Optogenetics has been applied in humans to restore vision in patients with retinitis pigmentosa, an eye disease causing severe vision impairment. A viral vector containing a channelrhodopsin protein is injected into the patient's eyeball, making their retinal ganglion cells optogenetically active. The patient wears goggles that detect changes in light intensity and send corresponding light pulses into the retina. The cited study demonstrates that the subject was able to successfully perform visual tasks with this procedure.

University of Michigan - Ann Arbor

* Epilepsy
* Vision restoration 
* Parkinson's disease 
* Chronic pain

Therapeutic applications of optogenetics

Sahel, JA., Boulanger-Scemama, E., Pagot, C. et al. Partial recovery of visual function in a blind patient after optogenetic therapy. Nat Med 27, 1223–1229 (2021). https://doi.org/10.1038/s41591-021-01351-4

Partial recovery of visual function in a blind patient after optogenetic therapy (2021)

In vitro and in vivo experiments have shown that drug-resistant seizures can be optogenetically treated by activating halorhodopsin to inhibit excitatory neurons, or by activating channelrhodopsin-2 to stimulate inhibitory neurons. This strategy has been used to create a closed-loop system in which the onset of a seizure is detected by an EEG device, which prompts an optical fiber to activate the opsin and suppress seizure activity. 

Although optogenetic therapy offers a high amount of precision, it is highly invasive. Furthermore, reliable methods to detect seizures are yet to be developed, attenuation limits how deep the light spreads, and tissue can become heated - altering behavior.

Optogenetics and epilepsy

Optogenetics has been used to study regions of the brain involved in Parkinson's disease, inspiring new methods of treatment. 
The optogenetic inhibition of DYSK-SPNs, a small subset of D1-SPNs, with halorhodopsin in mice has been shown to reduce involuntary movements - a side effect of L-DOPA medication. 
Optogenetic stimulation of cortical neurons projecting to the subthalamic nucleus reduces motor deficits, helping to explain the mechanism by which deep brain stimulation works.
Additionally, optogenetics has been used to identify a subset of external globus pallidus (GPe) neurons that result in long-lasting restoration of movement when stimulated.

Optogenetics and Parkinson's disease

Selective opsin expression is more difficult to implement in the peripheral nervous system, which is more spread out than the brain, to investigate pain using optogenetics. Although peripheral  organs are less stable to hold optical fibers than the skull, these devices have been adapted for use in the periphery. In the future, the optogenetic inhibition of nociceptive neurons, in vivo, could be a strategy to treat humans with chronic pain.

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