Trophic interactions between sensory nerves and their targets.

Neurotrophins are target-derived trophic factors essential for the survival and maintenance of neurons. Among these, nerve growth factor (NGF) and neurotrophin-3 (NT-3) are particularly important for sensory neurons. The actions of neurotrophins are through the p75 low-affinity receptor and the high-affinity receptor tyrosine kinase (trk). Each neurotrophin has its preferred receptor, i.e. trkA for NGF, and trkC for NT-3. The primary sensory neurons in the dorsal root ganglion are classified into two categories, namely, the large and small sensory neurons based on their size. The large sensory neurons with the expression of trkC depend on NT-3 for development and subserve the function of position sensations. Some of the small sensory neurons express trkA and are NGF-dependent. They are responsible for nociceptive sensation, the detection of painful and thermal stimuli. A more intriguing observation is the bidirectional interactions between nociceptive nerves and their target, the skin. The peripheral processes of small sensory neurons innervate the epidermis of the skin as 'free nerve endings'. In denervated skin, there is a drastic reduction in the epidermal thickness, a finding corroborated by the phenomenon of trophic change, the shining and thinning of the skin, in the disorders of peripheral nerves. The performance of animals with peripheral nerve disorders improved after administration of neurotrophic factors. Based on these results, the therapeutic potentials of neurotrophic factors in human are under investigation.