Neuropeptides

Neuropeptides are small protein-like molecules (peptides) used by neurons to communicate with each other. They are neuronal signaling molecules that influence the activity of the brain and the body in specific ways. Different neuropeptides are involved in a wide range of brain functions, including analgesia, reward, food intake, metabolism, reproduction, social behaviors, learning and memory.

Neuropeptides are related to peptide hormones, and in some cases peptides that function in the periphery as hormones also have neuronal functions as neuropeptides. The distinction between neuropeptide and peptide hormone has to do with the cell types that release and respond to the molecule; neuropeptides are secreted from neuronal cells (primarily neurons but also glia for some peptides) and signal to neighboring cells (primarily neurons). In contrast, peptide hormones are secreted from neuroendocrine cells and travel through the blood to distant tissues where they evoke a response. Both neuropeptides and peptide hormones are synthesized by the same sets of enzymes, which include prohormone convertases and carboxypeptidases that selectively cleave the peptide precursor at specific processing sites to generate the bioactive peptides.

Neuropeptides modulate neuronal communication by acting on cell surface receptors. Many neuropeptides are co-released with other small-molecule neurotransmitters. The human genome contains about 90 genes that encode precursors of neuropeptides. At present about 100 different peptides are known to be released by different populations of neurons in the mammalian brain. Neurons use many different chemical signals to communicate information, including neurotransmitters, peptides, and gasotransmitters. Peptides are unique among these cell-cell signaling molecules in several respects. One major difference is that peptides are not recycled back into the cell once secreted, unlike many conventional neurotransmitters (glutamate, dopamine, serotonin). Another difference is that after secretion, peptides are modified by extracellular peptidases; in some cases, these extracellular cleavages inactivate the biological activity, but in other cases the extracellular cleavages increase the affinity of a peptide for a particular receptor while decreasing its affinity for another receptor. These extracellular processing events add to the complexity of neuropeptides as cell-cell signaling molecules.

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