Zebrafish May Provide Clues to Visual Thinking

The zebrafish looks like any other run of the mill aquarium-inhabitant—a small, glittery swimmer suited to the fishbowl world of a childhood pet. But to Gund Professor of Neuroscience John E. Dowling, this fish represents a living window into the complex interactions between vision and the brain.

As Dowling and his laboratory researchers have discovered, study of retinal activity and development in zebrafish provides clues to the connection between what we see and how we act.

“I’ve long been interested in the visual system,” Dowling says. He has always used the retina as a model for research in the brain, in part due to the variety of cells involved in retinal actions. Aside from photoreceptors, five other types of neurons in the retina connect with the brain to help start the process of visual imaging.

By the time we become aware of what we’re looking at, two main stages of processing have occurred. The first, in which spatial analysis and form recognition occur, takes place in the outer realm of the retina, while the second, in the retina’s inner region, involves temporal analysis and movement detection.

“What we’re trying to ask is, ‘How does the retina achieve this?’” Dowling explains, and his research thus far has attempted to focus on the circuitry of the retina. He investigates how the individual neurons connect and then respond to light stimuli, recording these as electrical responses.

In simpler terms, Dowling says he is ultimately seeking to find out how cells “talk to each other.”

In one step towards this goal, Dowling says he has recently started researching the genetics and development of the retina, using zebrafish as subjects.

He says zebrafish are particularly ideal subjects because a biologist can use them for “forward genetics”—identifying genes by inducing mutations and then finding the specific gene related to the produced phenotype, or appearance.

Dowling describes his subjects with excitement, noting, “We haven’t yet had a practical vertebrate to do forward genetics with.”

“Our objective is really to find new genes. The zebrafish has a particularly beautiful retina—it has four types of cones, and then the rods—which is more than even in humans,” he adds.

Dowling says he has a particular interest in working with behaviorally mutant zebrafish.

In a vivid demonstration, Dowling attaches GFP protein to the promoter regions of photoreceptor genes, so that a highly fluorescent protein is produced. This serves as a marker for how the rod cells develop and, as Dowling notes, “it enables us to find mutants who have specific defects in the development of photoreceptor cells.” This marker helps to mutant behavior to specific structural defects.

The result of this treatment, he says, is zebrafish with eyes that look akin to fluorescent green nightvision goggles.

“It’s very dramatic to watch,” Dowling chuckles, “sort of like fish swimming around with their headlights on.”

Recently, Dowling obtained a National Institute on Drug Abuse (NIDA) grant to study place-conditioning in zebrafish with relation to addiction to cocaine. The experiment starts with placing the zebrafish for a day in a normal tank, followed by a day in a tank with a cocaine-soaked piece of filter paper dangling at one end—a “zebrafish crackhouse,” the scientist quips. The third day, Dowling found that most fish would nearly immediately return to the end of the tank which previously housed the cocaine-soaked paper.

But some of the fish seemed to not respond appropriately and didn’t return to this end of the tank. Dowling says he wondered whether the fish just didn’t remember the cocaine correctly. He theorized that the defect might be a result of mutations in genes affecting signaling between the retina and the brain.

For many reasons, the match between Dowling’s research and the zebrafish has proved one made in biological heaven. Dowling is quick to note the varied attributes which make the organism ideal for such experiments. The fish are very small and can thus be raised in large numbers; in Dowling’s small facility, he can keep 30,000 fish at a time. The fish’s fast generation time also proves helpful in developmental studies; by the time a fish is just five days old, one can measure its visual function very effectively by using the optokinetic reflex—the same test used to test vision of human babies. Finally, Dowling notes, “They’re highly responsive visually—for my purposes, perfect.”

Though the zebrafish lacks some more complex features present in mammals, Downling says the fish will serve his research purposes well for the future.

As Dowling concludes, “For studying basic developmental processes, the zebrafish is ideal.”

—Staff writer Rebecca M. Milzoff can be reached at milzoff@fas.harvard.edu.