Both the neuronal and inducible isoforms contribute to upregulation of retinal nitric oxide synthase activity by brain-derived neurotrophic factor

Although neurotrophins are best known for their trophic functions, growing evidence suggests that neurotrophins can also be neurotoxic, for instance b y enhancing excitotoxic insults. We have shown recently that brain-derived neurotrophic factor (BDNF) limits its neuroprotective action on axotomized rat retinal ganglion cells (RGCs) by upregulating nitric oxide synthase (NO S) activity (Klocker et al., 1998). The aim of the present study was to inv estigate this interaction of BDNF and NOS in the lesioned adult rat retina in more detail. We used NOS immunohistochemistry and NADPH-diaphorase (NADP H-d) reaction to characterize morphologically retinal NOS expression and ac tivity. Using reverse transcription-PCR and Western blot analysis, we were able to identify the NOS isoforms being regulated. Six days after optic ner ve lesion, we observed an increase in neuronal NOS (NOS-I) mRNA and protein expression in the inner retina. This did not lead to a marked increase in overall retinal NOS activity. Only RGC axons displayed strong de novo NADPH -d reactivity. In contrast, intraocular injection of BDNF resulted in a mar ked upregulation of NOS activity in NOS-I-immunoreactive structures, leavin g the level of NOS-I expression unchanged. In addition, an induction of ind ucible NOS (NOS-II) was found after BDNF treatment. We identified microglia l cells increasing in number and being activated by BDNF, which could serve as the cellular source of NOS-II. In summary, our data suggest that BDNF u pregulates retinal NOS activity by both a post-translational regulation of NOS-I activity and an induction of NOS-II. These findings might be useful f or developing pharmacological strategies to improve BDNF-mediated neuroprot ection.