Probabilistic secretion of quanta from nerve terminals at synaptic sites on muscle cells: non-uniformity, autoinhibition and the binomial hypothesis

A model of the secretion of a quantum at a release site is proposed in which, following the influx of calcium ions, synaptic vesicles are made available for release by the activation of kappa phosphorylation steps with rate alpha. At any time during this process the vesicles may become unavailable for secretion at rate gamma. On completion of the kappa phosphorylation steps the vesicles participate in the formation of a fusion pore with the terminal membrane to give exocytosis at rate delta. Changes in alpha, delta and kappa are shown to produce characteristic changes in the number and timecourse of quantal secretions following a nerve impulse, which are similar to those observed following drug treatments that are thought to act selectively on each of these processes. The number of quanta secreted from nerve terminals that consist of many release sites does not fluctuate much during a low frequency train of impulses: the variance is small compared with the mean level, so secretion follows binomial rather than Poisson statistics. A theory is derived that shows that variations in the probability of secretion amongst these release sites of any particular kind fails to reduce the variance of the total secretion from the terminal; Poisson rather than binomial statistics then still apply. The theory shows that an interaction between release sites is required to reduce this variance and such an effect is provided if secretion at a site inhibits secretion at nearby sites. Simulations show that incorporating this process of autoinhibition into the model reproduces the experimental observations on the effects of calcium ions on the binomial parameters p and n as well as on the relative constancy of p during facilitation and depression of quantal secretion. Methods for estimating the timecourse of changes in the probability of secretion at release sites following an impulse, by using either the time of occurrence of first, second, third or later quantal latencies, are given. These procedures show that current methods for estimating the time-dependent probability changes are inadequate for detecting interaction between release sites, such as autoinhibition, unless this is relatively large. Therefore, estimates from third quantal latencies are used.     

Presynaptic receptors regulating the time course of neurotransmitter release from vertebrate nerve endings

A number of different types of presynaptic receptors was revealed in central and peripheral chemical synapses activated both by main mediator and co-mediators released simultaneously. Physiological significance and mechanisms of functioning of these receptors are not clear yet. They are assumed to provide negative or positive feedback decreasing or increasing the number of neurotransmitter quanta released in response to nerve impulse and thus regulating synaptic transmission. At the same time, there is one more way of secretion process modulation associated with the changes of timing of transmitter release. This mechanism was shown to contribute to the efficiency of synaptic transmission. The role of presynaptic receptors in regulation of the kinetics of quanta release is one of the interesting questions of modern neurophysiology. This paper overviews the results obtained by the authors that demonstrate the contribution of presynaptic receptors of different types into the regulation of temporal parameters of quantal secretion at the vertebrates neuromuscular junction. It was shown that activation of the cholinergic nicotinic receptors leads to a decrease of the amplitude of postsynaptic response not only due to reduction of the quantity of released quanta but also due to increased the level of asynchronous release. On the contrary, the facilitating effect of catecholamines on the neuromuscular synapse is the result of activation of presynaptic β₁-adrenoreceptors which leads to greater synchronization of release process and, consequently, to the increase of the amplitude of the postsynaptic response. Presynaptic purine receptors, involved in the modulation the intensity of secretion, are also capable of alteration of the time course of secretion. Activation of ryanodine receptors results in the increase of the number of quanta released with prolonged latencies leading to appearance of the phase of delayed asynchronous neurotransmitter release.     

The facilitated probability of quantal secretion within an array of calcium channels of an active zone at the amphibian neuromuscular junction

A Monte Carlo analysis has been made of the phenomenon of facilitation, whereby a conditioning impulse leaves nerve terminals in a state of heightened release of quanta by a subsequent test impulse, this state persisting for periods of hundreds of milliseconds. It is shown that a quantitative account of facilitation at the amphibian neuromuscular junction can be given if the exocytosis is triggered by the combined action of a low-affinity calcium-binding molecule at the site of exocytosis and a high-affinity calcium-binding molecule some distance away. The kinetic properties and spatial distribution of these molecules at the amphibian neuromuscular junction are arrived at by considering the appropriate values that the relevant parameters must take to successfully account for the experimentally observed amplitude and time course of decline of F1 and F2 facilitation after a conditioning impulse, as well as the growth of facilitation during short trains of impulses. This model of facilitation correctly predicts the effects on facilitation of exogenous buffers such as BAPTA during short trains of impulses. In addition, it accounts for the relative invariance of the kinetics of quantal release due to test-conditioning sequences of impulses as well as due to change in the extent of calcium influx during an impulse.     

