In multiple sclerosis (MS), a candidate downstream mechanism for neuronal injury is glutamate (Glu)‐induced excitotoxicity, leading to toxic increases in intraneuronal Ca²⁺. Here, we used in vivo two‐photon imaging in the brain of TN‐XXL transgenic Ca²⁺ reporter mice to test whether promising oral MS therapeutics, namely fingolimod, dimethyl fumarate, and their respective metabolites fingolimod‐phosphate and monomethyl fumarate, can protect neurons against acute glutamatergic excitotoxic damage. We also assessed whether these drugs can protect against excitotoxicity in vitro using primary cortical neurons, and whether they can directly inhibit Glu release from pathogenic T‐helper 17 lymphocytes. In vivo, direct and acute (1 h) administration of 100 mM Glu to the brainstem resulted in a rapid and significant up‐regulation in neuronal Ca²⁺ signaling as well as morphological excitotoxic changes that were attenuated by the NMDA‐receptor antagonist MK801. Direct CNS administration of MS drugs prior to Glu significantly delayed or reduced, but did not prevent the neuronal Ca²⁺ increase or morphological changes. In vitro, prolonged (24 h) treatment of primary neurons with the fumarates significantly protected against neurotoxicity induced by Glu as well as NMDA, similar to MK801. Furthermore, monomethyl fumerate significantly reduced Glu release from pathogenic T‐helper 17 lymphocytes. Overall, these data suggest that MS drugs may mediate neuroprotection via excitotoxicity modulating effects.

Evidence suggests MS pathogenesis may involve neuronal excitotoxicity, induced by local release of glutamate. However, current MS drugs, including dimethyl fumerate (DMF) and fingolimod (FTY720) are largely anti‐inflammatory and not yet fully tested for their neuroprotective potential. Here, we show that the drugs, in particular DMF metabolite monomethyl fumerate (MMF), protect neurons by excitotoxicity modulating effects. Th17, T‐helper 17.