The effect of repetitive stimulation on facilitation of transmitter release at the frog neuromuscular junction

Abstract

1. End-plate potentials (e.p.p.s) were recorded from frog neuromuscular junctions blocked with high Mg and/or low Ca to characterize the processes underlying increased transmitter release during repetitive stimulation.2. There was a progressive increase in the amplitude of successive e.p.p.s during repetitive stimulation. Increasing the frequency or duration of stimulation increased this facilitation of e.p.p. amplitudes. Facilitation is defined as the fractional increase in amplitude of a test e.p.p. over a control.3. By assuming that each impulse in a train contributes an identical increment of facilitation that sums linearly with the facilitation contributed by the previous impulses, estimates of the facilitation contributed by a single impulse, f(t), were made from the incremental increase in e.p.p. amplitudes during repetitive stimulation. The average value of f(t) contributed by the first impulse in the train during stimulation at 20/sec is given by f(t) = 0.8 e(-t/50) + 0.12 e (-t/300) + 0.025 e(-t/3000),where t is in msec. The first two terms in this equation were independent of the stimulation rate used to determine f(t) while the coefficient of the third term was a function of the stimulation rate, decreasing 2 to 3 times when the stimulation rate was decreased from 20/sec to 1/sec.4. This linear facilitation model predicted growth of e.p.p. amplitudes during the first several hundred msec of repetitive stimulation. Thereafter, e.p.p. amplitudes were typically facilitated more than predicted by the linear model.5. Several new methods are presented which can be used to obtain estimates of the magnitude and time course of facilitation contributed by specific impulses during repetitive stimulation.6. It is found that the value of short-term f(t) in the tested range of 25-300 msec progressively increases during repetitive stimulation while its time course of decay remains unchanged. After 9 sec of stimulation at 20/sec, the short-term f(t) increased to 1.4 times control.7. The increase in short-term f(t) was independent of whether it was determined from a step increase or decrease in total facilitation, excluding the possibility that the observed increase in short-term f(t) resulted from a change in the rate of decay of facilitation.8. It is suggested with supporting data from the following paper (Magleby, 1973) that each impulse contributes two types of facilitation that are responsible for the growth of e.p.p.s during repetitive stimulation: a short-term facilitation with linear summation properties described by the first two terms in the expression in paragraph 3 and a long-term cumulative facilitation approximated by the third term. The long-term facilitation is expressed as an increase in both the short-term facilitation and in the base level of transmitter release. The relative contribution of these two expressions of the long-term facilitation to the third term is a function of the stimulation rate and is given by the ratio of facilitation to the base level of transmitter release.

Augmentation Increases Vesicular Release Probability in the Presence of Masking Depression at the Frog Neuromuscular Junction

“…Figures 1 and 2 show stimulation-induced changes in transmitter release using identical stimulation patterns for conditions of low quantal content with negligible depression ( During the conditioning train, the EPP amplitudes rapidly increased and then decayed back to control level after the train, with multiple apparent time constants, as has been reported previously (Magleby, 1979;Zengel and Magleby, 1982;Regehr and Stevens, 2001;Zucker and Regehr, 2002). The immediate drop in EPP amplitude during the first second following the train is primarily attributable to the rapid decay of facilitation, which has a time course of Ͻ1 sec (Mallart and Martin, 1967;Magleby, 1973a;Zengel and Magleby, 1982;Zucker and Regehr, 2002). After the decay of facilitation, the EPP amplitudes decay with two components, augmentation and potentiation (Magleby and Zengel, 1976a).…”

“…Endplate potential (EPP) amplitude was used as a measure of evoked transmitter release, as in previous studies (Fatt and Katz, 1951;Del Castillo and Katz, 1954;Magleby, 1973a;Wu and Betz, 1998). EPPs were recorded from cutaneous pectoris nervemuscle preparations Betz, 1996, 1998) from both northern and southern varieties of the frog Rana pipiens and also in a few experiments from the sartorius nerve-muscle preparation (Fatt and Katz, 1951).…”

“…The surface electrode was filled with the same solution as in the bath and positioned over an endplate region to obtain a maximal positive or negative amplitude response while just lightly touching the muscle surface. Surface recording averages the response from many endplates and gives a good measure of average intracellular activity (Magleby, 1973a). Such averaging reduces the number of trials required to average quantal fluctuation.…”

The Number of Components of Enhancement Contributing to Short-Term Synaptic Plasticity at the Neuromuscular Synapse during Patterned Nerve Stimulation Progressively Decreases As Basal Release Probability Is Increased from Low to Normal Levels by Changing Extracellular Ca2+

“…EPP amplitudes were recorded from the sartorius nerve-muscle preparation of northern grass frogs (Rana pipiens) of either sex with a surface electrode, as described previously (Magleby, 1973a). Animal protocols were approved by the Institutional Animal Care and Use Committee at the University of Miami Miller School of Medicine and are in accordance with National Research Council Guidelines for the Care and Use of Laboratory Animals.…”

“…Components of STP that increase transmitter release (enhancement) include first (F1) and second (F2) components of facilitation with time constants of 50 and 300 ms (Mallart and Martin, 1967;Magleby, 1973a;Zengel and Magleby, 1982;Goda and Stevens, 1994;Bennett et al, 2007), augmentation ( A) with a time constant of 4 -7 s (Magleby and Zengel, 1976a;Zengel and Magleby, 1982;Stevens and Wesseling, 1999), and potentiation ( P) with a time constant of tens of seconds to minutes (Liley and North, 1953;Magleby, 1973b). For low basal vesicular release probability, prob 0 , all four enhancement processes, F1, F2, A, and P, make major contributions to STP during repetitive nerve activity .…”