Researchers use optogenetics to produce pain relief by shutting off neurons with light.
Scientists at the Montreal Neurological Institute and Hospital of McGill University and the McGill University Health Centre have demonstrated a precise, non‑pharmacological approach to pain relief using light. Their work highlights optogenetics as a promising, highly focused alternative to conventional pain medications, with the potential to provide targeted, on‑demand analgesia without systemic side effects.
In this study, researchers created a transgenic mouse model in which peripheral sensory neurons that transmit pain signals were made light‑sensitive. Specifically, Nav1.8+ nociceptors—peripheral neurons known to carry pain information—were engineered to express light‑activated proteins called opsins. When these opsins are illuminated with yellow light, they transport ions across the neuronal membrane and hyperpolarize the cell, reducing its bioelectric activity and effectively silencing the pain signal.
“The opsins we added to these neurons were initially isolated from archaebacteria and sense yellow light,” explains Professor Philippe Séguéla of the Montreal Neurological Institute and Hospital, the study’s senior author. “When we transfer these to neurons, we can control their responses simply by illuminating the skin with innocuous yellow light.”
The researchers applied yellow light transdermally to the hind paw of the modified mice. Light exposure rapidly reduced sensitivity to mechanical and thermal stimuli in that localized area. The intensity and duration of analgesia were directly controllable by the timing and length of illumination, illustrating the high spatial and temporal precision of the method.
This optogenetic approach produced both immediate and lasting reductions in pain sensitivity. Acute illumination reduced mechanical allodynia under inflammatory conditions without changing baseline mechanical sensitivity, while longer periods of optical silencing produced post‑stimulation analgesia that outlasted the light application. The findings indicate that peripheral neuronal input, especially from Nav1.8+ fibers, contributes not only to the onset of pain hypersensitivity but also to its maintenance, and that peripheral circuits retain plasticity amenable to intervention after sensitization has occurred.
Compared with current treatments, such as opiates that act systemically and often require escalating doses due to tolerance, targeted optogenetic therapy could deliver localized pain relief without the widespread side effects associated with systemic medications. Patients could potentially control pain at the affected site by applying light as needed, offering a precise, reversible alternative to long‑term drug use.
Translating this approach to humans will require further advances. One proposed route is the transient delivery of inhibitory opsins to selected human neurons using a benign viral vector that does not produce harmful effects. Such a delivery system would need to be safe, reversible, and specific to avoid off‑target effects. The authors emphasize that additional neuroscience research and careful safety testing are essential steps before clinical application is possible.
Chronic pain is a widespread problem. According to population surveys, roughly one in ten Canadians between ages 12 and 44—about 1.5 million people—experience long‑lasting pain that can persist for months or years. Chronic pain accompanies many medical conditions, including diabetes, arthritis, cancer, shingles, and sciatica, and often reduces daily functioning while contributing to sleep disruption, mood disorders, and diminished quality of life.
“Chronic pain is an increasingly big problem clinically and for many years we’ve relied only on opiates,” Séguéla notes. “It’s hard to treat because of tolerance, making it necessary to increase dosages, which leads to serious side effects. Optogenetic therapy could be a highly effective way to relieve chronic pain while avoiding the side effects of traditional pain medication.”
Funding: The study, published in the journal eNeuro, received support from the Canadian Institutes of Health Research, the Natural Sciences and Engineering Research Council, the Quebec Pain Research Network, and the Louise and Alan Edwards Foundation.
Source: Shawn Hayward, McGill University. Image credit: Image adapted from the McGill press release. Original research: Abstract for “Optogenetic silencing of Nav1.8‑positive afferents alleviates inflammatory and neuropathic pain” by Ihab Daou, Hélène Beaudry, Ariel R. Ase, Jeffrey S. Wieskopf, Alfredo Ribeiro‑da‑Silva, Jeffrey S. Mogil, and Philippe Séguéla, published in eNeuro (2016).
Abstract
Optogenetic silencing of Nav1.8‑positive afferents alleviates inflammatory and neuropathic pain
The authors report a novel transgenic mouse model in which peripheral nociceptor terminals can be optogenetically silenced with high spatio‑temporal precision, producing alleviation of inflammatory and neuropathic pain. Inhibitory archaerhodopsin‑3 (Arch) proton pumps were targeted to Nav1.8+ primary afferents. Arch expression encompassed both peptidergic and non‑peptidergic nociceptors, and yellow light reliably blocked electrically induced action potentials in dorsal root ganglion neurons. Acute transdermal illumination of the hind paw in Nav1.8‑Arch+ mice significantly reduced mechanical allodynia under inflammatory conditions while leaving basal mechanical sensitivity intact. Arch‑driven hyperpolarization of nociceptive terminals prevented channelrhodopsin‑2 (ChR2)‑mediated mechanical and thermal hypersensitivity in double transgenic Nav1.8‑ChR2+‑Arch+ mice. Prolonged optical silencing in anesthetized Nav1.8‑Arch+ mice produced post‑stimulation analgesia with significant decreases in mechanical and thermal hypersensitivity under both inflammatory and neuropathic conditions. These results underscore the role of peripheral neuronal input, particularly Nav1.8+ afferents, in the initiation and maintenance of pain sensitization and demonstrate that selective optical inhibition of genetically identified sensory fibers can serve as a precise analgesic strategy that overcomes limitations of genetic ablation approaches.
Significance statement: Selective optical activation or inhibition of peripheral nociceptors enables precise control of pain transmission and perception. The Nav1.8‑Arch+ transgenic line shows that inhibitory opsins can efficiently silence Nav1.8+ nociceptive afferents and reduce mechanical and thermal hypersensitivity in inflammatory and neuropathic models. This optogenetic method can be applied to investigate other sensory neuron subsets with high temporal resolution, and safe genetic delivery of inhibitory opsins may have future clinical relevance.