Quantum Content Of End-plate Potentials Research Articles

Postsynaptic Potentiation in Mouse Motor Synapses Induced by ATP Accumulation in Synaptic Cleft.

In mouse motor synapses, a non-selective purinoceptor antagonist suramin increased the quantum content of endplate potentials (EPP) without changing the time course of synaptic potentials. An ectonucleotidase inhibitor ARL 67156 had no effect on the amplitude and quantum content of EPP and miniature endplate potentials (mEPP) evoked by single stimuli, but significantly prolonged their duration. Long-term high-frequency stimulation of the nerve in the presence of ARL 67156 persistently increased the amplitude and duration of EPP during the train of impulses, but did not change their quantum content. ATP-γ-S, a non-hydrolyzed ATP analogue, significantly increased the amplitudes and prolonged the rising and falling phases of EPP and mEPP. The ATP-induced postsynaptic potentiation in neuromuscular transmission can result from the increase in ATP content and its longer presence in the synaptic cleft.

Characterization of neuromuscular transmission in mice with progressive motoneuronopathy

Progressive motoneuronopathy (PMN) is an autosomal recessive mouse disease, which is characterized by the development of hind limbs paralysis rapidly progressing to the anterior parts of the body, muscular atrophy, respiratory depression, and death at 6-7 postnatal weeks. Here, we recorded the resting membrane potential (RMP), spontaneous miniature endplate potentials (MEPPs), and quantum content of endplate potentials (EPP) at the diaphragm muscle fibers in controls and PMN mice aged 18 to 43 days. In control animals, there was a progressive increase in RMP, MEPP frequency and EPP quantum content, as well as a decrease in mean MEPP amplitude. In PMN mice, the developmental increase in frequency and decrease in the amplitude of MEPPs was practically stopped at the postnatal day 18, whereas RMP increased but only until the age of 31 days and then progressively decreased. The distribution histogram of RMP in PMN mice older than 35 days revealed the existence of two subpopulations of muscle fibers: one showing a denervation-like decrease in RMP and the second, which was matching controls. In addition, EPP quantum content was significantly attenuated in older PMN animals. These results indicate that neurotransmission is severely affected in advanced, but not in early stage of disease, which is apparently due to a partial denervation of the muscles.

Calcium channel peptide can cause an autoimmune-mediated model of Lambert-Eaton myasthenic syndrome in rats.

The Lambert-Eaton myasthenic syndrome (LEMS) is a disorder of neuromuscular transmission characterized by the reduced quantal release of acetylcholine from the motor nerve terminal, wherein the P/Q-type of voltage-gated calcium channel (VGCC) and is attacked by a majority of LEMS antibodies. Using the molecular structure of the alpha1 subunit (consisting of 4 domains) of the P/Q-type VGCC as a reference, we synthesized the extracellular region (S5-S6 linker) of the domain III, known as the segment which plays an important role in channel functions. Six of the ten Lewis rats immunized with this synthetic peptide conjugated with carrier protein showed moderate weakness (grade 1 in a 3-graded scale, for myasthenic weakness in experimental animals) and a reduction in acetylcholine quantum content of end-plate potentials. Antipeptide antibodies raised in test rats reacted with omega-conotoxin MVIIC-sensitive cerebellar extract (P/Q-type VGCC) and the domain III peptide inhibited the binding of rat antibodies to VGCCs. Our findings suggest the identification of one of the potential epitopes of LEMS antibodies.

Calcitonin gene-related peptide prevents disuse-induced sprouting of rat motor nerve terminals

Calcitonin gene-related peptide (CGRP) coexists with acetylcholine (ACh) in motor nerve terminals. Externally applied CGRP has been shown to increase the synthesis of ACh receptors in cultured myotubes by a mechanism independent of muscle activity. Thus, CGRP is suggested to be a neurotrophic factor that may regulate the expression of several long-term events occurring at the neuromuscular junction. We have examined the effect of CGRP on the sprouting of motor nerve terminals induced by chronic block of nerve-muscle activity in adult rats. Daily treatment with CGRP suppressed the disuse-induced terminal sprouting in a dose-dependent manner, whereas the morphology of motor nerve terminals in active muscles was unaffected by CGRP. CGRP may be a possible candidate for an antisprouting agent which has been postulated to exist in nerve terminals. The disuse-induced outgrowth of terminal sprouts was accompanied by an increase in the mean quantum content of end-plate potentials, as well as in the frequency of spontaneous miniature end-plate potentials. This increased transmitter release was still maintained at the junctions in which disuse-induced terminal sprouting had been suppressed by CGRP. It is suggested that the formation of terminal sprouts per se is not responsible for the plastic change of transmitter release induced by prolonged disuse of the neuromuscular junction.

