Expression of Trk A receptors in the mammalian inner ear.

Tyrosine kinase receptors, including Trk A, Trk B and Trk C, participate in many different biological processes that are regulated by neurotrophic factors. Nerve growth factor (NGF)-triggered Trk A signaling is involved in growth, survival and differentiation of neurons in the central nervous system and in neural crest-derived cells. Trk A, Trk B and Trk C expression has been reported in the rat ventral cochlear nucleus. In the present study, we explored the immunocytochemical distribution of Trk A in the rodent inner ear. Rat and mouse cochleae were immunolabeled with a rabbit anti-Trk A polyclonal antibody (Chemicon) that has no reported cross-reactivity with Trk B and Trk C. In embryonic day 16 mice, no Trk A immunolabeling could be detected in the developing neuroepithelium. At postnatal day 6, weak Trk A labeling could be observed in both inner and outer hair cells. At postnatal day 12, enhanced punctate Trk A immunoexpression was present in hair cells. In adult mice and rats, intense Trk A labeling was observed in outer and inner hair cell bodies, in supporting cell bodies throughout the cochlea, and in spiral ganglion neurons. Trk A was not observed in stria vascularis, hair cell stereocilia, nor in the Trk B- and Trk C-rich cerebellum. This distribution pattern of Trk A suggests that its ligand, NGF, exerts significant trophic effects in the rodent inner ear.