Bilirubin Inhibits Transport of Neurotransmitters in Synaptic Vesicles

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Abstract
Uptake of neurotransmitters into synaptic vesicles occurs through specific transport proteins which are driven by an ATPase-generated electrochemical force consisting of a proton gradient and a membrane potential. In this study we examined the effects of bilirubin, a well known neurotoxic agent, on the vesicle uptake both of [3H]dopamine (which is driven mostly by the proton gradient) and [3H]glutamate (which is driven mostly by the membrane potential), and compared these to the vesicular proton gradient, which was estimated by analyzing the uptake of [14C]methylamine. Bilirubin inhibited the uptake of both dopamine and glutamate (p< 0.01), with an identical dose-response curve for both transmitters. Inhibition was detected readily at 75 µM. The effects of bilirubin were dependent on the concentration of vesicles in the assay, suggesting that the concentration of bilirubin in the membranes and not the water phase was important. Bilirubin also decreased uptake-dependent efflux of dopamine from the vesicles. In contrast, bilirubin had no effect on the vesicular proton gradient, as measured by methylamine uptake. Our results show that bilirubin has essentially identical inhibitory effects on the uptake of both a monoamine transmitter and an amino acid transmitter into synaptic vesicles, but does not influence the vesicular H+-ATPase or proton translocation. Our data suggest an inhibitory interaction between bilirubin and several transport proteins in synaptic vesicle membranes.