Electrochemistry at the cell membrane/solution interface

Electrochemistry in ultra-small environments has emerged as an increasingly important technique for fundamental studies of diffusionally restricted reactions, low sample volume analysis, and single cell neurochemistry. Development of electrochemical methods for detection of neurotransmitters began with the ground-breaking work of Adams1 and has progressed to the point where it is now possible to detect the release of a neurotransmitter from a single vesicle, first demonstrated in the seminal work by Wightman and co-workers.2,3 In the pioneering experiments, a carbon fiber electrode 5 mm in diameter was placed adjacent to a bovine adrenal chromaffin cell isolated in a culture dish, and the cell was stimulated to release either by chemical or mechanical means. Understanding the chemistry and structure of single cells is an area of great interest in the biological and medical sciences. Indeed, books have been written on this broad topic.4 In neuroscience, knowledge of the chemical composition and dynamics of single nerve cells should lead to better models of the cellular neurotransmission process. The key dynamic event in neuronal communication is exocytosis. This is a process that has been investigated extensively for several decades.5,6 The process of exocytosis can be summarized as the docking of vesicles (storage compartments) to the cell membrane and subsequent release of the contents by fusion of the vesicle and cell membranes. This process allows the conversion of an electrical signal (action potential) to a chemical signal (messenger release and receptor recognition), which is necessary for exocytotic communication between cells. Methods to observe and quantify individual exocytotic events have traditionally revolved around electron microscopy and patch-clamp capacitance measurements.7 In 1990, Wightman and co-workers showed that they could directly monitor individual exocytotic events involving easily oxidized messengers and occurring on the millisecond time scale by use of amperometric measurements at microelectrodes.3 This was first applied by Wightman’s group to adrenal chromaffin cells,2 and later by Neher’s group.8 This chapter will focus on the development of electrochemical and related methods to analyze exocytosis events at cells, as well as artificial cells. These methods, emphasizing amperometry at microelectrodes, will be reviewed in terms of different cell types examined and discussed for representative and specific studies of neuroscience.