Intracellular Acidification Suppresses Synaptic Vesicle Mobilization in the Motor Nerve Terminals.

Intracellular protons play a special role in the regulation of presynaptic processes, since the functioning of synaptic vesicles and endosomes depends on their acidification by the H+-pump. Furthermore, transient acidification of the intraterminal space occurs during synaptic activity. Using microelectrode recording of postsynaptic responses (an indicator of neurotransmitter release) and exo-endocytic marker FM1-43, we studied the effects of intracellular acidification with propionate on the presynaptic events underlying neurotransmitter release. Cytoplasmic acidification led to a marked decrease in neurotransmitter release during the first minute of a 20-Hz stimulation in the neuromuscular junctions of mouse diaphragm and frog cutaneous pectoris muscle. This was accompanied by a reduction in the FM1-43 loss during synaptic vesicle exocytosis in response to the stimulation. Estimation of the endocytic uptake of FM1-43 showed no disruption in synaptic vesicle endocytosis. Acidification completely prevented the action of the cell-membrane permeable compound 24-hydroxycholesterol, which can enhance synaptic vesicle mobilization. Thus, the obtained results suggest that an increase in [H+]in negatively regulates neurotransmission due to the suppression of synaptic vesicle delivery to the sites of exocytosis at high activity. This mechanism can be a part of the negative feedback loop in regulating neurotransmitter release.

The vesicle cycle in motor nerve endings of the mouse diaphragm.

Experiments on the mouse diaphragm muscle using intracellular microelectrode recordings and fluorescence microscopy were performed to study the dynamics of transmitter secretion and synaptic vesicle recycling processes (the exocytosis-endocytosis cycle) in motor nerve endings (NE) during prolonged rhythmic stimulation (20 impulses/sec). During stimulation, there were triphasic changes in the amplitude of endplate potentials (EPP): an initial rapid reduction, followed by prolonged (1-2 min) stabilization of amplitude, i.e., a plateau, and then a further slow decrease. Restoration of EPP amplitude after stimulation for 3 min occurred over a period of several seconds. Loading of synaptic vesicles with the fluorescent endocytic stain FM1-43 showed that rhythmic stimulation led to a gradual (over 5-6 min) decrease in NE fluorescence, demonstrating exocytosis of synaptic vesicles. Quantum analysis of the electrophysiological data and comparison of these data with results from fluorescence studies suggested that mouse NE have a high rate of endocytosis and reutilization of synaptic vesicles (the mean recycling time was about 50 sec), which may support the maintenance of reliable synaptic transmission during prolonged high-frequency activity. The sizes of the release-ready and recycling pools of synaptic vesicles were determined quantitatively. It is suggested that vesicle recycling in mouse NE occurs via a short, rapid pathway with incorporation into the recycling pool. Vesicles of the reserve pool are not used for transmitter secretion in the stimulation conditions used here.