Neurotrophins and the development of cochlear innervation.

The rapid progress in the past few years concerning neurotrophic factor research, has greatly stimulated advances in developmental neurobiology of hearing. We have summarized evidence that neurotrophins are expressed by auditory sensory epithelia during the time at which ganglion cells with neurotrophin receptors send their processes to these epithelia. Recent findings have led to the identification of BDNF and NT3 as responsible substances. Since no NGF mRNA nor the NGF high affinity receptor component trkA mRNA were detectable during the development of cochlear structures, this factor is not likely to be an important neurotrophin at this level. By their biological activity, neurotrophins could be responsible for chemotrophic, differentiation, survival, and maintenance functions at the afferent as well as at the efferent level of the inner ear development.