Estimation of the number of synaptic vesicles in asymmetric synapses between hippocampal neurons

Within the framework of the quantum hypothesis of synaptic transmission, the amount of a neurotransmitter released in a unitary event of calcium-dependent exocytosis corresponds to the content of a synaptic vesicle (SV). The number of these organelles in the presynaptic terminal is an important index characterizing the functional state of the given synapse. The technique of estimation of the dimension of the total SV pool, which is based on mathematical modeling and is realized in a computer experiment, is described. This technique allows one to interpret quantitative estimations obtained in the course of the analysis of images of random ultrathin sections of presynaptic terminals in the terms of 3D space. The capabilities of this technique are illustrated using an example of estimation of the size of the total SV pool in asymmetric synapses between neurons of the radial layer of the murine hippocampal CA1 area. Neirofiziologiya/Neurophysiology, Vol. 38, No. 3, pp. 219–223, May–June, 2006.     

The Skok legacy and beyond: Molecular mechanisms of slow synaptic excitation in sympathetic ganglia

Vladimir Skok and his colleagues did much of the pioneering work on fast excitatory synaptic transmission in sympathetic ganglia and on nicotinic acetylcholine receptors that mediate fast transmission. I and my colleagues (including Alex Selyanko, one of Vladimir’s protégés) have studied the additional process of slow synaptic excitation that is mediated by the action of acetylcholine on muscarinic receptors. This results primarily from the closure of “M-channels,” a subset of voltage-gated potassium channels composed of Kv7.2 and Kv7.3 channel subunits. These channels require membrane phosphatidylinositol-4,5-bisphosphate (PIP₂) for their opening, and their closure by muscarinic receptor activation is now thought to result from the reduction in PIP₂ levels that follows receptor-induced PIP₂ hydrolysis. The dynamics of these two forms of synaptic excitation are compared. Neirofiziologiya/Neurophysiology, Vol. 39, Nos. 4/5, pp. 284–289, July–October, 2007.     

Electrophysiological Effects of Three Groups of Glutamate Metabotropic Receptors in Rat Piriform Cortex

1. The effects of three metabotropic glutamate receptor (mGluR) agonists were tested in two pathways of rat piriform cortex. The group I, II and III mGluR agonists used were RS-3,5-dihydroxyphenenylglycine (DHPG) (10–100 μM), (2S,1′S,2′S)-2-Carboxycyclopropyl (L-CCG) (20–100 μM) and L(+)-2-amino-4-phosphonobutyric acid (L-AP4) (5–500 μM), respectively.2. The effects of the three groups of agonists on synaptic transmission in the two piriform cortex pathways also were examined. All three agonists reduced the amplitude of the monosynaptic EPSPs generated by stimulation of the lateral olfactory tract (LOT) or of the association fiber pathway (ASSN). This was always accompanied by an increase in paired pulse facilitation.3. Group I and II mGluR agonists had similar synaptic effects on the two pathways, while the group III mGluR agonist suppressed the LOT pathway more than the association pathway.4. The group II and III mGluR agonists had no effect on passive membrane properties of pyramidal neurons. Group I agonists depolarized the pyramidal neuron membrane potential, and enhanced both membrane resistance and noise.5. Our data suggest that all three types of mGluRs modulate synaptic transmission in both of these pathways in piriform cortex. Only group I agonists alter post-synaptic membrane properties, while all three types of receptor regulate synaptic transmission. Groups I and II are equally potent in the LOT and association fiber pathways, while group III receptors are more potent in the LOT than the association fiber pathways.     

Pre- and Post-Synaptically Induced Short-Term Plasticity of GABA-ergic Synaptic Transmission

In this communication, the published data and some results of studies of the authors dealing with the problem of short-term plasticity of GABA-ergic synaptic transmission in cerebral structures are described. Neirofiziologiya/Neurophysiology, Vol. 37, No. 3, pp. 294–307, May–June, 2005.     

