Neurotrophin and neurotrophin receptor mRNA expression in developing inner ear

Receptors which bind the neurotrophins NGF, BDNF, NT-3 and NT-4/5 were shown to be present in cochlear and vestibular ganglion cells during development, implying a neurotrophic role for these molecules in the inner ear. We have found by in situ hybridization that cochlear and vestibular sensory epithelial cells express BDNF and NT-3 mRNAs, but neither NGF or NT-4 mRNAs, in mouse embryos from embryonic day (E)11.5 through postnatal day (P)1. NT-3 mRNA was expressed throughout the sensory epithelium whereas BDNF mRNA appeared to be localized in hair cells (vestibular) and epithelial precursors of hair cells (cochlea). BDNF mRNA was also expressed in a subpopulation of cells in the cochleovestibular ganglion at E11.5 and E12.5. Additionally, cochlear and vestibular neurons contained mRNAs encoding the neurotrophin receptors p75 and trkB. TrkA mRNA was transiently expressed in cochleovestibular ganglion cells at E12.5. These data suggest that BDNF and NT-3 play a role in cochleovestibular neuron survival and neurite outgrowth during development in the inner ear.