Stress, like many other things, is good in moderation. Acute stressors such as a deadline or a race motivate us to work hard. However, severe stress not only impairs our physical and mental health, but can also lead to depression – a condition characterized by a lack of motivation.
Why is it that stress can sometimes motivate us, but other times disable us? Lemos et al at University of Washington conducted an interesting experiment to try to answer this question.
When response to stress, our hypothalamus releases something called corticotropin releasing factor (CRF), which stimulates secretion of adrenocorticotropic releasing hormone (ACTH) in our anterior pituitary, which in turn triggers adrenal cortex to release cortisol. To make this less complicated, cortisol is the stress hormone that’s responsible for most of the bad stuff that happen when we are stressed out, such as suppressing the immune system and causing us to gain weight.
However, CRF is not only responsible for releasing the stress hormone, but also the motivating chemical. In fact, in response to acute stressors, CRF is also released into a brain region (nucleus accumbens) that’s associated with motivational things such as drugs and sex, causing the brain to release dopamine, a type of neurotransmitter that’s associated with reward and motivation.
It now seems that, during acute stress, the hypothalamus releases CRF, which in turn stimulates the secretion of both the stress hormone and the motivation chemical, making the person both stressed and motivated. To see exactly how it works, Lemos et al put a mouse in a box with two compartments. They consistently give the mouse saline while it’s on one side of the box, and CRF while it’s on the other side, so the mouse makes associations between the two compartments with the two substances. Then, the researchers put the mouse in between, and see where it chooses to go. As predicted, the mouse prefers the CRF compartment, presumably because of the CRF’s motivating effect.
Next, the researchers expose the poor mouse to a severe stress situation by letting it swim for an hour before putting it back to the box. This time, the mouse prefers the saline compartment over CRF. Using a technique called voltammetry, the researchers also find that the dopamine that previously existed in response to CRF has now disappeared. It seems that prolonged stress not only changes the mouse’s preference between saline and CRF, but also something substantial in the brain.
Why is it? Well, we are not sure yet. But at least now we know that acute and sever stress don’t just feel different – they are different processes within the brain.
