Augmentation: A process that acts to increase transmitter release at the frog neuromuscular junction.

Abstract

SUMMARY1. End-plate potentials (e.p.p.s) were recorded from frog neuromuscular junctions bathed in Ringer solution containing increased Mg and decreased Ca to reduce transmitter release. Conditioning and testing stimulation was applied to the nerve to study a previously uncharacterized process which acts to increase e.p.p. amplitudes. We will refer to this process as augmentation.2. Following repetitive stimulation augmentation decayed approximately exponentially over most of its time course with a mean time constant of about 7 see (range 4-10 see) which is intermediate in duration between the time constants for 'the decay of facilitation and potentiation.3. The magnitude of augmentation increased with the duration of the conditioning stimulation. Assuming a multiplicative relationship between augmentation and potentiation, values of the magnitude of augmentation ranged from 0 3 to 0-6 following 50 impulses at 20/sec to 0-5-7-8 following 600 impulses at 20/sec. (An augmentation of 0 3 and 7-8 would increase e.p.p. amplitudes 1-3 and 8-8 times, respectively.) 4. The time constant characterizing the decay of augmentation remained relatively constant as the duration of the conditioning stimulation was increased.5. Augmentation as well as facilitation and potentiation resulted from an increase in the number of quanta of transmitter released from the nerve terminal.6. Augmentation decayed faster at higher temperatures with a mean temperature coefficient, Q10, of about 3-8. The corresponding Q10 for the decay of potentiation was found to be about 2'4. 7. It is concluded that augmentation can be a significant factor in increasing transmitter release and will therefore have to be accounted for when studying the effects of repetitive stimulation on the function of the nerve terminal or when formulating models of transmitter release.

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

“…In some trials, large erratic changes in EPP amplitude would occur. This was assumed to result from nerve failure, and these trials were discarded, as in previous studies of this type (Magleby and Zengel, 1976a).…”

“…Such short-term synaptic plasticity has been divided into processes of synaptic enhancement that increase transmitter release and synaptic depression (D) that decrease release. Components of enhancement include facilitation, augmentation (A), and potentiation (P), with time constants of Ͻ1 sec, ϳ7 sec, and tens of seconds to minutes, respectively (Mallart and Martin, 1967;Rosenthal, 1969;Magleby and Zengel, 1976a;McNaughton, 1982;Poage and Zengel, 1993). Components of depression include very fast, fast, and slow components with time constants of recovery of Ͻ0.5 sec, ϳ6 sec, and tens of seconds to minutes, respectively (Takeuchi, 1958;Magleby, 1973b;Dittman and Regehr, 1998;Wu and Betz, 1998;Stevens and Wesseling, 1999b;Wesseling and Lo, 2002) In this report, we study the interaction between augmentation and the fast (ϳ6 sec) component of depression to determine whether augmentation continues to enhance transmitter release when depression dominates the response.…”

A prelude to long-term potentiation

“…These appear to be common to virtually all excitatory synapses, and have been particularly well studied by Magleby & Zengel (1975, 1976, 1982. After a single synaptic activation, excitatory synapses remain in a state that allows the synapse to release more transmitter for a period, peaking at ca.…”

Effects of diacylglycerol and phorbol ester on acetylcholine release and action at the neuromuscular junction in mice

“…The early decline in the amplitude of e.p.ps in the presence of (+)-tubocurarine (Tc) at 20Hz stimulation may be due to the depletion of a store of synaptic vesicles which are immediately available for release (Elmqvist & Quastel, 1965). However, the decline is probably at least partly due to a presynaptic effect of Tc (see for example, Gibb & Marshall, 1984 (Magleby & Zengel, 1976). Neither PDB nor OAG had a significant effect on the amplitude or quantal content of the e.p.p.…”