An injection into the eye could one day restore sight to the blind, scientists say.
The jab has already been found to repair sight in blind mice, leading to hopes for new treatments for human patients.
It allowed genetically programmed sightless animals to temporarily see. The researchers are now working on a better compound that could eventually cure people with degenerative blindness.
It could help those with the genetic disease retinitis pigmentosa – the most common inherited form of blindness – as well as AMD (age-related macular degeneration).
In both diseases the light sensitive cells in the retina – the rods and cones – die, leaving the eye without functional photoreceptors.
Professor Richard Kramer, of California University in Berkeley, said the chemical called AAQ acts by making the remaining, normally ‘blind’ cells in the retina sensitive to light.
AAQ (acrylamide-azobenzene-quaternary ammonium) is a photoswitch that binds to proteins on the surface of retinal cells. When switched on by light AAQ activates brain cells in much the same way as rods and cones are triggered.
Prof Kramer said: ‘This is similar to the way local anaesthetics work – they embed themselves in ion channels and stick around for a long time so you stay numb for a long time.
Because the chemical eventually wears off it may offer a safer alternative to other experimental approaches for restoring sight – such as gene or stem cell therapies – which permanently change the retina. It’s also less invasive than implanting light-sensitive chips in the eye.
Prof Kramer said: ‘The advantage of this approach is it is a simple chemical which means you can change the dosage, you can use it in combination with other therapies or you can discontinue the therapy if you don’t like the results.
‘As improved chemicals become available you could offer them to patients. You can’t do that when you surgically implant a chip or after you genetically modify somebody.’
Co-researcher Dr Russell Van Gelder, an ophthalmologist at Washington University in Seattle, said: ‘This is a major advance in the field of vision restoration.’
After injecting very small amounts of AAQ into their eyes, light sensitivity was restored because the mice’s pupils contracted in bright light.
The mice showed light avoidance – a typical rodent behavior, impossible without the animals being able to see some light.
Prof Kramer whose study is published in Neuron is hoping to conduct more sophisticated vision tests in rodents injected with the next generation of the compound.
Dr Van Gelder said: ‘The photoswitch approach offers real hope to patients with retinal degeneration.
‘We still need to show these compounds are safe and will work in people the way they work in mice but these results demonstrate this class of compound restores light sensitivity to retinas blind from genetic disease.’
The current technologies being evaluated for restoring sight include injection of stem cells, gene therapy to insert a photoreceptor into blind neurons to make them sensitive to light and installation of electronic prosthetic devices to stimulate blind neurons.
Eight years ago his researchers developed an optogenetic technique to chemically alter potassium ion channels in blind neurons so a photoswitch could latch on.
Potassium channels normally open to turn a cell off but with the attached photoswitch they were opened when hit by ultraviolet light and closed when hit by green light – activating and deactivating the neurons.
Prof Kramer said new versions of AAQ now being tested activate neurons for days rather than hours using blue-green light of moderate intensity.
These photoswitches naturally deactivate in darkness so a second color of light is not needed to switch them off.
He said: ‘This is what we are really excited about.’
‘However, clearly it is still at an early stage and more extensive trials are needed to confirm the safety and effectiveness of this kind of treatment.’