Effect of lanthanum ions on the amplitude distributions of miniature endplate potentials and on synaptic vesicles in frog neuromuscular junctions

Miniature endplate potential (MEPP) amplitude distributions, MEPP frequencies and percentages of small MEPPs were determined as well as synaptic vesicle diameters and numbers in the frog neuromuscular junction during La3+ treatment. MEPP frequencies initially increased by two orders of magnitude and then fell to very low values. La3+ treatment had an initial postsynaptic effect making the MEPPs larger. Prolonged treatment had a variable effect on MEPP amplitudes. There were considerable variations in MEPP frequencies in adjacent junctions so single junctions on edge muscle fibers were recorded for the duration of many experiments and later identified in the electron microscope. Therefore, the physiological and morphological conditions of a given identified junction could be compared. There was a loss of synaptic vesicles and no change in mean diameter during depletion. During high MEPP frequencies infoldings occurred on the axolemma and these disappeared when MEPP frequencies decreased towards the end of the La3+ treatment. After 3-4 h of La3+ treatment, the overall frequency of MEPPs dropped and many were composed of a small class of MEPPs. It is suggested that the morphological correlate of small MEPPs, as well as the classical bell-MEPPs is likely to be synaptic vesicles.     

Spontaneous potentials and fine structure of identified frog denervated neuromuscular junctions

Morphological changes at the neuromuscular junction of the frog during nerve degeneration were correlated with the occurrence of either nerve or Schwann cell-induced miniature endplate potentials in identified edge muscle cells that were subsequently examined in the electron microscope. The initial stages of nerve deterioration produced showers of nerve-induced miniature endplate potentials but showed no apparent change in the vesicle population in nerve terminals. When the miniature endplate potentials ceased, the nerve ending appeared dense and exhibited clumped vesicles, swollen and disrupted mitochondria and membrane whorls. Nerve death was followed by movement and growth of the Schwann cell into the space previously occupied by the nerve terminal. The Schwann cell-muscle junctions were initially silent and no obvious morphological change accompanied the generation of miniature endplate potentials. Schwann cell-induced miniature endplate potentials had frequencies of 10–100/min which were comparable to the frequencies of nerve-induced miniature endplate potentials from similar sized neuromuscular junctions. Some histograms of Schwann cell-induced miniature endplate potentials showed a peak demonstrating a mean quantal size.Only a few vesicles were found in Schwann cells, including the high frequency junctions, so the morphological storage site for the production of miniature endplate potentials is not obvious. The frequencies of miniature endplate potentials evoked by Schwann cells were usually altered in the opposite direction to those evoked by nerves following various challenges, suggesting that the release mechanism for acetylcholine is quite different.     

Effects of baclofen on junctional transmission in rat phrenic diaphragm preparation.

The pre- and postsynaptic effects of the antispastic drug β(p-chlorophenyl)-γ-aminobutyric acid (baclofen, Lioresal) on the rat phrenic diaphragm neuromuscular junction were studied. At very high concentrations (2.10^?4 M) baclofen decreases the efficiency of synaptic transmission, as indicated by a decrease in the size of endplate potentials). More detailed electrophysiological examination indicated that this depression of transmission is caused by both a presynaptic change (the quantal content decreases) and postsynaptic changes (the input resistance of the muscle fibres is decreased and the decay phase of the end plate current is shortened). At somewhat lower concentrations of baclofen (2.10^?5 M) the depression of transmission is less pronounced. At the lowest concentrations used in this study (2.10^?6 M) baclofen increases the efficiency of synaptic transmission (the endplate potentials increase in size). This seems to be caused by an increase in quantal content with no significant postsynaptic action.It is concluded that at concentrations likély to be reached after therapeutic administration, the action of baclofen on the neuromuscular junction does not explain its antispastic action and probably does not contribute significantly to it.     

