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Doctors have long thought that vision impairment from diabetic retinopathy was primarily owed to blood vessel damage in the eyes.
New research does not challenge this but does point to a significant role the neurotransmitter dopamine plays in the eye’s retina.
"What’s new here is we can restore dopamine levels and improve visual function in an animal model of diabetes,” said researcher Machelle Pardue, Ph.D., of Emory University. This research is excellent news for the more than 600,000 individuals in the U.S. at risk for retinopathy.
The retinopathy research was done using a group of mice made diabetic by giving them a substance (streptozocin) toxic to insulin-manufacturing pancreas cells. The onset of diabetes-related vision problems was delayed, and the severity of eventual eye damage lessened, by giving the mice injections of L-DOPA, a precursor to dopamine. L-DOPA is commonly used to treat Parkinson’s, a disease caused by the death of brain cells that produce dopamine.
You are right in assuming the rodents' eyesight was not tested by having them read tiny letters of the alphabet projected on a wall. Instead, the mice were put on a platform inside a rotating cylinder with a pattern of vertical lines projected onto it. Visual acuity was determined by whether, and how far, the mice moved their heads in response to the moving lines on the cylinder.
The scientists also discovered that the visual improvements from L-DOPA were largely, if not completely, owed to changes in the retina. Using electroretinography, it was determined that the treatment enhanced retinal responses to levels near those of the control mice.
Some of the diabetic mice were given dopamine receptor agonists instead of L-DOPA. Dopamine agonists are drugs that bind themselves to dopamine receptors in the brain and mimic dopamine effects. One agonist acted on the D1R receptor and helped mice see fine lines with greater acuity. A different agonist docked on D4R receptors, enhancing the rodent’s sensitivity to visual contrast.
"This is important because it shows that treatments targeting dopamine could be beneficial to patients with established diabetes," said Michael Iuvone, Ph.D., of Emory Eye Center. "It should be straightforward to try L-DOPA or dopamine receptor agonist treatment in adults, although L-DOPA could have complications in children."
One dopamine agonist called bromocriptine is currently FDA-approved for treatment of type 2 diabetes. Researchers are waiting to see whether the same receptors regulate visual contrast and acuity in people as in mice.
Source: Science Daily
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