brain and oxidative stress

Hystopatalogical alterations in brain causes oxidativa stres.Mental distress is an important result of oxidative stress and you know bipolar disorders is one of the mental disorders.recently studies indicated that lithium which treats bipolar disorders decrease oxidative stress in the brain.but this positive effects of lithium can't be seen in other organs.why?what is the mechanism for lithium which working in brain positive contrary to the other organs?