Membrane depolarization evokes neurotransmitter release in the absence of calcium entry

Abstract

THE discovery that Ca²⁺ is necessary for the release of neurotransmitter, the primary means by which nerve cells communicate, led to the calcium hypothesis of neutransmitter release^1–4, in which release is initiated after an action potential only by an increase in intracellular Ca²⁺ concentration near the release sites and is terminated (1–2 ms) by the rapid removal of Ca²⁺. Since then, the calcium-voltage hypothesis has been proposed^5,6, in which the depolarization of the presynaptic terminals has two functions. First, in common with the calcium hypothesis, the Ca²⁺ conductance is increased, thereby permitting Ca²⁺ entry. Second, a confor-mational change is induced in a membrane molecule that renders it sensitive to Ca²⁺, and then binding of Ca²⁺ to this active form triggers release of neurotransmitter. When the membrane is repolarized, the molecule is inactivated and release is terminated, regardless of the local Ca²⁺ concentration at that moment. This hypothesis, in contrast to the calcium hypothesis, accounts for the insensitivity of the time course of release to experimental manipulations of intracellular Ca²⁺ concentation^7–11, Furthermore, it explains rapid termination of release after depolarization, even though Ca²⁺ concentration may still be high. Here we describe experiments that distinguish between these two hypotheses and find that our results support the calcium voltage hypothesis.