Kainic acid and synaptic transmission in the stellate ganglion of the squid.

Kainate, a conformational analogue of glutamate, blocks synaptic transmission across the giant synapse of the squid. In the presence of blocking doses of kainate, impulses continue to propagate into the nerve terminal, but action potentials are slightly reduced in size and the subsequent hyperpolarization is greatly diminished. Kainate depolarizes the postsynaptic axon. Since the depolarizing action of kainate is confined to the postsynaptic membrane, it appears that kainate can combine with the receptors which are normally activated by the transmitter. This results in a diminished effect of the transmitter released by a presynaptic nerve impulse.     

The effect of calcium ions on the secretion of quanta evoked by an impulse at nerve terminal release sites

A study has been made of the effects of calcium ions on the number of quanta secreted from all the release sites at an amphibian motor nerve terminal recorded with an intracellular microelectrode (m) compared with the number secreted simultaneously from a small number of release sites recorded with an extracellular microelectrode (me). If the endplate potential was made subthreshold by lowering the external calcium concentration ([Ca]o less than or equal to 0.4 mM), it was possible to find small groups of release sites for which me was comparable to m, indicating considerable nonuniformity in the probability of release of a quantum at different groups of release sites (Pe) in a given [Ca]o. Increasing [Ca]o in the range from 0.25 to 0.4 mM increased the probability of release of a quantum at groups of release sites (Pe), independent of the initial value of Pe, and the dependence of Pe on [Ca]o followed a fourth power relationship. A conditioning impulse enhanced the probability of release of a quantum by a subsequent test impulse at release sites, if Pe was less than 1.0 during the conditioning impulse. It is shown that the present observations regarding the dependence of Pe on [Ca]o and on conditioning impulses can be quantitatively predicted from previous observations regarding the dependence of the binomial parameters m, p, and n on [Ca]o and on conditioning impulses determined with intracellular electrodes, if the probability of secretion of a quantum at a release site (Pj) is different for different release sites and Pj is distributed as a beta random variable.     

The topography and dynamics of cholinergic transmission in the myenteric plexus-smooth muscle preparation of the guinea-pig ileum; the effect of ketocyclazocine, a kappa opiate ligand

Output of acetylcholine (ACh), neurogenic electromyogram (NEMG) and contractions of guinea-pig ileum preparations were studied during stimulation by high-frequency trains of impulses. Under control conditions the output of ACh per impulse after 2nd to 4th impulses during train stimulation (30 Hz) was higher by 20-40% than the level of ACh output during the first impulse. In the presence of ketocyclazocine (KTZ, 80 nmol x l-1) the output of ACh evoked by the first impulse was more effectively inhibited than that after impulses 2 to 4 so that the increase was higher (80-170%). NEMG, a direct consequence of the localized action of released transmitter (ACh), was recorded in the longitudinal muscle 4 and 10 mm aborally from the focal stimulation site. The incidence of NEMG responses was higher at the proximal than at the distal site and was proportional to the number of impulses in a train (100 Hz). At the distal site KTZ suppressed the appearance of NEMG responses to single impulses whereas at the proximal site its effect was much less; and so was its effect at either site during train stimulation. It is concluded that in the course of train stimulation, sites of transmission more distant from the stimulation focus were recruited, and consequently the secretion of ACh in succeeding impulses was enhanced. KTZ might preferentially inhibit the propagation of excitation by the very first impulse.     