Effects of mibefradil on synaptic transmission in the hippocampus and on voltage-dependent currents in isolated hippocampal and thalamic neurons of the rat

The effects of a novel anti-hypertensive drug, mibefradil, on voltage-dependent currents in isolated thalamic and hippocampal neurons, as well as on synaptic transmission in the hippocampus have been studied. Mibefradil exerted a potent inhibitory action on low-threshold calcium currents in thalamic neurons (IC₅₀=160 nM). In higher concentrations (1–20 μM), this drug blocked not only low-threshold calcium current but also voltage-dependent sodium and delayed potassium currents in pyramidal hippocampal neurons. The amplitude of population action potentials in hippocampal slices decreased by 55% in the presence of 20μM mibefradil. All of the effects of mibefradil were almost completely reversible. In our experiments, the sensitivity of low-threshold calcium channels in thalamic neurons to mibefradil was higher than that observed on other objects. The ability of mibefradil to block not only calcium currents but also other types of voltage-dependent ion conductances in hippocampal neurons may be considered an essential factor that determines the specificity of the pharmacological profile of this drug.     

Role for calcium in heat shock‐mediated synaptic thermoprotection in Drosophila larvae

Chemical synaptic transmission is the mechanism for fast, excitation‐coupled information transfer between neurons. Previous work in larval Drosophila has shown that transmission at synaptic boutons is protected by heat shock exposure from subsequent thermal stress through pre‐ and postsynaptic modifications. This protective effect has been, at least partially, ascribed to an up‐regulation in the inducible heat shock protein, hsp70. Effects of hsp70 are correlated with changes to intracellular calcium handling, and the dynamics of intracellular calcium regulate synaptic transmission. Consistent with such a relationship, synaptic plasticity increases at locust neuromuscular junctions following heat shock, suggesting an effect of heat shock on residual presynaptic calcium. Intracellular recording from single abdominal muscle fibers of Drosophila larvae showed that prior heat shock imparts thermoprotection by increasing the upper temperature limit for synaptic transmission. Heat shock exposure enhances short‐term synaptic plasticity and increases its thermosensitivity. Increasing extracellular calcium levels eliminates the physiological differences between control and heat shock preparations; excess calcium itself induces thermoprotection at elevated concentrations. These data support the hypothesis that stress‐induced neuroprotection at the nerve terminal acts, at least partially, through an alteration to the physiological effects of residual presynaptic calcium. © 2003 Wiley Periodicals, Inc. J Neurobiol 56: 360–371, 2003     

Neurotrophins improve synaptic transmission in the adult rodent diaphragm

Neurotrophins are usually viewed as secreted proteins that control long-term survival and differentiation of neurons. However, recent studies have established that among the most important functions of neurotrophins is their capacity to regulate synaptic functions and plasticity. When altering synaptic function, neurotrophins are able to produce two types of outcomes, an immediate effect on synaptic transmission and long-term control of synaptic structure and function. The first effect occurs within seconds or minutes after the neurotrophic factor has been applied and usually involves acute modification of synaptic transmission. The second effect takes hours and days, as protein synthesis is required to complete the structural changes. Neurotrophins and their receptors are expressed within the neuromuscular system, making these agents ideal candidates for the short-and long-term regulation of skeletal muscle function. For instance, neurotrophins can alter neuromuscular function acutely, by modulating the amount of neurotransmitter released with each nerve impulse, or chronically, by changing postsynaptic properties or the content and size of synaptic vesicles. It is obvious that the effects of neurotrophins depend on the specific neurotrophin involved (four neurotrophins have been found in mammals; these are nerve growth factor, brain-derived neurotrophic factor, and neurotrophins-3 and-4) and on the specific synapse being studied. Growing evidence highlights the role of neurotrophins in the development and function of neuromuscular synapses. This review will examine the role of neurotrophins in the regulation of neuromuscular transmission. Neirofiziologiya/Neurophysiology, Vol. 39, Nos. 4/5, pp. 327–337, July–October, 2007.     

