Membrane Potential Imaging in the Nervous System Methods and Applica
Membrane Potential Imaging in the Nervous System: Methods and Applications describes optical techniques to monitor neuronal membrane potential signals. The knowledge upon which voltage-imaging technique is based was generated over several decades and the
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Monitoring Membrane Voltage Using Two-Photon Excitation of Fluorescent Voltage-Sensitive Dyes Jonathan A.N. Fisher and Brian M. Salzberg
11.1 Introduction Voltage-sensitive dyes, or potentiometric probes, are molecular voltmeters that insert into, but do not cross cell membranes, where they intercalate among the phospholipids that compose either leaflet of the bilayer. There, by mechanisms that are not entirely understood, they sense a representative portion of the membrane electric field and alter their light absorption and/or emission in response to changes in that field, thereby providing the physical basis for optical measurement of membrane potential (Salzberg et al. 1973; Cohen and Salzberg 1978). From its inception in the early 1970s, optical measurement of electrical activity promised advantages under circumstances where electrodes were difficult or impossible to use for reasons of size, complexity, or membrane topology. But it was the prospect of VSDs functional imaging of nervous systems, whether invertebrate ganglia (Salzberg et al. 1977) or mammalian brains (Grinvald 1985; Orbach et al. 1985; Obaid et al. 1992) that really impelled the field of optical recording of electrical activity. Indeed, beginning with some of the earliest reports of molecular probes of membrane potential, ambitious ideas for the application of these nanometer scale voltmeters to the study of the central nervous system were never far from consciousness. The tantalizing metaphor of Sherrington’s “enchanted loom where millions of flashing shuttles weave a dissolving pattern, always a meaningful pattern though never an abiding one; a shifting harmony of sub patterns” (Sherrington 1951) inspired scientists to try to find new ways of exploiting potentiometric probes to learn more about the brain. Since the functional architecture of the mammalian brain is emphatically three-dimensional, it is the goal of potentiometric neuroimaging to accurately resolve neuronal electrical activity in all dimensions with high spatial- and temporal-resolution. Further, the work of Llinás (1988) and Llinás et al. (1998) has clearly demonstrated that mesoscale aspects of brain function can emerge from the electrical properties of individual neurons, emphasizing the need for cellular and subcellular scale imaging in three dimensions. Only recently, however, has the promise of functional imaging of electrical activity in three dimensions using VSDs seemed close to being fully realized. The use of voltage-sensitive (potentiometric) dyes (VSDs) as molecular voltmeters (Salzberg et al. 1973; Cohen and Salzberg 1978; Salzberg 1983) is presently the only optical technique
enabling direct measurement of rapid changes in neuronal membrane potential, and optical detection of transmembrane electrical events offers numerous advantages over more conventional measurement techniques. Because cell membranes are not physically breached and mechanical access is not required, the methods are relatively noninvasive, and considerable latitude is possible in the choice
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