Nerve growth factor (NGF) and its receptor TrkA —their relations with pain—

Abstract

In this review we introduce the nature of NGF molecule, NGF receptor TrkA, NGF producing cells, role of NGF in differentiating nociceptors, and influences of NGF on axonal properties and terminal branching. NGF is essential for the survival and differentiation of sympathetic neurons and neural crest–originated small DRG neurons to nociceptors in fetal life. Expression of TrkA changes along the day after birth: More than 80% DRG neurons express TrkA in fetus, but this percentage decreases after birth to 45% in 14 days, instead percentage of isolectin B4–binding neurons increases from 0% on birth to 40% in 14 days after birth. TrkA is expressed mainly in small DRG neurons, and percentages of neurons expressing TrkA are different depending on tissues ⁄ organs. Notably larger neurons (possibly innervating proprioceptors) also express TrkA in skeletal muscle innervating DRG neurons, and majority of bone innervating DRG neurons are presumed to express TrkA. NGF influences axonal properties such as maximal frequency to follow, axon potential duration and activity dependent slowing. Altogether NGF has deep influences on nociception.