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Quantal Release (quantal + release)
Selected AbstractsL-type calcium channels are involved in fast endocytosis at the mouse neuromuscular junctionEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 6 2008Paula P. Perissinotti Abstract We used fluorescence microscopy of FM dyes-labeled synaptic vesicles and electrophysiological recordings to examine the functional characteristics of vesicle recycling and study how different types of voltage-dependent Ca2+ channels (VDCCs) regulate the coupling of exocytosis and endocytosis at mouse neuromuscular junction. Our results demonstrate the presence of at least two different pools of recycling vesicles: a high-probability release pool (i.e. a fast destaining vesicle pool), which is preferentially loaded during the first 5 s (250 action potentials) at 50 Hz; and a low-probability release pool (i.e. a slow destaining vesicle pool), which is loaded during prolonged stimulation and keeps on refilling after end of stimulation. Our results suggest that a fast recycling pool mediates neurotransmitter release when vesicle use is minimal (i.e. during brief high-frequency stimulation), while vesicle mobilization from a reserve pool is the prevailing mechanism when the level of synaptic activity increases. We observed that specific N- and L -type VDCC blockers had no effect on evoked transmitter release upon low-frequency stimulation (5 Hz). However, at high-frequency stimulation (50 Hz), L -type Ca2+ channel blocker increased FM2-10 destaining and at the same time diminished quantal release. Furthermore, when L -type channels were blocked, FM2-10 loading during stimulation was diminished, while the amount of endocytosis after stimulation was increased. Our experiments suggest that L -type VDCCs promote endocytosis of synaptic vesicles, directing the newly formed vesicles to a high-probability release pool where they compete against unused vesicles. [source] Negative cross-talk between presynaptic adenosine and acetylcholine receptorsEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 1 2006A. V. Shakirzyanova Abstract Functional interactions between presynaptic adenosine and acetylcholine (ACh) autoreceptors were studied at the frog neuromuscular junction by recording miniature end-plate potentials (MEPPs) during bath or local application of agonists. The frequency of MEPPs was reduced by adenosine acting on presynaptic adenosine A1 receptors (EC50 = 1.1 µm) or by carbachol acting on muscarinic M2 receptors (EC50 = 1.8 µm). However, carbachol did not produce the depressant effect when it was applied after the action of adenosine had reached its maximum. This phenomenon implied that the negative cross-talk (occlusion) had occurred between A1 and M2 receptors. Moreover, the occlusion was receptor-specific as ATP applied in the presence of adenosine continued to depress MEPP frequency. Muscarinic antagonists [atropine or 1-[[2-[(diethylamino)methyl)-1-piperidinyl]acetyl]-5,11-dihydro-6H-pyrido [2,3-b][1,4]benzodiazepine-6-one) (AFDX-116)] had no effect on the inhibitory action of adenosine and adenosine antagonists [8-(p -sulfophenyl)theophylline (8-SPT) or 1,3-dipropyl-8-cyclopentylxanthine (DPCPX)] had no effect on the action of carbachol. These data suggested that membrane,delimited interactions did not occur between A1 and M2 receptors. Both carbachol and adenosine similarly inhibited quantal release triggered by high potassium, ionomycin or sucrose. These results indicated a convergence of intracellular pathways activated by M2 and A1 receptors to a common presynaptic effector located downstream of Ca2+ influx. We propose that the negative cross-talk between two major autoreceptors could take place during intense synaptic activity and thereby attenuate the presynaptic inhibitory effects of ACh and adenosine. [source] The Effect of Carbachol and ,-Bungarotoxin on the Frequency of Miniature Endplate Potentials at the Frog Neuromuscular JunctionEXPERIMENTAL PHYSIOLOGY, Issue 2 2000Ela Bukharaeva The effects of an acetylcholine analogue, carbachol (CCh), and a purified irreversible nicotinic antagonist, ,-bungarotoxin (BTX), on the frequency of the miniature endplate potentials (mEPPs) at the neuromuscular junction of the frog were tested at 20 and 10°C. CCh (5 ± 10-6 m) reduced the frequency of mEPPs to about 60%; this reduction was not affected by 1 ± 10-7 g ml-1 BTX. BTX also reversibly decreased the mEPP frequency by 40%, but not in the presence of CCh or in Ringer solution with 0 or 8 mM Ca2+. The present data show that BTX, which inhibits a class of nicotinic ACh receptors, does not block the decrease of mEPP frequency evoked by CCh and can itself suppress the frequency of spontaneous quantal release. [source] Glutamate spillover augments GABA synthesis and release from axodendritic synapses in rat hippocampusHIPPOCAMPUS, Issue 1 2010Misty M. Stafford Abstract Tight coupling between gamma-aminobutyric acid (GABA) synthesis and vesicle filling suggests that the presynaptic supply of