© Cambridge University Press 2017. Optogenetic Mapping of Local Neural Circuits The Birth of the “Opto” in “Optogenetics”: Circuit Mapping via Glutamate Uncaging A description of the structure of a neural network and the potential of plasticity at its nodes is prerequisite for a mechanistic conception of how neural signals flow within the circuit and ultimately realize the circuit's computational function (Martin, 2009). However, this endeavor is hampered by many problems. For example, descriptions of axonic and dendritic morphology (Binzegger et al., 2004; Helmstaedter et al., 2009a; Helmstaedter et al., 2009b; Helmstaedter et al., 2009c) do not capture the actual physiological connectivity (e.g. synaptic strength) between neurons. In contrast, approaches exclusively using electrophysiology (Thomson et al., 1996; Gupta et al., 2000; Thomson et al., 2002; Brown and Hestrin, 2009) can only look at few neurons at a time (Scanziani and Häusser, 2009); they thus remain blind to the organization of the network as a whole. Contrastingly, optical activation of neurons in combination with the electrophysiological recording of optically evoked inputs arriving at a postsynaptic cell has the potential to overcome these obstacles. It allows us to “scan” the connections in a circuit by moving an activating light beam from point to point and thereby probe the connections from hundreds of neurons (or small clusters of neurons) sequentially within a single experiment. The key technology that enabled this optical mapping approach was the development of so-called caged neurotransmitters; that is, neurotransmitters covalently attached to chemical groups that inhibit their binding to endogenous synaptic receptors. When such biologically inert caged precursors are exposed to ultraviolet light, the “cage” group is photolytically removed and the transmitter freed in its active form. Although multiple caged agonists and antagonists of synaptic receptors have been developed, the most widely used compound deployed for mapping studies is caged glutamate. Its photo-cleavage leads to a sudden surge of the local glutamate concentration that activates all synapses and neurons in its immediate vicinity, irrespective of neuronal subtype. This method has been refined in order to optically activate local populations of neurons (Walker et al., 1986; Wilcox et al., 1990; Callaway and Katz, 1993; Wieboldt, et al., 1994a; Wieboldt, et al., 1994b) and map tens or hundreds of connections between neurons at high speed by rastering neural tissue with an uncaging light beam (Callaway and Katz, 1993; Katz and Dalva, 1994).
Optogenetics: From Neuronal Function to Mapping and Disease Biology
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