Neurotrophic factors: important regulators of nociceptive function.

Neurotrophic factors have an established developmental role in regulating the survival and specification of sensory neurons. However, these factors continue to exert an important influence on sensory neurons throughout the postnatal period and into adult life. In adulthood, approximately one-half of nociceptors are dependent on nerve growth factor (NGF) for trophic support, whereas the other half are sensitive to glial cell line-derived neurotrophic factor (GDNF). It is now known that many chronic pain states are maintained by widespread changes in the anatomy, neurochemistry, and function of the sensory nervous system both at the level of the primary sensory neuron and the dorsal horn of the spinal cord. Trophic factors appear to orchestrate many of these dynamic changes. This review highlights some of the key roles played by these molecules and in particular the role of NGF in the peripheral sensitization of nociceptors and brain-derived neurotrophic factor (BDNF) as a central pain modulator.