Which organ in the body has 100 million neurons and the greatest amount of the nerve transmitter serotonin?

Surprise: It is not the brain, but the gut!

This interesting tidbit about the body organ that deals with the messier things of life comes from a researcher who specializes in it. He is Michael Gershon, Ph.D., chair of anatomy and cell biology at Columbia University College of Physicians and Surgeons, who presented a lecture at APA’s 2001 annual meeting in New Orleans in May.

Aside from dazzling his audience with the gut’s army of neurons and lavish quantities of serotonin, Gershon also touted some of its other virtues. To wit: Although the gut receives inputs from nerves without, it is largely self-sustained by its own nerve network. So, if the vagus nerve connecting the brain and gut is cut, the gut can still function. Or if a person blows into a gut that has been removed from the human body, the gut will blow back at him!

What’s more, Gershon pointed out, the gut can have a potent impact on the brain. For example, nearly all the nerve fibers in the vagus nerve ferry messages from the gut to the brain—say, sensations of pain or bloating. True, "not all that runs up the vagus nerve reaches consciousness," Gershon said, but the impulses are sent nonetheless.

And indeed, Gershon declared, the gut contains every class of nerve-transmitting chemical that has been found in the brain. Take, for instance, serotonin. Epithelial cells that line the gut secrete serotonin. Serotonin then excites sensory nerves in the gut and in turn triggers gut reflexes that signal the brain about pain, nausea, and so forth. Such a copious amount of serotonin is released into the gut every day, in fact, that it would be lethal if the excess were not disposed of by certain cells in the gut lining that contain serotonin transporters.

Gershon also elaborated on the role that the gut plays in some digestive disorders that have traditionally been thought to be largely or even exclusively provoked by the psyche.

For instance, he suggested that the gut might be able to take at least some of the credit for anorexia nervosa. The reason, he said, is that if serotonin is injected into a rat while it is eating, it will stop eating immediately, suggesting a possible link between gut serotonin and anorexia.

"We now know that ulcerative colitis is not psychologically based," he declared. "Anybody chained by bloody diarrhea to a toilet seat is going to be psychologically impaired. Yes, there is an ulcerative colitis personality, but it comes from physical impairment."

He even made the case that the gut is totally responsible for irritable bowel syndrome. One reason, he said, is that the syndrome can occur in infants, as well as in children and adults, suggesting a physiological rather than psychological cause. Another is that colicky infants often grow up to have irritable bowel syndrome, once again implying a physiological cause. A third reason is that he and his colleagues have created a breed of mice that have irritable bowel syndrome. The mice lack the gene that codes for serotonin transporters in the gut.

Nonetheless, Gershon did note that these mice also lack serotonin transporters in their brains, suggesting that irritable bowel syndrome might be due to a paucity of such transporters in the brain as well as in the bowel. If such were the case, it might plunk some of the blame for the syndrome back into the psyche, or at least into the brain.

And, yes, for all its formidable talents, the gut is vulnerable in still other ways to the psyche and things psychiatric, Gershon confessed.

For instance, the brain can influence the gut via the vagus nerve just as the gut can influence the brain via this nerve.

Doses of serotonin reuptake inhibitors used to treat depression and other psychiatric disorders are well known to be capable of triggering nausea, diarrhea, and constipation in patients. They do so because they increase the activity of serotonin in the gut and thus put the gut into high gear.

Also intriguingly, whereas standard SSRI doses used to treat depression sometimes trigger nausea, diarrhea, or constipation, lower SSRI doses—less than 20 mg—can be effectively used to treat the nausea, diarrhea, and constipation caused by the irritable bowel syndrome. ▪