Classically, temporally precise excitation of membrane potential in neurons 
within intact tissue can be achieved by direct electrical stimulation or 
indirect electrical stimulation induced by changing magnetic fields. Both of 
these approaches have a predetermined selectivity based on the biophysical 
properties of the nervous tissue and membrane in the region of the stimulation. 
A recent advance in selective excitation of neurons is the "optogenetic" 
approach utilizing channelrhodopsins (ChRs). By expressing the light-responsive 
ChR in neurons using cell-type selective promoters or other methods, specific 
neurons can be depolarized by light in a temporally precise manner with 
millisecond resolution even if their membrane biophysical properties are less 
favorable for electrical stimulation. In addition, ChRs can be used to 
depolarize nonneuronal cells in the nervous tissue, and to sustain 
depolarization over a prolonged period of time, both of which cannot be achieved 
with electrical or magnetic stimulations. To conduct an experiment with ChR, 
experimenters need to make the correct choices on the three main components to 
such an experiment: the expression system, the illumination source, and the ChR 
variant used. This chapter aims to provide some discussions on the current 
developments of these aspects of the experiments. To express ChR in neurons, the 
common expression systems include viral vectors, in utero electroporation, and 
transgenic animals, each with their advantages and limitations regarding the 
cost, expression pattern, and the required effort. In terms of the 
instrumentation, an illumination source that is capable of providing the desired 
wavelength with high intensity is crucial for the success of the experiment. The 
important factors regarding the light source used include the cost, light 
density output, efficiency for fiber coupling for in vivo rodent experiments, 
and the available methods to control light intensity and onset/termination. The 
third component of the experiment is the choice of the appropriate variants of 
ChR. Many novel ChR variants with unique properties have been engineered, and it 
can be difficult for the experimenters to choose the right variant with the 
desired properties for their experiments, as some information necessary for the 
experimenter to make the right selection is often incomplete or unavailable. 
Currently, the available variants for neuroscientific research are wild-type 
ChR2, ChR2+H134R, ChETA, VChR1, SFO, ChD, ChEF, ChIEF, ChRGR, CatCh, and TC. The 
features and limitations of these different variants are presented here. Lastly, 
this chapter will provide some suggestion for the future development of the 
light source, expression system, and the development of the "next" generation of 
ChRs.