Papers of the Week

Nerve growth factor (NGF) and glial cell line-derived neurotrophic factor (GDNF) are essential for neuronal development and survival in embryo. However, after birth they play pivotal roles in generation of hyperalgesia in many painful conditions. Both factors are believed to act on different groups of primary afferents, and no interaction has been studied. Here we show a synergism of both factors. Intramuscular injection of a mixture of both factors of low concentration, each of which alone had no effect, induced a significant muscular mechanical hyperalgesia in rats. We show synergism occurs in the primary afferent neurons and find about 25 % primary afferents innervating the muscle express both TrkA (NGF receptor) and GFRα1 (GDNF receptor). We show by pharmacological means that afferent neurons with TrkA and GFRα1 express both TRPV1 and ASICs. Our data establish a basis for synergism of NGF and GDNF.