Transmitter quantal size in Torpedo electrocytes is determined by frequency of release

Miniature end-plate potentials (MEPPs) were focally recorded from the cytoplasmic surface of electrocytes in isolated columns of the Torpedo electric organ. Double electrode studies showed that the junctional area was restricted to 12 μm 2. MEPP frequencies ranging from 1/min to 400/s were controlled with electrode advancement against the cytoplasmic surface. Stable membrane potentials and noise levels indicated constant intracellular, focal recording conditions. Focal MEPPs are only 1–3 mV and MEPP amplitudes smoothly decreased with an increase in MEPP frequency which demonstrates a process that meters quantal size at moment of release. Thus, release if not from a prepackaged store. MEPP interval analyses showed that events are weakly interactive at low frequencies and periodic at higher frequencies. The interdependency of MEPP amplitudes and intervals indicates that the mechanism of release controls both rate and quantal size. We propose that the amplitude and frequency dependencies of MEPPs at the Torpedo nerve–electrocyte junction are best described by a membrane channel (e.g., mediatophore, Israël and Dunant, Neurochem. Int. 28 (1996) 1–9) that meters transmitter from a presynaptic store.