Interactions of bilirubin with isolated presynaptic nerve terminals: Functional effects on the uptake and release of neurotransmitters

1. The functional effects of bilirubin:albumin solutions (10:1, mol/mol) on several synaptosomal functions were investigated using rat cortical, striatal, and hippocampal synaptosomes prepared by iso-osmotic Percoll/sucrose gradient centrifugation. 2. Bilirubin (10-80 microM) depolarized synaptosomes in a tetrodotoxin-insensitive manner as assessed by the equilibrium distribution of tetra-[3H]phenylphosphonium. Depolarization induced by bilirubin was of a lesser magnitude than that caused by KCl or veratridine. Steady-state pH gradients across the synaptosomal membrane were determined using the transmembrane distribution of [14C]methylamine. Bilirubin (20-40 microM) did not modify the intracellular pH in physiological buffers. The pigment effected a 0.14 delta pH change when the synaptosomes were suspended in a Ca2+ and Na+ free choline medium containing ouabain. 3. Bilirubin (20-80 microM) had no effect of its own on [7,8-3H] dopamine release from striatal synaptosomes. In contrast, it inhibited the initial rate of synaptosomal uptake of the catecholamine and its intrasynaptosomal content at 10 min. The pigment (20 and 40 microM) reduced the 35 mM KCl-induced release of endogenous acetylcholine from hippocampal synaptosomes by 20 and 36%, respectively. 4. The association of bilirubin with synaptic plasma membrane vesicles was characterized by a chloroform:methanol 2:1 (v/v) extraction method. At total concentrations of 10 to 80 microM bilirubin, the molar percentage of the pigment in synaptic plasma membrane phospholipids was 1-4%. 5. It is proposed that the two main functional consequences of the bilirubin-nerve ending interaction are an impairment of specific membrane-bound neurotransmitter uptake mechanisms and a reduction of the response to depolarizing stimuli. This may be the basis for rapid alterations in synaptic transmission documented in early reversible bilirubin encephalopathy.