Focal, extracellular recording of slow miniature junctional potentials at the mouse neuromuscular junction.

Miniature endplate potentials (MEPPs) with slow rising phase can be attributed either to burst of transmitter releases or to distortion of conduction from remote releasing sites. The spontaneous activity of neuromuscular junctions recorded extracellularly at mouse diaphragms using sharp electrodes was analyzed to test these two hypotheses. The miniature junctional potentials (MEJPs) frequencies observed intracellularly as compared to MEPP frequency measured intracellularly in controls indicate that most events recorded extracellularly are induced by the presence of the electrode. All types of MEPPs (bell-MEPPs, skew-MEPPs, slow-, and giant MEPPs) previously described with intracellular recording methods (Vautrin and Kriebel, Neuroscience 41:71-88, 1991) were observed extracellularly and showed similar characteristics. This means that the presynaptic and postsynaptic zones that generate these synaptic events are restricted within areas of a few micrometers squared of synaptic contact. Long rise times of extracellularly recorded synaptic spontaneous events may be explained by multiple transmitter releases at intervals shorter than the rise time of individual events, which postsynaptic responses fuse into a single peak.