Optogenetics: An attempt to dissect neuronal pathways that regulate behavior

This review is an introduction to a recently developed technology “optogenetics” that allows researchers to directly manipulate the activity of aimed neurons with millisecond (ms) order in a behaving animal. Two types of microbial opsin, channel rhodopsin 2 (ChR2) and halorhodopsin (NpHR), are commonly used as the tools for optogenetics. ChR2 responds to blue light to induce neuronal firing via cation influx, whereas NpHR responds to yellow light to inhibit neuronal activity via Cl- influx. This review first introduces these and other opsins that have been used for optogenetics. Next, three methods to introduce these foreign genes into mouse nervous system are going to be explained: 1) viral infection, 2) in utero electrophoresis, and 3) transgenic mouse. Then, this review illustrates how neuron-type specific expression of the opsin gene can be achieved, and also how the optic stimulation of opsins that expresses in the deep brain structure can be accomplished. Finally, how the optogenetic technique has been used for behavioral neuroscience will be discussed by focusing on the studies about amygdala microcircuit that mediates conditioned fear.