Blood Monoamine Metabolism in Huntington's Disease

• In 25 patients with Huntington's disease (HD), the mean blood concentration of serotonin (5-HT) and percentage of plasma free tryptophan were significantly increased while plasma concentrations of total and protein-bound tryptophan were significantly decreased. The pattern of changes in tryptophan concentrations was related to clinical severity but not to 5-HT levels. Platelet monoamine oxidase (MAO) activity was significantly increased in patients with HD; kinetic and marker enzyme studies suggested an increased enzyme concentration. Offspring at risk for HD also had elevated platelet MAO activity but normal concentrations of blood 5-HT and plasma tryptophan. In ten patients, plasma epinephrine concentrations were significantly increased; plasma dopamine and norepinephrine concentrations were positively related to MAO activity. The finding of peripheral neurotransmitter abnormalities in HD raises the question of an interaction between CNS and peripheral processes or a systemic disorder of neurotransmitter metabolism.