In its broadest sense, optogenetic tools include both optical indicators of neuronal activity, such as genetically encoded calcium or voltage sensors, as well as optical actuators of neuronal activity, such as light-activated membrane channels and pumps.
One of the most exciting recent advances in experimental neuroscience has been the development of genetically encoded light-sensitive proteins, giving rise to the burgeoning field of optogenetics. Given the wealth of information transgenic mice have yielded in past studies of neural circuits, it is not surprising that considerable efforts have been expended to establish lines in which the activity of populations of neurons can be both easily observed and reliably and reversibly manipulated. Further, using a strategy for combinatorial expression of fluorescent proteins, BrainBow mice have enabled the simultaneous mapping of projections and connectivity among multiple neurons ( Livet et al., 2007). For example, the strong, neuronally-restricted expression of fluorescent reporter in Thy1-EYFP mice has made possible studies of morphology, connectivity, electrophysiology, and mRNA content of a single neuron and has permitted long-term in vivo imaging of neurons ( Feng et al., 2000 Micheva et al., 2010 Sugino et al., 2006). In functional studies of the mouse brain, a variety of transgenic strategies have been used to inactivate or over-express particular genes, label specific cell populations or their subcellular compartments, and manipulate the activity or function of specific cell populations ( Luo et al., 2008). Perhaps the greatest advantage of using a transgenic approach in such studies is that cell population-restricted transgene expression can be achieved using specific promoters, and this restricted pattern of expression can be passed on to subsequent generations fairly reproducibly. Transgenic mice have been widely used in neuroscience research to facilitate the deciphering of gene and cellular functions. In this chapter, we review the state of these efforts and consider aspects of the current technology that would benefit from additional improvement.
Not surprisingly then, recent years have seen substantial efforts directed towards generating transgenic mouse lines that express functionally relevant levels of optogenetic tools. Transgenic mice engineered to express optogenetic tools in a cell type-specific manner offer a powerful approach for examining the role of particular cells in discrete circuits in a defined and reproducible way. Recent advances in the field of optogenetics now enable researchers to monitor and manipulate the activity of genetically defined cell populations with the speed and precision uniquely afforded by light. Though informative for many kinds of studies, these approaches are not sufficiently fine-tuned for examining the functionality of specific cells or cell classes in a spatially or temporally-restricted context. Dissection of such complex networks has typically relied on disturbing the activity of individual gene products, perturbing neuronal activities pharmacologically, or lesioning specific brain regions, to investigate the network’s response in a behavioral output. A major challenge in neuroscience is to understand how universal behaviors, such as sensation, movement, cognition, and emotion, arise from the interactions of specific cells that are present within intricate neural networks in the brain.
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