glucose transport proteins Even earlier in Stoeckenius and O

Even earlier, in 1971, Stoeckenius and Oesterhelt showed that bacteriorhodopsin acts as an ion pump that may be rapidly activated by visible-light photons [7]. Later other members of this family, halorhodopsin (1977) and channelrhodopsin (2002), were discovered [8]. Still, the consensus for a long time had been that this combination of optical and genetic methods would not produce the desired effect: firstly, because the foreign membrane proteins introduced to the cell could be toxic; secondly, many scientists assumed the light-induced currents to be too low. Additionally, to absorb photons, bacterial opsins need a chemical cofactor, all-trans-retinal. In the summer of 2005, a study was published demonstrating it was possible to use a bacterial opsin without adding any other parts, components or reagents [1], with the neurons rendered photosensitive. In the following years, other researchers found that bacteriorhodopsin and halorhodopsin, as well as channelrhodopsin, are capable of turning neurons on and off rapidly and without any risks to cells in response to being irradiated by light of varying wavelengths. Vertebrate tissues already contain all-trans-retinal, and therefore optogenetic control is possible in intact brain tissues and even in freely moving animals.
Modern advances A number of supremely interesting experiments using the new technology have been carried out in the recent seven years. New opsins are under development with the goal of applying optogenetics to a wide array of studies on various organisms. For example, in 2008, channelrhodopsin VChR1 sensitive to yellow light instead of blue was derived from Volvox carteri algae [9]. By concurrently using several types of channelrhodopsins, an experimenter may simultaneously control mixed cell populations: some commands may be given to cells of the first type by yellow light, and others to the second type by blue light. The so-called ‘fast’ and ‘slow’ opsins were also created, allowing to control glucose transport proteins duration. The former opsins are capable of creating action potentials up to 200 times per second [10]. Opsins have already been designed that are sensitive to light whose frequency is at the boundary between the visible and the infra-red regions. Waves with this frequency penetrate the tissues deeper and are more easily focused. One of the most interesting possibilities of optogenetic applications is controlling not only the electrical events in a neuron but also certain biochemical events. A lot of medicinal drugs are known to function through the interaction with the family of membrane receptors (GPCR). These receptors transduce external signals from some compound (medication) into cells, thus changing the intracellular signaling, e.g., the calcium ion levels. If a photosensitive rhodopsin domain is added to a GPCR, it is possible to obtain receptors that are sensitive to green light. These receptors have been termed optoXRs [11]. When a single-component optoXR gene was delivered via a virus into the brains of laboratory animals, cell-specific control by light over certain biochemical signal transmission pathways was successfully exercised [11]. The first optogenetic studies on freely moving animals were aimed at examining the neurons synthesizing the hypocretin neurotransmitter [12]. These cells are considered to be responsible for the narcolepsy sleep disorder. It was these cells that were discovered to exhibit specific types of electrical activity leading to awakening. Optogenetics also helped prove that dopaminergic neurons are responsible for the sense of joy [13]. Research into the newest methods for treating Parkinson\'s disease [14,15] has produced some of the best-known optogenetics experiments. This condition is characterized by impaired transmission of information in the Substantia nigra pars compacta neurons responsible for the motor function. Deep brain stimulation has been used to treat Parkinson\'s disease since the 1990s. This procedure involves alternating electrical pulses being sent to specific brain areas using implanted tools. The potential efficiency of the treatment strategy is, however, severely limited, since the electrodes non-selectively stimulate individual brain cells. A fundamental insight into this treatment method has been gained through optogenetics. When different types of neurons were activated, unexpected results were uncovered in parkinsonian mice. Apparently, the greatest therapeutic effort was achieved not by stimulating a certain type of cells but by controlling the activity of connecting axons.