20] Calcium influx during an action potential

Abstract

Action potentials open voltage-sensitive calcium channels in excitable cells, leading to an influx of calcium ions. Calcium ions may control, among others, cell excitability, neurotransmitter release, or gene transcription. This chapter discusses two different methods that can be used to quantify the Ca 2+ flows into a cell or cell compartment during an action potential. Quantification of Ca 2+ influx may be a first step toward the construction of a model for the calcium dynamics of cells. It may also serve as a reference to study pathological processes such as cell death during ischemia or amyotrophic lateral sclerosis, where increases in calcium influx have generally been implicated. The two methods that are compared in this chapter are a voltage-clamp method, in which cells are voltage clamped with an action potential waveform command, and a fluorescence method, in which cells are loaded with a high concentration of a calcium-sensitive fluorescent dye. Both methods are applied to a giant axosomatic terminal, called the “calyx of Held,” in a slice preparation of the rat brain stem. Each principal cell in the medial nucleus of the trapezoid body (MNTB) is contacted by a single calyx-type terminal. This synapse is part of a fast auditory pathway that is involved in the localization of sound. This chapter is interested in the relationship between calcium influx and neurotransmitter release at this glutamatergic synapse. Both methods gave similar values for the number of calcium ions that enter the presynaptic terminal during an action potential. It first discusses the action potential waveform voltage-clamp method.