Transmitters and peptides: Basic principles

Signaling between cells plays an important role in all multicellular organisms, both in the nervous system as well as other tissues. Most intercellular signaling involves chemical mediators such as neurotransmitters and neuropeptides; these are collectively referred to as chemical transmitters. A large number of drugs affect cell-cell signaling, and an understanding of these signaling molecules is therefore an essential aspect of neuropharmacology. Neurotransmitters and neuropeptides share a number of key properties. Both types of chemical transmitters are synthesized and secreted from the signaling cell. The synthesis and/or secretion of the molecules is highly regulated. Another common feature of chemical transmitters is that the molecules produce a physiological response, often mediated by binding to a receptor on the cell surface. Finally, the response is terminated by removal of the signaling molecule from the extracellular space, either by chemical transformation of the neurotransmitter or neuropeptide into an inactive form or by uptake of the neurotransmitter into a cell. These general properties are shared by classical neurotransmitters (such as acetylcholine and dopamine), nonclassical neurotransmitters (such as nitric oxide and the endocannabinoids), and neuropeptides (such as enkephalin and neuropeptide Y). However, fundamental differences exist between these various classes of signaling molecules, and collectively, these distinct chemical transmitters contribute to the diversity of signaling within brain and other organs.