Why & How?
Optical stimulation of electrically excitable
cells is superior to classical activation by microelectrodes. The reason for
this is due to its high temporal and spatial resolution. Optical stimulation
can be achieved by using caged compounds, e.g. caged ATP, caged Glutamate,
whereby the substrates for depolarizing ion channels are delivered to membranes
and activated by pulses of UV-light to the chemical photolabile cage in the
micro and millisecond time scale. An improved approach is the application of chemical
photo-switches attached to ion channels, which allows to depolarize cells in a
reversible manner with high temporal and spatial resolution. These elegant
techniques, named chemical genetics, are applicable for neural cells in culture
as well as in small animals like drosophila and zebrafish. The discovery of channelrhodopsin2 (ChR2)
from the unicellar alga Chlamydomonas reinhardtii was the starting point
for the optogenetic approach. When
transfected into mammalian cells and activated by blue light (λmax 470nm) ChR2
acts as an inwardly rectifying cation channel, thus depolarizing the cells.
Together with the light-activated inwardly directed, hyperpolarizing Cl-pump
Halorhodopsin (NphR, λmax 580nm) from the archea Natronomonas pharaonis, the
two form an ideal pair for the activation and inactivation of neural cells (at
two different wavelengths). ChR2 activates the cells with blue light by
depolarization, whereas NphR inactivates the cells with yellow light by
hyperpolarization of the cells (fig1.). In contrast to the chemical approach
the microbial rhodopsins do not need any extra addition of chemicals, because
the light sensing cofactor retinal is produced by the host cells and binds
correctly to the opsins, forming the functional rhodopsins. ChR2 and NphR can
be expressed in a more selective manner in certain cell types of the brain by
use of finer tuned promoters and molecular biology methods. Because ChR2 and
NphR and their variants can be easily expressed in neural cells or used to form
transgenic animals, these microbial rhodopsins are long sought tools for
neurobiological research. (Source: http://www.mpg.de/36227/bm06_Optogenetics-basetext.pdf)