Twr. Hansen et al., Bilirubin inhibits Ca2+-dependent release of norepinephrine from permeabilized nerve terminals, NEUROCHEM R, 24(6), 1999, pp. 733-738
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.