Stress hormones lower threshold for aggression and aggression raises stress hormones

Oct 4 2004

Scientists may be learning why it’s so hard to stop the cycle of violence. The answer may lie in the nervous system.

There appears to be a fast, mutual, positive feedback loop between stress hormones and a brain-based aggression-control center in rats, whose neurophysiology is similar to ours. It may explain why, under stress, humans are so quick to lash out and find it hard to cool down. The findings, which could point to better ways to prevent pathological violence, appear in the October issue of Behavioral Neuroscience, which is published by the American Psychological Association (APA).

In five experiments using 53 male rats, behavioral neuroscientists from the Netherlands and Hungary studied whether stimulating the brain’s aggression mechanism raised blood levels of a stress hormone and whether higher levels of the same hormone led to the kind of aggression elicited by that mechanism. The results showed a fast-acting feedback loop; the mechanism works in both directions and raising one variable raises the other. Thus, stress and aggression may be mutually reinforcing, which could explain not only why something like the stress of traffic jams leads to road rage, but also why raging triggers an ongoing stress reaction that makes it hard to stop.

In the study, the scientists electrically stimulated an aggression-related part of the rat hypothalamus, a mid-brain area associated with emotion. The rats suddenly released the stress hormone corticosterone (very like cortisol, which humans release under stress) -- even without another rat present. Normally, rats don’t respond like that unless they face an opponent or another severe stressor.

Says lead author Menno Kruk, PhD, “It is well known that these stress hormones, in part by mobilizing energy reserves, prepare the physiology of the body to fight or flee during stress. Now it appears that the very same hormones ‘talk back’ to the brain in order to facilitate fighting.”

To study the hypothesized feedback loop from the other direction, the scientists removed the rats’ adrenal glands to prevent any natural release of corticosterone. Then researchers injected the rats with corticosterone. Within minutes of injection, the hormone facilitated stimulation-evoked attack behavior.

Thus, in rapid order, stimulating the hypothalamic attack area led to higher stress hormones and higher stress hormones led to aggression – evidence of the feedback loop within a single conflict. Write the authors, “Such a mutual facilitation may contribute to the precipitation and escalation of violent behavior under stressful conditions.”

They add that the resulting vicious cycle “would explain why aggressive behavior escalates so easily and is so difficult to stop once it has started, especially because corticosteroids rapidly pass through the blood-brain barrier.” The findings suggest that even when stress hormones spike for reasons not related to fighting, they may lower attack thresholds enough to precipitate violent behavior. That argument, if extended in research to humans, could ultimately explain on the biological level why a bad day at the office could prime someone for nighttime violence toward family members.

It is speculated that the findings may help also to explain why people who are not typically violent become violent in settings previously associated with aggression: Their stress hormones rise, facilitating the onset of aggression and making them more likely to become violent in seemingly benign settings.

The adrenocortical (hormonal) and hypothalamic (aggressive) responses are ancient, inbred and found across many mammalian species including rats, cats and monkeys. As a result, scientists cannot help but wonder whether the stress-aggression feedback loop could well operate in the real world, not just the lab, and – most importantly -- in humans. Further study is clearly required.

The authors comment that their findings, if extended, could explain why “the adrenocortical stress response that accompanies conflict may effectively cancel out the effect of therapies intended to reduce violent behavior. Regulation of the stress response may offer a novel approach to the understanding and control of violent behavior.” They speculate that medications, perhaps as-yet undeveloped anxiety-reducers that regulate the stress response, might conceivably help to lower acute stress-precipitated violence.