@Contraceptionismyfriend
Researchers refer to stress that happens in unpredictable ways and at unpredictable times as “chronic unpredictable stress,” and they have been studying its effects on animal development for decades—long before Felitti and Anda’s investigation into ACEs first began. In classic studies, investigators expose animals to different types of stressors for several weeks, to see how those stressful stimuli affect their behavior. In one experiment, McCarthy and her postdocs exposed male and female rats to three weeks of chronic unpredictable mild stress. Every day, rats were exposed to a few low-grade stressors: their cage was rotated; they were given a five-minute swim, their bedding was dampened; they went for a day without food; they were physically restrained for thirty minutes; or they were exposed to thirty minutes of strobe lights. At the end of the three weeks, McCarthy’s team examined the rats to evaluate brain differences. In the group exposed to chronic unpredictable mild stress, she and her team found significant changes in the receptors in the brain’s hippocampus—an area of the brain associated with emotion, which would normally help modulate stress hormone production and put the brakes on feelings of stress and anxiety after a stressor has passed. The rats who’d been exposed to chronic unpredictable stress weren’t able to turn off the stress response, but the control group that experienced no stress showed no brain changes. However, when stress is completely predictable, even if it is more traumatic—such as giving a rat a regularly scheduled foot shock accompanied by a sharp, loud sound—the stress does not create these exact same brain changes. “Rats exposed to a much more traumatic stressor get used to it if it happens at the same time and in the same way every day,” says McCarthy. “They manage. They know it’s coming, then it’s over.” Moreover, she says, “They don’t show signs of these same brain changes, or inflammation, or illness.” On the other hand, she adds, “if you introduce more moderate but unpredictable stressful experiences at a different time each day, with different levels of intensity, adding in different noises, such as loud clapping at unpredictable intervals, those rats show significant changes to the brain. And they get physically sick; they get ulcers.” This is why researchers believe that it is the unpredictability of stress that is particularly damaging. On a walking tour of her lab, McCarthy points out the metal stand on which rodents’ cages can be gently shaken for a short time. “Even the most mild unpredictable stressors, something as simple as gently shaking the cage, playing rock music, putting a new object in the cage that they aren’t used to, all these cause very specific changes in the brain when we do them without warning.” The bottom line, McCarthy says, is that the brain can “tolerate severely stressful events if they are predictable, but you cannot tolerate even mild stressful events if they are very unpredictable.”
No charge. McCarthy is Margaret McCarthy (PhD), professor of neuroscience at University of Maryland School of Medicine.