Lecture on the Motor Behaviors of Zebrafish During Phototaxis

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Harold Burgess, Head of the Unit on Behavioral Neurogenetics at the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), visited St. Mary’s on Monday, March 5 to share his research on the motor behaviors of both normal and genetically altered zebrafish during phototaxis.

Phototaxis is the observed behavior of animals moving from a darkened place towards a patch of light and then aggregating there. Phototaxis in zebrafish is especially interesting to study because zebrafish, during their larval stage, are transparent. “What is amazing about this animal…is you can stare down the microscope and see the brain. You can watch neurons fire in real time,” said Burgess.

Burgess was looking at two complex behaviors of zebrafish, scooting and turning, in relation to phototaxis. In order to more simply study these complex behaviors, they were broken down into their more simple, stereotyped movements. Scooting (a forward movement) and turning (a reorientation movement) are present in zebrafish after four to five days of development.

Similar behaviors of phototaxis have been heavily studied in other organisms, including fruit flies. In order to navigate toward a target light source, fruit flies use a simple strategy. If they sense light coming from the right, they move their left side more quickly, causing their bodies to turn to the right. Once the fly is directed toward the light source, both sides of their body will move at the same rate, causing them to move forward. This process is similar to the motion of rowing a boat.

Burgess and his colleagues wondered if this same process held true for phototaxis in zebrafish. So, to do this, they measured how frequently zebrafish initiated the stereotyped movements of scooting and turning while being exposed to a target light source in a Petri dish.

“They show a really high frequency of turn movements when facing away from the light source,” said Burgess. He continued, “It’s not simply a blind turn where they’re hoping they’ll end up facing the right way.”

These results would suggest zebrafish behave similarly to fruit flies, but this is not actually the case. Burgess discovered that phototaxis occurs even when the zebrafish were genetically mutated to have no eyes. “It’s not a very impressive phototaxis, but there does still seem to be aggregation on the light side,” said Burgess.

Burgess and his colleagues assumed zebrafish would be motionless when exposed to a light source without eyes. “It’s kind of shocking that the fish was still slowly moving to that side of the dish even though we removed its eyes… This was supposed to be a control,” he said.

So, Burgess began to research the question of how the zebrafish were orienting without eyes. Instead of fruit flies, Burgess tried comparing zebrafish to woodlice. This small invertebrate crustacean uses the process of kinesis to locate patches of damp ground. Woodlice need damp places to survive, or else they will desiccate.

Burgess described the process of kinesis by saying, “If [the woodlice sense the area is] dry, they move in a really fast and somewhat random fashion. But if it’s damp, they slow down and hang out where they are.” So, the rule is that speed of movement is inversely proportional to stimulus intensity (how dry the area is); that is,  as the woodlice approach dampness, their speed decreases. In woodlice, motion is never purposefully oriented towards a damp area, it is simply random.

Burgess found that when zebrafish are put in total darkness, they become hyperactive. “They are about twice as active if you turn the lights off,” he said. Even larvae without eyes showed hyperactivity in response to dark, so photokinesis was still intact.

In recent research, Burgess had read about a photoreceptor protein called melanopsin, which is expressed in the hypothalamus of the brain. Burgess found that zebrafish mutated to not produce opta, a transcription factor that leads to melanopsin production in certain brain regions, showed a strong reduction in photokinesis.

“The fish have not just one mechanism for phototaxis, they have two,” concluded Burgess. Zebrafish can use both their eyes and proteins in their brain to detect light.

“Dr. Burgess’ material connected well with the material we teach in Sensory Biology and the Neuroscience Seminar,” said Assistant Professor of Biology John Ramcharitar. “He showed how photoreceptors play a role in the eye, but also in the hypothalamus and the pineal system.”

“We had read [an article written by Burgess] before for class,” said senior William Ehrig. “I really liked the presentation, and after seeing it, it cleared up some of my questions.”

“I thought [the lecture] was pretty informative,” said sophomore Utsav Gyawali. “It would be interesting to see whether or not [melanopsin is] positive in animals as well [as zebrafish].”

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