Although serotonin is well known as a brain neurotransmitter, it is estimated that 90 percent of the bodys serotonin is made in the digestive tract1. In fact, altered levels of this peripheral2 serotonin have been linked to diseases such as irritable3 bowel4 syndrome5, cardiovascular disease, and osteoporosis. New research at Caltech, published in the April 9 issue of the journal Cell, shows that certain bacteria in the gut6 are important for the production of peripheral serotonin. More and more studies are showing that mice or other model organisms with changes in their gut microbes exhibit altered behaviors, explains Elaine Hsiao, research assistant professor of biology and biological engineering and senior author of the study. We are interested in how microbes communicate with the nervous system. To start, we explored the idea that normal gut microbes could influence levels of neurotransmitters in their hosts.Peripheral serotonin is produced in the digestive tract by enterochromaffin cells and also by particular types of immune cells and neurons. Hsiao and her colleagues first wanted to know if gut microbes have any effect on serotonin production in the gut and, if so, in which types of cells. They began by measuring peripheral serotonin levels in mice with normal populations of gut bacteria and also in germ-free mice that lack these resident microbes.The researchers found that the EC cells from germ-free mice produced approximately 60 percent less serotonin than did their peers with conventional bacterial7 colonies. When these germ-free mice were recolonized with normal gut microbes, the serotonin levels went back up--showing that the deficit8 in serotonin can be reversed.EC cells are rich sources of serotonin in the gut. What we saw in this experiment is that they appear to depend on microbes to make serotonin--or at least a large portion of it, says Jessica Yano, first author on the paper and a research technician working with Hsiao.The researchers next wanted to find out whether specific species of bacteria, out of the diverse pool of microbes that inhabit the gut, are interacting with EC cells to make serotonin.After testing several different single species and groups of known gut microbes, Yano, Hsiao, and colleagues observed that one condition--the presence of a group of approximately 20 species of spore-forming bacteria--elevated serotonin levels in germ-free mice. The mice treated with this group also showed an increase in gastrointestinal motility compared to their germ-free counterparts, and changes in the activation10 of blood platelets, which are known to use serotonin to promote clotting11.Wanting to home in on mechanisms12 that could be involved in this interesting collaboration13 between microbe and host, the researchers began looking for molecules14 that might be key. They identified several particular metabolites--products of the microbes metabolism--that were regulated by spore-forming bacteria and that elevated serotonin from EC cells in culture. Furthermore, increasing these metabolites in germ-free mice increased their serotonin levels.Previous work in the field indicated that some bacteria can make serotonin all by themselves. However, this new study suggests that much of the bodys serotonin relies on particular bacteria that interact with the host to produce serotonin, says Yano. Our work demonstrates that microbes normally present in the gut stimulate15 host intestinal9 cells to produce serotonin, she explains.
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1 tract n.传单，小册子，大片 参考例句： He owns a large tract of forest.他拥有一大片森林。 He wrote a tract on this subject.他曾对此写了一篇短文。
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