Giant miniature endplate potentials induced by 4-aminoquinoline

In experiments on the isolated extensor digitorum longus muscle of the rat it was shown that 4-aminoquinoline (125–250 μM) altered the amplitude distribution of spontaneous miniature endplate potentials to include a large portion of giant miniature endplate potentials with slow rise and decay times. Similar, slow-rising giant miniature endplate potentials were induced by the drug at neuromuscular junctions with regenerating nerve terminals, i. e. in a condition where spontaneous as well as evoked transmitter release is depressed. The appearance of giant miniature endplate potentials was not correlated with inhibition of cholinesterase since neostigmine (3 μM) failed to induce such potentials. Nerve impulse evoked endplate potentials of amplitudes similar to the spontaneous giant miniature endplate potentials had a faster and more uniform rise time. The results suggest that 4-aminoquinoline, by a direct action on the nerve terminal, causes the release of larger than normal quanta of acetylcholine. Quantitative assays of acetylcholine released before and in the presence of 4-aminoquinoline gave similar values showin that the amounts of acetylcholine which give rise to the giant miniature potentials contribute little to the total amount of acetylcholine liberated.     

Ultrastructural and physiological effects of the ionophore A23187 at identified frog neuromuscular junctions

The physiological and morphological effects of the calcium ionophore A23187 (calimycin) at the frog neuromuscular junction in vitro were examined. Miniature endplate potentials were recorded intracellularly during exposure to the ionophore. Preparations fixed 15 or 30 min after adding the drug to the incubating medium, which exhibited a greatly increased miniature endplate potential frequency, showed no obvious morphological differences when compared to controls with regard to synaptic vesicle number or distribution of vesicles within the terminal. However, after 45-60 min of exposure to the ionophore, when miniature endplate potential frequency had declined almost to zero, most of the nerve endings appeared devoid of synaptic vesicles and other organelles while the plasma membrane was intact. It is suggested that the apparent depletion of vesicles from the terminal induced by the calcium ionophore is a consequence of irreversible changes at the terminal.     

Changes in the distribution of the extrajunctional acetylcholine sensitivity along muscle fibers during development and following cordotomy in the rat

Acetylcholine sensitivity along the entire length of muscle fibers was studied during postnatal development and following transection of the spinal cord in the rat. During postnatal development, the acetylcholine sensitivity in the soleus and extensor digitorum longus muscles decreased faster at the juxtajunctional region than near the tendons. Thus, the adult pattern of low acetylcholine sensitivity at the extrajunctional membrane was achieved through the uneven change of acetylcholine sensitivity during normal development. This uneven pattern of the sensitivity was found to appear in both muscles in older rats after cordotomy, and is in striking contrast to the uniform pattern in denervated muscles. The uneven appearance of the sensitivity could not be explained by changes in input resistance or resting membrane potential. In the soleus muscle whose nerve was implanted at an ectopic site, the lowest sensitivity also appeared at the ectopic juxtajunctional region after cordotomy. These results indicate that the motor nerve exerts regionally different effects along a fiber with respect to the appearance of acetylcholine receptors.     

An analysis of the mechanisms underlying the non-quantal release of acetylcholine at the mouse neuromuscular junction

The hyperpolarization produced by the application of 10^?5 M d-tubocurarine to the end-plate of mouse diaphragm-phrenic nerve preparation pretreated with an irreversible anticholinesterase was studied. This hyperpolarization has been attributed to a non-quantal release of acetylcholine by the nerve terminals. The usual hyperpolarization effect of about 10 mV was mimicked by adding 10^?7 M acetylcholine to the bath which contained 15mM Ca²⁺ to block the non-quantal release. The hyperpolarization effect was maximal (9.2 ± 0.8mV) in saline solution with the normal concentrations of Ca²⁺ (2mM) and K⁺ (5mM). Reducing [Ca²⁺] in the bath decreased the hyperpolarization effect. In Ca-free solution with 4.10^?4 M ethleneglycoltetra-acetate the hyperpolarization effect was3.8 ± 0.6mV. An increase of [Ca²⁺] also reduced the hyperpolarization effect and it was absent in 15mM of Ca²⁺. When the external concentration of Ca²⁺ was kept constant (2mM), both decreasing and increasing K⁺ concentration from 5 mM diminished the hyperpolarization effect. However, the decrease at elevated K⁺ concentrations was due to a reduction of membrane resistance and when the hyperpolarization effect was corrected for the change in size of the miniature endplate potential, it was unchanged in solutions with increased [K⁺] up to 13 mM. No traces of non-quantal release were observed during repetitive stimulation of the phrenic nerve in a medium containing 0.3 mM Ca²⁺. The hyperpolarization effect was not found in denervated muscles that lacked signs of neuromuscular transmission. The hyperpolarization effect was blocked by botulinum toxin while some evoked release of ACh was present. The time required for the block by 1 μg/ml toxin was shorter than for 0.25 μg/ml. The application of α-bungarotoxin produced a hyperpolarization of the membrane in the junctional region. The addition of d-tubocurarine failed to produce the hyperpolarization effect. The hyperpolarization effect was also absent when muscles were incubated with the desensitization potentiating drug, SKF-525A. The hyperpolarization effect was unchanged by replacement of Na⁺ with guanidine and was absent following the replacement of Na⁺ with arginine.The data suggest the channel responsible for the hyperpolarization effect is the same as that giving the endplate potential. The results are in agreement with the hypothesis that the hyperpolarization effect reflects a sustained non-quantal release of acetylcholine from the nerve terminal into the synaptic cleft.     

