Modulation of Synaptic Vesicle Exocytosis in Muscle-Dependent Long-Term Depression at the Amphibian Neuromuscular Junction

Abstract

We have labeled recycling synaptic vesicles at the somatic Bufo marinus neuromuscular junction with the styryl dye FM2-10 and provide direct evidence for refractoriness of exocytosis associated with a muscle activity-dependent form of long-term depression (LTD) at this synapse. FM2-10 dye unloading experiments demonstrated that the rate of vesicle exocytosis from the release ready pool (RRP) of vesicles was more than halved in the LTD (induced by 20 min of low frequency stimulation). Recovery from LTD, observed as a partial recovery of nerve-evoked muscle twitch amplitude, was accompanied by partial recovery of the refractoriness of RRP exocytosis. Unexpectedly, paired pulse plasticity, another routinely used indicator of presynaptic forms of synaptic plasticity, was unchanged in the LTD. We conclude that the LTD induces refractoriness of the neuromuscular vesicle release machinery downstream of presynaptic calcium entry.

A Novel Striated Muscle-Specific Myosin-Blocking Drug for the Study of Neuromuscular Physiology

“…At this concentration, muscle action potentials (APs) in response to nerve stimulation were much smaller than expected (10–20 mV vs. 70–90 mV; van Lunteren and Moyer, 1998; n = 5 from three mice; data not shown). At higher doses of BTS, such as those used in previous studies of the toad NMJ (50 μM; Etherington et al, 2014), nerve-evoked muscle potentials decreased below 5 mV ( n = 5 from three mice; Supplementary Figure 1). …”

“…Finally, the analysis of neurotransmission in embryonic mouse mutants could be assessed for the first time, since Na v 1.5 channels, which are resistant to μ-CTX (Wilson et al, 2011), are expressed in fetal muscle, rather than μ-CTX-sensitive Na v 1.4 channels (Lupa et al, 1993). Recently, a drug originally identified in a skeletal muscle-specific myosin ATPase-inhibitor screen, N -benzyl- p -toluene sulfonamide (BTS; Cheung et al, 2002), was used to measure muscle APs and ionic currents in adult mouse skeletal muscle cells Wang et al, 1999; Woods et al, 2004), as well as to measure nerve stimulation-induced muscle APs at the cane toad NMJ (Etherington et al, 2014). …”