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What is depression?
There are probably many different kinds of depression, so that the disease is slightly different in many (or all) people and dogs. As a result, getting a handle on the mechanisms of depression and its treatments is difficult. So studies about depression and antidepressants have to speak in generalities, such as “this is true for 50% of people with depression.”
In general, then, depression is triggered by chronic stress, which results in increased levels of stress hormones. The major stress hormone in humans and dogs is cortisol, so that’s the term I’ll use in this post. The major stress hormone in mice and rats is the closely-related corticosterone, so if you delve into more of the research in this area, you may find that hormone mentioned as well. It’s basically the same as cortisol. Note that while I’ll talk about depression in this post, in dogs we more commonly perceive stress-related problems as behavior problems, such as shyness or aggression. These problems, in certain cases, can be very successfully treated with antidepressants in combination with training.
Depression and the hippocampus
The area of the brain which is the most sensitive to increased levels of cortisol is the hippocampus, a part of the limbic system which is involved in learning, memory, and emotion. The cells of the hippocampus are armed with little widgets called glucocorticoid receptors, or GR, which grab cortisol molecules as they float by. Once a GR has attached itself to some cortisol, it becomes active, and takes itself over to the cell’s DNA. Here it tells the cell to activate some genes and deactivate other genes. This is how cortisol effects stress-related changes in our body: by telling the massive recipe-book inside our cells which genes to cook up and which ones to leave idle.
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So the first generality about depression is that it results in more cortisol than normal. The second generality is that it also results in a smaller hippocampus than normal. We can guess, though we’re not sure, that this is somehow related to all that GR activation. We know that in depression, fewer new neurons are born in the hippocampus, so that is probably part of the answer. However, it’s not the whole answer, because even in healthy people, new neurons are created at a very low rate, not fast enough to explain this decrease in size of the hippocampus. Another possibility is that the shape of individual neurons changes. Neurons branch out like trees to touch lots of other neurons, and the neurons of depressed people have fewer branches. So the problem could be caused by fewer neurons, or by neurons that have fewer branches and therefore less communication with other neurons. We’re not sure which, but either way, the changes are significant.
Antidepressants and the GR
Antidepressants affect many substances in the brain (most famously, the class of antidepressants of which Prozac is a member affect serotonin levels). We have trouble picking out cause and effect here. We assume that antidepressants aren’t affecting all of these substances directly; we assume that some of these affects are indirect, in other words, side effects. We’d like to know which substances antidepressants directly affect in the brain, in other words, what their mechanisms are, but we’re still not sure.
We do know that they affect the GR, though we don’t know if they do so directly or indirectly. So far, we haven’t found any direct effects on the GR. One theory is that antidepressants affect another widget, one which pumps cortisol out of cells so that the GR can’t grab it and become active. The decrease in number of active GR, then, causes the changes in the brain which result in mood improvement.
We do know that depressed people on antidepressants start to have increased birth of new neurons in their hippocampus, which itself increases in size. Why does this affect mood? We don’t know, but we can hypothesize that improved ability to make new mental connections and learn is at the heart of the change. This helps us understand why antidepressants typically take several weeks to work: our brains are changing, growing, and that takes time.
Antidepressants and dogs
As usual, all the research on how this stuff works was done in rats, mice, and humans. But we do think that the mechanisms are similar in dogs, and indeed in most or all mammals. Many dogs are on antidepressants with positive effects, including my shy dog Jenny, who receives both buspirone and lots of counter-conditioning. As research continues to get us more answers about how these medications actually work in the brain, we will do better and better at understanding which kinds of antidepressants are better for which individuals, when to start them, and when to stop them.
Anacker C., Livia A. Carvalho & Carmine M. Pariante (2011). The glucocorticoid receptor: Pivot of depression and of antidepressant treatment?, Psychoneuroendocrinology, 36 (3) 415-425. DOI: http://dx.doi.org/10.1016/j.psyneuen.2010.03.007
For further reading, check out related posts by Scicurious: