Elevated Hcy reduces claudin-5 expression of cerebral microvascular endothelium, which is rescued by NMDAr inhibition with memantine. Western blotting in vivo (A) and in vitro (B) demonstrates significant loss of claudin-5 expression in the mouse model (cbs^+/−) and cell culture model (bEnd.3 cells, 20μM) of elevated Hcy. This loss was completely recovered by 2 weeks of treatment with the NMDAr antagonist memantine in vivo (A) and fully inhibited by concomitant memantine treatment in vitro (B). (C) Expression of the tight junction-associated protein ZO-1 was not affected by HHcy. Brains of hyperhomocysteinemic mice (cbs^+/−) showed the same expression levels of ZO-1 as WT littermates (WT; cbs^+/+), and brain microvascular ECs (bEnd.3) expressed the same amount of ZO-1 whether treated for 3 days with Hcy (20μM) or vehicle control (Ctrl, PBS).