Effects of heating and cooling on nerve terminal impulses recorded from cold-sensitive receptors in the guinea-pig cornea

An in vitro preparation of the guinea-pig cornea was used to study the effects of changing temperature on nerve terminal impulses recorded extracellularly from cold-sensitive receptors. At a stable holding temperature (31-32.5 degrees C), cold receptors had an ongoing periodic discharge of nerve terminal impulses. This activity decreased or ceased with heating and increased with cooling. Reducing the rate of temperature change reduced the respective effects of heating and cooling on nerve terminal impulse frequency. In addition to changes in the frequency of activity, nerve terminal impulse shape also changed with heating and cooling. At the same ambient temperature, nerve terminal impulses were larger in amplitude and faster in time course during heating than those recorded during cooling. The magnitude of these effects of heating and cooling on nerve terminal impulse shape was reduced if the rate of temperature change was slowed. At 29, 31.5, and 35 degrees C, a train of 50 electrical stimuli delivered to the ciliary nerves at 10-40 Hz produced a progressive increase in the amplitude of successive nerve terminal impulses evoked during the train. Therefore, it is unlikely that the reduction in nerve terminal impulse amplitude observed during cooling is due to the activity-dependent changes in the nerve terminal produced by the concomitant increase in impulse frequency. Instead, the differences in nerve terminal impulse shape observed at the same ambient temperature during heating and cooling may reflect changes in the membrane potential of the nerve terminal associated with thermal transduction.     

Pooling of impulse sequences,with emphasis on applications to neuronal spike data

Summary A mathematical model is presented that is supposed to describe those types of neuronal discharges which show a preponderance of short intervals, as well as one or more preferred intervals of a longer duration. It is assumed that via two channels impulses impinge upon a nerve cell and that each impulse gives rise to a response. The intervals between impulses in one channel are distributed according to an exponential, or an exponential-like, function; those in the other channel are distributed according to a monomodal, or a multimodal, function.The interval distributions and the expectation density (auto-correlation) functions of the model are in particular compared with data on thalamic neuron discharge patterns reported in the literature.The properties of superimposed time series of events would seem to be of a wider interest, stretching beyond the field of theoretical neurophysiology. It is indicated how the theory is of use in the detection of hidden rhythms in records which are composed of a mixture of different signals.     

The role of presynaptic ryanodine receptors in regulation of the kinetics of the acetylcholine quantal release in the mouse neuromuscular junction

To determine the role of presynaptic ryanodine receptors in the regulation of the kinetics of neurotransmitter quantum secretion caused by a nerve impulse in the experiments on the mouse neuromuscular junction, temporal parameters of phase synchronous and asynchronous delayed release of acetylcholine under the conditions of ryanodine receptors block and rhythmic stimulation were examined. The analysis of histograms of synaptic delays of the uni-quantal end-plate currents registered within 50 ms after the onset of the presynaptic action potential showed that ryanodine receptor blockers ryanodine, TMB-8 and dantrolene reduced the intensity of both phase synchronous and delayed asynchronous release of the mediator. The proportion of quanta released synchronously increased at the expense of the reduction of quantum numbers forming the delayed asynchronous release, i.e., there was a redistribution of quanta between synchronous and asynchronous phases of secretion. A block of ryanodine receptors also reduced the fluorescence intensity of the specific fluorescent calcium-sensitive dye Fluo-3 AM, which indicates a decrease in the intracellular calcium ion concentration. Thus, the presynaptic ryanodine receptors control the intracellular content of calcium ions under repetitive stimulation of the nerve endings and contribute to the modulation of the time parameters of the evoked release of the neurotransmitter quanta by increasing the intensity of the delayed asynchronous release of neurotransmitters.     

Excitation properties of the squid axon membrane and model systems with current stimulation. Statistical evaluation and comparison.

The space-clamped squid axon membrane and two versions of the Hodgkin-Huxley model (the original, and a strongly adapting version) are subjected to a first order dynamic analysis. Stable, repetitive firing is induced by phase-locking nerve impulses to sinusoidal currents. The entrained impulses are then pulse position modulated by additional, small amplitude perturbation sinusoidal currents with respect to which the frequencies response of impulse density functions are measured. (Impulse density is defined as the number of impulses per unit time of an ensemble of membranes with each membrane subject to the same stimulus). Two categories of dynamic response are observed: one shows clear indications of a corner frequency, the other has the corner frequency obscured by dynamics associated with first order conductance perturbations in the interspike interval. The axon membrane responds with first order perturbations whereas the unmodified Hodgkin-Huxley model does not. Quantitative dynamic signatures suggest that the relaxation times of axonal recovery excitation variables are twice as long as those of the corresponding model variables. A number of other quantitative differences between axon and models, including the values of threshold stimuli are also observed.     

