PATTERN OF TRKB PROTEIN-LIKE IMMUNOREACTIVITY IN-VIVO AND THE IN-VITRO EFFECTS OF BRAIN-DERIVED NEUROTROPHIC FACTOR (BDNF) ON DEVELOPING COCHLEAR AND VESTIBULAR NEURONS

The cochleo-vestibular ganglion (CVG) contains the neurons connecting the sensory epithelia of the inner ear to the cochlear and vestibular nuclei in the medulla. Expression of trkB protein-like immunoreactivit y was studied in the developing CVG, using both Western blot and immun ocytochemistry on tissue sections. Specific immunoreactivity was obser ved in the CVG from the 12th gestation day (gd) to the first postnatal week, reflecting the presence of high-affinity receptors for brain-de rived neurotrophic factor (BDNF), a member of the NGF family of neurot rophins. Whole explants and dissociated cell cultures of cochlear (CG) and vestibular ganglion (VG) from mouse embryos and postnatal specime ns were grown in neurotrophin-free medium to assay changes in neurite outgrowth and neuronal survival in response to the addition of physiol ogical concentrations (0-5 ng/ml) of BDNF. Exogenous BDNF (2 ng/ml) pr omoted neurite outgrowth and neuronal survival in explants of both CG and VG, and the effects were stage-dependent. The onset of the respons e to BDNF occurred at gd 11-12. The response then reached a maximum be tween 14 and 18 gd and subsequently decreased, although it remained si gnificantly present during the first postnatal week. BDNF-induced resp onse was no longer observed in the mature cochlear and vestibular gang lion (after 30 postnatal days). The effects of BDNF on neuronal differ entiation and survival were dose-dependent, starting at 0.5 ng/ml, wit h saturation at 2 ng/ml and half-maximal effect occurring between 1 an d 1.5 ng/ml. On the basis of our results, we propose that BDNF may be physiologically involved in the control of both neuronal differentiati on, and central and peripheral target-dependent neuronal death, in the CVG of embryos and early postnatal mice. BDNF may act alone or in coo peration with other neurotrophins to establish the afferent innervatio n of the inner ear sensory epithelium.