The nature of the presynaptic effects of (+)-tubocurarine at the mouse neuromuscular junction.

1. The effects of (+)-tubocurarine (TC) on tetanic run-down and quantum content of end-plate potentials (EPPs) were investigated in cut-fibre preparations of mouse diaphragm. 2. (+)-Tubocurarine, 0.15 microM, halved the amplitude of spontaneous miniature EPPs (MEPPs) and steepened the tetanic run-down of EPPs evoked at 10 Hz by increasing the quantum content of the first EPP of the train while having no effect on quantum content of plateau EPPs. With stimulation at 1 Hz, there was little run-down and the quantum content of all EPPs was increased by TC. 3. The use of binomial statistics to analyse release indicated that after TC the increase in the quantum content of the first EPP in the train at 10 Hz was due to an increase in n and that during the run-down there was a decrease in p so that plateau EPP quantum content at 10 Hz was not different from control. 4. To elucidate a possible role of cholinoreceptors in the presynaptic effects of TC, studies were made on the effects of pancuronium or of alpha-bungarotoxin (BTX), with concentrations and exposure times where they had postsynaptic effects equal to 0.15 microM-TC. The run-down of EPPs was unaffected by BTX, while pancuronium steepened it to a lesser extent than TC. 5. The anticholinesterase, ecothiopate, decreased the quantum content of plateau EPPs only at high frequencies of stimulation (50 Hz) and did not affect the presynaptic effects of TC at 10 Hz. 6. At concentrations which reduced MEPP amplitude, atropine (10 microM) or hexamethonium (50 microM) had no effect on EPP run-down. 7. These results indicate that TC could have presynaptic effects via a presynaptic acetylcholine receptor, but that such a receptor may not have the same binding specificities as the postsynaptic receptor.

Facilitatory effects of dexamethasone on neuromuscular transmission

The beneficial effects of glucocorticoids in myasthenia gravis are attributed to their immunosuppressive actions. There are also studies reporting direct facilitatory as well as depressant effects of glucocorticoids on neuromuscular transmission. The effects of dexamethasone on neuromuscular transmission were studied by intracellular and extracellular microelectrode recording techniques in the mouse phrenic nerve-diaphragm preparation. Creatinine had to be added to the bathing media to prevent precipitation of the glucocorticoid with Ca 2+ and Mg 2+; creatinine had no effect. One hour of perfusion with dexamethasone (10 −4 to 10 −3 M) increased the frequency of miniature end-plate potentials (MEPPs), as well as the amplitude and quantum content of end-plate potentials (EPPs), but did not change MEPP amplitude, suggesting an increase in acetylcholine release. Dexamethasone also enhanced presynaptic facilitation and potentiation during repetitive stimulation. It had no effect on muscle resting membrane potential but increased the amplitude, overshoot, and rate of rise of muscle action potentials. The amplitudes of nerve terminal action potentials were also enhanced by dexamethasone. These findings suggest that glucocorticoids have a direct facilitatory action on neuromuscular transmission by a presynaptic action.

Antagonism of botulinum toxin paralysis by low temperature.

AbstractDecreasing the temperature from 37 C to 20 C in vitro reactivates muscle twitch in response to nerve stimulation in a muscle previously paralyzed by botulinum toxin. Transmitter release calculated as quantum content of endplate potentials has an extremely enhanced sensitivity to temperature in botulinum paralysis (Q10 about 45) compared to normal muscle blocked by high magnesium concentration (Q10 about 2). This difference is not caused by increased efficacy of the calcium mechanism of transmitter release in botulinum poisoning at low temperature, as demonstrated by unaltered calcium‐dependence of quantum content at 22 C compared to 35 C for both types of neuromuscular block. The results give exciting clues to the mode of action of botulinum toxin.

