Competition of some biogenic amines for uptake into synaptic vesicles of the striatum

Summary

Synaptic vesicles were isolated from the caudate nucleus of the pig by differential centrifugation and incubated with labelled monoamines in the absence or in the presence of ATP-Mg²⁺. Addition of ATP-Mg²⁺ enhanced the uptake of ¹⁴C-dopamine into the vesicles. Serotonin competitively inhibited the ATP-Mg²⁺-dependent uptake of ¹⁴C-dopamine without influencing the uptake which took place in the absence of ATP-Mg²⁺. Likewise, dopamine caused a dose-dependent inhibition of the ATP-Mg²⁺-dependent uptake of ¹⁴C-serotonin without inhibiting the uptake in the absence of ATP-Mg²⁺. Incubation of the vesicles with equal concentrations of ³H-dopamine and ¹⁴C-serotonin revealed that the presence of the one amine competitively inhibited the ATP-Mg²⁺-dependent uptake of the other. Tyramine competitively inhibited the ATP-Mg²⁺-dependent uptake of ¹⁴C-dopamine, ¹⁴C-serotonin and ¹⁴C-noradrenaline into the vesicles; the uptake of the amines which took place in the absence of ATP-Mg²⁺ was not impaired by tyramine. Analysis of the amine uptake by the ABC test showed that a mutual inhibition exists between dopamine and serotonin for the uptake into the synaptic vesicles. GABA did not influence the uptake of ¹⁴C-dopamine either in the absence, or in the presence of ATP-Mg²⁺.

The uptake of ¹⁴C-tyramine or ¹⁴C-β-phenylethylamine was not increased by ATP-Mg²⁺, while that of ³H-octopamine was greatly enhanced. Tyramine inhibited the ATP-Mg²⁺-dependent uptake of ³H-octopamine without influencing the uptake in the absence of ATP-Mg²⁺. During incubation with tyramine virtually no conversion to octopamine took place.