Serotonin synthesis with rat brain synaptosomes. Effects of L-dopa, L-3-methoxytyrosine and catecholamines

Experiments were done with a fraction of rat brain containing mitochondria and synaptosomes. L-DOPA∥ (K_t 0·3 mM) and L-3-methoxytyrosine (K_t 0·5 mM) were found to be competitive inhibitors of tryptophan accumulation, while dopamine and l-noradrenaline had no effect on the accumulation of tryptophan. Furthermore, L-DOPA (K_t 5·6 μM) was about 100 times more potent than L-3-methoxytyrosine in inhibiting the synthesis of 5HT from tryptophan. The inhibition of 5HT synthesis by L-DOPA appeared to be competitive to tryptophan and was not linear at concentrations of L-DOPA higher than 1 μM. L-DOPA also interfered with the rate of deamination of 5HT synthesized in vitro. Dopamine and l-noradrenaline also inhibited the synthesis of 5HT.