S1PR1 and relapsing-remitting multiple sclerosis: The importance of S1P—and its receptor S1PR1—in the pathogenesis of PD might be also inferred by the fact that a few studies that reported a beneficial effect of fingolimod (or FTY720)—an S1PR1 antagonist/modulator and the first oral treatment ever patented in the therapy for relapsing-remitting multiple sclerosis—ameliorates the clinical phenotype in murine models of PD [122].