Mechanism of presynaptic filament stabilization by the bacteriophage T4 UvsY recombination mediator protein

UvsY is the recombination mediator protein (RMP) of bacteriophage T4, which promotes homologous recombination by facilitating presynaptic filament assembly. The results of previous studies suggest that UvsY promotes the assembly of presynaptic filaments in part by stabilizing interactions between T4 UvsX recombinase and single-stranded DNA (ssDNA). To test this hypothesis, we studied the interactions of UvsX and UvsY with a fluorescein-derivatized oligonucleotide. This assay distinguishes between bipartite UvsX- or UvsY-ssDNA and tripartite UvsX-UvsY-ssDNA complex formation via differential fluorescence quenching effects. Salt stabilities of the three complexes were measured at equilibrium in the presence and absence of various nucleotide ligands of the UvsX protein and also under steady-state conditions for UvsX-catalyzed ssDNA-dependent ATP hydrolysis. The results demonstrate that UvsY globally stabilizes UvsX-ssDNA complexes, consistent with an increase in the apparent equilibrium binding affinity, K(ss)omega, of the UvsX-ssDNA interactions. The UvsY-mediated affinity increase is observed at equilibrium in the presence of ADP, ATPgammaS, or in the absence of the nucleotide and also at steady-state in the presence of ATP. Intriguingly, the stabilizing effects of UvsY and ATPgammaS on UvsX-ssDNA interactions are synergistic, indicating nonredundant mechanisms for UvsX-ssDNA complex stabilization by RMP versus nucleoside triphosphate effectors. Experiments with UvsY missense mutants defective in ssDNA binding demonstrate that UvsY-ssDNA interactions are of major importance in stabilizing UvsX-ssDNA complexes, whereas UvsY-UvsX protein-protein interactions provide residual stabilization energy. Together, the data is consistent with a mechanism in which UvsY stabilizes presynaptic filaments by organizing the ssDNA lattice into a structure that is favorable for UvsX-ssDNA interactions.     

Effect of single impulses on cochlear microphone potentials of the guinea-pig cochlea]

The influence of highly intensive single impulses on the cochlea of guinea pig was studied in an acute experiment. Very short impulses of less than or equal to 0.1 ms duration were produced by a sparknoise generator. The cochlear microphonics (CM) to a test stimulus (sinus tone, 3150 Hz) were recorded from the round window and measured prior to, during, and following impulse treatment. During the impulse treatment, the greatest amplitude reduction of CM occurred after the first impulse, while the further impulses caused a decreasing reduction. At first the number of impulses was varied: 1, 3, and 5 impulses were applied at intervals of 15 s each, at an impulse sound level of 164 dB sound pressure level re. 0.002 mubar (SPL). After these impulse treatments, in all cases a continual decrease of CM amplitudes up to a constant end value without recovery was found within a 2-hrs period of observation. The height of the end value depends on the number of impulses applied. Subsequently, at an exposure to 5 impluses the impulse sound level was stepwise reduced (164, 153, 144, 139 and 133 dB SPL). Again, a characteristic decrease of CM amplitudes was observed during the 2-hrs period of observation. The height of the end value is now dependent on the impluse sound level. Impulses of 164, 153 and 144 dB SPL cause a strong decrease of CM while the effect of impulses of 139 and 133 dB SPL is distinctly lower.     

A hypothesis of the code of nerve impulses

There is probably only one information system in living nature — the macromolecular system including DNA, RNA and protein. Its unity for the genetic and nervous activity can be followed in the storage of information (heredity, memory) and in its processing (recombination and selection of both genetic and mental information). According to the hypothesis of the code of nerve impulses, nucleotide triplets of the nucleus, or more likely amino acids of the surface protein of the impulse generating area of a neuron, generate a limited variety of interspike intervals so that each amino acid corresponds to a certain interspike interval and this particular interval initiates by means of a specific neurotransmitter, the synthesis of the same amino acid (or nucleotide triplet) in the postsynaptic neuron. Thus, a series of impulses produces in the postsynaptic neuron a sequence of amino acids in a form of a polypeptide identical to the polypeptide of the presynaptic neuron.     

