Should I stay or should I go? In the zebrafish (Danio rerio), two competing neural circuits determine whether an animal swims or turns tail and escapes. Social status can tip the balance between these circuits, leading dominant and subordinate animals to behave differently, according to a recent study in the Journal of Neuroscience.
Social status and its effects on the nervous system have been well-studied in other fish such as the African cichlid (Astatotilapia burtoni), which forms a complex and hierarchical society. But in recent years, researchers have noticed that zebrafish, a popular animal model, also establish social ranks among themselves. “If you put two males together, like two boxers in a ring, they’ll duke it out, and within a few hours, you’ll have one animal emerging as dominant and another as subordinate,” says senior author Fadi Issa, a neuroscientist at East Carolina University in Greenville, North Carolina. By studying this phenomenon in zebrafish, Issa and others see the potential for leveraging genetic tools that exist to manipulate the animal’s nervous system.
In their latest study, Issa and his colleagues placed male zebrafish together in a tank, two at a time, for two weeks. Through bouts of biting and chasing, a dominant fish soon emerged from each pair. Dominant fish took over much of the tank, swimming more frequently and over a wider range. Subordinate fish, meanwhile, reduced their swimming and retreated to a bottom corner of the tank. When the researchers later separated the fish, the animals continued to behave according to their respective ranks.
To study the neurons behind these lasting changes, the researchers stuck electrodes in the water and picked up distinct electrical signatures that the “swim” and the “escape” circuits have previously been shown to emit. Compared to control animals (housed in large groups), the dominant fish of the pair showed greater activity of the swim circuit, while the subordinate fish showed lower-than-normal activity in those neurons. To look for changes in the escape circuit, Issa’s team played popping sounds at different volumes through speakers placed near the tank. In subordinate fish, they found that these neurons had become more sensitive, activating more quickly and at lower volumes than in control or dominant animals.
“It’s amazing that they could find enough of a signal from external recordings to make the case that this was from two different classes of neurons,” says Russell Fernald, a neuroscientist at Stanford University in California, who was not involved in the study. “I think it’s a good start, but it’s going to be more complicated than this. Now they have to go in and record and look at individual cells—how do they recognize social dominance and go on to change?”
To begin to investigate how this process occurs, Issa’s team simulated more than 2,000 different scenarios on a simplified computer model of the two circuits and their connections. They were able to mimic their experimental observations by changing the excitability of two types of cells: command neurons of the escape circuit (called Mauthner cells) and inhibitory neurons connecting the swim and escape circuits. The model also suggested that a neurochemical called 2-AG might be mediating this change. This doesn’t prove that these mechanism are at work in the real animal, Issa emphasizes, but does point to a plausible pathway that his group is now investigating experimentally.
“People have made models of this circuitry before, but very few studies have done this in the context of social status,” says Hans Hofmann, a systems neuroscientist at the University of Texas at Austin, who was not involved in the study. “It opens up a door, an avenue of research that was not really available before.”