Excess target-derived brain-derived neurotrophic factor preserves the transient uncrossed retinal projection to the superior colliculus

During early postnatal development, a widespread ipsilateral projection to the superior colliculus is secondarily restricted to a small topographicall y defined region by elimination of ipsilaterally projecting retinal ganglio n cells. Brain-derived neurotrophic factor (BDNF) has been proposed as the target-derived neurotrophic factor for retinal ganglion cells in several st udies. Here we investigated the long-term effects of excess BDNF in the ret inal ganglion cell target on naturally occurring retinal ganglion cell (RGC ) elimination and on the restriction of the ipsilateral projection. To this end, sustained overexpression of BDNF was achieved in the postnatal superi or colliculus using an adenoviral vector. While the total number of retinal ganglion cells in the adenovirus-BDNF treated animals was unchanged, a muc h higher proportion of RGCs retained a projection to the ipsilateral superi or colliculus. We conclude that an excess of target-derived BDNF does not r educe the net amount of naturally occurring cell death in the retino-collic ular system, but prevents the negative selection of retinal ganglion cells making inappropriate topographic connections.