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 neu rotrophin-3 (NT-3) are particularly important for sensory neurons. The acti ons of neurotrophins are through the p75 low-affinity receptor and the high -affinity receptor tyrosine ki nase (trk). Each neurotrophin has its prefer red receptor, i,e. trkA for NGF, and trkC for NT-3. The primary sensory neu rons in the dorsal root ganglion are classified into two categories, namely , the large and small sensory neurons based on their size. The large sensor y neurons with the expression of trkC depend on NT-3 for development and su bserve the function of position sensations. Some of the small sensory neuro ns 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 a nd their target, the skin. The peripheral processes of small sensory neuron s innervate the epidermis of the skin as 'free nerve endings'. In denervate d 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 anim als with peripheral nerve disorders improved after administration of neurot rophic factors. Based on these results, the therapeutic potentials of neuro trophic factors in human are under investigation.