Why do neurotransmitters act like growth factors?

It is now well established that neurotransmitters act as growth-regulatory signals for neuronal and non-neuronal cells of both primitive and higher or ganisms, where they control cell proliferation, motility, survival, growth, differentiation, and gene expression. Many of these actions are reminiscen t of the actions of other growth-regulatory signals such as growth factors, neurotrophins, and proto-oncogenes. How, then, do neurotransmitters exert these effects? Although some information is available concerning second mes sengers activated by these neurotransmitters in developing cells, little is known about subsequent steps involving signal transduction cascades leadin g to their final outcomes. This review attempts to provide testable hypothe ses regarding possible cellular and molecular mechanisms downstream of seco nd messengers activated by neurotransmitters, based on recent insights into signal transduction cascades activated by classical growth-regulatory sign als. In many cases, there are clear points of convergence between these pat hways, raising the interesting possibility that neurotransmitters and other growth-regulatory signals may cooperate to regulate developmental function s of cells and tissues.