Presynaptic Dopamine Autoreceptors Control Tyrosine Hydroxylase Activation in Depolarized Striatal Dopaminergic Terminals

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Abstract
The possible control of tyrosine hydroxylase (TH) activity by dopaminergic receptor-dependent mechanisms was investigated using rat striatal slices or synaptosomes incubated in the presence of various 3,4-dihydroxyphenylethylamine (dopamine or DA) agonists and antagonists. Under "normal" conditions (4.8 mM K+ in the incubating medium), the DA agonists apomorphine, 6,7-dihydroxy-N,N-dimethyl-2-aminotetralin (TL-99), 7-hydroxy-N,N-dipropyl-2-aminotetralin (7-OH-DPAT), Trans-(-)-4,4a,5,6,7,8,8a,9-octahydro-5-propyl-2H-pyrazolo-3,4- quinoline, and 3-(3-hydroxyphenyl)-N-n-propylpiperidine decreased TH activity in soluble extracts of incubated tissues. In the case of the catechol-containing drugs apomorphine and TL-99, this effect was partly due to a direct inhibition of the enzyme, but in all other cases it appeared to depend on the stimulation of presynaptic DA autoreceptors. No effect of DA antagonists was detected on TH activity under "normal" conditions. In contrast, when tissues were incubated in a K+ -enriched (60 mM) medium, (-)-sulpiride and other DA antagonists enhanced TH activation due to depolarization whereas DA agonists were ineffective. Because (-)-sulpiride also increased the enzyme activity in striatal slices exposed to drugs inducing release of DA, such as veratridine and d-amphetamine, it is concluded that the stimulating effect of the DA antagonist resulted in fact from the blockade of the negative control of TH normally triggered by endogenous DA acting on presynaptic autoreceptors. In contrast to TH activation due to K+ -induced depolarization, the activation evoked by tissue incubation with dibutyryl cyclic AMP was unaffected by the typical agonist 7-OH-DPAT or the antagonist (-)-sulpiride. This would suggest that TH control via presynaptic DA autoreceptors normally concerns possible modulations of the cyclic AMP-dependent phosphorylation of the enzyme.