The control of dopamine and serotonin synthesis in rat brain synaptosomes

Abstract

The synthesis of dopamine and serotonin in rat brain striatal synaptosomes has been studied by direct measurement of dopamine and serotonin by HPLC employing electrochemical detection. In both cases the presence of dibutyryl cyclic AMP stimulated the rate of synthesis and this effect was prolonged in the presence of theophylline. Theophylline alone had no effect. The stimulating effect of 1 mM dibutyryl cyclic AMP on dopamine synthesis was prevented by 1 mM EDTA but not by NaF. EDTA alone did not effect dopamine synthesis but 20 mM NaF alone caused a marked stimulation (increase of 33%) comparable to that produced by 1 mM dibutyryl cyclic AMP. In contrast 1 mM EGTA caused a marked inhibition (30% decrease) of the synthesis of dopamine in this system. The stimulation of serotonin synthesis by 1 mM dibutyryl cyclic AMP was inhibited by 1 mM EDTA and 20 mM NaF. In contrast to dopamine synthesis, serotonin synthesis was markedly inhibited (40%) by 1 mM EDTA on its own, but not affected by 20 mM NaF. However, 1 mM EGTA caused a similar inhibition of serotonin synthesis to that of dopamine synthesis. Exogenously added noradrenaline, adrenaline and serotonin (1–100 μM) markedly inhibited dopamine synthesis in a non linear fashion as did noradrenaline and adrenaline on serotonin synthesis. The addition of imipramine together with noradrenaline rendered the noradrenaline inhibition of both dopamine and serotonin synthesis less effective. The results are discussed with respect to the mechanisms of feed back inhibition of dopamine and serotonin synthesis by products and the stimulation of their synthesis by cyclic AMP linked systems. It is concluded that, whilst the cyclic AMP stimulation of dopamine synthesis in striatal synaptosomes is consistent with activation of tyrosine hydroxylation by phosphorylation, the cyclic AMP stimulation of serotonin synthesis occurs by an alternative mechanism, possibly at the level of the synaptosomal membrane.