Neuropeptides are small proteins produced by neurons that act on G protein-coupled receptors and are responsible for slow-onset, long-lasting modulation of synaptic transmission. Neuropeptides often coexist with each other or with other neurotransmitters in single neurons. According to their chemical nature, coexisting messengers are localized to different cell compartments: neuropeptides are packaged in large granular vesicles (LGVs), whereas low-molecular weight neurotransmitters are stored in small synaptic vesicles. This compartmentalization allows selective release in response to stimuli. Individual LGVs may store several different neuropeptides, at times with monoamines or neurotrophic factors. LGVs release their neuropeptide cargo by cavicapture or full-fusion exocytosis.