Neuropeptides and coexistence

Neuropeptides, small molecules composed of 3–100 amino-acid residues, can be localized to discrete cell populations of the central and peripheral nervous systems. Commonly they coexist in neurons with low-molecular-weight neurotransmitters. Co-storage and coexistence of these two classes of neuroactive molecules are responsible for pre- and postsynaptic functional interactions. Neuropeptides are selectively stored, singularly or more frequently in combinations, within large granular vesicles. Release occurs through mechanisms different from those of the classical calcium-dependent exocytosis at the synaptic cleft. Thus neuropeptides account for slow synaptic and/or nonsynaptic communication in neurons, and for modulation of fast neurotransmission.