Analysis of Presynaptic Ca²⁺ Influx and Transmitter Release Kinetics during Facilitation at the Inhibitor of the Crayfish Neuromuscular Junction

Abstract

The inhibitory synapse of the crayfish neuromuscular junction was used to examine mechanisms underlying the F2 component of synaptic facilitation. Because previous studies have shown accelerated transmitter release during facilitation, we examined whether an activity-dependent plasticity in I Ca could underlie this acceleration. We established that fluorescent transients generated by Magnesium Green can resolve small differences in presynaptic Ca 2ϩ influx that correlate with changes in IPSC waveform. However, there was no change in Ca 2ϩ transients associated with the accelerated release. Analyzing the initial rise of IPSC and the duration of the presynaptic spike yielded a depolarization-release coupling plot that captures the impact of spike waveform on the initial rate of release. We conclude that accelerated release during F2 facilitation cannot be attributed to plasticity of I Ca or modulation of spike waveform. Kinetic analysis showed a reduction in synaptic delay during facilitation only when broad action potentials were used. In unfacilitated release, synaptic delay increased as spike duration lengthened. We propose that small single Ca 2ϩ channel currents during the plateau phase of broad action potentials raise local Ca 2ϩ concentration only enough to fill a high-affinity site. Occupation of this site in itself, or events downstream, would convert a vesicle from control to facilitated state. If the conversion were a slow process, it could explain the changes in synaptic delay reported here. This hypothesis can also account for a number of observations related to Ca 2ϩ cooperativity and synaptic facilitation.

Desynchronization of Multivesicular Release Enhances Purkinje Cell Output

“…The frequency dependent desynchronization we describe appears similar to the activity-induced prolongation of the phasic vesicle release rate at the calyx of Held (Fedchyshyn and Wang, 2007; Scheuss et al, 2007), hippocampus (Diamond and Jahr, 1995), cortex (Boudkkazi et al, 2007), and other synapses (Auger et al, 1998; Vyshedskiy et al, 2000; Waldeck et al, 2000). Yet activity-dependent desynchronization at CF synapses does not recruit delayed release, as described at other synapses following repetitive activation (Atluri and Regehr, 1998; Lu and Trussell, 2000; Hefft and Jonas, 2005; Iremonger and Bains, 2007).…”

“…It is harder, however, to predict the significance of desynchronized vesicle release on the millisecond timescale. This may explain why the physiological consequences of desynchronized release have remained largely unexplored, despite the demonstration that this form of release occurs throughout the brain (Diamond and Jahr, 1995; Auger et al, 1998; Vyshedskiy et al, 2000; Waldeck et al, 2000; Fedchyshyn and Wang, 2007; Scheuss et al, 2007). It is proposed that regulation of release latency on the millisecond timescale provides a temporal code that may enrich the storage capacity of neural networks (Boudkkazi et al, 2007).…”

Activity‐dependent changes in temporal components of neurotransmission at the juvenile mouse calyx of Held synapse

“…Little is known regarding whether the various temporal components of neurotransmission show activity‐dependent plasticity when synapses are presented with more physiological stimuli such as high frequency trains. Indeed, when this issue was investigated in non‐mammalian synapses using other protocols such as paired‐pulse stimuli or trains, it was demonstrated that SD could be modified under appropriate conditions (Vyshedskiy et al 2000; Waldeck et al 2000). At the calyx of Held‐MNTB synapse, increases in both AP latency and EPSC onset have also been observed following prolonged tetanic stimulation (Habets & Borst, 2005; Kim et al 2007).…”

“…However, recent evidence from other synapses, including the goldfish Mauthner cell synapse and crayfish NMJ, suggests that synaptic delay is modifiable. Paired‐pulse depression in the Mauthner cell synapse is associated with a prolongation in synaptic delay (Waldeck et al 2000), while facilitation at the crayfish NMJ leads to an activity‐dependent shortening in synaptic delay (Vyshedskiy et al 2000). These studies raise the possibility that the temporal components of neurotransmitter release, including synaptic delay, may undergo changes, either in parallel with or independently of, changes in synaptic strength.…”

An antibody to synaptotagmin I facilitates synaptic transmission

“…Vyshedskiy et al . (2000) showed that increases in the basal intracellular [Ca 2+ ] level reduced the delay and peak time of synaptic current at a tonic inhibitory synapse.…”