EXPRESSION OF NEUROTROPHINS AND TRK RECEPTORS IN THE DEVELOPING, ADULT, AND REGENERATING AVIAN COCHLEA

We studied the expression of neurotrophins and their Trk receptors in the chicken cochlea. Based on in situ hybridization, brain-derived neu rotrophic factor (BDNF) is the major neurotrophin there, in contrast t o the mammalian cochlea, where neurotrophin-3 (NT-3) predominates. NT- 3 mRNA labeling was weak and found only during a short time period in the early cochles. During embryogenesis, BDNF mRNA was first seen in e arly differentiating hair cells. Afferent cochlear neurons expressed t rkB mRNA from the early stages of gangliogenesis onward. In accordance , in vitro, BDNF promoted survival of dissociated neurons and stimulat ed neuritogenesis from ganglionic explants. High levels of BDNF mRNA i n hair cells and trkB mRNA in cochlear neurons persisted in the mature cochlea. In addition, mRNA for the truncated TrkB receptor was expres sed in nonneuronal cells, specifically in supporting cells, located ad jacent to the site of BDNF synthesis and nerve endings. Following acou stic trauma, regenerated hair cells acquired BDNF mRNA expression at e arly stages of differentiation. Truncated trkB mRNA was lost from supp orting cells that regenerated into hair cells. High levels of BDNF mRN A persisted in surviving hair cells and trkB mRNA in cochlear neurons after noise exposure. These results suggest that in the avian cochlea, peripheral target-derived BDNF contributes to the onset and maintenan ce of hearing function by supporting neuronal survival and regulating the (re) innervation process. Truncated TrkB receptors may regulate th e BDNF concentration available to neurites, and they might have an imp ortant role during reinnervation. (C) 1997 John Wiley & Sons, Inc.