Non-invasive visual evoked potentials to assess optic nerve involvement in the dark agouti rat model of experimental autoimmune encephalomyelitis induced by myelin oligodendrocyte glycoprotein

Experimental autoimmune encephalomyelitis (EAE) is the primary disease model of multiple sclerosis (MS), one of the most diffused neurological diseases characterized by fatigue, muscle weakness, vision loss, anxiety and depression. EAE can be induced through injection of myelin peptides to susceptible mouse or rat strains. In particular, EAE elicited by the autoimmune reaction against myelin oligodendrocyte glycoprotein (MOG) presents the common features of human MS: inflammation, demyelination and axonal loss. Optic neuritis affects visual pathways in both MS and in several EAE models. Neurophysiological evaluation through visual evoked potential (VEP) recording is useful to check visual pathway dysfunctions and to test the efficacy of innovative treatments against optic neuritis. For this purpose, we investigate the extent of VEP abnormalities in the dark agouti (DA) rat immunized with MOG, which develops a relapsing-remitting disease course. Together with the detection of motor signs, we acquired VEPs during both early and late stages of EAE, taking advantage of a non-invasive recording procedure that allows long follow-up studies. The validation of VEP outcomes was determined by comparison with ON histopathology, aimed at revealing inflammation, demyelination and nerve fiber loss. Our results indicate that the first VEP latency delay in MOG-EAE DA rats appeared before motor deficits and were mainly related to an inflammatory state. Subsequent VEP delays, detected during relapsing EAE phases, were associated with a combination of inflammation, demyelination and axonal loss. Moreover, DA rats with atypical EAE clinical course tested at extremely late time points, manifested abnormal VEPs although motor signs were mild. Overall, our data demonstrated that non-invasive VEPs are a powerful tool to detect visual involvement at different stages of EAE, prompting their validation as biomarkers to test novel treatments against MS optic neuritis.