precursor glutamate could dynamically regulate inhibitory synapses. Although the neuronal glutamate transporter excitatory amino acid transporter 3 (EAAT3) has been proposed to mediate such a metabolic role, highly efficient astrocytic uptake of synaptically released glutamate normally maintains low-extracellular glutamate levels. We examined whether axodendritic inhibitory synapses in stratum radiatum of hippocampal area CA1, which are closely positioned among excitatory glutamatergic synapses, are regulated by synaptic glutamate release via presynaptic uptake. Under conditions of spatially and temporally coordinated release of glutamate and GABA within pyramidal cell dendrites, blocking glial glutamate uptake enhanced quantal release of GABA in a transporter-dependent manner. These physiological findings correlated with immunohistochemical studies revealing expression of EAAT3 by interneurons and uptake of D-asparate into putative axodendritic inhibitory terminals only when glial uptake was blocked. These results indicate that spillover of glutamate between adjacent excitatory and inhibitory synapses can occur under conditions when glial uptake incompletely clears synaptically released glutamate. Our anatomical studies also suggest that perisomatic inhibitory synapses, unlike synapses within dendritic layers of hippocampus, are not capable of glutamate uptake and therefore transporter-mediated dynamic regulation of inhibition is a unique feature of axodendritic synapses that may play a role in maintaining a homeostatic balance of inhibition and excitation. © 2009 Wiley-Liss, Inc. [source] Kinetics of both synchronous and asynchronous quantal release during trains of action potential-evoked EPSCs at the rat calyx of HeldTHE JOURNAL OF PHYSIOLOGY, Issue 2 2007V. Scheuss We studied the kinetics of transmitter release during trains of action potential (AP)-evoked excitatory postsynaptic currents (EPSCs) at the calyx of Held synapse of juvenile rats. Using a new quantitative method based on a combination of ensemble fluctuation analysis and deconvolution, we were able to analyse mean quantal size (q) and release rate (,) continuously in a time-resolved manner. Estimates derived this way agreed well with values of q and quantal content (M) calculated for each EPSC within the train from ensemble means of peak amplitudes and their variances. Separate analysis of synchronous and asynchronous quantal release during long stimulus trains (200 ms, 100 Hz) revealed that the latter component was highly variable among different synapses but it was unequivocally identified in 18 out of 37 synapses analysed. Peak rates of asynchronous release ranged from 0.2 to 15.2 vesicles ms,1 (ves ms,1) with a mean of 2.3 ± 0.6 ves ms,1. On average, asynchronous release accounted for less than 14% of the total number of about 3670 ± 350 vesicles released during 200 ms trains. Following such trains, asynchronous release decayed with several time constants, the fastest one being in the order of 15 ms. The short duration of asynchronous release at the calyx of Held synapse may aid in generating brief postsynaptic depolarizations, avoiding temporal summation and preserving action potential timing during high frequency bursts. [source] ,-Latrotoxin increases spontaneous and depolarization-evoked exocytosis from pancreatic islet ,-cellsTHE JOURNAL OF PHYSIOLOGY, Issue 3 2005Amelia M. Silva ,-Latrotoxin (,-LT), a potent excitatory neurotoxin, increases spontaneous, as well as action potential-evoked, quantal release at nerve terminals and increases hormone release from excitable endocrine cells. We have investigated the effects of ,-LT on single human, mouse and canine ,-cells. In isolated and combined measurements, ,-LT, at nanomolar concentrations, induces: (i) rises in cytosolic Ca2+, into the micromolar range, that are dependent on extracellular Ca2+; (ii) large conductance non-selective cation channels; and (iii) Ca2+ -dependent insulin granule exocytosis, measured as increases in membrane capacitance and quantal release of preloaded serotonin. Furthermore, at picomolar concentrations, ,-LT potentiates depolarization-induced exocytosis often without evidence of inducing channel activity or increasing cytosolic Ca2+. These results strongly support the hypothesis that ,-LT, after binding to specific receptors, has at least two complementary modes of action on excitable cells. (i) ,-LT inserts into the plasma membrane to form Ca2+ permeable channels and promote Ca2+ entry thereby triggering Ca2+ -dependent exocytosis in unstimulated cells. (ii) At lower concentrations, where its channel forming activity is hardly evident, ,-LT augments depolarization-evoked exocytosis probably by second messenger-induced enhancement of the efficiency of the vesicle recruitment or vesicle fusion machinery. We suggest that both modes of action enhance exocytosis from a newly described highly Ca2+ -sensitive pool of insulin granules activated by global cytosolic Ca2+ concentrations in the range of ,1 ,m. [source] | ||||||||||||||||||||||