Scientists at the University of California, San Francisco (UCSF) have found the first evidence that, despite their appearance, synaptic vesicles in two distinct pools in neurons have distinct identities and fates that are defined by the particular proteins on their surfaces.
Electrical impulses flowing through a neuron cause it to release tiny vesicle sacs, spilling their neurochemical contents into the synapse (a gap between the nerve ending and the next neuron). These chemicals then seep over to the adjoining neuron, sometimes triggering it to fire in turn.
But there appear to be two distinct pools of synaptic vesicles (tiny sacs filled with neurotransmitters) in an average neuron.
The smaller pool, found at the extreme end of the neuron, holds the vesicles that release neurotransmitters when an electrical impulse arrives. After release, the vesicles are quickly recycled for continued use, and for this reason scientists have called this the “recycling” pool of vesicles.
The second pool of vesicles can be much larger, accounting for up to 80 percent of all the vesicles at a synapse. Surprisingly, these vesicles do not respond to electrical impulses. Instead they sit dormant when the signal arrives and, because of this, scientists have dubbed this the “resting” pool.
Glowing proteins tell the story
Using a technique for labeling proteins with glowing molecules derived from jellyfish, the scientists were able to show that a protein called VAMP7 is present at high levels in the resting pool rather than the recycling pool, which contains more of other synaptic vesicle proteins.
Resting vesicles are involved in a separate, not-well-understood process in which neurons spontaneously release vesicles, which may help them adjust the types of connections they make with each other as well as the strength of those connections. The scientists said this process may play a role in neurological diseases such as Parkinson’s, many of which are characterized by changes in the type and strength of synapses.
According to UCSF professor Robert Edwards, the observation gives new insight into the function of the brain at the most basic, microscopic level and has far-reaching implications for our understanding of how neurotransmitters are packaged, transported and released from neurons.
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