Mechanoreception and synaptic transmission of hydrozoan nematocytes

Mechanoelectric transduction and its ultrastuctural basis were studied in the cnidocil apparatus of stenotele nematocytes of marine and freshwater Hydrozoa (Capitata and Hydra) as a paradigm for invertebrate hair cells with concentric hair bundles. The nematocytes respond to selective deflection of their cnidocil with phasic-tonic receptor currents and potentials, similar to vertebrate hair cells but without directional dependence of sensitivity. Ultrastructural studies and the use of monoclonal antibodies allowed correlating the mechanoelectric transduction with structural components of the hair bundle. Two other types of depolarising current and voltage changes in nematocytes are postsynaptic, as concluded from their ionic and pharmacological characteristics. One of these types is induced by mechanical stimulation of distant nematocytes and sensory hair cells. It is graded in amplitude and duration, but different from the presynaptic receptor potential. Adequate chemical stimulation of the stenoteles strongly increases the probability of discharge of their cnidocyst, if the chemical stimulus precedes the mechanical one. Simultaneously, the probability of synaptic signalling induced by mechanical stimulation is increased, reaching nearly 100%. The chemoreception of the phospholipids used could be localized in the shaft of the cnidocil, because of the water-insolubility of the stimulant. This chemical stimulation itself does not cause a receptor potential; its action is classified as a modulatory process. Electron microscopy of serial sections of the tentacular spheres of Coryne revealed synapses that are efferent to nematocytes and hair cells besides neurite–neurite synapses, each containing 3–10 clear and/or dense-core vesicles of 70–150 nm diameter. The only candidates to explain the graded afferent signal transmission of nematocytes and hair cells are regularly occurring cell contacts associated with 1(–4) clear vesicles of 160–1100 nm diameter. Transient fusion and partial depletion of stationary vesicles are discussed as mechanisms to reconcile functional and structural data of many cnidarian synapses. Review contributed to the Symposium on Neuro-Anatomy and -Physiology of Coelenterates; 7th International Conference on Coelenterate Biology, Lawrence, Kansas, USA; July 6–11, 2003.     

Are steady-state temporal correlations between evoked stochastic releases of synaptic vesicles as informative as suggested?

Using a stochastic model, we found that the steady-state temporal correlation between synaptic responses evoked by successive presynaptic spikes under conditions of high-frequency repetitive stimulation (50–100 sec⁻¹) is always negative. Therefore, the sign of this correlation cannot be used as a criterion that allows one to distinguish the univesicular vs multivesicular modes of neurotransmitter release in an active zone or the univesicular releases with low vs high probabilities of vesicle release, as suggested earlier [7]. For lower stimulation frequencies (15–20 sec⁻¹), positive correlation between release events evoked by consecutive stimuli is observed only in those cases where the number of ready-releasable vesicles and the time constant of recovery from depression are sufficiently large. Neirofiziologiya/Neurophysiology, Vol. 38, Nos. 5/6, pp. 412–415, September–December, 2006.     

Temporal correlation based learning in neuron models

We study a learning rule based upon the temporal correlation (weighted by a learning kernel) between incoming spikes and the internal state of the postsynaptic neuron, building upon previous studies of spike timing dependent synaptic plasticity (Kempter, R., Gerstner, W., van Hemmen, J.L., Wagner, H., 1998. Extracting Oscillations: Neuronal coincidence detection with noisy periodic spike input. Neural computation 10, 1987–2017; Kempter, R., Gerstner, W., van Hemmen, J.L., 1999. Hebbian learning and spiking neurons. Physical Reviewm E59, 4498–4514; van Hemmen, J.L., 2001. Theory of synaptic plasticity. In: Moss, F., Gielen, S. (Eds.), Handbook of biological physics. vol. 4, Neuro Informatics, neural modelling, Elsevier, Amsterdam, pp. 771–823. Our learning rule for the synaptic weight w _ij is where the t _j,μ are the arrival times of spikes from the presynaptic neuron j and the function u(t) describes the state of the postsynaptic neuron i. Thus, the spike-triggered average contained in the inner integral is weighted by a kernel Γ(s), the learning window, positive for negative, negative for positive values of the time difference s between post- and presynaptic activity. An antisymmetry assumption for the learning window enables us to derive analytical expressions for a general class of neuron models and to study the changes in input-output relationships following from synaptic weight changes. This is a genuinely non-linear effect (Song, S., Miller, K., Abbott, L., 2000. Competitive Hebbian learning through spike timing dependent synaptic plasticity. Nature Neuroscience 3, 919–926).     

