Neuropeptides as synaptic transmitters.

Neuropeptides are small protein molecules (composed of 3-100 amino-acid residues) that have been localized to discrete cell populations of central and peripheral neurons. In most instances, they coexist with low-molecular-weight neurotransmitters within the same neurons. At the subcellular level, neuropeptides are selectively stored, singularly or more frequently in combinations, within large granular vesicles. Release occurs through mechanisms different from classical calcium-dependent exocytosis at the synaptic cleft, and thus they account for slow synaptic and/or non-synaptic communication in neurons. Neuropeptide co-storage and coexistence can be observed throughout the central nervous system and are responsible for a series of functional interactions that occur at both pre- and post-synaptic levels. Thus, the subcellular site(s) of storage and sorting mechanisms into different neuronal compartments are crucial to the mode of release and the function of neuropeptides as neuronal messengers.