Vesicles containing neuropeptide neurotransmitters are referred to as large dense-core vesicles, as compared to the small clear-core vesicles containing small molecule neurotransmitters. This refers to the larger amount of material inside the dense-core vesicles, which contain not only neurotransmitters, but also proteases and other peptide chains that have been cleaved from the active neurotransmitter. Large dense core vesicles also tend to be stored, in cages in the bouton, farther from the synapse than the small clear-core vesicles; this means that the release of neuropeptide neurotransmitters generally has a longer delay than the release of small molecule neurotransmitters. When it arrives at the terminal, the necessary neurotransmitter has been fully synthesized and processed and is ready for release.
The vesicles are held in place by the interaction of synapsin and actin molecules. Synapsin I, found on the membrane of the pre-synaptic vesicles, binds to actin filaments, microtubules, spectrin, and other elements of the cytoskeleton to hold the vesicle in a "cage," awaiting an action potential. For a more in depth report on the energy requirements and kinetics of synapsin's binding to synaptic vesicles, go to http://physiology.cup.cam.ac.uk/JPhysiol/1997/504p3/7005/7005