Methylene blue is a cationic thiazine dye that has diverse biological activities.^1,2,3It inhibits the catalytic function of inducible and constitutive nitric oxide synthasesviaoxidation of enzyme-bound ferrous iron.¹Methylene blue also inhibits monoamine oxidase (MAO) and increases the activity of mitochondrial cytochrome C oxidase (complex IV).^2,3In vivo, methylene blue stabilizes systemic circulation without increasing peripheral resistance, decreases lipid peroxidation, and reduces anoxic tissue injury in brain and heart in a porcine model of global ischemia and reperfusion injury.¹It inhibits MAO activity in brain and reduces the age-related decline in grip strength and spatial memory in old mice when administered in drinking water at a concentration of 250 μM.²Methylene blue (15, 30, and 60 mg/kg) also reduces immobility time in the forced swim test in mice, indicating antidepressant-like activity.³Methylene blue is commonly used to stain cells and tissues in research.⁴

1.Wiklund, L., Basu, S., Miclescu, A., et al.Neuro- and cardioprotective effects of blockade of nitric oxide action by administration of methylene blueAnn. N.Y. Acad. Sci.1122(1)231-244(2007) 2.Gharib, A., and Atamna, H.Methylene blue induces mitochondrial complex IV and improves cognitive function and grip strength in old miceNeurodegeneration: Theory, disorders and treatments63-85(2011) 3.Harvey, B.H., Duvenhage, I., Viljoen, F., et al.Role of monoamine oxidase, nitric oxide synthase and regional brain monoamines in the antidepressant-like effects of methylene blue and selected structural analoguesBiochem. Pharmacol.80(10)1580-1591(2010) 4.Hu, X., Laguerre, V., Packert, D., et al.A simple and efficient method for preparing cell slides and staining without using cytocentrifuge and cytoclipsInt. J. Cell Biol.2015:813216813216(2015)