The mobility of synaptic vesicles

Abstract

The synaptic vesicles are small organelles containing the neurotransmitters used to communicate between neurons, and between neurons and other cells. The vesicles fuse with the plasma membrane to release neurotransmitter and are then reformed and prepared for new rounds of release, in what is termed vesicle recycling. In order to fulfill their function, the vesicles need to be mobile: they need to reach the release sites (active zones) for fusion, and then need to be transported back inside the synapse. Moreover, the vesicles move also from synapse to synapse and from the synapse to the cell body of the neuron. We review here the current knowledge on synaptic vesicle movement - a complex and rather confusing field. We present the main fluorescence imaging techniques which have been used to investigate vesicle movement within synapses, and we discuss the general conclusion that, paradoxically, the vesicles are actually not very mobile, both at rest and during stimulation. We discuss also the evidence dealing with the involvement of different cytoskeletal components in vesicle dynamics, with the conclusion that both actin and synapsin may be essential proteins, although, paradoxically again, no difficulties in synaptic vesicle recycling have been noted in several studies where these proteins have been severely perturbed. Finally, we speculate on the mobility of synaptic vesicle components in the plasma membrane, a subject which has never been directly addressed. We also conclude that not all vesicles may be identical, with some being mobile and others stable - a conclusion which may help to explain some of the complex observations in the field.