For both projects, which were conducted while Schaaf and Faught were employed at Leiden University, the biologists examined the effects of stress on zebrafish. Schaaf explains: ‘The hormonal systems of zebrafish and humans are very similar. When under stress, they produce the same hormone – cortisol – and have the same receptors that cause the effects of the hormone.’ In their first study, the zebrafish were placed in a tube that was swirled around, disrupting their sense of balance. Faught notes: ‘It didn’t hurt them, but it did cause stress. We then examined the effects of that stress on their immune system.’
Transparent
One of the advantages of using zebrafish is that they are transparent. This allowed the researchers to literally see what was happening to the immune cells inside the fish. Faught explains: ‘We saw that after the stress stimulus, some of the immune cells moved outward, toward the skin, as if preparing to protect the fish. In addition, these cells also took samples from the surrounding water, as though they were monitoring the environment.’ Schaaf continues: ‘This means that short-term stress actually activates the immune system, whereas we know that long-term stress weakens it. We also saw a tipping point: if the period of stress lasted longer than two hours, the stress didn’t stimulate the immune system anymore, but suppressed it.’
Receptors at Work
The second study showed that zebrafish behavior under stress depends on two different receptors (glucocorticoid (GR) and mineralocorticoid (MR)) that must work closely together. Schaaf explains: ‘We already knew that two receptors have to bind to each other to be active, but we always assumed that two identical receptors have to work together – what we’ve now demonstrated is that two different receptors must connect and cooperate for cortisol to affect certain behaviors.’
To test this, the zebrafish were exposed to alternating periods of five minutes of darkness and five minutes of light. Faught says: ‘Normally, without stress, zebrafish are inactive in the light and move around a lot in the dark. But when stressed, they were also very active in the light.’ By having the receptors selectively form couples of identical or different receptors, the researchers discovered that the fish only showed stressed behavior when the two different receptors were able to work together.
Schaaf sees promising applications for the future: ‘This could have major implications for drugs such as prednisone, which mimics cortisol. That drug suppresses inflammatory responses, and is therefore successfully used to treat disease such as asthma and rheumatoid arthritis, but it also causes side effects such as sleep disruption and behavioral changes. Now that we understand how these receptors influence cortisol’s effects, it may be possible to develop a drug that avoids those side effects.’