Uptake, localization and release of serotonin in the chick retina

1. High pressure liquid chromatography and electrochemical detection have shown that serotonin exists in the chick retina (127 ng/g wet weight). No other indoleamine was identified. 2. Immunofluorescent histological studies showed that the endogenous serotonin was localized apparently in those cell bodies and processes which took up exogenous [3H]serotonin as revealed by autoradiography. These serotonergic neurones can be destroyed by injecting kainic acid into the eye. 3. Isolated chick retina accumulated exogenous [3H]serotonin. Kinetic analysis revealed the presence of two saturable uptake systems: a 'high affinity' mechanism with an apparent Km of 5.9 X 10(-8) M and a Vmax of 0.143 X 10(-13) mol/mg wet weight . min and a low affinity mechanism with an apparent Km2 of 1.8 X 10(-3) M and Vmax of 0.12 X 10(-9) mol/mg wet weight . min. 4. The uptake of serotonin was temperature-sensitive and sodium-dependent and Lilly 110140 and chlorimipramine were potent inhibitors of the amine uptake. 5. Autoradiographic studies indicated that neuronal processes associated with the innermost and outermost areas of the inner plexiform layer and perikarya situated in the inner nuclear layer are the sites which accumulated exogenous [3H]serotonin. 6. [3H]Serotonin accumulated in the retina was released by increasing the external K+ concentration. This release was Ca2+-dependent. Additionally, autoradiographic studies show that [3H]serotonin taken up by the serotonin neurones was also released by Ca2+-dependent K+ depolarization of the retina.