- Most people think of stress as a subjective feeling of pressure or tension.
- Scientists think of stress response as a cascade of events our bodies trigger in response to a "stressor" or threat, real or perceived.
- Our bodies' stress response is an attempt to get back into equilibrium or homeostasis.
- How effectively our bodies react to stress can determine our well-being.
- Long-term alcohol intake can "blunt" the body's ability to respond to stress.
"Homeostasis is a state of equilibrium that maintains our bodily parameters within certain limits," said Catherine Rivier, lead author and Professor at The Salk Institute. Sustaining homeostasis - the body's relatively steady internal status - is essential for survival. "Anything that throws these parameters off is dangerous and needs to be dealt with," she explained. "An infection that gives you a fever, for example, is a threat to this homeostasis. Our systems will recognize this challenge and do whatever is necessary to bring the body back to where it used to be."
Homeostasis is a delicate balance of biochemical and physiological functions that are constantly challenged by a variety of stressors. These can range from illness and injury to depression and fear, and even include exposure to extreme temperatures or too much exercise. Once a stress is recognized, the body will mobilize a 'counter-offensive' composed of an extensive array of both physiological and behavioral changes, all for the purpose of returning to homeostasis, all occurring very quickly.
"Let's say that I'm walking outside and I hear a gunshot," said Dipak Kumar Sarkar, professor and chair of the Department of Animal Sciences at Rutger's University. "My body will immediately react to that. The impulse will go immediately into the brain, and the hypothalamus will initiate what some people call the 'fight or flight reaction.'"
In response to the sound of gunshot, for example, a part of the brain called the hypothalamus initiates the stress response by secreting hormones called corticotropin-releasing factor (CRF) and, to a lesser degree, vasopressin (VP). CRF is what Rivier calls "the central stress hormone;" it coordinates the stress response by triggering an integrated series of physiological and behavioral reactions. CRF and VP are transported from the brain to the pituitary gland (located at the base of the brain), where they cause the release of adrenocorticotropin (ACTH). ACTH then enters the body's circulation, travelling down to the adrenal glands (located at the top of the kidneys), causing them to produce corticosteroids. The corticosteroids then enter the blood circulation, where they 'guide' the body's redirection of nutrients - including glucose - to those parts of the body that are under stress.
"The purpose of all of the above," said Sarkar, "is to find a way to maintain body homeostasis, which allows for normal body function. If this stress continues, however, it can cause a lot of problems." Under conditions of chronic stress, for example, the body may fail to compensate, which can cause suppression of growth, immune system dysfunction, or brain cell damage resulting in impaired learning and memory. Sarkar explained that "stress can bring sickness by altering the body's immune function, like when students get sick during an exam or when people have a death in the family." As Rivier's study has discovered, long-term alcohol can also blunt the stress axis' ability to respond to additional stressors.
The study used rats, after they were exposed to alcohol vapors in such a way that mimicked long-term alcohol intake by humans, to test for response to stressors. In the absence of alcohol exposure, ACTH and corticosteroid levels remained at normal levels. In those rats exposed to alcohol, however, hormonal and neuronal responses were significantly blunted. The results suggest that long-term alcohol exposure can decrease the stress-prompted release of CRF and VP in the brain, which can then have a domino-like effect on the release of ACTH and corticosteroids, ultimately blunting the body's ability to respond to stressors.
"People who abuse alcohol have all kinds of health-related problems," said Rivier, "What we showed here is that, in an animal model, the body loses part of its ability to respond to additional stressors. Alcoholics may therefore become unable to activate their stress axis appropriately when they are faced with a challenge, and that can be damaging. In this study, we tried to understand why this axis becomes blunted, why its activity becomes diminished."
"Eventually, if we understand the mechanisms," she said, "then we can develop therapies. You can't develop therapies if you don't know what the problem is."
Co-authors of the Alcoholism: Clinical & Experimental Research paper included: Soon Lee of the Clayton Foundation Laboratories for Peptide Biology, The Salk Institute; Donné Schmidt and Fred Tilders of Graduate School Neurosciences Amsterdam, Research Institute Neurosciences Vrije Universiteit, Faculty of Medicine, Department of Pharmacology in The Netherlands; and Maury Cole and Amanda Smith of La Jolla Alcohol Research in California. The study was funded in part by the National Institutes of Health, the Foundation for Research, and the Dutch Foundation for Scientific Research.