Effects of adenosine on Ca2+ entry in the nerve terminal of the frog neuromuscular junction

1999 ◽  
Vol 77 (9) ◽  
pp. 707-714 ◽  
Author(s):  
Richard Robitaille ◽  
Sébastien Thomas ◽  
Milton P Charlton
Keyword(s):  
Neuromuscular Junction ◽  
Nerve Terminal ◽  
Nerve Stimulation ◽  
High Frequency ◽  
Transmitter Release ◽  
Synaptic Depression ◽  
Presynaptic Terminal ◽  
Nerve Terminals ◽  
Frog Neuromuscular Junction ◽  
Induced Changes

This study aimed to test whether nerve-evoked and adenosine-induced synaptic depression are due to reduction in Ca2+ entry in nerve terminals of the frog neuromuscular junction. Nerve terminals were loaded with the fluorescent Ca2+ indicator fluo 3 (fluo 3-AM) or loaded with dextran-coupled Ca2+ green-1 transported from the cut end of the nerve. Adenosine (10-50 µM) did not change the resting level of Ca2+ in the presynaptic terminal, whereas it induced large Ca2+ responses in perisynaptic Schwann cells, indicating that adenosine was active and might have induced changes in the level of Ca2+ in the nerve terminal. Ca2+ responses in nerve terminals could be induced by nerve stimulation (0.5 or 100 Hz for 100 ms) over several hours. In the presence of adenosine (10 µM), the size and duration of the nerve-evoked Ca2+ responses were unchanged. When extracellular Ca2+ concentration was lowered to produce the same reduction in transmitter release as the application of adenosine, Ca2+ responses induced by nerve stimulations were reduced by 40%. This indicates that changes in Ca2+ responsible for the decrease in release should have been detected if the mechanism of adenosine depression involved partial block of Ca2+ influx. Ca2+ responses evoked by prolonged high frequency trains of stimuli (50 Hz for 10 or 30 s), which caused profound depression of transmitter release, were sustained during the whole duration of the stimulation, and adenosine had no effect on these responses. These data indicate that neither adenosine induced synaptic depression nor stimulation-induced synaptic depression are caused by reductions in Ca2+ entry into the presynaptic terminal in the frog neuromuscular junction.Key words: adenosine, Ca2+, nerve terminal, transmitter release, synaptic depression.

scholarly journals Changes in miniature endplate potential frequency during repetitive nerve stimulation in the presence of Ca2+, Ba2+, and Sr2+ at the frog neuromuscular junction.

1981 ◽  
Vol 77 (5) ◽  
pp. 503-529 ◽  
Author(s):  
J E Zengel ◽  
K L Magleby
Keyword(s):  
Nerve Stimulation ◽  
Transmitter Release ◽  
Neuromuscular Junctions ◽  
Fourth Power ◽  
Bathing Solution ◽  
Frog Neuromuscular Junction ◽  
Repetitive Nerve Stimulation ◽  
Time Constants ◽  
Endplate Potential ◽  
Induced Changes

Miniature endplate potentials (MEPPs) were recorded from frog sartorious neuromuscular junctions under conditions of reduced quantal contents to study the effect of repetitive nerve stimulation on asynchronous (tonic) quantal transmitter release. MEPP frequency increased during repetitive stimulation and then decayed back to the control level after the conditioning trains. The decay of the increased MEPP frequency after 100-to 200-impulse conditioning trains can be described by four components that decayed exponentially with time constants of about 50 ms, 500 ms, 7 s, and 80 s. These time constants are similar to those for the decay of stimulation-induced changes in synchronous (phasic) transmitter release, as measured by endplate potential (EPP) amplitudes, corresponding, respectively, to the first and second components of facilitation, augmentation, and potentiation. The addition of small amounts of Ca2+ or Ba2+ to the Ca2+-containing bathing solution, or the replacement of Ca2+ with Sr2+, led to a greater increase in the stimulation-induced increases in MEPP frequency. The Sr-induced increase in MEPP frequency was associated with an increase in the second component of facilitation of MEPP frequency; the Ba-induced increase with an increase in augmentation. These effects of Sr2+ and Ba2+ on stimulation-induced changes in MEPP frequency are similar to the effects of these ions on stimulation-induced changes in EPP amplitude. These ionic similarities and the similar kinetics of decay suggest that stimulation induced changes in MEPP frequency and EPP amplitude have some similar underlying mechanisms. Calculations are presented which show that a fourth power residual calcium model for stimulation-induced changes in transmitter release cannot readily account for the observation that stimulation-induced changes in MEPP frequency and EPP amplitude have similar time-courses.

Intraterminal Ca2+ and Spontaneous Transmitter Release at the Frog Neuromuscular Junction

2001 ◽  
Vol 85 (1) ◽  
pp. 287-294 ◽  
Author(s):  
J. K. Angleson ◽  
W. J. Betz
Keyword(s):  
Neuromuscular Junction ◽  
Phorbol Ester ◽  
Intracellular Recording ◽  
Transmitter Release ◽  
The Other ◽  
Nerve Terminals ◽  
Frog Neuromuscular Junction ◽  
Tetraacetic Acid ◽  
Ratiometric Imaging ◽  
The Relationship

