Striatal synaptosomal dopamine synthesis: Evidence against direct regulation by an autoreceptor mechanism

Regulation of the rate-limiting step in dopamine (DA) synthesis was estimated in striatal synaptosomes by measuring the rate of hydroxylation of L-4-[³H]phenylalanine, a substrate of tyrosine hydroxylase (TH). DA inhibited hydroxylation with an IC₅₀ of 0.2 μM. The concentration-response curve of DA-induced inhibition was not affected by the presence of 1 μM chlorpromazine, a phenothiazine DA antagonist. Sulpiride and haloperidol, DA antagonists of the benzamide and butyrophenone classes respectively, also failed to alter the inhibition of substrate hydroxylation by 1 μM DA, even at cocnentrations up to 10 μM. In contrast, a parallel 15 fold shift to the right in the concentration-response curve of DA-induced inhibition of hydroxylation was obtained when 10 μM nomifensine, a competitive DA uptake inhibitor, was added. Even in the presence of nomifensine, 1μM chlorpromazine had no effect on the DA concentration-response curve. The addition of DMPH₄, an artificial cofactor for TH, completely blocked DA-induced inhibition of enzymatic activity. These data suggest that direct autoreceptor control of synaptosomal TH activity does not exist in vitro, and that DA-induced inhibition of TH occurs subsequent to reuptake via classical feedback inhibition, presumably by competitive displacement of the necessary endogenous cofactor.