EPP.S. V. Kurashov Medical Institute, Ministry of Health of the RSFSR, Kazan'. Translated from Neirofiziologiya, Vol. 17, No. 2, pp. 152–160, March–April, 1985.     

Role of the adenylate cyclase system in cholinergic modulation of synaptic transmission in the hippocampus

Involvement of the adenylate cyclase system in cholinergic modulation of synaptic transmission was investigated in area CA1 in rat hippocampal slices. Microiontophoretic application of acetylcholine as well as addition of carbachol to the superfusate or of tolbutamide (a cAMP-dependent protein kinase inhibitor) depressed transmission at synapses formed by Schaffer collaterals and commissural fibers with dendrites of pyramidal cells belonging to hippocampal area CA1. Both numbers of free quanta of neurotransmitter and the likelihood of transmitter release decreased following carbachol action. Atropine suppressed the inhibitory action of carbachol on synaptic transmission. Dibutyryl cAMP and forskolin increased the amplitude of synaptic potentials and suppressed, either partially or in full, the inhibitory effects of cholinomimetics on synaptic potentials. It was concluded that cholinomimetics and activators of the adenylate cyclase system exert opposing effects on neurotransmission at synapses formed between Schaffer collaterals/commissural fibers and dendrites of pyramidal neurons belonging to hippocampal area CA1.Institute of Biophysics, Academy of Sciences of the USSR, Pushchino. Translated from Neirofiziologiya, Vol. 21, No. 4, pp. 435–442, July–August, 1989.     

Effects of tractotomy on reflex activity in the lumbar segment of the rat spinal cord previously disrupted by severing the sciatic nerve

Evoked electrical discharges in the spinal cord roots and dorsal surface ipsilateral to the previously severed sciatic nerve (as well as on the contralateral side) were investigated in rats one, three, seven, and 14 days after tractotomy. Monosynaptic reflex discharges in the ventral roots were found to return to 20–40% of the level of this parameter as measured on the contralateral side within seven and 14 days after tractotomy. Mean amplitude of antidromic dorsal root discharges, afferent peak, and the N₁ component of potential(s) at the dorsal surface ipsilateral to the severed nerve barely altered, remaining significantly lower than on the contralateral side. Mechanisms are suggested for the increase in monosynaptic reflex ventral root discharges ipsilateral to the severed nerve following tractotomy — thought to be largely due to raised sensitivity to transmitter at the motoneuronal membrane resulting from degeneration of synapses of descending pathways.Medical Institute of the Ukrainian Ministry of Health, Dnepropetrovsk. Translated from Neirofiziologiya, Vol. 21, No. 3, pp. 366–371, May–June, 1989.     

Action of morphine on guinea-pig myenteric plexus and mouse vas deferens studied by intracellular recording.

Morphine reduces the output of transmitter from the myenteric plexus-longitudinal muscle preparation of the guinea-pig ileum and from the mouse vas deferens. Intracellular recordings were made from ganglion cells of the myenteric plexus and smooth muscle cells of the vas deferens. Synaptic transmission within the myenteric plexus was blocked by hexamethonium. Morphine did not change the properties of the ganglion cells, nor did it affect synaptic potentials. 5-Hydroxytryptamine inhibited acetylcholine release at intraganglionic synapses by an action which was unaffected by morphine. In the vas deferens, excitatory junction potentials were elicited by stimulation of postganglionic adrenergic nerve fibres. The junction potentials were depressed by morphine and levorphanol but not by dextrorphan. This depression was reversed by naloxone. The results indicate that morphine acts directly to reduce transmitter release at the neuro-effector junctions in the myenteric plexus-longitudinal muscle preparation and in the vas deferens in these species.     

Contribution of homosynaptic depression to stabilization of spinal cord potentials

The following were measured during experiments on spinal anesthetized cats: firstly, variations in the amplitude of dorsal root potentials produced by applying single or regular stimuli in 120–150 trails to hindlimb cutaneous nerves and dorsal surface of the spinal cord and secondly, numbers of extracellular discharges in neurons involved in generating these potentials. A reduction in the variation between these parameters was found when applying stimulation at the rate of 0.1–5.0 Hz. The authors attribute the effect observed to the influence of homosynaptic depression.Institute of Biology, State University Commemorating 300th Anniversary of Russian-Ukrainian Reunion, Dnepropetrovsk. Translated from Neirofiziologiya, Vol. 20, No. 2, pp. 180–185, March–April, 1988.     

