Abstract

Repeated administrations of lithium salts (Li₂CO₃ or LiCl, 60 and 85 mg/kg, respectively, twice a day for five days) to rats increased brain 5-hydroxyindoleacetic acid (5-HIAA) by about 80% and brain serotonin by 15 to 20%. The changes were not due to an inhibition of 5-HIAA transport from brain, but to an increase in the rate of synthesis of brain serotonin. Rate of serotonin synthesis was measured by multiplying the rate constant of 5-HIAA decline, after inhibition of monoamine oxidase, by the steady-state level of 5-HIAA. The calculation indicated that lithium increased the synthesis rate of brain serotonin by about 80%. Moreover, chronic treatment with lithium salts also increased the levels of brain tryptophan by about 70%, suggesting that this could be the mechanism by which lithium stimulates serotonin synthesis.