Colocalization of amino acid signal molecules in neurons and endocrine cells

During the last 20 to 30 years, numerous examples have been provided of neurons and endocrine cells that are able to produce, store, and in many cases release more than one type of signal molecule. Recent models propose that neurons often employ an amino acid, an amine, and one or more neuroactive peptides, and that endocrine cells may release more than one peptide hormone. In neurons, the different classes of transmitter convey fast, intermediate, and slow signalling respectively. However, a series of studies demonstrates that neurons may colocalize more than one neuroactive amino acid, and that endocrine cells may contain an amino acid along with their peptide hormone. These forms of colocalization seem to add new levels of complexity to the role of amino acids in cell signalling, suggesting that, in neurons, amino acids may interact at the receptor level, modifying the effect of each other, and that, in endocrine cells, amino acids may act together with or parallel to a peptide hormone in a paracrine or autocrine manner.