MPP+ injected as an aqueous solution into the bloodstream causes no symptoms of Parkinsonism in test subjects, since the highly charged molecule is unable to diffuse through the blood-brain barrier.[1] Furthermore, MPP+ shows little toxicity to cells other than dopaminergic neurons, suggesting that these neurons have a unique process by which they can uptake the molecule, since, being charged, MPP+ cannot readily diffuse across the lipid bilayer that composes cellular membranes.[9]