The tiny spheres inside brain cells that ferry chemical messengers into the synapse make their rounds much more expeditiously than once assumed, National Institute of Mental Health (NIMH) - funded researchers have discovered. They used a dye to track the behavior of such synaptic vesicles in real time, in rat brain cells. Rather than fusing completely with the cell membrane and disgorging their dye contents all at once, brain vesicles more often remained intact, secreting only part of the tracer cargo in each of several repeated, fleeting contacts with the membrane, report Richard Tsien, D.Phil., Stanford University, and colleagues Alex Aravanis and Jason Pyle, in the June 5, 2003 Nature. Dubbed "kiss-and-run" recycling, this allows for more efficient communication between brain cells, suggest the researchers.
Brain cells communicate in a process that begins with an electrical signal and ends with a neurotransmitter binding to a receptor on the receiving neuron. It lasts less than a thousandth of a second, and is repeated billions of times daily in each of the human brain’s 100 billion neurons. Much of the action is happening inside the secreting cell. There, electrical impulses propel vesicles into the cell wall to spray the neurotransmitter into the synapse. Likened to soap bubbles merging, or bubbles bursting at the surface of boiling water, this process of membrane fusion (*RealPlayer format) may hold clues about what goes wrong in disorders of thinking, learning and memory, including schizophrenia and other mental illnesses thought to involve disturbances in neuronal communication.
Neurons must recycle a finite number of vesicles. In "classical" membrane fusion, the vesicle totally collapses and mixes with the cell membrane, requiring a complex and time-consuming and retrieval and recycling process. Yet, Tsien and colleagues point out that this process was discovered in huge neurons, such as those in squid giant synapses, with tens or hundreds of thousands of vesicles per nerve terminal. By contrast, they find that the comparatively tiny nerve terminals of the mammalian brain must make do with only about 30 functional vesicles – hardly enough to keep up with the split-second demands of synaptic communication if vesicles can only be replenished via the, one-shot classical process, they argue. Hence, the "kiss-and-run" hypothesis.
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