Paracrine and autocrine actions of neurotrophic factors.

Neurotrophic factors are proteins that promote the survival and growth of neurons in the vertebrate nervous system. Although it is well known that many neurons obtain these factors from the regions to which their axons project, studies of the sites of neurotrophic factor synthesis have raised the possibility that at least some neurons may obtain these factors from other sources. Alternative sources of neurotrophic factors include cells along a neuron's axon shaft and cells or other axons terminals within the vicinity of a neuron's cell body and dendritic arbour. In addition, recent experimental studies have shown that at certain stages of development neurotrophic factor autocrine loops operate in some neurons. The evidence for and the potential physiological significance of these different modes of action of neurotrophic factors will be discussed.