The effect of potassium on exocytosis of transmitter at the frog neuromuscular junction.

Abstract

SUMMARY1. Electrophysiology and morphology have been combined to investigate the time course of the exocytosis of quanta of neurotransmitter induced by elevated concentrations of K+ at the frog neuromuscular junction.2. Replicas of freeze-fractured resting nerve terminals fixed in the presence of 20 mM-K+ showed images of fusion of synaptic vesicles with the presynaptic axolemma which were closely associated with the active zones. After 1 min in 20 mm-K+ fusions appeared also outside the active zones, and by 5 min they became uniformly distributed over the presynaptic membrane.3. The average total density of fusions was not significantly different at the various times examined since it decreased at the active zones while it increased over the rest of the membrane.4. Resting terminals fixed in 20 mM-K+ released 33000-45000 quanta after the addition of fixative; terminals stimulated by 20 mM-K+ for 1-5 min released 50000-100000 quanta during fixation. The fixative potentiated K+-induced transmitter release.5. Fusions were uniformly distributed in terminals pre-incubated for 5 min in 20 mM-K+ without added Ca2+, stimulated by adding Ca2`for 30 s, and then fixed. Conversely, after 5 min stimulation in hypertonic Ringer solution fusions remained predominantly located near the active zones. A similar distribution was observed after 15 min stimulation by a lower concentration of K+ (15 mM).6. At all concentrations of K+ tested (10, 15, 20, 25 mM) miniature end-plate potential (MEPP) rate attained a steady-state value within 10-15 min. Values from a single junction were generally lower at higher concentrations of K+, which indicates partial inactivation of the secretion-recycling process.7. The data indicate that K+ initially activates exocytosis at the active zones.Subsequently, ectopic exocytosis is activated while sites at the active zones appear to undergo partial inactivation. These phenomena are not related to the intensity or to the amount of previous secretion.

High-Concentration Rapid Transients of Glutamate Mediate Neural-Glial Communication via Ectopic Release

“…Ectopic release produces high concentrations of transmitter out- side of the synaptic cleft. The existence of such ectopic release has been suggested by freeze-fracture studies of the neuromuscular junction (Ceccarelli et al, 1979(Ceccarelli et al, , 1988, -4-(4-(dibutylamino)styryl) pyridinium dibromide] imaging studies of retinal bipolar cells (Zenisek et al, 2000(Zenisek et al, , 2003, ultrastructural studies of hair cells (Lenzi et al, 2002), and electrophysiological recordings of transmission between CFs and BGs Jahr, 2003, 2004).…”

Correlation between quantal secretion and vesicle loss at the frog neuromuscular junction.

“…Two possible explanations of this difference are: (1) that the fixative itself causes a large discharge of quanta when it is applied to secreting terminals (Ceccarelli, Fesce, Grohovaz & Haimann, 1988), and (2) that the difference arises randomly from the variations in the richness of the QUANTA AND SYNAPTIC VESICLES population of vesicles in terminals from different individual frogs, or from different groups of frogs. These variations can be quite large, and we have seen the average number of synaptic vesicles per cross-section range from 71 vesicles/section (Haimann et al 1985) to the 188 vesicles/section obtained in this work (control for groups E and F).…”

Differential Control of Synaptic and Ectopic Vesicular Release of Glutamate

“…Freeze-fracture studies of the neuromuscular junction (Ceccarelli et al, 1979(Ceccarelli et al, , 1988Grohovaz et al, 1989), FM1-43 imaging studies that use total internal reflection fluorescence microscopy of retinal bipolar cells (Zenisek et al, 2000(Zenisek et al, , 2003, and ultrastructural studies of hair cells (Beutner et al, 2001;Lenzi et al, 2002) all support the existence of ectopic release. How ectopic release is controlled and regulated remains primarily unknown.…”