Uptake of Neurotransmitter Candidates by Pigeon Optic Tectum

Abstract —The kinetics of the uptake of various compounds into the crude mitochondrial fraction (P2‐fraction) of the pigeon optic tectum were studied. Aspartate, GABA, glutamate, glycine, proline and choline were taken up by high and low affinity systems. Only low affinity uptake was found for leucine and dopamine. The uptake for taurine was not saturable. The uptakes for all of the above compounds were temperature and sodium dependent. The high affinity system was more effected by sodium withdrawal than the low affinity system. Continuous sucrose gradients from 0.4 to 1.5 m were run with incubated P2 fractions. Particle‐bound radioactivity sedimented in a density range of 1.0 to 1.2 m‐sucrose. Additional experiments with glutamate showed that its uptake is competitively inhibited by aspartate. The addition of a 1000‐fold excess of glutamate to a P2 fraction incubated at a concentration of 10–6m‐glutamate led to a massive decrease of particle‐bound radioactivity, suggesting a coupled action of uptake and efflux.