An excess of one type of serotonin receptor in the center of the brain may explain why antidepressants fail to relieve symptoms of depression for 50 percent of patients, a new study from researchers at Columbia University Medical Center shows.
...Most antidepressants -- including the popular SSRIs -- work by increasing the amount of serotonin made by cells -- called raphe neurons -- deep in the middle of the brain. Serotonin relieves symptoms of depression when it is shipped to other brain regions.
But too many serotonin receptors of the 1A type on the raphe neurons sets up a negative feedback loop that reduces the production of serotonin, Dr. Hen and his colleagues discovered. "The more antidepressants try to increase serotonin production, the less serotonin the neurons actually produce, and behavior in mice does not change," Dr. Hen says.
Seeing as anti-depressant therapy hasn't shown much promise in the treatment of anorexia nervosa (although it does appear to help treat co-morbid conditions like depression and anxiety), this research could help with the development of new treatments for AN. It also seemed like a good a time as any to discuss the links between serotonin levels and eating disorders. In a 2005 review article, titled "Serotonin alterations in anorexia and bulimia nervosa," Walter Kaye wrote that people with either anorexia and/or bulimia showed alterations of brain functioning in specific neural areas:
Importantly, such disturbances are present when subjects are ill and persist after recovery, suggesting that these may be traits that are independent of the state of the illness. Emerging data point to a dysregulation of serotonin pathways in cortical and limbic structures that may be related to anxiety, behavioral inhibition, and body image distortions...Alterations of these circuits may affect mood and impulse control as well as the motivating and hedonic aspects of feeding behavior. Such imaging studies may offer insights into new pharmacology and psychotherapy approaches.
The serotonin/anorexia connection has been researched over the years (searching PubMed for "serotonin anorexia" gives you over 700 results), and the most recent thinking goes something like this. People with anorexia are generally thought to have unusually high levels of serotonin in their brains, and high levels of brain serotonin have been linked to anxiety and obsessionality. An old BBC article titled "Genetic clues to eating disorders" has a quote from Janet Treasure that explains some of the link:
People with high levels of serotonin are prone to anxiety. Dr Janet Treasure, director of the eating disorders unit at the Maudsley, believes this could be behind anorexic patients' ability to suppress appetite. She said: "In anorexia nervosa the drive to eat can be inhibited, but we know that in normal people who are starved they will kill each other and do all sorts of morally repugnant things, and eat all sorts of foodstuffs that you wouldn't normally touch.
"Yet that doesn't happen in anorexia nervosa, so there's some aspect of the appetite system that isn't working."
The unit looked at the biology of stress mechanisms, in particular the fight or flight response. This is where the body prepares itself for action when confronted by a stressful situation. Heart rate and blood pressure rise and two of what are usually humans' highest priorities, eating and reproducing, are put on hold. It is possible that anorexic people are chronically in an acute state of stress reaction - they are constantly in a fight or flight state of mind.
And by restricting food intake, people with anorexia can lower the amount of serotonin their bodies can make (serotonin is ultimately derived from the essential amino acid tryptophan). This actually makes people with anorexia feel better. However, the brain begins to sense the decreased serotonin production and tries to maintain homeostasis by increasing the number of serotonin receptors. Thus the brain is back at Square One, as it is producing less serotonin but is using the decreased amount much more efficiently. So restricting doesn't feel as good, and the (obvious!) solution is to eat even less. And thus that negative cycle is born and the anorexic becomes trapped by their own brain chemistry.
Refeeding would then increase the amount of serotonin in the brain before the brain has a chance to decrease the number of serotonin receptors. This could be the neurological equivalent of All Hell Breaking Loose and could very well explain why refeeding is so distressing, although I don't think there has been any formal research done on the subject.
In bulimia, the serotonin problem is reversed. People with BN appear to have much lower than average levels of serotonin in the brain, which may be temporarily increased by binge eating.* Purging increases levels of vasopressin, which can have a euphoric and sedating effect, thus making the binge/purge cycle addictive much in the same way that starvation becomes addictive in AN. The chronic low levels of serotonin in BN also explain why SSRIs can be effective at reducing the urges to binge and purge.
Of course, plenty of people cross over from anorexia to bulimia, and I haven't the slightest idea of how serotonin might affect that crossover. So many brain systems are thrown out of whack during an ED that I don't know an exact answer will ever be found.
*The story is, as usual, a little more complicated than this, but the basic idea is the same.