BILIRUBIN (B) INHIBITS CA++-DEPENDENT RELEASE OF NOREPINEPHRINE (NE) FROM PERMEABILIZED NERVE TERMINALS (SYN). † 346

Abstract

Background: In vitro physiological studies in transverse hippocampal slices indicate that B inhibits synaptic activation. This might be due e.g. to inhibition of neurotransmitter release, and/or to changes in membrane potential, but the mechanism involved has remained unclear. Ca++-dependent exocytotic transmitter release can be studied independently from changes in membrane potential in SYN permeabilized with streptolysin O (SL-O). We have used this model to examine the effects of B on exocytotic release of NE. Methods: SYN were isolated from rat cerebral cortex by differential centrifugation, loaded with 3H-NE, washed and incubated at 37°C in buffer (pH 7.4) containing SL-O [2 U/mL], in the absence or presence of B [10-320 μM] with Ca++ [1 mM] used to evoke exocytosis or EGTA [1 mM] to obtain background values. The reactions were stopped after 5 min by addition of EGTA [10 mM]. Samples were rapidly transferred to a microplate filter unit, and the filtrates were collected by vacuum filtration. 3H-NE released from SYN was quantitated by liquid scintillation counting. Results: As shown in thefigure, exocytotic release of NE evoked by Ca++ was inhibited by B in a dose-dependent fashion. Discussion: These results document for the first time an inhibitory effect of B on Ca++-dependent exocytotic transmitter release. In permeabilized SYN, where changes in membrane potential have been eliminated, the effects of B may include either regulation of Ca++-induced fusion of synaptic vesicle with the plasma membrane, and/or subsequent release of contents into the synaptic cleft. Several aspects of this reaction appear sensitive to protein phosphorylation systems, many of which are inhibited by B. Therefore, B-induced depression of neuronal function might be partly explained by the ability of B to inhibit nerve terminal protein phosphorylation.