Stimulation of dopamine synthesis and activation of tyrosine hydroxylase by phorbol diesters in rat striatum

Abstract

In rat striatal synaptosomes, 4 ß -phorbol 12-myristate 13-acetate (PMA) and 4 ß -phorbol 12, 13-dibutyrate (PDBu), two activators of Ca 2+-phospholipid-dependent protein kinase (protein kinase C) increased dopamine (DA) synthesis measured by following the release of 14CO 2 from L-[1- 14C] tyrosine. Maximal stimulation (21–28% increase of basal rate) was produced by 0.5 μM PMA and 1 μM PDBu. 4 ß -Phorbol and 4 ß -phorbol 13-acetate, which are not activators of protein kinase C, were ineffective at 1 μM. PMA did not change the release of 14CO 2 from L-[1- 14C] DOPA. Addition of 1 mM EGTA to a Ca 2+-free incubation medium failed to affect PMA stimulation. KCl (60 mM) enhanced DA synthesis by 25%. Exposure of synaptosomes to either PMA or PBDu prior to KCl addition resulted in a more than additive increase (80–100%) of DA synthesis. A similar synergistic effect was observed when the phorbol diesters were combined with either veratridine or d-amphetamine but not with forskolin and dibutyryl cyclic AMP. Pretreatment of striatal synaptosomes with phorbol diesters produced an activation on of tyrosine hydroxylase (TH) associated with a 60% increase of the Vmax and a decrease of the Km for the pterine cofactor 6-methyl-5, 6, 7, 8-tetrahydropterin. These results indicate that protein kinase C participates in the regulation of striatal TH in situ and that its activation may act synergistically with DA releasing agents in stimulating DA synthesis.