Is inhibition of striatal synaptosomal tyrosine hydroxylation by dopamine agonists a measure of dopamine autoreceptor function?

Rat striatal synaptosomal tyrosine hydroxylation was inhibited dose- and pH dependently by a number of dopamine agonists. The catecholic agonists apomorphine and (-)N-n-propylnorapomorphine inhibited synaptosomal tyrosine hydroxylase completely, with IC50 values of around 0.3 mumol/l at pH 6.6. The noncatechol agonists pergolide and bromocriptine and the putative dopamine autoreceptor agonists 3-PPP(-), 3-PPP(+), HW-165 and B-HT 920 produced only partial inhibition of synaptosomal tyrosine hydroxylation at high concentrations. Comparison of the inhibition of synaptosomal and soluble tyrosine hydroxylase indicated that the inhibition produced by apomorphine could be ascribed to a direct effect on the enzyme, whereas this was not the case for the noncatechol agonists. The inhibition produced by pergolide and 3-PPP(-) was not antagonised by either dopamine receptor or alpha-adrenoceptor antagonists. The present results have been compared with results reported in the literature for inhibition of synaptosomal tyrosine hydroxylation and for two other tests of dopamine autoreceptor agonist activity (inhibition of dopamine release from striatal slices in vitro, and inhibition of the gamma-butyrolactone induced increase in dopamine synthesis in vivo). It is concluded that inhibition of striatal synaptosomal tyrosine hydroxylation by dopamine agonists does not fulfil the criteria required for it to be considered as a useful measure of dopamine autoreceptor function.