KillerRed protein (red) expressed in the zebrafish habenula
Animals quickly learn to avoid danger, but if they are first exposed to a highly stressful stimulus, they become ‘helpless’ and do not avoid danger even if they experience continued pain. A brain structure called the habenula has been implicated in this avoidance response, but its exact role is controversial. Suresh Jesuthasan from the A*Star Duke–NUS Neuroscience Research Partnership and co-workers have conducted a study on zebrafish that clarifies the role of the habenula in avoidance response. The findings could provide a better understanding of human anxiety disorders.
The researchers placed zebrafish larvae in a rectangular box equipped with electrodes and red lights at each end. When the fish swam to one end of the box, they were shown the light and given a mild shock five seconds later. With repeated trials, the fish learned to associate the two stimuli, and to avoid the shock by moving away from that end of the box when presented with the light alone. But they failed to do so if first exposed to an electric shock from which they cannot escape.
The fish was then genetically modified to express a photoreactive protein called KillerRed in neurons projecting from the ventrolateral forebrain to the habenula (see image). The technique, known as photobleaching, enabled the researchers to selectively destroy the neurons by illuminating them with green light, which causes KillerRed to release reactive oxygen atoms that damage the cell membrane.
The researchers found that fish with damaged habenula input neurons did not display the avoidance response. They also appeared more anxious. To confirm the role of the habenula in avoidance, they then generated a new line of zebrafish expressing tetanus toxin in dorsal habenula cells. The toxin silences cells by blocking their transmission of nervous impulses. These larvae also failed to avoid the red light when it was shown to them, confirming that the learning of the avoidance response is dependent on the neural circuitry in the dorsal habenula.
The researchers suggest that the habenula could be involved in signaling stressful stimuli, with the dorsal habenula transmitting information about whether a stressful stimulus can be controlled by a specific action. And because the habenula has a similar pattern of neural connections in mammals, they further suggest that damage to the habenula could play a role in mental disorders characterized by anxiety and helplessness.
“One implication is that stimulating the habenula may reduce anxiety,” says Jesuthasan. “If we know how the brain naturally switches off anxiety, we will have a different way of helping people with anxiety disorders.”
The A*STAR-affiliated researchers contributing to this research are from the A*Star Duke–NUS Neuroscience Research Partnership and the Institute of Molecular and Cell Biology
