Introduction: Optic neuritis is an inflammatory demyelinating (i.e. loss of insulation around nerve fibres) disease of the optic nerve, and it is a common presenting symptom in patients with multiple sclerosis (MS).  The visual deficits caused by acute optic neuritis, which can include blindness, often appear to resolve over time, but permanent residual deficits typically remain due to unresolved demyelination and to axonal degeneration.  The acute deficits are typically attributed to the demyelination, but more recent evidence raises the possibility that the deficits could be due to inflammatory hypoxia.  Thus the optic nerve may have insufficient oxygen to support the mitochondrial function essential for impulse activity and to prevent axonal degeneration.  We have therefore examined whether the inflamed optic nerve is hypoxic, using an animal model of MS, experimental autoimmune encephalomyelitis (EAE).    

Method: EAE was induced in female Dark Agouti rats by immunization with recombinant myelin oligodendrocyte glycoprotein and adjuvant.  Control animals were “immunized” with adjuvant only.  Tissue hypoxia was assessed using an immunohistochemical probe, pimonidazole, injected intravenously, and by labelling for endogenous hypoxia-inducible factor-1a (HIF1a).  Oxidative and nitrative stress was assessed by labelling for inducible nitric oxide synthase (iNOS) and 3-nitrotyrosine (3NT), as well as by the fluorescence products of dihydroethidium (DHE) previously injected intravenously.  Vascular density and diameter were also examined histologically.    

Results: Out of 57 nerves collected from animals with EAE, 48 exhibited optic neuritis and were identified as EAE with optic neuritis (EAE-ON), while only 9 showed no sign of inflammation and were designated EAE-NON.  The inflamed optic nerves (EAE-ON) were found to be hypoxic, with significantly higher labelling for HIF1a (p=0.002) and pimonidazole (p=0.001) compared with nerves from control animals (IFA, n=29 nerves).  Hypoxia is known to trigger oxidative and nitrative stress, revealed in our study by significantly higher labelling for iNOS (p<0.001) and 3NT (p=0.003), and fluorescence due to DHE (p=0.055) in the EAE-ON nerves compared with nerves from IFA animals.  EAE-ON nerves also showed significant dilation of capillaries compared with IFA (p<0.001).    

Conclusion: We conclude that the inflamed optic nerve is hypoxic, associated with oxidative and nitrative stress and damage, as well as alterations in the vasculature.  This finding has implications for the treatment of optic neuritis, and provides insights on the early detection and treatment of MS.