Flupirtine has been shown to function as a
neuroprotectant and is presently used in man to treat a number of conditions. The aim of this study was to investigate the specific
antioxidant properties of
flupirtine in relation to
oxidant-induced damage to retinal photoreceptors. Initial in vitro studies on brain membranes showed that
flupirtine was approximately 20 times more potent than
trolox (
vitamin E analogue) and 8 times more potent than
metipranolol at attenuating lipid peroxidation caused by the
nitric oxide donor,
sodium nitroprusside (SNP). Subsequent immunohistochemical studies revealed that following an
intraocular injection of SNP, retinal photoreceptors are the only
retinal cell types that appear to be clearly affected. This was supported by electroretinogram (ERG) recordings which showed both the a- and b-wave amplitudes to be significantly reduced. Western blotting techniques showed that SNP caused a significant decrease in photoreceptor-specific markers (RET-P1,
rhodopsin kinase), an increase in cleaved
caspase-3, Bcl-2, and cleaved PARP
proteins that are associated with apoptosis and no change in the
ganglion cell specific marker, neurofilament (NF-L). This was supported by RT-PCR data where
rhodopsin (photoreceptor specific)
mRNA was reduced while Thy-1 and NF-L (
ganglion cell specific) mRNAs were unaffected. In addition SNP caused an elevation of glial cell response mRNAs primarily associated with Müller cells (GFAP,
CNTF, bFGF) as well as
caspase-3 and Bcl-2. Importantly, when
flupirtine was co-injected, the effects to the retina caused by SNP on
retinal proteins and mRNAs were in most cases significantly blunted. The conclusion reached from this study is that
flupirtine is a powerful
antioxidant and when injected into the eye with SNP attenuates the detrimental influence of SNP to retinal photoreceptors. Since oxidative stress has been implicated in
retinal diseases like
age-related macular degeneration (AMD) this study provides "proof of principle" for the idea that
flupirtine may help individuals suffering from such
retinal diseases.