Existence of different populations of unitary evoked postsynaptic potentials at the frog neuromuscular junction]

The postsynaptic response to monoquantic evoked transmitter release (mean quantal content about 0.3) has been studied at temperatures from 10 to 23 degrees C. The delay between nerve stimulation (1 to 10 Hz) and the unitary postsynaptic potentials fluctuates by steps. The existence of preferential delay sites can always be detected (mean number 13.5 +/- 3.1). Identical delay unitary postsynaptic potentials often shows identical amplitude and identical time to peak. These results suggest that few emitting sites are preferentially activated along the nerve terminal at low level release during long lasting stimulation. The "single process" assumption used in statistical studies of transmitter release is probably oversimplified.     

Activity increases quantal size at the frog neuromuscular junction

Two h of nerve stimulation at 10 Hz or of elevated spontaneous release in hypertonic solution increased the size of miniature end-plate potentials (m.e.p.p.'s) and currents at the frog neuromuscular junction, probably by increasing the amount of acetylcholine in a quantum. Increases in quantal size may modulate synaptic transmission.     

Activity increases quantal size at the frog neuromuscular junction

Summary Two h of nerve stimulation at 10 Hz or of elevated spontaneous release in hypertonic solution increased the size of miniature end-plate potentials (m.e.p.p.'s) and currents at the frog neuromuscular junction, probably by increasing the amount of acetylcholine in a quantum. Increases in quantal size may modulate synaptic transmission.Acknowledgments. Supported by grant 10320 from the NINCDS and by a grant from the MDA. We are grateful to Dr I.S. Cohen for participating in some of the experiments and for helpful criticism, and to Drs C. Clausen and G. Baldo for help with the computer. Some of the results were presented at a meeting of the Physiological Society.     

Spontaneous miniature activity and gradation of transmission at the neuromuscular junction

Zusammenfassung Aus den bisher veröffentlichten experimentellen Befunden über presynaptische, für die neuromuskuläre Erregungsübertragung massgeblich erscheinende Vorgänge wird geschlossen, dass die Förderung der Erregungsübertragung in direkter Beziehung zur Grösse des Aktionspotentials in den motorischen Nervenendigungen steht, während die Frequenz der spontanen Miniaturentladungen an sich kein eindeutiges Mass für die Intensität des Übertragungsvorganges von Nervimpulsen zum Muskel darstellt.     

Evolution in time of transmitter release at frog neuromuscular junction]

It has previously been shown that the unitary quantal end-plate potentials observed at synapses blocked by low Ca++ high Mg++ ringer, belong to distinct clusters according to their amplitude, time to peak and latency characteristics. These clusters correspond probably to distinct releasing units dispersed along the presynaptic terminal branches. The distribution versus time of the occurence of unitary potentials belonging to one latency cluster has been studied over long lasting evoked nerve activity (stimulation frequencies: 1 to 10 Hz). It was observed that transmitter release at one releasing site is statistically periodic with emitting periods separated by rest periods. At a given end-plate the period of emitting activity seems to be independent from one emitting site to another.     

Changes in transmitter release at frog neuromuscular junction induced by 4-aminopyridine]

4-aminopyridine (4-AP) at micromolar concentrations, increases the end-plate potential amplitude in curarized preparations and the mean quantal content in every preparation tested, but the spontaneous release is not modified by 4-AP. These results can explain the anticurare activity observed in the wole animal or in vitro. 4-AP prolongs the falling phase of the muscle action potential without change in the muscle membrane potential.     

Spontaneous miniature potentials in primary afferent fibres.

Intracellular recording from primary afferent fibres of the isolated frog spinal cord revealed the existence of spontaneous synaptic activity, related probably to the firing of presynaptic inhibitory fibres.     

Morphological evidence for tracer uptake at the active zones of stimulated frog neuromuscular junction

Lanthanum or ferritin added to the fixative were found in small and non-coated vesicles located at active zones in nerve-muscle preparations stimulated by potassium during cold aldehyde fixation. The presence of labeled vesicles at active zones supports the hypothesis that a double process of exo-endocytosis might occur under moderate stimulation conditions.     

The effect of charatoxin, 4-methylthio-1,2-dithiolane,on the frog sartorius neuromuscular junction

Summary The insecticide 4-methylthio-1,2-dithiolane, named charatoxin, blocks the frog muscular twitches elicited through the neuromuscular junction. The activity level and the course of inhibition is comparable to that of 4-dimethyl-amino-1,2-dithiolane, nereistoxin.The authors are grateful to Dr N. Jacobsen, Cheminova A/S for synthesizing the toxins and the 2 analogues.     

The effect of charatoxin, 4-methylthio-1,2-dithiolane, on the frog sartorius neuromuscular junction

The insecticide 4-methylthio-1,2-dithiolane, named charatoxin, blocks the frog muscular twitches elicited through the neuromuscular junction. The activity level and the course of inhibition is comparable to that of 4-dimethyl-amino-1,2-dithiolane, nereistoxin.     

Spontaneous miniature potentials in primary afferent fibres

Summary Intracellular recording from primary afferent fibres of the isolated frog spinal cord revealed the existence of spontaneous synaptic activity, related probably to the firing of presynaptic inhibitory fibres.     

Dantrolene and the effect of temperature on the spontaneous release of transmitter at the frog neuromuscular junction

The characteristic effect of temperature on m.e.p.p. frequency at the amphibian neuromuscular junction is unaltered by the presence of Dantrolene (an agent that is believed to reduce the efflux of Ca2+ from intracellular stores) or by changes in [Ca2+)o. It is concluded that temperature affects the release system directly, with a transition temperature at about 16 degrees C.     

Dantrolene and the effect of temperature on the spontaneous release of transmitter at the frog neuromuscular junction

Summary The characteristic effect of temperature on m.e.p.p. frequency at the amphibian neuromuscular junction is unaltered by the presence of Dantrolene (an agent that is believed to reduce the efflux of Ca²⁺ from intracellular stores) or by changes in [Ca²⁺]_o. It is concluded that temperature affects the release system directly, with a transition temperature at about 16°C.     

Formation of the neuromuscular junction in cultures of rat embryonic cells]

The morphological evidence of the primary nerve muscle contacts are described. They consist of areas of cholinesterase activity (detected histochemically) localized on the myotube membranes and of mutiple clusters of ACh receptors whose 125I-alpha-bungarotoxin binding sites are revealed by radio-autography. After the stage of the primary nerve muscle contacts, some of which seem transient, characteristic neuromuscular junctions appear. These neuromuscular junctions which possess subneural infoldings are similar to the end-plates of the Rat in vivo.     

Depletion of synaptic vesicles at the frog (Rana pipiens) neuromuscular junctions by tetraphenylboron.

At the frog neuromuscular junction tetraphenylboron produced a decrease in miniature endplate potential amplitude, accompanied by a decrease in the number but not the size of synaptic vesicles.     

Effects of calcium and magnesium ions on the frequency of miniature end-plate potential discharges in amphibian muscle in the presence of ethyl alcohol

Zusammenfassung Während bekanntlich Kalzium- und Magnesiumionen keinen Einfluss auf die Frequenz der Spontanrhythmik (m.e.p.p.) der Kaltblütermuskeln in der normalen Ringer-Lösung haben, konnte experimentell festgestellt werden, dass die Frequenz der Spontanrhythmik in einer Äthanol-Ringer-Lösung durch Zusatz von Kalziumionen vermehrt, von Magnesiumionen vermindert wird.