A new type of transmitter release at the neuromuscular junction

Examination of spontaneous miniature endplate potentials (MEPPs) in murine skeletal muscle has revealed that in conditions such as botulinum poisoning, during nerve terminal regeneration or in the presence of the drug 4-aminoquinoline, two types of acetylcholine release are responsible for the MEPPs. In addition to the MEPPs which correspond to the quantal component of a nerve impulse-evoked endplate potential a second type of acetylcholine release occurs. The latter type of transmitter release gives rise to MEPPs with a more prolonged time-to-peak and frequently a larger than normal amplitude. It is unaffected by nerve terminal depolarization and transmembrane Ca2+ fluxes. The relationship between MEPP frequency and temperature has a Q10 of about 12 compared to 2-3 for normal MEPPs. In botulinum-poisoned muscles this secretory type of transmitter release dominates, being exclusively present in muscles where nerve stimulation fails to release transmitter. In normal muscle such a release is induced by 4-aminoquinoline which may cause up to 45% of all the spontaneous MEPPs to be of that kind. It is suggested that the described spontaneous secretion of acetylcholine serves in inductory and neurotrophic function.     

The relationship between the membrane potential of neurosecretory nerve endings,as measured by a voltage-sensitive dye,and the release of neurohypophysial hormones

The membrane potential of isolated rat neurohypophyses and isolated neurosecretosomes (neurosecretory nerve endings) was monitored with the voltage sensitive fluorescent probe diS-C₃-(5). K ions, in contrast to Na or Cl ions, give rise to large changes of the fluorescent signal. The fluorescent response is linearly related to log[K⁺]₀ at values higher than 10 mM, whereas at lower [K⁺]₀ the permeability of the membrane for Na ions has to be taken into account. Veratridine increases the fluorescent signal only in the presence of external sodium; this effect is blocked by tetrodotoxin. After prolonged K-induced depolarisation, addition of veratridine to the medium gives a further change in fluorescence of diS-C₃-(5) associated with release of vasopressin.Vasopressin release from isolated neurohypophyses started to increase significantly only above 25 mk [K⁺]₀, while the depolarization of the membrane was linearly related to log[K⁺]₀. The results are consistent with the view that neurosecretory nerve endings have voltage-dependent calcium channels that regulate the amount of hormone released during depolarisation.     

Stroboscopic illumination and dark rearing block the sharpening of the regenerated retinotectal map in goldfish

Blocking activity with intraocular tetrodotoxin prevents the sharpening of the retinotectal map formed during regeneration of the optic nerve. If (under normal conditions) the initially diffuse map sharpens because of correlated activity in neighboring but not distant ganglion cells, then sharpening should also be prevented merely by disrupting the spatiotemporal correlation in the pattern of activity. To test this idea, fish were exposed during regeneration to stroboscopic illumination in a featureless environment, or were maintained in complete darkness. The regenerating cells remained visually responsive after axotomy, and the xenon strobe effectively drove each ganglion cell at a constant latency. The maps formed in the strobe-reared fish were normally oriented, but the multiunit receptive fields were greatly enlarged, averaging 32 degrees. In control regenerates, multiunit receptive fields averaged only 11-12 degrees, nearly the same as for single units. Dark rearing, which allows only spontaneous activity, also resulted in enlarged multiunit receptive fields, averaging more than 28 degrees. Both effects parallel those reported previously with tetrodotoxin block. The mature projection did not become diffuse as a result of the strobe rearing, and the sensitive period corresponded to the early stage of synaptogenesis (20-34 days). Periods of normal visual exposure after 35 days produced very little sharpening of the diffuse maps produced during either strobe or dark rearing. The results are attributed to an activity-dependent stabilization of developing synapses. The correlated firing of neighboring ganglion cells could allow postsynaptic summation of their responses, and the retention of those more effective, retinotopically placed synapses might then occur via a Hebbian mechanism.     