Visualizing recycling of synaptic vesicle proteins

The use of modern techniques (in particular, novel fluorescence markers of a few molecular participants of the exo-and endocytotic processes, including pH-sensitive agents, immuno-electron and laser-scanning microscopy) allows experimenters to visualize different stages of recycling of synaptic vesicle proteins. Neirofiziologiya/Neurophysiology, Vol. 39, Nos. 4/5, pp. 350–351, July–October, 2007.     

Involvement of L-type calcium channels and mitochondria in post-tetanic potentiation: Is it a general rule for different types of synapses?

Our experiments and studies of a few other authors demonstrated that L-type calcium channels and mitochondria are involved in the induction of post-tetanic potentiation (PTP) in a number of preparations (Aplysia central nervous system, hippocampal cell cultures, crayfish neuromuscular junctions, etc.). We extend this conclusion on cortical synapses by the demonstration that inhibitors of mitochondrial Ca²⁺ uptake and release suppress PTP in rat neocortical cell cultures. Neirofiziologiya/Neurophysiology, Vol. 39, Nos. 4/5, pp. 403–404, July–October, 2007.     

Age-related changes in synaptic vesicle pools of axo-dendritic synapses on hippocampal CA1 pyramidal neurons in mice

The ultrastructure of symmetric (putatively inhibitory) axo-dendritic synapses on the membrane of hippocampal CA1 pyramidal neurons was investigated in young (20-day-old) and adult (1-year-old) mice. It was shown that synapses of adult animals contain, on average, fewer synaptic vesicles (SVs), and resting SVs of the reserve pool are mostly responsible for this difference. At the same time, in the synapses of adult mice SVs are localized closer to active zones, and the readily releasable pool of SVs is larger in these animals than in young mice. The observed changes in the spatial structure of SV pools presumably demonstrate the age-associated adaptation of inhibitory synapses providing the maintenance of adequate functional properties of hippocampal neuronal networks. Neirofiziologiya/Neurophysiology, Vol. 38, Nos. 5/6, pp. 407–411, September–December, 2006.     

Key role of clathrin-mediated endocytosis in synaptic vesicle recycling

Using novel fluorescent markers, virus-induced modulation of amphiphysin 1 expression, and electron microscopy, we demonstrated that clathrin-mediated endocytosis is the main mechanism of synaptic vesicle retrieval; a hypothesis on the role of a fast “kiss-and-run” mechanism has not been supported. Neirofiziologiya/Neurophysiology, Vol. 39, Nos. 4/5, pp. 388–389, July–October, 2007.     

Modulation of ion currents and regulation of transmitter release in short-term synaptic plasticity: The rise and fall of the action potential

Up and down-regulation of calcium and potassium conductances are associated with several forms of short-term synaptic modulation. Detailed investigation of synaptic plasticity in the marine gastropodAplysia, and in other mollusks, indicates that synaptic transmission can be influenced in a number of ways by modulatory neurotransmitters acting through several second-messenger cascades. Modulation at the synapse itself occurs by means of the regulation of calcium current as well as through effects on processes directly involved in transmitter mobilization and exocytosis. Modulation of potassium current plays a major role in controlling neuronal excitability and may contribute to a lesser extent to the regulation of transmitter release through actions on the resting potential and on action potential configuration.     

槲皮素通过抑制胞吞增强短时程抑制效应

目的 槲皮素是一种广泛分布于药用植物中的黄酮类化合物,传统被认为具有神经保护作用。在本研究中,我们利用位于大鼠脑干的花萼状突触的突触前神经末梢的进行膜片钳记录,研究槲皮素调控突触传递和可塑性的突触前机制。方法 利用全细胞膜片钳结合膜电容记录,在突触后记录微小兴奋性突触后电流（mEPSC）,在突触前神经末梢记录钙內流和神经囊泡的释放、回收以及可立即释放库（RRP）的恢复动力学。并且利用纤维刺激在轴突给予5~200 Hz的刺激,诱发突触后EPSC,记录突触后短时程抑制（STD）。结果 100 μmol/L槲皮素不影响突触后mEPSC的振幅、频率以及AMPA受体的动力学特征。在突触前神经末梢,槲皮素不改变钙内流或囊泡的释放,但显著抑制胞吐后的网格蛋白依赖的慢速胞吞。抑制胞吞会导致突触前囊泡动员的减慢,降低RRP的补充速率,并且增强高频刺激下的短时程可塑性STD。结论 本研究为槲皮素调控中枢神经突触传递提供全新的突触前神经机制,槲皮素有助于抑制中枢神经过度兴奋,进而发挥神经保护作用。     

