In addition to the classic "fight or flight" stress response, there is a separate system for chronic stress, centered around a group of nerve cells in the paraventricular nucleus (PVN), a part of the brain inside the hypothalamus, which uses corticotropin-releasing factor (CRF) as a neurotransmitter.

"When they removed the adrenal glands of genetically engineered mice, and thus prevented the production of cortisol, the receptors did not appear on the PVN nerve cell walls, while injecting synthetic stress hormones caused them to appear and restart the chain reaction."

"They compared mice genetically engineered to lack the receptor with a control group and exposed them to different kinds of stress, testing the hormones in their blood afterward. When the mice experienced acute stress, both groups reacted in a similar manner, and their hormone levels were also similar. But chronic stressors told a different story: The genetically engineered mice stayed calmer and had lower levels of the cortisol-like hormone."
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