Vesicular neurotransmitter transport and the presynaptic regulation of quantal size.

Specific transport activities package classical neurotransmitters into secretory vesicles for release by regulated exocytosis, but the proteins responsible for the vesicular transport of neurotransmitters are still being identified. One family of proteins includes vesicular transporters for monoamines and acetylcholine. Genetic manipulation in cells and in mice now shows that changes in the expression of these proteins can alter the amount of neurotransmitter stored per synaptic vesicle, the amount released and behavior. Although the mechanisms responsible for regulating these transporters in vivo remains unknown, recent work has demonstrated the potential for regulation by changes in intrinsic activity and in location. In addition, a recently identified vesicular transporter for GABA defines a novel family of proteins that mediates the packaging of amino acid neurotransmitters.