Calcium dependence of the rate of exocytosis in a synaptic terminal

Abstract

RAPID calcium-dependent exocytosis underlies neurotransmitter release from nerve terminals. Despite the fundamental importance of this process, neither the relationship between presynaptic intra-cellular calcium ion concentration ([Ca²⁺]_i) and rate of exocytosis, nor the maximal rate of secretion is known quantitatively. To provide this information, we have used flash photolysis of caged Ca²⁺ to elevate [Ca²⁺]_i rapidly and uniformly in synaptic terminals, while measuring membrane capacitance as an index of exocytosis and monitoring [Ca²⁺]_i with a Ca²⁺-indicator dye. When [Ca²⁺]_i was abruptly increased to >10 µM, capacitance rose at a rate that increased steeply with [Ca²⁺]_i. The steepness suggested that at least four calcium ions must bind to activate synaptic vesicle fusion. Half-saturation was at 194 µM, and the maximal rate constant was 2,000–3,000 s^–1. A given synaptic vesicle can exocytose with high probability within a few hundred microseconds, if [Ca²⁺]_i rises above lOOµM. These properties provide for the extremely rapid signalling required for neuronal communication.