A ROLE FOR BDNF IN EARLY POSTNATAL RAT VESTIBULAR EPITHELIA MATURATION - IMPLICATION OF SUPPORTING CELLS

The early development of the inner ear is largely determined by two me mbers of the neurotrophic family: brain-derived neurotrophic factor (B DNF) and neurotrophin 3 (NT-3). Little information is available on the role of these neurotrophins during the late stages of vestibular deve lopment in the rat which take place during the first postnatal weeks. At this period where terminal synaptogenesis and maturation occur, we have investigated the expression and the activity of BDNF, the most im portant neurotrophin in the vestibular system. Using different experim ental approaches, we show that BDNF is released by vestibular epitheli a on postnatal day 3 (P3) and continues to have a trophic effect on ve stibular neurones in vitro. Immunocytochemistry coupled to confocal mi croscopy revealed a remarkable evolution in BDNF localization during l ater stages of development. Whereas BDNF is present in both supporting cells and hair cells at P3, its distribution gradually changed and is highly compartmentalized within the upper part of supporting cells at P8 and P15. In parallel, we observed the presence of a truncated form of the BDNF receptor in sensory hair cells. These results suggest an original role for supporting cells, which could be involved in the rel ease of BDNF during the late stages of synaptogenesis in mammalian ves tibular epithelia. In particular, BDNF could participate to the set up of the calyx, a specific nerve structure surrounding type I vestibula r hair cells.