The effect of prolonged, reversible block of nerve impulses on the elimination of polyneuronal innervation of new-born rat skeletal muscle fibers.

The soleus muscle was reversibly paralyzed in 9- and 10-day-old rats by implantation of a plastic plug containing slowly releasable tetrodotoxin under the epineurium of the sciatic nerve. After different periods of paralysis we determined the frequency of polyneuronally innervated fibers in the muscle by intracellular recording of end-plate potentials to nerve stimulation. During the first 2–4 days of nerve block the elimination of polyneuronal innervation, occurring in rat muscles of this age, proceeded nearly as normally. After longer periods of block there was an increased frequency of polyneuronally innervated muscle fibers presumably due to motor nerve sprouting induced by the paralyzed muscle.     

Substance P acts by releasing 5-hydroxytryptamine from enteric neurons in the stomach of the rainbow trout, Salmo gairdneri

The effect and mode of action of substance P was studied in a perfused stomach preparation and on isolated strip preparations of the stomach wall from the rainbow trout, Salmo gairdneri. Substance P was excitatory on the stomach muscle wall in a dose-dependent manner. Two other tachykinins, physalaemin and eledoisin, excited the preparations in a similar manner and in the same dose range. The effect of substance P was not antagonized by the substance P analogues [D-Pro2, D-Phe7, D-Trp9]substance P and [D-Pro2, D-Trp7,9]substance P (both 10(-5) M). Tetrodotoxin reduced or abolished the effect of substance P, while no reduction of the response was obtained after atropine, chlorisondamine or phentolamine (all 10(-6) M). 5-Hydroxytryptamine excited the stomach and this effect was not antagonized by tetrodotoxin, suggesting that the action of 5-hydroxytryptamine was direct on the smooth muscle. The 5-hydroxytryptamine antagonist methysergide, in a concentration which selectively blocked the response to 5-hydroxytryptamine, also blocked the response to substance P (10(-9)-10(-8) M). The outflow of 5-[3H]hydroxytryptamine from a preloaded perfused stomach was clearly increased by substance P, and this release was blocked by tetrodotoxin. Immunohistochemistry revealed the presence of nerve fibres and ganglion cells showing 5-hydroxytryptamine-like immunoreactivity in the myenteric plexus and smooth muscle layers of the stomach wall. The immunoreactive cells and nerve fibres were particularly abundant in the pyloric part of the stomach. It is concluded that the main effect of substance P on the stomach wall of the rainbow trout is indirect, via activation of a non-adrenergic, non-cholinergic neuron. The results are compatible with the view that this neuron exerts its action by release of 5-hydroxytryptamine. Supramaximal concentrations (greater than or equal to 10(-7) M) of substance P may in addition have a direct effect on the gastric smooth muscle.     

Independent release of supranormal acetylcholine quanta at the rat neuromuscular junction

This electrophysiological study deals with the occurrence and with the mode of release of unusually large miniature end-plate potentials at the rat neuromuscular junction during physiological conditions. A specific limit for the normal miniature end-plate potential amplitude at each cell studied was determined after fitting the observed frequency-amplitude histogram to a Gaussian distribution. The relative abundance of giant miniature end-plate potentials was 4.15% at room temperature. The occurrence of giant miniature end-plate potentials was temperature dependent. The percentage of giant miniature end-plate potentials was 5.8% and 0.61% at 35 degrees C and at 16 degrees C, respectively. The amplitude-independence of the intervals between miniature end-plate potentials was demonstrated at room temperature as well as at 35 degrees C and at 16 degrees C. The results of this study show that giant miniature end-plate potentials are produced by acetylcholine packets which are released independently and that they are not a consequence of the synchronous release of several normal-sized quanta. Moreover, the results indicate that during physiological conditions a minor but regular proportion of the spontaneous release of acetylcholine is made up of larger packets, which produce miniature end-plate potentials of supranormal amplitude.     