Effect of substances blocking calcium channels (verapamil,D-600, manganese ions) on transmitter release from motor nerve endings in frog muscle

Experiments on isolated frog nerve-muscle preparations showed that manganese ions (0.4–5.0 mM) inhibit evoked transmitter release by reducing the quantum composition of the end-plate potentials, and they intensify spontaneous transmitter release to a certain extent by increasing the frequency of miniature potentials. Verapamil (1 · 10^–6–5·10^–5 g/ml) and D-600 (2.5·10^–5 g/ml), by contrast with manganese ions, do not inhibit evoked release, but also intensify spontaneous release of the transmitter. All the agents tested prevent the potentiating effect of imidazole (3 mM). During repetitive stimulation, verapamil disturbs action potential generation in the motor nerve. Manganese ions had no such action. It is concluded that between the calcium channels of motor nerve endings and the calcium channels of heart muscle or the neuron soma there are molecular differences, expressed as sensitivity to the blocking action of verapamil and D-600.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 9, No. 4, pp. 415–422, July–August, 1977.     

Action of thiamine on neuromuscular transmission in the frog

The action of thiamine on neuromuscular transmission in the frog sartorius muscle was investigated. It was found that thiamine at a concentration of 1×10^–14 to 1×10^–4 M increases transmitter secretion at the nerve endings. This is demonstrated by the increased frequency, amplitude, and quantal content of miniature endplate potentials, and is due to the enhanced likelihood of transmitter release. The role of thiamine in regulating synaptic transmission and the mechanism of its interaction with thiamine-sensitive receptors are examined.A. V. Palladin Institute of Biochemistry, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 17, No. 6, pp. 794–800, November–December, 1985.     

Synaptic transmission in the sixth ganglion of the cockroach: action of 4-aminopyridine.

1. Study was made of the action of 4-aminopyridine (5 X 10(-5) M) on synaptic transmission in the last abdominal ganglion of Periplaneta americana. The 'oil-gap' technique was used to record postsynaptic events in a single giant axon. 2. 4-AP quickly increased the 'background' of postsynaptic activity, which consisted of 'spontaneous' unitary EPSPs and IPSPs. Postsynaptic spikes were also propagated. 3. Both evoked EPSPs (stimulation of cercal nerve XI) and evoked IPSPs (stimulation of cercal nerve X) were greatly increased in amplitude although their duration (half-time) was unaltered. 4. 4-AP triggered presynaptic action potentials in the cercal nerves (recorded with external electrodes). These 'antidromic' potentials appeared singly or sometimes repetitively, especially after electrical stimulation of the cercal nerves. They were often in monosynaptic correlation with unitary EPSPs. 5. Neither the resting potential nor the postsynaptic membrane resistance was modified. 6. There were no changes in the equilibrium potentials of the ions involved in postsynaptic events. 7. The results may be essentially explained by an increase in transmitter release after 4-AP treatment, which may be partly the result of a rise in presynaptic terminal excitability, and partly the result of a lengthening of the presynaptic action potentials.     

Paired pulse facilitation of summated intracellular synaptic potentials in single retinotectal fibers in the frog

Facilitation of the second of two consecutive test EEG quanta (the summated monosynaptic potentials of the synapses of one axon arborizing in layer F of the frog tectum) was investigated in the normal and under conditions of raised extracellular Ca²⁺ and Mg²⁺ concentration. Intensification of paired-pulse facilitation (×1.4–2.4) was observed at the shortest interstimulus intervals (of 2.5–5 msec). The distribution of maximum levels for facilitation of EEG quanta was bimodal at levels 1.95 and 1.65, on the basis of which two groups were identified, one potentiating EEG quanta more than the other. The time course of paired-pulse facilitation of both groups of EEG quanta can be broken down into two exponential components with time constants of 5–6, 140–150 and 6–8, 60–70 msec respectively. Bimodal distribution of maximum paired-pulse levels in the normal, together with findings from experiments involving raised Ca²⁺ and Mg²⁺ concentrations would indicate that facilitation of frog retinotectal synapses is dependent on the quantal release of transmitter; it may thus be postulated that this release reaches near-saturation point in the normal. It is suggested that two types of axonal terminal arborizations whose synapses differ in the quantal content of transmitter release are present in layer F of the frog tectum. These axonal arbors could well originate from different class 3 and 5 retinal detectors.Z. Yanushkevichyus Institute of Physiology and Pathology of the Cardiovascular System of the Kaunas Medical Institute. Translated from Neirofiziologiya, Vol. 18, No. 1, pp. 45–55, January–February, 1986.