Bilirubin inhibits Ca2+-dependent release of norepinephrine from permeabilized nerve terminals

Although the well-known neurotoxic agent bilirubin can induce alterations i n neuronal signaling, direct effects on neurotransmitter release have been difficult to demonstrate. In the present study we have used permeabilized n erve terminals (synaptosomes) from rat brain prelabeled with [H-3]norepinep hrine to examine the effects of bilirubin on transmitter release. Rat cereb rocortical synaptosomes were permeabilized with streptolysin-O (2 U/ml) in the absence or presence of bilirubin (10 mu M-320 mu M) and Ca2+ (100 mu M) , and the amount of radiolabeled transmitter released during 5 min to the m edium was analysed. Low levels of bilirubin decreased Ca2+-evoked release i n a dose-dependent manner, with half-maximal effect at approx 25 mu M bilir ubin. Higher levels of bilirubin (100-320 mu M) increased [H-3]norepinephri ne efflux in the absence of Ca2+, suggesting that high bilirubin levels ind uced leakage of transmitter from vesicles. The nontoxic precursor biliverdi n had no effect on Ca2+-dependent exocytosis. Our data indicate that biliru bin directly inhibits both exocytotic release and vesicular storage of brai n catecholamines.