Electrical Characteristics of Insect Mechanoreceptors

External direct coupled recordings from the neurons of the mechanosensory hairs of insects show nerve impulses and graded slow potentials in response to deformation of the hair. These slow potentials or receptor potentials are negative going, vary directly with the magnitude of the stimulus, and show no overshoot when returning to baseline. The impulses have an initial positive phase which varies in size directly with the amplitude of the receptor potential. The receptor potential is related to the generator potential for the impulse in that it must attain some critical level before impulses are produced, and the frequency of impulses varies directly with amplitude of the receptor potential. The receptor potential does not return to the baseline after each impulse. In some receptors static deformation of the hair will maintain the receptor potential. It appears likely that both the receptor potential and the variation in size of the impulses are caused by a change in conductance of the cell membrane at the receptor site, and that the receptor potential originates at a site which is not invaded by the propagated impulses.     

Selective stabilization of muscle innervation during development: A mathematical model

The biochemical model presented concerns a critical step of the development of skeletal muscle innervation. After invasion of the muscle by exploratory motor axons, several nerve terminals converge from different motoneurons onto each muscle fibre at a single endplate. During the folloing weeks the redundant innervation disappears: a single nerve ending per muscle fibre becomes stabilized. The model is based on the assumption that the numbers of motoneurons and of muscle fibres remain constant during this evolution and that the selective stabilization of the adult connectivity results from the competition of the active nerve terminals for a postsynaptic retrograde factor μ. At the peak of the multiple innervation, the synthesis of μ by the muscle fiber stops, possibly as a consequence of muscle electrical and/or mechanical activity. The stock of μ becomes limited; a retrograde trans-synaptic diffusion of μ from the muscle to the nerve endings takes place. Within each nerve ending, μ enters into a chemical autocatalytic reaction which results in the production of a presynaptic stabilization factor s. The nerve impulses reaching the nerve terminal initiate this reaction. Any given nerve terminal become stabilized when the concentration of s reaches a threshold value. The mathematical analysis of the model shows that there exists a unique solution which is physically acceptable. Its application and computer simulation predict that only one nerve terminal becomes stabilized per muscle fibre. The model accounts for the experimental observations that the reduction in size of the motor units is not necessarily accompanied by a reduction in the variability of their size. The model also accounts for the acceleration or delay in regression which follows modifications of the chronic activity of the nerve endings and for the variability of the pattern of innervation observed in isogenic organisms. Plausible biochemical hypotheses concerning the factors engaged in the “selective stabilization” of the nerve-endings are discussed.     

Probabilistic Firing of Neurons Considered as a First Passage Problem

A formalized neuron receiving unitary excitatory impulses at random is considered. Each impulse provokes an effect of equal magnitude and of a duration not constant for each impulse, but which varies according to an exponential distribution. The effects sum until a threshold is reached when a response occurs. The distribution of intervals between successive responses is computed and compared with those obtained from a model in which the effects decay exponentially with time. Upon introducing inhibitory impulses also, the theory is applied to data on discharge characteristics of driven and spontaneously active thalamic neurons reported in the literature.     

A model of neural network for spatiotemporal pattern recognition

A model of neural network to recognize spatiotemporal patterns is presented. The network consists of two kinds of neural cells: P-cells and B-cells. A P-cell generates an impulse responding to more than one impulse and embodies two special functions: short term storage (STS) and heterosynaptic facilitation (HSF). A B-cell generates several impulses with high frequency as soon as it receives an impulse. In recognizing process, an impulse generated by a P-cell represents a recognition of stimulus pattern, and triggers the generation of impulses of a B-cell. Inhibitory impulses with high frequency generated by a B-cell reset the activities of all P-cells in the network.Two examples of spatiotemporal pattern recognition are presented. They are achieved by giving different values to the parameters of the network. In one example, the network recognizes both directional and non-directional patterns. The selectivities to directional and non-directional patterns are realized by only adjusting excitatory synaptic weights of P-cells. In the other example, the network recognizes time series of spatial patterns, where the lengths of the series are not necessarily the same and the transitional speeds of spatial patterns are not always the same. In both examples, the HSF signal controls the total activity of the network, which contributes to exact recognition and error recovery. In the latter example, it plays a role to trigger and execute the recognizing process. Finally, we discuss the correspondence between the model and physiological findings.     

