You’ve likely never considered how to give an eye exam to a zebra fish. Here’s a hint, a chart with a big letter E on the top won’t get you very far.
For Vikki Connaughton, visual neuroscientist, fish eye exams are part of everyday research. Since the zebra fish has a retinal structure very much like that found in humans — if a bit better organized — her hope is to use these fish as a model for how various factors can impact our retinal health.
Testing Hyperglycemic Fish
In humans, uncontrolled blood sugar from diabetes can cause myriad reactions in cells located in the kidneys, feet, and eyes. One common complication is diabetic retinopathy — the leading cause of blindness among working age Americans. Studies on diabetic human eyes have identified changes in retinal anatomy, physiology, and vasculature. If zebra fish can act as a stand-in, there’s hope that disorders like diabetic retinopathy can be better, and more rapidly, understood.
To prove that the minnow-sized fish with their silvery eyes can be a proxy for human retinal damage, Connaughton needs to mimic spikes in blood sugar.
“We can’t make the fish diabetic. But, we can make them hyperglycemic,” she says.
Her first goal was testing whether the zebra fish would exhibit retinal changes comparable to people with hyperglycemia. To do this, Connaughton and her team of student researchers spend a month moving fish daily between plain water and 2 percent sugar solution.
Prior to the study, the fish go through optomotor testing (fish eye exams) to verify that vision is normal. To do this, each fish is placed in a dish, around which rotates a white cylinder with a black bar on its surface. Normal fish will swim to follow the cylinder as though tracking a school of other fish. After spiking and dropping sugar levels for a month, the fish are tested again. Connaughton has found their response rate — how they do on the eye exam — changes after just that month. Later, physiological tests show an anatomical change that parallels human retinal thinning.
Having established the possibility of modeling human retinal change using zebra fish, the lab plans further study, including extending the experiment. If one month in sugar solution shows these changes in the zebra fish, the question is, what are the long-term consequences of uncontrolled blood sugar — here, in the fish, over two or four months.
The Tricky Case of Methylmercury Poisoning
It becomes clear why it might be preferable to test out the consequences of certain health factors using zebra fish, when one considers the case of methylmercury poisoning.
A substance that gets into our water supply through acid rain, methylmercury has the ability to bioaccumulate. That means, if a small fish contains methylmercury, and a larger fish eats it, that bigger fish takes on the amount from the smaller fish in addition to its own exposure. This continues up the food chain, compounding as each animal, and eventually humans, eat.
Connaughton is working on a joint study with University of Wisconsin — Milwaukee scientists Michael Carvan III (School of Freshwater Sciences) and Daniel Weber (Children’s Environmental Health Sciences Center), focusing upon methylmercury bioaccumulation in fish. Native American Ojibwa populations eat fish from northern Wisconsin lakes. Of particular concern is the fact that if a mother eats a diet rich in methylmercury, the substance can cross the placental barrier, leading to possible neurological and sensory deficits — specifically related to vision.
The Wisconsin lab ran experiments exposing zebra fish embryos to different concentrations of methylmercury for the first 24 hours postfertilization, then allowed them to develop to adulthood in normal water. The animals showed decreased visual response compared to unexposed fish when given optomotor exams.
This is where Connaughton comes in. Her expertise in zebra fish retinology allows her to get to the physiologic changes in the ocular system — caused by methylmercury exposure — that result in those poor eye exams.
In both the human and zebra fish retina, there is a simple three-cell nerve pathway leading from rods and cones (which detect light and color), these relay signals to bipolar cells, which relay signals to ganglion cells. This last step forms axons with the optic nerve which takes signals directly to the brain.
Connaughton calls the bipolar cells “functionally quite important.” They are the middlemen in the visual relay system. If the bipolar cells go off track, sight can deteriorate.
Evaluating nerve cell activity in the bipolar cells of zebra fish exposed to methylmercury, Connaughton has found changes that correlate with the poor vision tests found in the University of Wisconsin lab. She’s confirming the anatomic impact of the exposure and helping explain a cause for depleted vision.
Again, it comes back to what humans have in common with zebra fish. This study in particular is important because the zebra fish are only exposed during embryotic development — and that exposure was enough to induce an adult deficit.
Collaborating scientist Weber explains the significance of modeling zebra fish in this experiment: “By demonstrating alterations in at least one physiological mechanism important to this behavior, we have prepared a foundation for future work to identify fundamental biological process that are sensitive to methylmercury exposures that will have relevance for pregnant women.”
Those are some pretty hefty flags to be raised by such tiny fish.