Two distinct pools of synaptic vesicles appear to be involved in the spontaneous release of neurotransmitters and in neurotransmission triggered by a stimulus, researchers report in Neuron this week. Their findings raise questions about a fundamental theory of neurotransmission developed by Bernard Katz.

Ege T. Kavalali and colleagues from the University of Texas (UT) Southwestern Medical Center studied the spontaneous fusion of synaptic vesicles in rat hippocampal cells, using dyes that fluoresce when inside a cell membrane.

The researchers found that when vesicles were allowed to fill in the absence of network activity and then subjected to stimulation, loss of dye was slow. When activity was induced, vesicle destaining showed an initial rapid phase, then slowed to the rate of spontaneously loaded vesicles.

Their conclusion was that there may be two types of recycling vesicles. Spontaneously recycling vesicles are reluctant to release neurotransmitters during stimulation, but do so readily...

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