Enhanced [³H]-noradrenaline release in synaptosomes from ethanol-tolerant animals: The role of nerve terminal calcium ion concentrations

Cortical slices from brains of ethanol-tolerant rats have previously been shown to release a greater fraction of uptaken [³H]-noradrenaline ([³H]-NA) on K+ depolarisation than slices from control animals. In the present experiments, when stimulated with the Ca²⁺ ionophore A23187 (30 μM), untreated cortical synaptosomes release only a very small fraction of uptaken [³H]-NA. Under these conditions synaptosomal preparations from ethanol-tolerant animals released a greater fraction of uptaken [³H]-NA but the difference was not significant. However, after incubation with ouabain (100 μM, to increase intrasynaptosomal [Ca²⁺]) A23187 now produced a much greater enhancement of [³H]-NA release in preparations from ethanol-tolerant rats. Further, after superfusion of the synaptosomal preparation with 1mM EGTA and A23187 for 30 min (a procedure which should reduce intrasynaptosomal [Ca²⁺]) the reintroduction of Ca²⁺ to the superfusing fluid caused a marked release of [³H]-NA which was also significantly greater in preparations from ethanol-tolerant animals. Ca²⁺/Mg²⁺-ATPase activity was also higher in these synaptosomes but no difference could be detected in the release of [³H]-NA from a combined synaptic vesicle: synaptic plasma membrane preparation. The results suggest that the development of ethanol tolerance is associated with a fundamental change in the dynamic control of Ca²⁺ concentrations within the nerve terminal which potentiates the depolarisation-induced release of neurotransmitters.