The Stanford neuroscientist Karl Deisseroth and his graduate student, Ed Boyden, were intrigued by the observation that certain algae could convert light into electrical energy, as photoreceptors in your eyes do. If these photoreceptive properties could be transferred to specific neurons in the brain, then those neurons could be controlled by light. By isolating the DNA coding the photoreceptive properties in the algae, these scientists were able to transfer the genes into specific rat neurons. The genes produced opsins (light-sensitive proteins) in the neurons of specific targeted brain areas, enabling researchers to control the firing of specific populations of neurons by shining light on them, which alters the transport of ions (e.g., sodium, potassium) across the cell membrane (Mikulak, 2012). Consequently, researchers can use this technique to stimulate or inhibit activity in light-sensitized brain areas, including those thought to be involved in sleep and waking.
This technique, optogenetics, is similar to electrical brain stimulation, but more specific in the cells it targets. Optogenetics can be viewed as the perfect blend of an electrode with very specific timing and a genetic probe that can target specific types of neurons (e.g., dopaminergic, serotonergic). Further, different opsins respond to different colors of light. Certain opsins such as chanelrhodopsin- 2 can be used to activate neurons, whereas others such as halorhodopsin can be used to inhibit neural activity (Adamantidis, Carter, & Lecea, 2010). If both of these opsins are infused into a specific area of the brain, the neurons can be turned either on or off depending on the color of light.
This discovery has been applied to the sleep–wake system. For example, to evaluate the role of hypocretin-producing neurons in the transition from sleep to wakefulness, researchers used a virus to deliver channelrhodopsin-2 to hypocretin-producing neurons in mice. Once the DNA for the opsin was delivered to the targeted cells and incorporated into their genetic code, the neurons could be activated with light. When the hypocretin-producing neurons were activated in this manner, it increased the probability of the mice waking up from either REM or non- REM sleep (Adamantidis, Zhang, Aravanis, Deisseroth, & de Lecea, 2007).
Optogenetics is one of the most exciting techniques that has been introduced to the field in the past half-century. More and more labs are using the technique for research questions beyond those related to sleep. If a certain brain area is suspected to play a role in a certain behavior or mental condition, that area can be both activated and suppressed in animal models using optogenetic technology.
What about humans? Will we be able to use optogenetics to treat neurological and psychiatric illnesses in humans in the future? Researchers are currently exploring these questions in appropriate animal models for conditions such as depression, Parkinson’s disease, and blindness caused by retinitis pigmentosa (a hereditary condition that affects photoreceptors; Albert, 2014; Busskamp & Roska, 2011; Kravitz et al., 2010; Manfredsson, Bloom, & Mandel, 2012). Further, this technique has also been used to examine the role of brainstem stimuli in anxiety, a pervasive response that affects many diverse behaviors (Masseck et al., 2014). Thus, although researchers should proceed with caution since this is an invasive procedure, there may be approved clinical applications for this technique in the future.