Expression patterns of neurotrophin and their receptor mRNAs in the rat inner ear

In situ hybridization was used to study the expression of mRNAs of nerve growth factor (NGF), brain-derived neutrophic factor (BDNF), neurotrophin-3 (NT-3), neurotrophin-5 (NT-5) and the components of their high-affinity receptors in the early postnatal and adult rat inner ears. NGF or NT-5 transcripts were not detected in the inner ear neuroepithelium or in the innervating neurons. NT-3 mRNA was intensely expressed over the one-week-old and adult inner hair cells (IHCs) but in the outer hair cells (OHCs) and vestibular maculae only during the early postnatal period. BDNF mRNA was expressed in the IHCs and OHCs of the early postnatal cochlea but not in the adult organ of Corti. High levels of BDNF transcripts were observed in the sensory epithelia of all vestibular end organs. mRNAs of low affinity NGF receptor, trkB and trkC, but not of trk, were expressed in the spiral and vestibular ganglia. In addition, the non-catalytic form of trkB mRNA localized to the sensory epithelia of maculae utriculi and sacculi. The present results show that of the neurotrophins examined, NT-3 is the predominant neurotrophin in the adult organ of Corti and BDNF is that in vestibular organs. The expression patterns of NT-3 and BDNF mRNAs suggest that these neurotrophins may participate in the maintenance of mature cochleovestibular neurons and they may be involved in the survival response of injured neurons.