RIKEN BRAIN SCIENCE INSTITUTE (RIKEN BSI)

A fish fight club reveals how the brains of winners and losers differ

Competing neural circuits determine outcomes of conflict

April 1, 2016

Amanda Alvarez
This post originally appeared on Neurographic

 

The two opponents circle each other, showing off their scaly might and impressive stripes. Suddenly one darts forward, biting the other near the dorsal fin — and the fish fight is in full swing. The biting and dodging can go on for up to 20 minutes, but this isn’t a fight to the death. Zebrafish fight to maintain a social hierarchy, and eventually one will concede and withdraw to the bottom of the tank while the victorious opponent circles above.

These winner and loser behaviors, it turns out, are all in the mind, and zebrafish can be turned into winners or losers by tweaking certain brain circuits. According to a new study published in Science, a part of the zebrafish brain called the habenula has two competing neural pathways whose signals are tied to either a winning or losing outcome. Researchers turned off either pathway by genetically engineering fish. Zebrafish with silenced “winning” circuits tended to lose their fights, while fish with impaired “losing” circuits were more likely to win.

The researchers behind this study were actually interested in whether the habenula and nearby brain areas are involved in fight-or-flight behaviors. They had previously found that the ventral or lower part of the habenula helps zebrafish avoid danger by storing information about negative outcomes, like getting a shock whenever a red light turns on. The ventral habenula, it turns out, is also important for making sure losers behave like losers; more on that below.

The researchers behind this study were actually interested in whether the habenula and nearby brain areas are involved in fight-or-flight behaviors. They had previously found that the ventral or lower part of the habenula helps zebrafish avoid danger by storing information about negative outcomes, like getting a shock whenever a red light turns on. The ventral habenula, it turns out, is also important for making sure losers behave like losers; more on that below.


A schematic of the zebrafish brain shows the habenula circuits associated with winning and losing in red and green, respectively. The ventral habenula circuit is shown in blue.

A schematic of the zebrafish brain shows the habenula circuits associated with winning and losing in red and green, respectively. The ventral habenula circuit is shown in blue. Image: Hitoshi Okamoto/RIKEN Brain Science Institute

Taking the experiment a step further, they looked at fighting behaviors in genetically engineered fish where this sub-region had been knocked out. These fish were very likely to lose fights. On the other hand, turning off the opposite sub-region, the medial part of the dorsal habenula, predisposed fish to win their fights. The neural activity in these two opposing sub-regions thus seem decisive to fight outcomes.

And these signals don’t stop when the fight is over. Without the usual neural signals from the ventral habenula to a nearby area called the raphe nucleus, loser fish don’t withdraw, prompting further attacks from opponents. This first video shows how a losing fish normally behaves, skulking at the bottom of the tank. In the second video, a fish with reduced ventral habenular signaling just can’t stay away from the center of the tank and keeps getting attacked. Acting like a loser is an important component of repelling continued aggression, and for this the ventral habenula is essential.

 

On the face of it, losing may seem hard-wired into the habenula, at least for the genetically altered fish. Actually, according to research group leader Hitoshi Okamoto of the RIKEN Brain Science Institute, what’s at play is regulation of a continuum of fleeing, freezing, and fighting behaviors. By producing transgenic fish with different deactivated brain circuits, Okamoto and colleagues observed fish that behaved more aggressively and tenaciously like winners or were quicker to give up like losers. It is the interplay between these circuits — the strength of the winning or losing signals and the associated rewards and harms for continued aggression or surrender — that eventually steers the fish’s behavior. And much like in Fight Club, “Where is my mind?” is a fitting theme song for zebrafish: after 24 hours they can’t remember if they were winners or losers.

Citation:
Chou MY, Amo R, Kinoshita M, Cherng BW, Shimazaki H, Agetsuma M, Shiraki T, Aoki T, Takahoko M, Yamazaki M, Higashijima S, Okamoto H. (2016). Social conflict resolution regulated by two dorsal habenular subregions in zebrafish. Science, 352(6281), 87–90. DOI: 10.1126/science.aac9508