Inhibition by Triton X-100 of histamine liberation from mast cells induced by substance 48/80

The presynaptic action of carbacholine (Cch) was studied in experiments on the frog sartorius muscle neuromuscular preparation. Cch proved to decrease the quantum content of the end plate potential (EPP); this effect was induced by a direct Cch action on the motor nerve endings. d-tubocurarine decreased the sensitivity of the nerve endings to Cch significantly. Both d-tubocurarine and Cch had concurrent antagonistic interrelations in respect to their action on the quantum content of EPP. Atropine in low concentrations had no influence on the presynaptic effect of Cch. It was concluded that Cch decreased the quantum content of EPP through the N-cholinergic structures of the motor nerve endings.

Changes in total and quantal release of acetylcholine in the mouse diaphragm during activation and inhibition of membrane ATPase.

1. Acetylcholine (ACh) released from mouse diaphragm was gel filtrated and estimated by bio-assay and compared with electrophysiologically measured quantal release, expressed either as frequency of miniature end-plate potentials or quantum content of end-plate potentials. 2. Activation of Na+-K+-dependent membrane ATPase (membrane ATPase) in Na+-loaded muscles lowered the total amount of ACh released at rest to one tenth of the control value, but quantal release remained unchanged. 3. Inhibition of membrane ATPase by 2 X 10(-5) M-ouabain or by K-free solution led to an increase in total release and to a delayed progressive increase in quantal release. When Ca2+ was removed only the total release was enhanced. 4. Depolarization of the diaphragm by 8, 11 and 14 mM-K increased both total and quantal release only in the presence of Ca2+ in the perfusion medium. When Ca2+ was removed, no significant increase in release was observed. 5. The total and quantal release in response to 2 Hz stimulation of the preparation was increased 1.4 and 45 times, respectively. It is concluded that the total amount of ACh released at rest consists of two fractions, quantal and non-quantal, the former representing about 1% of the total release.

Myasthenia gravis: further electrophysiological and ultrastructural analysis of transmission failure in the mouse passive transfer model.

Using the mouse passive transfer model the mean amplitude of miniature endplate potentials and endplate potentials of mice treated with myasthenic immunoglobulins was markedly decreased. Miniature endplate potential frequency and quantum content of endplate potentials were normal, arguing against a major presynaptic disarrangement. Under electron-microscopy no gross structural alterations of endplates were demonstrated. It is concluded that the mouse passive transfer model closely resembles human myasthenia gravis of recent onset.

The effect of adenosine on the release of the transmitter from the phrenic nerve of the rat.

1. The action of adenosine on neuromuscular transmission has been studied on the rat phrenic nerve-diaphragm preparation.2. Adenosine (0.025-10 mM) reversibly reduced the quantum content of end-plate potentials and the frequency of miniature end-plate potentials to about half the control values in preparations in which transmission was blocked with high Mg(2+) and/or low Ca(2+) concentrations. Where transmission was blocked with tubocurarine, the amplitude of end-plate potentials was reduced to about half the control values by adenosine.3. In solutions with very low Ca(2+) concentrations (no added Ca(2+) and 1 mM-EGTA) adenosine had a smaller effect on the frequency of miniature end-plate potentials.4. The effect of adenosine (0.025 and 0.05 mM) was abolished by theophylline (1.8 mM).5. The results are discussed in relation to the increase in cyclic AMP in brain slices caused by adenosine and the abolition of this effect by theophylline (Sattin & Rall, 1970).

Reduction of the quantum content of endplate potentials by atropine.

Postsynaptische Potentiale der Muskelfasern (EPP) des isolierten Rattenzwerchfells wurden intrazellulär abgeleitet. Die statistische Analyse zeigt, dass der Mengenanteil von EPP bei Anwesenheit von Atropin vermindert ist und zwar simultan mit der postsynaptischen Blockade. Die Erholung des presynaptischen Effektes ist viel langsamer als diejenige des postsynaptischen.