The stress of life

[Note: This post is intended as reading material for my upcoming online course, "Canine Hormones: From molecules to behavior." This is an entirely online course offered through APDT, begins Februrary 11, and is worth 12 CEUs. I posted with more information. I encourage you to sign up!]

Stress isn't good or bad. Stress is life. Stress is some change in your environment that means your body has to work a little harder. Stress is a blast of cold, missing a meal, not getting enough sleep. But stress is also going for a run, seeing a loved one after a long absence, thinking through a hard problem and getting it right. All of these things might mean your body has to rev up: your heart might beat faster, you might spend less energy on digestion, your immune system might adapt to meet expected coming challenges. An extra challenge can be good or bad. If life were one long nap, it wouldn't be much of anything.

Your body uses the hormone often referred to as the stress hormone, cortisol, to manage its response to stress. Going to an agility trial today? Need a little more cortisol to deal with all the extra energy you're going to spend. Cortisol affects our bodies profoundly, regulating our immune systems, our metabolism, and our behavior. Without enough of it, we would die, and indeed there is a disease (called Addison's in humans and hypoadrenocorticism in dogs, but it is the same thing) which is simply a lack of sufficient cortisol. Without treatment, it is often fatal.

But we think of stress as a bad thing, and indeed when it goes on too long, it is. Our bodies have developed to expect brief, passing stressors. A predator's attack. A few days of icy weather. Then safety and warm sun. When stress goes on and on, our bodies try to adapt, but high cortisol levels over weeks or months have side effects. Our immune systems become suppressed -- look at any college campus during final exams and you'll see rampant sneezing and coughing as students' high cortisol levels leave them unable to fight off infections. Our metabolism changes — we store fat for famines that never come, and eventually succumb to diabetes. And our brains suffer: ongoing high levels of cortisol can actually cause certain parts of our brains (associated with learning and memory) to become smaller, and other parts (associated with fear) to become larger. Ongoing stress leads to depression.

The stress system is among the more complex of the various hormonal systems. It is called the hypothalamic-pituitary-adrenal (HPA) axis because of the three main organs which secrete its hormones: the hypothalamus (part of the brain), the pituitary (maybe part of the brain and maybe a little stalk hanging off of it, depending on who you ask), and the adrenals (tiny glands down by the kidneys). But there are all kinds of other parts to this system: other parts of the brain which feed in to it from the top level (the hippocampus, site of learning and memory and many other things; the amygdala, site of fear and many other things); corticosteroid binding globulin, the little carrier protein which carries cortisol around in the blood and is made by the liver. The reproductive hormones, estrogen and testosterone, also affect the HPA axis. So does serotonin, the chemical targetted by so many anti-depressants.

All of these different parts of the system work together to regulate the amount of cortisol in the bloodstream: up when there is stress (good or bad), down when there is not. Our bodies are a chemical soup of hormones and these hormones are both cause and effect: cortisol rises when we experience stress. But high levels of cortisol also seem to cause us to feel distress. The stress system is enormously complicated, as mysterious in some ways as the brain itself, and yet a huge part of what makes each of us who we are. Different cortisol profiles (usually high, usually low, very reactive, very unreactive) are associated with different personality types in animals: bold, shy, proactive, reactive. The human research has some more complex findings but the basic truth remains that our personalities are, in part, chemical. We are our hormones.

[The title of this post, The Stress of Life, is also the title of a book by Hans Selye, who first isolated cortisol.]