Excess transport vesicles circulate constantly through the axon and nerve terminal as though on a conveyer belt, based on work from Dinara Shakiryanova, Arvonn Tully, and Edwin Levitan (University of Pittsburgh, Pittsburgh, PA). The constant stream gives active synapses immediate access to a boost in neuropeptides.
Synapses were thought to be at the mercy of the cell soma for the delivery of vesicles containing neuropeptides, which are released upon synapse activation. Levitan and others wondered how this set up allows synapses to be dynamic, since the soma can be far from a nerve terminal. “It can take days,” says Levitan, “to get stuff shipped down there, even with fast axonal transport.”
The new results “recast the relationship between the terminal and the soma,” says Levitan. “The soma sends out excess resources, and terminals decide for themselves how much to use.”
Vesicles entered synapses in constant numbers, but after synapse activation their transport out of the synapse was inhibited. Very little is known about regulated retrograde transport, but the authors suggest that the dynein motor might release vesicles—or any cargo, for that matter—upon activity-induced calcium influx.
The excess vesicle flow might seem like a waste of energy, but Levitan compares the scheme to a water plant running water from house to house so everyone gets immediate access. “Each synapse can tap in and get extra vesicles immediately whenever they need it.”