THE UPTAKE AND RELEASE OF [³H]DOPAMINE IN THE GOLDFISH RETINA

Abstract— The uptake and release of [³H]dopamine was studied in the goldfish retina with the following results: (1) when goldfish retinas were incubated with 2 ± 10^‐7m‐[³H]dopamine for less than 20min and processed for autoradiography. most of the label was associated with dopaminergic terminals that contact certain horizontal cells. Biochemical analysis showed that > 93% of this label was [³H]‐dopamine. (2) [³H]dopamine uptake saturated with increasing dopamine concentration and followed Michaelis‐Menten kinetics. This uptake could be explained by a single ‘high‐affinity’ mechanism with a K_m of 2.61 ± 0.41 ± 10^‐7m and a V_max of 66 ± 12 ± 10^‐12 mol/min/mg protein. (3) [³H]dopamine uptake was temperature‐dependent with a temperature coefficient of 1.7 and an energy of activation of 11.4 kcal/mol. (4) The initial rate of uptake was unaffected by the absence of Ca²⁺ or the presence of Co²⁺; however, more than 85, uptake was blocked in the absence of external Na⁺. (5) Neither 1 mm‐cyanide nor 5 mm‐iodoacetate blocked more than 30% of uptake individually; however, in combination > 70% of uptake was blocked. (6) Centrally acting drugs benztropine and diphenylpyraline inhibited at least 60–70% of [³H]dopamine uptake. (7) [³H]dopamine in the retina could be released by increasing the external K⁺ concentration. This release was Ca²⁺ ‐dependent and was blocked by 10mm‐Co²⁺ or 2Omm‐Mg²⁺. The amount of [³H]dopamine released was not affected by the presence of benztropine, diphenylpyraline or fluphenazine in the incubation medium. These studies add further support for dopamine as a neurotransmitter used by interplexiform cells of the goldfish retina.