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 a single neuron. According to their chemical nature, coexisting messengers are localized to different cell compartments: neuropeptides are packaged in large dense core vesicles, whereas low-molecular weight neurotransmitters are stored in small synaptic vesicles.