Modern life is stressful, and as Svante Winberg explains, living under stress for a long time affects more than just your mood. For fish living in the cramped conditions of an aquaculture unit there is no escape from the stress. Winberg knew that if he could begin to untangle the stressed-out fishes' physiology, he might be able to help them overcome some of the nastier side effects of stress. Taking a dietary approach to stress relief, Winberg and his team began testing how fish responded to stress after they had been fed a key neurological precursor, tryptophan. To their surprise, they found that a tryptophan supplement changes they way that fish control their hormonal stress response, possibly protecting them from the negative effects of stress(p. 3679).
One of the earliest physiological responses to physical stress is when the neurotransmitter, serotonin, is released deep in the brain, triggering a neurological cascade that culminates several minutes later in the release of the stress hormone, cortisol, into the blood. Animals that suffer continually high levels of stress, live with high levels of cortisol all the time, but the beneficial effects of the hormone begin to fail after several days, and more sinister side effects of the hormone begin to appear. Winberg wondered if he could somehow reduce the cortisol response to stress, and consequently the damaging effects of long-term cortisol exposure.
Winberg already had a clue that tryptophan might help. He knew that fish fed on a tryptophan-supplemented diet for a week became less aggressive, and that tryptophan is the key building block of serotonin. Could tryptophan in the diet affect the fishes' cortisol response too?
Together with Oliver Lepage and Olof Tottmar, Winberg transferred individual trout to the least stressful environment possible for a fish; their own private tanks. Once the fish had adjusted to their solitude, the team began feeding the fish different doses of tryptophan, long enough for the fish to become completely de-stressed. Only then did they raise the stress, by dropping the water level.
Winberg explains that he had expected to see the stressed fishes' cortisol levels rise as the water level fell. To their amazement, these fish hardly changed their cortisol levels at all, even though they were producing and using more serotonin!
But how had the tryptophan diet affected fish that hadn't been stressed?Had the excess amino acid in their diet affected their cortisol levels too? It had. Winberg explains that even though the effect was subtle, the unstressed fish on the tryptophan diet had slightly higher cortisol levels.
At first sight this didn't make sense, why didn't the extra serotonin in the stressed fishes' brains produce a huge cortisol response when the fish needed it? Winberg thinks the answer could be in the way that the hormone levels are controlled.
As well as initiating the release of cortisol, serotonin is thought to switch the response off too. Winberg explains that it is possible that that higher levels of serotonin in the brains of tryptophan-dosed fish might reduce the cortisol response to stress, rather than increasing it. So tryptophan could be an ecological alternative to Prozac for keeping farmed fish calm.