The effect of carbachol and alpha-bungarotoxin on the frequency of miniature endplate potentials at the frog neuromuscular junction

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.     

Effect of tetraalkylammonium derivative of 6-methyluracil on amplitude and temporal parameters of miniature endplate potentials in frog neuromuscular junction

The effect of C-547, a tetraalkylammonium derivative of 6-methyluracil, a novel highly selective acetylcholinesterase inhibitor, on frog neuromuscular junction was studied. In concentrations 10^-9-10^-7 M the preparation increased the amplitude and temporal parameters of miniature endplate potentials. In contrast to the effect of C-574 on purified acetylcholinesterase from mammals, the effect of this agent on frog neuromuscular junction was reversible. In a concentration of 10^-6 M the preparation apart from anticholinesterase activity produced a parasympatholytic effect manifested in a decrease in amplitude and decay time constant of miniature endplate potentials accompanied by a decrease in spontaneous transmitter secretion. After washout, the parasympatholytic effect recovered more slowly, but disappeared more rapidly compared to anticholinesterase activity. These findings suggest that parasympatholytic effect of C-547 results from direct action on receptor-channel complexes in the endplate membrane.     

Two types of miniature endplate potentials in Xenopus nerve-muscle cultures

The frequency distribution of miniature endplate potential (mepp) amplitudes was examined in Xenopus nerve-muscle cultures. In the early days of nerve-muscle co-culture (1-2 days) the mepp amplitude histogram was skewed in the majority of muscle cells and had a peak approximately at 0.6 mV. Innervated muscle cells in older cultures (3-4 days), however, often had two peaks. The distribution at the smaller amplitude was skewed and the peak was approximately at 0.6 mV, whereas the peak at the larger amplitude was more-or-less symmetrical and about 18 times greater than the small one. The time-to-peak value of mepps in the peak at the smaller amplitude was slightly smaller than those in the peak at the larger amplitude. Small mepps were not due to electrotonic spread of mepps generated at distant sites nor due to focal development of acetylcholinesterase. Two peaks were also observed in the amplitude histogram of extracellularly recorded miniature endplate currents. Therefore, these two types of mepps must be generated practically at the same site. Mepps in the peak at the smaller amplitude had similar properties as sub-miniature endplate potentials (s-mepps) described in the adult animal. The second symmetrical peak in older cultures may correspond to classical mepps (c-mepps). The age-dependent shift from the amplitude distribution with one peak to that with two peaks has also been reported in developing and regenerating neuromuscular junctions. Therefore, I suggest that during development in culture, mepps appear at first as s-mepps and subsequently c-mepps develop, forming the second peak in the amplitude histogram.     

终板蛋白多糖的变化对颈椎间盘力学性能的影响

目的:通过测定退变颈椎间盘压缩性能的改变,同时观测软骨终板基质蛋白多糖(proteoglycan PG)的变化,为退变的椎间盘显现异常力学特性作一分子机制上的探讨。方法:将24只家兔随机分为对照组和模型组,模型组家兔保持45°低屈曲住5小时/次·天,取C5-6椎间盘进行生物力学测定;同时生化测定椎间盘终板糖胺多糖(glycosaminoglycan,GAG)总量、硫酸软骨素(chongdroitin sulphate,CS)和硫酸角质素(keratan sulphate,KS)比值和透明质酸(hyaluronic acid,HA)含量;总结分析颈椎间盘退变时基质各成分的系统变化及其对椎间盘力学性能的影响。结果:模型组终板抗压强度、断裂时的最大形变、终板基质蛋白多糖总含量、硫酸软骨素/硫酸角质素比值、透明质酸含量都低于对照组(P<0.05),并随造模进程的深入而减小。结论:长时间的应力不均状态,造成椎间盘终板材料力学特性改变;颈椎间盘基质蛋白多糖含量和成分改变是颈椎间盘生物力学性能减退的主要原因之一。     

Changes in spontaneous and evoked release of transmitter induced by the crotoxin complex and its component phospholipase A2 at the frog neuromuscular junction.

