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Stimulus-Dependent Alterations in Quantal Neurotransmitter Release

Department of Neurobiology, Pharmacology, and Physiology, The University of Chicago, Chicago, Illinois

Submitted 6 January 2006; accepted in final form 16 August 2006

Neurotransmitter release is a steep function of the intracellular calcium ion concentration ([Ca²⁺]_i) at the release sites. Both the Ca²⁺ amplitude and the time course appear to be important for specifying neurotransmitter release. Ca²⁺ influx regulates the number of vesicles exocytosed as well as the amount of neurotransmitter each individual vesicle releases. In our study we stimulated mouse chromaffin cells in two different ways to alter Ca²⁺ presentation at the release sites. One method, digitonin permeabilization followed by exposure to Ca²⁺, allows for a large uniform global elevation of [Ca²⁺]_i, whereas the second method, application of nicotine, depolarizes chromaffin cells and activates voltage-dependent Ca²⁺ channels, thereby producing more phasic and localized changes in [Ca²⁺]_i. Using amperometry to monitor catecholamine release, we show that both kinds of stimuli elicit the exocytosis of similar quantities of neurotransmitter per large dense core vesicles (LDCVs) released. Even so, the release process was quite different for each stimulus; nicotine-elicited events were small and slow, whereas digitonin events were, in comparison, large and fast. In addition, the transient opening of the fusion pore, called the "foot," was essentially absent in digitonin-stimulated cells, but was quite common in nicotine-stimulated cells. Thus even though both strong stimuli used in this study elicited the release of many vesicles it appears that the differences in the Ca²⁺ levels at the release sites were key determinants for the fusion and release of individual vesicles.

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