We investigated the relationship between intraterminal Ca2+concentration ([Ca2+]i) and the frequency of miniature end plate potentials (MEPPs) at the frog neuromuscular junction by use of ratiometric imaging of fura-2-loaded nerve terminals and intracellular recording of MEPPs. Elevation of extracellular [KCl] over the range of 2–20 mM resulted in increases in [Ca2+]i and MEPP frequency. Loading terminals with the fast and slow Ca2+-buffers bis-( o-aminophenoxy)- N,N,N′,N′-tetraacetic acid-acetoxymethyl (BAPTA-AM) and EGTA-AM resulted in equivalent reductions in the KCl-dependent increases in MEPP frequency. The [Ca2+]i dependence of MEPP frequency determined by elevation of [Ca2+]i due to application of 0.1–10 μM ionomycin was similar to that determined when [Ca2+]i was raised by increasing extracellular KCl. Measurements in 10 mM extracellular KCl revealed that application of the phorbol ester phorbol 12 myristate 13-acetetate (PMA) caused an increase in MEPP frequency while the inactive analogue, 4α-PMA, did not. PMA application also caused an increase in [Ca2+]i. The relationship between [Ca2+]i and MEPP frequency in PMA was the same as was determined by the other methods of raising [Ca2+]i. Under all conditions tested, our data revealed a low [Ca2+]i threshold for activation of transmitter release and are consistent with a K d for [Ca2+]i on the order of 1 μM.

Synaptic depression in frog neuromuscular junction

1987 ◽  
Vol 58 (1) ◽  
pp. 230-246 ◽  
Author(s):  
M. I. Glavinovic
Keyword(s):  
Nerve Stimulation ◽  
High Frequency ◽  
Synaptic Depression ◽  
High Frequency Stimulation ◽  
Frog Neuromuscular Junction ◽  
Choline Uptake ◽  
Uptake System ◽  
End Plate ◽  
Quantal Size ◽  
Frequency Stimulation

1. The amplitudes of end-plate currents (EPCs) evoked by stimulating the nerve with frequencies ranging from 1 to 5 Hz and the amplitudes of miniature end-plate currents (MEPCs) gradually diminish if choline uptake is blocked by hemicholinium-3 (HC-3, 20 microM). This reduction of EPC amplitudes is predominantly of presynaptic origin, although an observed decrease in MEPC amplitudes suggests that some postsynaptic changes [due to direct action of HC-3 on acetylcholine (ACh) receptors or on open ACh channels] also occurs. 2. Shortening of both EPCs and MEPCs is observed during high-frequency stimulation (5 Hz) in the presence of cholinesterase inhibitor after impairment of ACh synthesis. Shortening of MEPCs probably results from a direct blocking action of HC-3 on open ACh channels, as well as from reduction in quantal size. Shortening of EPCs is more pronounced (EPCs eventually have shorter time courses than MEPCs) and usually does not result from a gradual reduction in the spatial overlap of quantal events (because of reduced quantal content) or from a diminished 'lingering ACh' (ACh that remains in the synaptic cleft between nerve impulses), but rather from a much reduced quantal size of nerve-evoked quanta. 3. It therefore appears that the quanta that are released by nerve stimulation are preferentially filled with newly synthesized ACh. In its absence nerve stimulation leads to secretion of only partially filled quanta. This occurs simultaneously with spontaneous secretion of almost normally filled quanta. Hence it seems that the quantal discharge is not strongly dependent, if at all, on its ACh content. Moreover, the correspondence between the quantal sizes of nerve-evoked and spontaneously released quanta does not remain valid during high-frequency prolonged stimulation. 4. Even with the choline uptake system intact, prolonged high-frequency stimulation leads to a gradual shortening of EPCs and, to a small extent, MEPCs. Shortening of EPCs appears to be mainly a result of a reduction of their quantal size. 5. It is estimated from the shortening of EPCs and the known EPC versus MEPC relationship that the reduction of the quantal sizes of nerve-evoked quanta probably contributes very significantly to synaptic depression that occurs during prolonged high-frequency nerve stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

scholarly journals Differential effects of Ba2+, Sr2+, and Ca2+ on stimulation-induced changes in transmitter release at the frog neuromuscular junction.

1980 ◽  
Vol 76 (2) ◽  
pp. 175-211 ◽  
Author(s):  
J E Zengel ◽  
K L Magleby
Keyword(s):  
Neuromuscular Junction ◽  
Time Constant ◽  
Transmitter Release ◽  
Repetitive Stimulation ◽  
Kinetic Properties ◽  
Bathing Solution ◽  
Frog Neuromuscular Junction ◽  
Induced Changes ◽  
Endplate Potentials ◽  
Frog Sartorius

Endplate potentials (EPP) were recorded from the frog sartorius neuromuscular junction under conditions of low quantal content to study the effect of Ba2+, Sr2+, and Ca2+ on the changes in evoked transmitter release that occur during and after repetitive stimulation. The addition of 0.1-1 mM Ba2+ or Sr2+ to the Ca2+-containing bathing solution, or the replacement of Ca2+ with 0.8-1.4 mM Sr2+, led to a greater increase in EPP amplitudes during and immediately after repetitive stimulation. These changes in release were analyzed in terms of the four apparent components of increased transmitter release that have previously been distinguished on the basis of their kinetic properties. The Ba2+-induced increase in EPP amplitudes was associated with an increase in the magnitude but not the time constant of decay of augmentation. Ba2+ had little effect on potentiation or the first and second components of facilitation. The Sr2+-induced increase in EPP amplitudes was associated with an increase in the magnitude and the time constant of decay of the second component of facilitation. Sr2+ had little effect on potentiation, augmentation, or the first component of facilitation. The selective effects of Ba2+ on augmentation and of Sr2+ on the second component of facilitation were reversible and could be obtained in the presence of the other ion. The addition of 0.1-0.3 mM Ca2+ to the bathing solution had little effect on potentiation, augmentation, or the two components of facilitation. These results provide pharmacological support for the proposal that there are four different components of increased transmitter release associated with repetitive stimulation and suggest that the underlying factors in the nerve terminal that give rise to these components can act somewhat independently of one another.

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