The organization of pulse trains in the dorsal hyperstriatum of hens in relation to the afferent input in ontogeny]

Analysis of background multicellular activity of neuronal populations in the dorsal hyperstriatum of chick embryos and baby-chicks, incubated either in darkness or under periodic illumination, revealed an impulse volley structure which is characterized by onset of discharges which follow each other in a form of small series at close intervals. These series originally may be observed in the background activity of the left dorsal hyperstriatum in "illuminated" chick embryos at the 19th day of incubation, and only at the 1st day after hatching--in the right dorsal hyperstriatum of embryos, "illuminated" although they are present in both left and right dorsal hyperstriatum in embryos incubated in darkness, the difference being presumably due to asymmetric input of visual afferentation to this structure. Series of impulse volleys with repeating intervals are considered as a reflection of the activity of local microsystems of neurons which exhibit cyclic structure and which are capable to maintain stable impulse activity in its intrinsic system of connections, which is one of the elements of a mechanism of synaptic stabilization and formation of organized neuronal complexes.     

A study of the change in conductivity of the neuronal membrane caused by a generator potential using a method of computer modeling

Injection of cAMP induces in snail neurons generator potential, which is related to an increase of sodium and decrease of potassium permeability of the neuron outer membrane. A model is proposed which takes into account cAMP diffusion inside the neuron from the injection place and interaction of these molecules with the intercellular system controlling permeability of the outer membrane. Resulting impulse generation induces calcium ions current through the outer membrane. The model also considers calcium diffusion toward cAMP and its effect on the rate of the enzyme work destroying cAMP. Agreement between the calculations of ionic current I(t) and the experiment permits determination of the model parameters and calculation of the observed change of time distribution of nerve impulses when calcium input is significant.     

Involvement of sphingosine-1-phosphate in glutamate secretion in hippocampal neurons

Neuronal activity greatly influences the formation and stabilization of synapses. Although receptors for sphingosine-1-phosphate (S1P), a lipid mediator regulating diverse cellular processes, are abundant in the central nervous system, neuron-specific functions of S1P remain largely undefined. Here, we report two novel actions of S1P using primary hippocampal neurons as a model system: (i) as a secretagogue where S1P triggers glutamate secretion and (ii) as an enhancer where S1P potentiates depolarization-evoked glutamate secretion. Sphingosine kinase 1 (SK1), a key enzyme for S1P production, was enriched in functional puncta of hippocampal neurons. Silencing SK1 expression by small interfering RNA as well as SK1 inhibition by dimethylsphingosine resulted in a strong inhibition of depolarization-evoked glutamate secretion. Fluorescence recovery after photobleaching analysis showed translocation of SK1 from cytosol to membranes at the puncta during depolarization, which resulted in subsequent accumulation of S1P within cells. Fluorescent resonance energy transfer analysis demonstrated that the S1P(1) receptor at the puncta was activated during depolarization and that depolarization-induced S1P(1) receptor activation was inhibited in SK1-knock-down cells. Importantly, exogenously added S1P at a nanomolar concentration by itself elicited glutamate secretion from hippocampal cells even when the Na(+)-channel was blocked by tetrodotoxin, suggesting that S1P acts on presynaptic membranes. Furthermore, exogenous S1P at a picomolar level potentiated depolarization-evoked secretion in the neurons. These findings indicate that S1P, through its autocrine action, facilitates glutamate secretion in hippocampal neurons both by secretagogue and enhancer actions and may be involved in mechanisms underlying regulation of synaptic transmission.