NGF, BDNF AND NT-3 PLAY UNIQUE ROLES IN THE IN-VITRO DEVELOPMENT AND PATTERNING OF INNERVATION OF THE MAMMALIAN INNER-EAR

Developing cochleovestibular ganglion (CVG) neurons depend upon intera ction with the otocyst, their peripheral target tissue, for both troph ic support and tropic guidance. RT-PCR of Ell through E14 otocyst-CVG RNA extracts have shown that NGF as well as BDNF and NT-3 are expresse d in the developing inner ear (in situ RT-PCR on tissue sections of E1 2 otocysts localized all three neurotrophins to the otocyst). To evalu ate the functional significance of NGF, BDNF and NT-3 expression, E10. 5 otocyst-CVG explants were treated with antisense oligonucleotides an d compared to sense treated and control cultures. Confocal microscopic analysis revealed that treatment with BDNF antisense resulted in exte nsive neuronal cell death, downregulation of NGF caused an inhibition of neuritogenesis and a decrease in the neuronal population of the CVG , whereas treatment with NT-3 antisense resulted in a loss of target d irected CVG neuritic ingrowth in this in vitro model. The effect of NG F or BDNF antisense treatment could be prevented by the simultaneous a ddition of the respective growth factor. These findings demonstrate th at each of the three neurotrophins have important roles during the ons et of neuritic ingrowth of the CVG neurons to the otocyst.