Use of rapid superfusion to differentiate the release of dopamine from striatal tissue induced by sympathomimetic amines from release induced by potassium.

A rapid superfusion system incorporating a continuous amperometric detector has been employed to study the effects of p-tyramine, d-amphetamine and cocaine on K+-stimulated release of dopamine (DA) from striatal tissue. Qualitative identification of the released substances is made by liquid chromatography with electrochemical detection and DA is established to be the principal substance released following exposure of the tissue to 60 mM K+ in the presence of Ca++. At a superfusion rate of 1 ml/min with a 20 microliter volume tissue holder, d-amphetamine and p-tyramine do not elicit detectable release of endogenous substances. These results are in direct contrast to the significant release of DA observed when striatal tissue is incubated with d-amphetamine for the same time period and suggest that the pools susceptible to release by the sympathomimetic amines are removed by superfusion. When striatal tissue is superfused with a buffer containing 60 mM K+ and cocaine or sympathomimetic amines, the effects of uptake blockade are manifested as an increase in the amount and duration of observed release over that obtained with 60 mM K+. In a competitive study of cocaine and p-tyramine with d-amphetamine, it appears that all three of these agents induce blockade at the same site(s). All of these data are in direct accord with the facilitated exchange diffusion mechanism proposed for the actions of sympathomimetic amines at DA nerve terminals.