The potential role of nerve growth factor, brain-derived neurotrophic factor and neurotrophin-3 in avian cochlear and vestibular ganglia development

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Abstract
The role of the nerve growth factor family of neurotrophins in the development of cochlear and vestibular ganglia is unclear. In order to predict the potential importance of nerve growth factor, brain-derived neurotrophic factor or neurotrophin-3, we examined the expression of neurotrophin mRNA and full-length neurotrophin receptor mRNA by in-situ hybridization and reverse transcription-polymerase chain reaction, as well as whether high affinity 125I-nerve growth factor binding was present, in cochlear and vestibular ganglia of the quail at several stages of development (stages 26, 31 and 36). Nerve growth factor, brain-derived neurotrophic factor and neurotrophin-3 mRNA was detected at all ages examined, suggesting that these neurotrophins may serve an autocrine or paracrine function, especially prior to target contact. In addition, we found full-length trkA and trkC mRNA was expressed, the products of which are the functional neuronal receptors for nerve growth factor and neurotrophin-3, respectively. Although full-length trkA mRNA was found, physiologically important high affinity 125I-nerve growth factor binding was not detected. Since nerve growth factor's effects on survival and neurite outgrowth are mediated through high affinity binding, nerve growth factor may serve an as yet unidentified role in this system. Full-length trkB mRNA, the product of which is the functional neuronal receptor for brain-derived neurotrophic factor, was not detected using reverse transcription-polymerase chain reaction, however, truncated (non-catalytic) trkB was present, at least in cochlear ganglia at stage 31. It is not known what function may be subserved by these truncated receptors.