Compartmentation of tyrosine hydroxylase at the nerve ending in rat brain striatum

The synthesis of dopamine in rat brain striatal synaptosomes has been studied by two different methods; a radiometric method based on the formation of labelled products from radioactive tyrosine, and an HPLC method based on the measurement of dopamine production. The synaptosomal tyrosine hydroxylase showed different sensitivities to high K[+] depolarization and removal of Ca[2]+ by EGTA when measured by 1]; a liquid chromatographic method [HPLC], or by 2]; a radiochemical assay. The pattern of the time course and extent of activation of the synaptosomal enzyme by dibutyryl-cyclic AMP as measured by the HPLC method was also different from that of radiochemical assay. The differences between the results obtained by these two techniques are interpreted as being consistent with the compartmentation of tyrosine hydroxylase in synaptosomes, i.e. a membrane-bound and a soluble form of the enzyme

Effects of catecholamines on dopamine and serotonin synthesis in rat brain striatal synaptosomes: the role of presynaptic receptors and the synaptosomal reuptake mechanism

The regulation of dopamine and serotonin synthesis in rat brain striatal synaptosomes has been studied using HPLC methods. Noradrenaline was shown to markedly inhibit both the synthesis of dopamine and serotonin. The response of the synaptosomes to the concentrations of noradrenaline appeared to be biphasic, a very effective inhibition occurring at low concentrations [1-5 microM] and a relatively ineffective further inhibition occurring at high concentrations [up to 100 microM]. The inhibition of dopamine and serotonin synthesis by noradrenaline was also studied in the presence of phenoxybenzamine [alpha adrenergic receptor blocker] and imipramine [reuptake inhibitor]. Phenoxybenzamine changed the pattern of inhibition of both dopamine and serotonin synthesis by noradrenaline by preventing the very effective inhibition previously seen at low [1-5 microM] noradrenaline concentrations. Imipramine, whilst showing marked inhibition of dopamine synthesis on its own, prevented any inhibition by noradrenaline. In the case of serotonin synthesis, however, imipramine alleviated some of the inhibition seen in the presence of noradrenaline alone. The results are discussed with respect to the role that presynaptic receptors and reuptake mechanisms play in the regulation of catecholamine and serotonin synthesis at the nerve ending