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Estimation of the presynaptic calcium level is a key task in studying synaptic transmission since calcium entry into the presynaptic cell triggers a cascade of events leading to neurotransmitter release. Moreover, changes in presynaptic calcium levels mediate the activity of many intracellular proteins and play an important role in synaptic plasticity. Studying calcium signaling is also important for finding ways to treat neurodegenerative diseases. The neuromuscular junction is a suitable model for studying synaptic plasticity, as it has only one type of neurotransmitter. This article describes the method for loading a calcium-sensitive dye through the cut nerve bundle into the mice's motor nerve endings. This method allows the estimation of all parameters related to intracellular calcium changes, such as basal calcium level and calcium transient. Since the influx of calcium from the cell exterior into the nerve terminals and its binding/unbinding to the calcium-sensitive dye occur within the range of a few milliseconds, a speedy imaging system is required to record these events. Indeed, high-speed cameras are commonly used for the registration of fast calcium changes, but they have low image resolution parameters. The protocol presented here for recording calcium transient allows extremely good spatial-temporal resolution provided by confocal microscopy.

In mouse neuromuscular junction, the amplitude of the presynaptic calcium (Ca2+) transient was measured and correlated with mediator release at different extracellular Ca2+ concentrations. Fluorescent calcium-sensitive dye Oregon Green 488 BAPTA 1 hexapotassium salt was used for Ca2+ transient registration. The quantal content of release was assessed by the amplitude of the endplate potentials (EPPs) and was measured using intracellular microelectrodes. The amplitude of the EPPs changed more significantly than the amplitude of the Ca2+ transient when the extracellular calcium concentration was changed. Linear approximation of the dependence of the quantal content on the amplitude of the Ca2+ transient on double logarithmic scale gave a slope showing that the biochemical cooperativity was 2.86. The obtained value is comparable with the data calculated earlier in the neuromuscular junction of the rat and other synapses using electrophysiological measurements. Our data suggest that the change of the Ca2+ transients recorded from the whole volume of the nerve terminal properly reflects the variation of calcium concentration responsible for the neurotransmitter release in active zone. Thus, analysis of the bulk Ca2+ transient can be used to evaluate the calcium entry into the nerve endings and compare it with the number of quanta released under different conditions.

Septins (Sept) are highly conserved Guanosine-5'-triphosphate (GTP)-binding cytoskeletal proteins involved in neuronal signaling in the central nervous system but their involvement in signal transmission in peripheral synapses remains unclear. Sept5 and Sept9 proteins were detected in mouse peripheral neuromuscular junctions by immunofluorescence with a greater degree of co-localization with presynaptic than postsynaptic membranes. Preincubation of neuromuscular junction preparations with the inhibitor of Sept dynamics, forchlorfenuron (FCF), decreased co-localization of Sept with presynaptic membranes. FCF introduced ex vivo or in vivo had no effect on the amplitude of the spontaneous endplate currents (EPCs), indicating the absence of postsynaptic effects of FCF. However, FCF decreased acetylcholine (ACh) quantal release in response to nerve stimulation, reduced the amplitude of evoked quantal currents and decreased the number of quanta with long synaptic delays, demonstrating the presynaptic action of FCF. Nevertheless, FCF had no effect on the amplitude of calcium transient in nerve terminals, as detected by calcium-sensitive dye, and slightly decreased the ratio of the second response amplitude to the first one in paired-pulse experiments. These results suggest that FCF-induced decrease in ACh quantal secretion is not due to a decrease in Ca2+ influx but is likely related to the impairment of later stages occurring after Ca2+ entry, such as trafficking, docking or membrane fusion of synaptic vesicles. Therefore, Sept9 and Sept5 are abundantly expressed in presynaptic membranes, and disruption of Sept dynamics suppresses the evoked synchronous and delayed asynchronous quantal release of ACh, strongly suggesting an important role of Sept in the regulation of neurotransmission in peripheral synapses.