Thyroid state and brain monoamine metabolism.

The rates at which rat brain synthesizes catecholamines and serotonin were estimated by measuring the accumulation of DOPA and 5-hydroxytryptophan (5-HTP) 45 min after ip administration of the decarboxylase inhibitor RO4-4602 (800 mg/kg BW). Following thyroparathy-roidectomy, hypothyroid rats showed a decreased accumulation of both precursor amino acids. On the other hand, hyperthyroidism (caused by administering 15 mug T4/100 g BW for 25 days) accelerated the accumulation of catecholamines and serotonin. The accumulation of 5-HTP correlated closely with brain tryptophan concentration in all treatment groups; DOPA accumulation, however, did not similarly correspond to brain tyrosine levels.