The time course and characteristics of the changes induced by the action of the crotoxin complex and of its component phospholipase A₂ on transmission at the frog neuromuscular junction was studiedin vitro at single endplates using intracellular recording techniques. Both the crotoxin complex and phospholipase A₂ induced variable changes in spontaneous transmitter release in which bursts of miniature endplate potentials alternated with periods when the frequency of miniature endplate potentials was stable but their amplitudes were markedly heterogeneous. The bursts of miniature endplate potentials were of sudden onset and of a high initial frequency, and their duration and incidence were highly variable both at one endplate and between endplates. These changes in the frequency and amplitude of miniature potentials occurred both before and after the abolition of neurally evoked endplate potentials and in the presence of tetrodotoxin. At a late stage in crotoxin intoxication, potassium depolarization of the motor nerve terminals failed to increase spontaneous transmitter release. Both the crotoxin complex and phospholipase A₂ reduced the amplitude of endplate potentials although phospholipase A₂ was less effective. The course of depression of the quantal content of evoked transmitter release induced by the crotoxin complex was occasionally interrupted by a brief facilitation of response; these irregularities in the rate of depression were more clearly seen when endplate potentials were recorded at partially curarized endplates exposed to either the crotoxin complex or to phospholipase A₂. At the stage of abolition of endplate potentials an increase in the external concentration of Ca^{2 +} enabled the evoked response to be restored temporarily. The crotoxin complex and phospholipase A₂ had no effect on the input resistance of the sarcolemmal membrane, but raised the threshold stimulus required to initiate an action potential at concentrations which affected transmitter release. Crotapotin did not affect any of the measured parameters even at high concentrations.The diversity of changes observed in the properties of spontaneous and evoked transmitter release, suggests that the crotoxin complex and phospholipase A₂ may induce two kinds of alterations in the structure of the presynaptic membrane as a result of the action of phospholipase A₂, viz (a) the formation of transient instabilities with an increased Ca^{2 +} inflow and (b) prolonged disturbance of the calcium channel and/or the synchronized release of subunits at the active zone.     

Ultrastructural changes accompanying the disturbances of neuromuscular transmission caused by Pardachirus toxin.

Toxic secretion from the Red Sea flatfish Pardachirus marmoratus (Soleidae) was applied to frog neuromuscular preparations while intracellular recording was done with microelectrodes. Endplates from treated preparations were examined in the electron microscope. Lyophilized Pardachirus toxin (0.01 mg/ml) caused a sudden great increase in spontaneous transmitter release lasting 20–120 min, a short-lived increase in the size of neurally evoked responses, and a short period (1–3 min) of spontaneous muscle tremor which could not be blocked completely by d-tubocurarine.Neuromuscular junctions viewed in the electron microscope revealed lamellar and occasionally concentric arrangements of Schwann cells surrounding and intruding into nerve terminals, while active zones on the presynaptic membrane remained in place opposite junctional folds. Synaptic vesicle aggregation and great density of neurofilaments were in evidence in many terminals, while muscle structure in preparations treated with higher concentrations (0.1 mg/ml) was radically disrupted.     

The vesicle cycle in motor nerve endings of the mouse diaphragm

Experiments on the mouse diaphragm muscle using intracellular microelectrode recordings and fluorescence microscopy were performed to study the dynamics of transmitter secretion and synaptic vesicle recycling processes (the exocytosis-endocytosis cycle) in motor nerve endings (NE) during prolonged rhythmic stimulation (20 impulses/sec). During stimulation, there were triphasic changes in the amplitude of endplate potentials (EPP): an initial rapid reduction, followed by prolonged (1–2 min) stabilization of amplitude, i.e., a plateau, and then a further slow decrease. Restoration of EPP amplitude after stimulation for 3 min occurred over a period of several seconds. Loading of synaptic vesicles with the fluorescent endocytic stain FM1-43 showed that rhythmic stimulation led to a gradual (over 5–6 min) decrease in NE fluorescence, demonstrating exocytosis of synaptic vesicles. Quantum analysis of the electrophysiological data and comparison of these data with results from fluorescence studies suggested that mouse NE have a high rate of endocytosis and reutilization of synaptic vesicles (the mean recycling time was about 50 sec), which may support the maintenance of reliable synaptic transmission during prolonged high-frequency activity. The sizes of the release-ready and recycling pools of synaptic vesicles were determined quantitatively. It is suggested that vesicle recycling in mouse NE occurs via a short, rapid pathway with incorporation into the recycling pool. Vesicles of the reserve pool are not used for transmitter secretion in the stimulation conditions used here. Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 94, No. 2, pp. 129–141, February, 2008.