Free Radical Scavenging and Inhibition of Nitric Oxide Synthase Potentiates the Neurotrophic Effects of Brain-Derived Neurotrophic Factor on Axotomized Retinal Ganglion CellsIn Vivo

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Abstract
Brain-derived neurotrophic factor (BDNF) partially promotes the survival of axotomized retinal ganglion cells (RGCs). In analogy within vitroexperiments (Koh et al., 1995; Samdami et al., 1996), we tested whether neuroprotection by BDNF is limited by adverse effects as a consequence of excessive free radical formation. First, we investigated whether BDNF and the free radical scavengerN-tert-butyl-(2-sulfophenyl)-nitrone (S-PBN) cooperate in protecting RGCs from axotomy-induced death. Although systemic S-PBN treatment alone did not influence RGC survival after axotomy, it potentiated the neuroprotective effects of BDNF significantly. Single BDNF treatment rescued 27% of the RGCs, which otherwise would have died 14 d after optic nerve transection, whereas a combined treatment of BDNF and S-PBN improved this rescue rate up to 68%. We then investigated whether the adverse effects of BDNF could be ascribed to activation of nitric oxide synthase (NOS). We found colocalization of NOS and the BDNF receptor TrkB in the retina. NADPH-diaphorase reactivity, a reliable marker for NOS in the rat retina, increased after chronic BDNF treatmentin vivo. Systemic application of the NOS-inhibitorN-ω-nitro-l-arginine-methylester (l-NAME) potentiated the neuroprotective action of BDNF (55% rescue rate). We conclude that activation of NOS is a pathological consequence of BDNF application, which reduces its neuroprotective potential. The observation that this adverse effect can be antagonized by systemic application of free radical scavengers could be of relevance for clinical applications of neurotrophins in human neurodegenerative diseases.