Cobalt and zinc salicylates as functional analogs of an initiating actor in the molluscan nervous system

We showed that applications of cobalt and zinc salicylates lead to restoration of the impulse activity of a PPa1 neuron of the snail, Helix pomatia, under conditions of the blockade of synaptic transmission by cadmium ions. In the case where a PPa1 neuron demonstrated no background activity and/or under conditions of total isolation of this cell, the above-mentioned salicylates initiate generation of action potentials, as well as exert an excitatory effect on “silent” non-identified cells of the parietal and visceral ganglia. Based on the data obtained, we conclude that the activating effect of cobalt and zinc salicylates on the PPa1 cell is similar to that of the so-called initiating factor (IF), which initiates generation of the burst activity. These effects are independent of the inward calcium current. Using an activator of cAMP phosphodiesterase, imidazole, we showed that the effects of the above salicylates (similar to the effect of IF) are related to the influence of these agents on the system of cyclic nucleotides. Neirofiziologiya/Neurophysiology, Vol. 38, No. 1, pp. 11–17, January–February, 2006.     

Depressing Effects of Caffeine at Crayfish Neuromuscular Synapses I. Dosage Response and Ca<Superscript>++</Superscript> Gradient Effects

The response of crayfish synaptic terminals to drugs began to be studied to characterize the terminal’s physiological characteristics. Caffeine, the first drug to be studied, was selected to enhance synaptic transmission because of its ability to increase calcium release from internal stores. 1. The largest excitor neuron to the superficial flexor muscle system of Procambarus clarkii was stimulated at 10 Hz while recording junction potentials from several lateral muscle fibers. 2. Caffeine unexpectedly decreased synaptic transmission in this system in a dosage-dependent manner. The depressing effect of caffeine was observed at 5 mM caffeine and junction potentials disappeared completely at 50 mM. Washing the preparation in fresh control Ringers did not restore the amplitudes of the junction potentials. 3. Changes in extracellular calcium concentrations delayed or depressed the caffeine effect depending on the calcium gradient across the membrane or the caffeine dosage. The data suggest that calcium is involved in caffeine’s response in this system in a way yet to be determined.     

Disturbance of synaptic vesicle recycling resulting from deletion of a mammalian actin-binding protein, mAbp1

Physiological and ultrastructural studies of synapses between hippocampal neurons of animals with knock-out of a mammalian actin-binding protein, mAbp1, demonstrated that recycling of synaptic vesicles undergoes, in this case, significant modifications. Thus, mAbp1 is rather important from this aspect, which can be related to the noticeable role of actin in clathrin-mediated endocytosis of synaptic vesicles. Neirofiziologiya/Neurophysiology, Vol. 39, Nos. 4/5, pp. 390–391, July–October, 2007.     

Role of synaptic proteins in neurotransmitter release-related vesicular trafficking

As is known, regulated exocytosis of synaptic vesicles constitutes a primary means of communication between neurons, and it is subjected to substantial alterations in a number of brain pathologies. Recent investigations showed that vesicular transport events in neuroendocrine cells and presynaptic terminals are realized by a family of specialized membrane proteins of the vesicle (v-SNAREs) and another family located in the target cytoplasmic membrane (t-SNAREs). A variety of such proteins has already been described in different preparations; however, their precise localization and role in vesicular trafficking during functional changes in the cells remain ambiguous. In addition, new synaptic proteins appear to be involved in the vesicular cycle; the functions of these proteins remain unclear. The role of synaptic proteins in the course of cell excitation, in particular functions of core SNARE synaptic proteins (vesicular synaptobrevin/VAMPs and plasma membrane syntaxins/SNAP-25), as well as those of novel presynaptic proteins (Munc-13, Munc-18, CAPS proteins, and others), are discussed in this review. Neirofiziologiya/Neurophysiology, Vol. 40, No. 2, pp. 155–159, March–April, 2008.