A neuropeptide is an endogenous protein molecule comprised of at least two amino acids, which are chained together by peptide bonds. They are found in large dense-core vesicles on nearly all areas of the neuron, most commonly the axon terminal at the synapse. Approximately 100 different neuropeptides have been identified.
The function of a neuropeptide often determines its classification as a neurotransmitter or hormone, but the terms are frequently intermixed. Generally, neuropeptides exert more prolonged and diverse effects on behavior than neurotransmitters, and act directly at G-coupled protein and metabotropic receptor sites.
Many populations of neurons have distinctive biochemical phenotypes. For example, in one subpopulation of about 3000 neurons in the arcuate nucleus of the hypothalamus, three anorectic peptides are co-expressed: α-melanocyte-stimulating hormone (α-MSH), galanin-like peptide, and cocaine-and-amphetamine-regulated transcript (CART), and in another subpopulation two orexigenic peptides are co-expressed, neuropeptide Y and agouti-related peptide (AGRP). These are not the only peptides in the arcuate nucleus; β-endorphin, dynorphin, enkephalin, galanin, ghrelin, growth-hormone releasing hormone, neurotensin, neuromedin U, and somatostatin are also expressed in subpopulations of arcuate neurons. These peptides are all released centrally and act on other neurons at specific receptors. The neuropeptide Y neurons also make the classical inhibitory neurotransmitter GABA.
Invertebrates also have many neuropeptides. CCAP has several functions including regulating heart rate, allatostatin and proctolin regulate food intake and growth, bursicon controls tanning of the cuticle and corazonin has a role in cuticle pigmentation and moulting.