Axonal degeneration is a major contributor to non-remitting deficits in
multiple sclerosis, and there is thus considerable current interest in the development of strategies that might prevent axonal loss in
neuroinflammatory disease. Dysregulation of
sodium ion homeostasis has been implicated in mechanisms leading to axonal degeneration, and several studies have shown that blockade of
sodium channels can ameliorate axon damage following anoxic, traumatic and
nitric oxide-induced CNS injury. Two
sodium channel blockers,
phenytoin and
flecainide, have been reported to protect axons in
experimental autoimmune encephalomyelitis (EAE) for 30 days, but long-term protective effects have not been studied. We demonstrate here that
oral administration of
phenytoin provides long-term (up to 180 days) protection for spinal cord corticospinal tract (CST) and dorsal column (DC) axons in both monophasic (C57/BL6 mice) and chronic-relapsing (Biozzi mice) murine EAE. Untreated C57/BL6 mice exhibit a 40-50% loss of CST and DF axons at 90 and 180 days post-EAE induction via
myelin-oligodendrocyte glycoprotein (MOG) injection. In contrast, only 4% of DF axons are lost at 90 days, and only 8% are lost at 180 days in
phenytoin-treated C57/BL6 mice with EAE; only 21-29% of CST axons are lost at 90 and 180 days in
phenytoin-treated C57/BL6 mice with EAE. Attenuation of dorsal column compound action potentials was ameliorated and clinical status was also significantly enhanced with
phenytoin treatment at 90 and 180 days in this model. In addition, inflammatory cell infiltration into the dorsal columns was reduced in
phenytoin-treated mice with EAE compared with untreated mice with EAE. Similar results were obtained in Biozzi mice with chronic-relapsing EAE followed for 120 days post-injection. These observations demonstrate that
phenytoin provides long-term protection of CNS axons and improves clinical status in both monophasic and chronic-relapsing models of
neuroinflammation.