Vision can be rebooted in adults with amblyopia, study suggests

Temporarily anesthetizing the retina briefly reverts the activity of the visual system to that observed in early development and enables growth of responses to the amblyopic eye, new research shows.

In the common vision disorder amblyopia, impaired vision in one eye during development causes neural connections in the brain’s visual system to shift toward supporting the other eye, leaving the amblyopic eye less capable even after the original impairment is corrected. Current interventions are only effective during infancy and early childhood while the neural connections are still being formed.

But a new study in mice by neuroscientists in The Picower Institute for Learning and Memory at MIT shows that if the retina of the amblyopic eye is temporarily and reversibly anesthetized just for a couple of days, the brain’s visual response to the eye can be restored even in adulthood.

The findings, published in Cell Reports, may improve the clinical potential of the idea of temporarily anesthetizing a retina to restore the strength of the amblyopic eye’s neural connections.

In 2021, the lab of Picower Professor Mark Bear and collaborators showed that anesthetizing the non-amblyopic eye could improve vision in the amblyopic one—an approach analogous in that way to the treatment used in childhood for patching the unimpaired eye. Those 2021 findings have now been replicated in adults of multiple species.

But the new evidence on how inactivation works suggests that the proposed treatment also could be effective when applied directly to the amblyopic eye, Bear said, though a key next step will be to again show that it works in additional species and, ultimately, people.

“If it does, it’s a pretty substantial step forward because it would be reassuring to know that vision in the good eye would not have to be interrupted by treatment,” said Bear, a faculty member in MIT’s Department of Brain and Cognitive Sciences. “The amblyopic eye, which is not doing much, could be inactivated and ‘brought back to life’ instead. Still, I think that especially with any invasive treatment, it’s extremely important to confirm the results in higher species with visual systems closer to our own.”

Madison Echavarri-Leet, a former graduate student whose doctoral thesis included this research, is the lead author of the study, which also demonstrates the underlying process in the brain that makes the potential treatment work.

A beneficial burst

Bear’s lab has been studying the science underlying amblyopia for decades, for instance, by working to understand the molecular mechanisms that enable neural circuits to change their connections in response to visual experience or deprivation. The research has produced ideas about how to address amblyopia in adulthood.

In a 2016 study with collaborators at Dalhousie University, they showed that temporarily anesthetizing both retinas could restore vision loss in amblyopia. Then five years later, they published a study showing that anesthetizing just the non-amblyopic eye produced visual recovery for the amblyopic eye.

Throughout that time, the lab weighed multiple hypotheses to explain how retinal inactivation works its magic. Lingering in the lab’s archive of results, Bear said, was an unexplored finding in the lateral geniculate nucleus (LGN) that relays information from the eyes to the visual cortex, where vision is processed: Back in 2008, they had found that blocking inputs from a retina to neurons in the LGN caused those neurons to fire synchronous “bursts” of electrical signals to downstream neurons in the visual cortex. Similar patterns of activity occur in the visual system before birth and guide early synaptic development.

The new study tested whether those bursts might have a role in the potential amblyopia treatments the lab was reporting. To get started, Leet and Bear’s team used a single injection of tetrodotoxin (TTX) to anesthetize retinas in the lab animals. They found that the bursting occurred not only in LGN neurons that received input from the anesthetized eye, but also in LGN neurons that received input from the unaffected eye.

From there they showed that the bursting response depended on a particular “T-type” channel for calcium in the LGN neurons. This was important because knowing this gave the scientists a way to turn it off. Once they gained that ability, then they could test whether doing so prevented TTX from having a therapeutic effect in mice with amblyopia.

Sure enough, when the researchers genetically knocked out the channels and disrupted the bursting, they found that anesthetizing the non-amblyopic eye could no longer help amblyopic mice. That showed the bursting is necessary for the treatment to work.

Aiding amblyopia

Given their finding that bursting occurs when either retina is anesthetized, the scientists hypothesized it might be enough to just do it in the amblyopic eye. To test this, they ran an experiment in which some mice modeling amblyopia received TTX in their amblyopic eye and some did not. The injection took the retina offline for two days.

After a week, the scientists then measured activity in neurons in the visual cortex to calculate a ratio of input from each eye. They found that the ratio was much higher even in mice that received the treatment vs. those left untreated, indicating that after the amblyopic eye was anesthetized, its input in the brain rose to be at parity with input from the non-amblyopic one.

Further testing is needed, Bear notes, but the team wrote in the study that the results were encouraging.

“We are cautiously optimistic that these findings may lead to a new treatment approach for human amblyopia, particularly given the discovery that silencing the amblyopic eye is effective,” the scientists wrote.

In addition to Leet and Bear, the paper’s authors are Tushar Chauhan, Teresa Cramer and Ming-fai Fong.